HPAVC/HPAVC CD-ROM.iso/pc/FAQSYS18.ZIP/FAQS.DAT/386INTEL.TXT

Text File | 1996-01-04 | 857.7 KB | 20,855 lines

INTEL 80386 PROGRAMMER'S REFERENCE MANUAL 1986

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Table of Contents

Chapter 1  Introduction to the 80386

1.1  Organization of This Manual
      1.1.1  Part I ── Applications Programming
      1.1.2  Part II ── Systems Programming
      1.1.3  Part III ── Compatibility
      1.1.4  Part IV ── Instruction Set
      1.1.5  Appendices

1.2  Related Literature
1.3  Notational Conventions
      1.3.1  Data-Structure Formats
      1.3.2  Undefined Bits and Software Compatibility
      1.3.3  Instruction Operands
      1.3.4  Hexadecimal Numbers
      1.3.5  Sub- and Super-Scripts

                     PART I    APPLICATIONS PROGRAMMING

Chapter 2  Basic Programming Model

2.1  Memory Organization and Segmentation
      2.1.1  The"Flat" Model
      2.1.2  The Segmented Model

2.2  Data Types
2.3  Registers
      2.3.1  General Registers
      2.3.2  Segment Registers
      2.3.3  Stack Implementation
      2.3.4  Flags Register
              2.3.4.1  Status Flags
              2.3.4.2  Control Flag
              2.3.4.3  Instruction Pointer

2.4  Instruction Format
2.5  Operand Selection
      2.5.1  Immediate Operands
      2.5.2  Register Operands
      2.5.3  Memory Operands
              2.5.3.1  Segment Selection
              2.5.3.2  Effective-Address Computation

2.6  Interrupts and Exceptions

Chapter 3  Applications Instruction Set

3.1  Data Movement Instructions
      3.1.1  General-Purpose Data Movement Instructions
      3.1.2  Stack Manipulation Instructions
      3.1.3  Type Conversion Instructions

3.2  Binary Arithmetic Instructions
      3.2.1  Addition and Subtraction Instructions
      3.2.2  Comparison and Sign Change Instruction
      3.2.3  Multiplication Instructions
      3.2.4  Division Instructions

3.3  Decimal Arithmetic Instructions
      3.3.1  Packed BCD Adjustment Instructions
      3.3.2  Unpacked BCD Adjustment Instructions

3.4  Logical Instructions
      3.4.1  Boolean Operation Instructions
      3.4.2  Bit Test and Modify Instructions
      3.4.3  Bit Scan Instructions
      3.4.4  Shift and Rotate Instructions
              3.4.4.1  Shift Instructions
              3.4.4.2  Double-Shift Instructions
              3.4.4.3  Rotate Instructions
              3.4.4.4  Fast"bit-blt" Using Double Shift
                         Instructions
              3.4.4.5  Fast Bit-String Insert and Extract

      3.4.5  Byte-Set-On-Condition Instructions
      3.4.6  Test Instruction

3.5  Control Transfer Instructions
      3.5.1  Unconditional Transfer Instructions
              3.5.1.1  Jump Instruction
              3.5.1.2  Call Instruction
              3.5.1.3  Return and Return-From-Interrupt Instruction

      3.5.2  Conditional Transfer Instructions
              3.5.2.1  Conditional Jump Instructions
              3.5.2.2  Loop Instructions
              3.5.2.3  Executing a Loop or Repeat Zero Times

      3.5.3  Software-Generated Interrupts

3.6  String and Character Translation Instructions
      3.6.1  Repeat Prefixes
      3.6.2  Indexing and Direction Flag Control
      3.6.3  String Instructions

3.7  Instructions for Block-Structured Languages
3.8  Flag Control Instructions
      3.8.1  Carry and Direction Flag Control Instructions
      3.8.2  Flag Transfer Instructions

3.9  Coprocessor Interface Instructions
3.10 Segment Register Instructions
      3.10.1  Segment-Register Transfer Instructions
      3.10.2  Far Control Transfer Instructions
      3.10.3  Data Pointer Instructions

3.11  Miscellaneous Instructions
       3.11.1  Address Calculation Instruction
       3.11.2  No-Operation Instruction
       3.11.3  Translate Instruction

                       PART II    SYSTEMS PROGRAMMING

Chapter 4  Systems Architecture

4.1  Systems Registers
      4.1.1  Systems Flags
      4.1.2  Memory-Management Registers
      4.1.3  Control Registers
      4.1.4  Debug Register
      4.1.5  Test Registers

4.2  Systems Instructions

Chapter 5  Memory Management

5.1  Segment Translation
      5.1.1  Descriptors
      5.1.2  Descriptor Tables
      5.1.3  Selectors
      5.1.4  Segment Registers

5.2  Page Translation
      5.2.1  Page Frame
      5.2.2  Linear Address
      5.2.3  Page Tables
      5.2.4  Page-Table Entries
              5.2.4.1  Page Frame Address
              5.2.4.2  Present Bit
              5.2.4.3  Accessed and Dirty Bits
              5.2.4.4  Read/Write and User/Supervisor Bits

      5.2.5  Page Translation Cache

5.3  Combining Segment and Page Translation
      5.3.1  "Flat" Architecture
      5.3.2  Segments Spanning Several Pages
      5.3.3  Pages Spanning Several Segments
      5.3.4  Non-Aligned Page and Segment Boundaries
      5.3.5  Aligned Page and Segment Boundaries
      5.3.6  Page-Table per Segment

Chapter 6  Protection

6.1  Why Protection?
6.2  Overview of 80386 Protection Mechanisms
6.3  Segment-Level Protection
      6.3.1  Descriptors Store Protection Parameters
              6.3.1.1  Type Checking
              6.3.1.2  Limit Checking
              6.3.1.3  Privilege Levels

      6.3.2  Restricting Access to Data
              6.3.2.1  Accessing Data in Code Segments

      6.3.3  Restricting Control Transfers
      6.3.4  Gate Descriptors Guard Procedure Entry Points
              6.3.4.1  Stack Switching
              6.3.4.2  Returning from a Procedure

      6.3.5  Some Instructions are Reserved for Operating System
              6.3.5.1  Privileged Instructions
              6.3.5.2  Sensitive Instructions

      6.3.6  Instructions for Pointer Validation
              6.3.6.1  Descriptor Validation
              6.3.6.2  Pointer Integrity and RPL

6.4  Page-Level Protection
      6.4.1  Page-Table Entries Hold Protection Parameters
              6.4.1.1  Restricting Addressable Domain
              6.4.1.2  Type Checking

      6.4.2  Combining Protection of Both Levels of Page Tables
      6.4.3  Overrides to Page Protection

6.5  Combining Page and Segment Protection

Chapter 7  Multitasking

7.1  Task State Segment
7.2  TSS Descriptor
7.3  Task Register
7.4  Task Gate Descriptor
7.5  Task Switching
7.6  Task Linking
      7.6.1  Busy Bit Prevents Loops
      7.6.2  Modifying Task Linkages

7.7  Task Address Space
      7.7.1  Task Linear-to-Physical Space Mapping
      7.7.2  Task Logical Address Space

Chapter 8    Input/Output

8.1  I/O Addressing
      8.1.1  I/O Address Space
      8.1.2  Memory-Mapped I/O

8.2  I/O Instructions
      8.2.1  Register I/O Instructions
      8.2.2  Block I/O Instructions

8.3  Protection and I/O
      8.3.1  I/O Privilege Level
      8.3.2  I/O Permission Bit Map

Chapter 9  Exceptions and Interrupts

9.1  Identifying Interrupts
9.2  Enabling and Disabling Interrupts
      9.2.1  NMI Masks Further NMls
      9.2.2  IF Masks INTR
      9.2.3  RF Masks Debug Faults
      9.2.4  MOV or POP to SS Masks Some Interrupts and Exceptions

9.3  Priority Among Simultaneous Interrupts and Exceptions
9.4  Interrupt Descriptor Table
9.5  IDT Descriptors
9.6  Interrupt Tasks and Interrupt Procedures
      9.6.1  Interrupt Procedures
              9.6.1.1  Stack of Interrupt Procedure
              9.6.1.2  Returning from an Interrupt Procedure
              9.6.1.3  Flags Usage by Interrupt Procedure
              9.6.1.4  Protection in Interrupt Procedures

      9.6.2  Interrupt Tasks

9.7  Error Code
9.8  Exception Conditions
      9.8.1  Interrupt 0 ── Divide Error
      9.8.2  Interrupt 1 ── Debug Exceptions
      9.8.3  Interrupt 3 ── Breakpoint
      9.8.4  Interrupt 4 ── Overflow
      9.8.5  Interrupt 5 ── Bounds Check
      9.8.6  Interrupt 6 ── Invalid Opcode
      9.8.7  Interrupt 7 ── Coprocessor Not Available
      9.8.8  Interrupt 8 ── Double Fault
      9.8.9  Interrupt 9 ── Coprocessor Segment Overrun
      9.8.10 Interrupt 10 ── Invalid TSS
      9.8.11 Interrupt 11 ── Segment Not Present
      9.8.12 Interrupt 12 ── Stack Exception
      9.8.13 Interrupt 13 ── General Protection Exception
      9.8.14 Interrupt 14 ── Page Fault
              9.8.14.1  Page Fault during Task Switch
              9.8.14.2  Page Fault with Inconsistent Stack Pointer

      9.8.15 Interrupt 16 ── Coprocessor Error

9.9  Exception Summary

9.10  Error Code Summary

Chapter 10  Initialization

10.1  Processor State after Reset
10.2  Software Initialization for Real-Address Mode
       10.2.1  Stack
       10.2.2  Interrupt Table
       10.2.3  First Instructions

10.3  Switching to Protected Mode
10.4  Software Initialization for Protected Mode
       10.4.1  Interrupt Descriptor Table
       10.4.2  Stack
       10.4.3  Global Descriptor Table
       10.4.4  Page Tables
       10.4.5  First Task

10.5  Initialization Example
10.6  TLB Testing
       10.6.1  Structure of the TLB
       10.6.2  Test Registers
       10.6.3  Test Operations

Chapter 11  Coprocessing and Multiprocessing

11.1  Coprocessing
       11.1.1  Coprocessor Identification
       11.1.2  ESC and WAIT Instructions
       11.1.3  EM and MP Flags
       11.1.4  The Task-Switched Flag
       11.1.5  Coprocessor Exceptions
                11.1.5.1  Interrupt 7 ── Coprocessor Not Available
                11.1.5.2  Interrupt 9 ── Coprocessor Segment Overrun
                11.1.5.3  Interrupt 16 ── Coprocessor Error

11.2  General Multiprocessing
       11.2.1  LOCK and the LOCK# Signal
       11.2.2  Automatic Locking
       11.2.3  Cache Considerations

Chapter 12  Debugging

12.1  Debugging Features of the Architecture
12.2  Debug Registers
       12.2.1  Debug Address Registers (DRO-DR3)
       12.2.2  Debug Control Register (DR7)
       12.2.3  Debug Status Register (DR6)
       12.2.4  Breakpoint Field Recognition

12.3  Debug Exceptions
       12.3.1  Interrupt 1 ── Debug Exceptions
                12.3.1.1  Instruction Address Breakpoint
                12.3.1.2  Data Address Breakpoint
                12.3.1.3  General Detect Fault
                12.3.1.4  Single-Step Trap
                12.3.1.5  Task Switch Breakpoint

       12.3.2  Interrupt 3 ── Breakpoint Exception

                          PART III    COMPATIBILITY

Chapter 13  Executing 80286 Protected-Mode Code

13.1  80286 Code Executes as a Subset of the 80386
13.2  Two Ways to Execute 80286 Tasks
13.3  Differences from 80286
       13.3.1  Wraparound of 80286 24-Bit Physical Address Space
       13.3.2  Reserved Word of Descriptor
       13.3.3  New Descriptor Type Codes
       13.3.4  Restricted Semantics of LOCK
       13.3.5  Additional Exceptions

Chapter 14  80386 Real-Address Mode

14.1  Physical Address Formation
14.2  Registers and Instructions
14.3  Interrupt and Exception Handling
14.4  Entering and Leaving Real-Address Mode
       14.4.1  Switching to Protected Mode

14.5  Switching Back to Real-Address Mode
14.6  Real-Address Mode Exceptions
14.7  Differences from 8086
14.8  Differences from 80286 Real-Address Mode
       14.8.1  Bus Lock
       14.8.2  Location of First Instruction
       14.8.3  Initial Values of General Registers
       14.8.4  MSW Initialization

Chapter 15  Virtual 8088 Mode

15.1  Executing 8086 Code
       15.1.1  Registers and Instructions
       15.1.2  Linear Address Formation

15.2  Structure of a V86 Task
       15.2.1  Using Paging for V86 Tasks
       15.2.2  Protection within a V86 Task

15.3  Entering and Leaving V86 Mode
       15.3.1  Transitions Through Task Switches
       15.3.2  Transitions Through Trap Gates and Interrupt Gates

15.4  Additional Sensitive Instructions
       15.4.1  Emulating 8086 Operating System Calls
       15.4.2  Virtualizing the Interrupt-Enable Flag

15.5  Virtual I/O
       15.5.1  I/O-Mapped I/O
       15.5.2  Memory-Mapped I/O
       15.5.3  Special I/O Buffers

15.6  Differences from 8086
15.7  Differences from 80286 Real-Address Mode

Chapter 16  Mixing 16-Bit and 32-Bit Code

16.1  How the 80386 Implements 16-Bit and 32-Bit Features
16.2  Mixing 32-Bit and 16-Bit Operations
16.3  Sharing Data Segments among Mixed Code Segments
16.4  Transferring Control among Mixed Code Segments
       16.4.1  Size of Code-Segment Pointer
       16.4.2  Stack Management for Control Transfers
                16.4.2.1  Controlling the Operand-Size for a CALL
                16.4.2.2  Changing Size of Call

       16.4.3  Interrupt Control Transfers
       16.4.4  Parameter Translation
       16.4.5  The Interface Procedure

                         PART IV    INSTRUCTION SET

Chapter 17  80386 Instruction Set

17.1  Operand-Size and Address-Size Attributes
       17.1.1  Default Segment Attribute
       17.1.2  Operand-Size and Address-Size Instruction Prefixes
       17.1.3  Address-Size Attribute for Stack

17.2  Instruction Format
       17.2.1  ModR/M and SIB Bytes
       17.2.2  How to Read the Instruction Set Pages
                17.2.2.1  Opcode
                17.2.2.2  Instruction
                17.2.2.3  Clocks
                17.2.2.4  Description
                17.2.2.5  Operation
                17.2.2.6  Description
                17.2.2.7  Flags Affected
                17.2.2.8  Protected Mode Exceptions
                17.2.2.9  Real Address Mode Exceptions
                17.2.2.10 Virtual-8086 Mode Exceptions

Instruction Sets

AAA
AAD
AAM
AAS
ADC
ADD
AND
ARPL
BOUND
BSF
BSR
BT
BTC
BTR
BTS
CALL
CBW/CWDE
CLC
CLD
CLI
CLTS
CMC
CMP
CMPS/CMPSB/CMPSW/CMPSD
CWD/CDQ
DAA
DAS
DEC
DIV
ENTER
HLT
IDIV
IMUL
IN
INC
INS/INSB/INSW/INSD
INT/INTO
IRET/IRETD
Jcc
JMP
LAHF
LAR
LEA
LEAVE
LGDT/LIDT
LGS/LSS/LDS/LES/LFS
LLDT
LMSW
LOCK
LODS/LODSB/LODSW/LODSD
LOOP/LOOPcond
LSL
LTR
MOV
MOV
MOVS/MOVSB/MOVSW/MOVSD
MOVSX
MOVZX
MUL
NEG
NOP
NOT
OR
OUT
OUTS/OUTSB/OUTSW/OUTSD
POP
POPA/POPAD
POPF/POPFD
PUSH
PUSHA/PUSHAD
PUSHF/PUSHFD
RCL/RCR/ROL/ROR
REP/REPE/REPZ/REPNE/REPNZ
RET
SAHF
SAL/SAR/SHL/SHR
SBB
SCAS/SCASB/SCASW/SCASD
SETcc
SGDT/SIDT
SHLD
SHRD
SLDT
SMSW
STC
STD
STI
STOS/STOSB/STOSW/STOSD
STR
SUB
TEST
VERR,VERW
WAIT
XCHG
XLAT/XLATB
XOR

Appendix A  Opcode Map

Appendix B  Complete Flag Cross-Reference

Appendix C  Status Flag Summary

Appendix D  Condition Codes


Figures

1-1     Example Data Structure

2-1     Two-Component Pointer
2-2     Fundamental Data Types
2-3     Bytes, Words, and Doublewords in Memory
2-4     80386 Data Types
2-5     80386 Applications Register Set
2-6     Use of Memory Segmentation
2-7     80386 Stack
2-8     EFLAGS Register
2-9     Instruction Pointer Register
2-10    Effective Address Computation

3-1     PUSH
3-2     PUSHA
3-3     POP
3-4     POPA
3-5     Sign Extension
3-6     SAL and SHL
3-7     SHR
3-8     SAR
3-9     Using SAR to Simulate IDIV
3-10    Shift Left Double
3-11    Shift Right Double
3-12    ROL
3-13    ROR
3-14    RCL
3-15    RCR
3-16    Formal Definition of the ENTER Instruction
3-17    Variable Access in Nested Procedures
3-18    Stack Frame for MAIN at Level 1
3-19    Stack Frame for Prooedure A
3-20    Stack Frame for Procedure B at Level 3 Called from A
3-21    Stack Frame for Procedure C at Level 3 Called from B
3-22    LAHF and SAHF
3-23    Flag Format for PUSHF and POPF

4-1     Systems Flags of EFLAGS Register
4-2     Control Registers

5-1     Address Translation Overview
5-2     Segment Translation
5-3     General Segment-Descriptor Format
5-4     Format of Not-Present Descriptor
5-5     Descriptor Tables
5-6     Format of a Selector
5-7     Segment Registers
5-8     Format of a Linear Address
5-9     Page Translation
5-10    Format of a Page Table Entry
5-11    Invalid Page Table Entry
5-12    80386 Addressing Mechanism
5-13    Descriptor per Page Table

6-1     Protection Fields of Segment Descriptors
6-2     Levels of Privilege
6-3     Privilege Check for Data Access
6-4     Privilege Check for Control Transfer without Gate
6-5     Format of 80386 Call Gate
6-6     Indirect Transfer via Call Gate
6-7     Privilege Check via Call Gate
6-8     Initial Stack Pointers of TSS
6-9     Stack Contents after an Interievel Call
6-10    Protection Fields of Page Table Entries

7-1     80386 32-Bit Task State Segment
7-2     TSS Descriptor for 32-Bit TSS
7-3     Task Register
7-4     Task Gate Descriptor
7-5     Task Gate Indirectly Identifies Task
7-6     Partially-Overlapping Linear Spaces

8-1     Memory-Mapped I/O
8-2     I/O Address Bit Map

9-1     IDT Register and Table
9-2     Pseudo-Descriptor Format for LIDT and SIDT
9-3     80386 IDT Gate Descriptors
9-4     Interrupt Vectoring for Procedures
9-5     Stack Layout after Exception of Interrupt
9-6     Interrupt Vectoring for Tasks
9-7     Error Code Format
9-8     Page-Fault Error Code Format
9-9     CR2 Format

10-1    Contents of EDX after RESET
10-2    Initial Contents of CRO
10-3    TLB Structure
10-4    Test Registers

12-1    Debug Registers

14-1    Real-Address Mode Address Formation

15-1    V86 Mode Address Formation
15-2    Entering and Leaving an 8086 Program
15-3    PL 0 Stack after Interrupt in V86 Task

16-1    Stack after Far 16-Bit and 32-Bit Calls

17-1    80386 Instruction Format
17-2    ModR/M and SIB Byte Formats
17-3    Bit Offset for BIT[EAX, 21]
17-4    Memory Bit Indexing


Tables

2-1      Default Segment Register Selection Rules
2-2      80386 Reserved Exceptions and Interrupts

3-1      Bit Test and Modify Instructions
3-2      Interpretation of Conditional Transfers

6-1      System and Gate Descriptor Types
6-2      Useful Combinations of E, G, and B Bits
6-3      Interievel Return Checks
6-4      Valid Descriptor Types for LSL
6-5      Combining Directory and Page Protection

7-1      Checks Made during a Task Switch
7-2      Effect of Task Switch on BUSY, NT, and Back-Link

9-1      Interrupt and Exception ID Assignments
9-2      Priority Among Simultaneous Interrupts and Exceptions
9-3      Double-Fault Detection Classes
9-4      Double-Fault Definition
9-5      Conditions That Invalidate the TSS
9-6      Exception Summary
9-7      Error-Code Summary

10-1     Meaning of D, U, and W Bit Pairs

12-1     Breakpeint Field Recognition Examples
12-2     Debug Exception Conditions

14-1     80386 Real-Address Mode Exceptions
14-2     New 80386 Exceptions

17-1     Effective Size Attributes
17-2     16-Bit Addressing Forms with the ModR/M Byte
17-3     32-Bit Addressing Forms with the ModR/M Byte
17-4     32-Bit Addressing Forms with the SIB Byte
17-5     Task Switch Times for Exceptions
17-6     80386 Exceptions


Chapter 1  Introduction to the 80386

────────────────────────────────────────────────────────────────────────────

The 80386 is an advanced 32-bit microprocessor optimized for multitasking
operating systems and designed for applications needing very high
performance. The 32-bit registers and data paths support 32-bit addresses
and data types. The processor can address up to four gigabytes of physical
memory and 64 terabytes (2^(46) bytes) of virtual memory. The on-chip
memory-management facilities include address translation registers,
advanced multitasking hardware, a protection mechanism, and paged virtual
memory. Special debugging registers provide data and code breakpoints even
in ROM-based software.


1.1  Organization of This Manual

This book presents the architecture of the 80386 in five parts:

   Part I      ── Applications Programming
   Part II     ── Systems Programming
   Part III    ── Compatibility
   Part IV     ── Instruction Set
   Appendices

These divisions are determined in part by the architecture itself and in
part by the different ways the book will be used. As the following table
indicates, the latter two parts are intended as reference material for
programmers actually engaged in the process of developing software for the
80386. The first three parts are explanatory, showing the purpose of
architectural features, developing terminology and concepts, and describing
instructions as they relate to specific purposes or to specific
architectural features.

Explanation                Part I   ── Applications Programming
                           Part II  ── Systems Programming
                           Part III ── Compatibility

Reference                  Part IV  ── Instruction Set
                           Appendices

The first three parts follow the execution modes and protection features of
the 80386 CPU. The distinction between applications features and systems
features is determined by the protection mechanism of the 80386. One purpose
of protection is to prevent applications from interfering with the operating
system; therefore, the processor makes certain registers and instructions
inaccessible to applications programs. The features discussed in Part I are
those that are accessible to applications; the features in Part II are
available only to systems software that has been given special privileges or
in unprotected systems.

The processing mode of the 80386 also determines the features that are
accessible. The 80386 has three processing modes:

   1.  Protected Mode.
   2.  Real-Address Mode.
   3.  Virtual 8086 Mode.

Protected mode is the natural 32-bit environment of the 80386 processor. In
this mode all instructions and features are available.

Real-address mode (often called just "real mode") is the mode of the
processor immediately after RESET. In real mode the 80386 appears to
programmers as a fast 8086 with some new instructions. Most applications of
the 80386 will use real mode for initialization only.

Virtual 8086 mode (also called V86 mode) is a dynamic mode in the sense
that the processor can switch repeatedly and rapidly between V86 mode and
protected mode. The CPU enters V86 mode from protected mode to execute an
8086 program, then leaves V86 mode and enters protected mode to continue
executing a native 80386 program.

The features that are available to applications programs in protected mode
and to all programs in V86 mode are the same. These features form the
content of Part I. The additional features that are available to systems
software in protected mode form Part II. Part III explains real-address
mode and V86 mode, as well as how to execute a mix of 32-bit and 16-bit
programs.

Available in All Modes             Part I ── Applications Programming

Available in Protected             Part II ── Systems Programming
Mode Only

Compatibility Modes                Part III ── Compatibility


1.1.1  Part I ── Applications Programming

This part presents those aspects of the architecture that are customarily
used by applications programmers.

Chapter 2 ── Basic Programming Model: Introduces the models of memory
organization. Defines the data types. Presents the register set used by
applications. Introduces the stack. Explains string operations. Defines the
parts of an instruction. Explains addressing calculations. Introduces
interrupts and exceptions as they may apply to applications programming.

Chapter 3 ── Application Instruction Set: Surveys the instructions commonly
used for applications programming. Considers instructions in functionally
related groups; for example, string instructions are considered in one
section, while control-transfer instructions are considered in another.
Explains the concepts behind the instructions. Details of individual
instructions are deferred until Part IV, the instruction-set reference.


1.1.2  Part II ── Systems Programming

This part presents those aspects of the architecture that are customarily
used by programmers who write operating systems, device drivers, debuggers,
and other software that supports applications programs in the protected mode
of the 80386.

Chapter 4 ── Systems Architecture: Surveys the features of the 80386 that
are used by systems programmers. Introduces the remaining registers and data
structures of the 80386 that were not discussed in Part I. Introduces the
systems-oriented instructions in the context of the registers and data
structures they support. Points to the chapter where each register, data
structure, and instruction is considered in more detail.

Chapter 5 ── Memory Management: Presents details of the data structures,
registers, and instructions that support virtual memory and the concepts of
segmentation and paging. Explains how systems designers can choose a model
of memory organization ranging from completely linear ("flat") to fully
paged and segmented.

Chapter 6 ── Protection: Expands on the memory management features of the
80386 to include protection as it applies to both segments and pages.
Explains the implementation of privilege rules, stack switching, pointer
validation, user and supervisor modes. Protection aspects of multitasking
are deferred until the following chapter.

Chapter 7 ── Multitasking: Explains how the hardware of the 80386 supports
multitasking with context-switching operations and intertask protection.

Chapter 8 ── Input/Output: Reveals the I/O features of the 80386, including
I/O instructions, protection as it relates to I/O, and the I/O permission
map.

Chapter 9 ── Exceptions and Interrupts: Explains the basic interrupt
mechanisms of the 80386. Shows how interrupts and exceptions relate to
protection. Discusses all possible exceptions, listing causes and including
information needed to handle and recover from the exception.

Chapter 10 ── Initialization: Defines the condition of the processor after
RESET or power-up. Explains how to set up registers, flags, and data
structures for either real-address mode or protected mode. Contains an
example of an initialization program.

Chapter 11 ── Coprocessing and Multiprocessing: Explains the instructions
and flags that support a numerics coprocessor and multiple CPUs with shared
memory.

Chapter 12 ── Debugging: Tells how to use the debugging registers of the
80386.


1.1.3  Part III ── Compatibility

Other parts of the book treat the processor primarily as a 32-bit machine,
omitting for simplicity its facilities for 16-bit operations. Indeed, the
80386 is a 32-bit machine, but its design fully supports 16-bit operands and
addressing, too. This part completes the picture of the 80386 by explaining
the features of the architecture that support 16-bit programs and 16-bit
operations in 32-bit programs. All three processor modes are used to
execute 16-bit programs: protected mode can directly execute 16-bit 80286
protected mode programs, real mode executes 8086 programs and real-mode
80286 programs, and virtual 8086 mode executes 8086 programs in a
multitasking environment with other 80386 protected-mode programs. In
addition, 32-bit and 16-bit modules and individual 32-bit and 16-bit
operations can be mixed in protected mode.

Chapter 13 ── Executing 80286 Protected-Mode Code: In its protected mode,
the 80386 can execute complete 80286 protected-mode systems, because 80286
capabilities are a subset of 80386 capabilities.

Chapter 14 ── 80386 Real-Address Mode: Explains the real mode of the 80386
CPU. In this mode the 80386 appears as a fast real-mode 80286 or fast 8086
enhanced with additional instructions.

Chapter 15 ── Virtual 8086 Mode: The 80386 can switch rapidly between its
protected mode and V86 mode, giving it the ability to multiprogram 8086
programs along with "native mode" 32-bit programs.

Chapter 16 ── Mixing 16-Bit and 32-Bit Code: Even within a program or task,
the 80386 can mix 16-bit and 32-bit modules. Furthermore, any given module
can utilize both 16-bit and 32-bit operands and addresses.


1.1.4  Part IV ── Instruction Set

Parts I, II, and III present overviews of the instructions as they relate
to specific aspects of the architecture, but this part presents the
instructions in alphabetical order, providing the detail needed by
assembly-language programmers and programmers of debuggers, compilers,
operating systems, etc. Instruction descriptions include algorithmic
description of operation, effect of flag settings, effect on flag settings,
effect of operand- or address-size attributes, effect of processor modes,
and possible exceptions.


1.1.5  Appendices

The appendices present tables of encodings and other details in a format
designed for quick reference by assembly-language and systems programmers.


1.2  Related Literature

The following books contain additional material concerning the 80386
microprocessor:

  ■  Introduction to the 80386, order number 231252

  ■  80386 Hardware Reference Manual, order number 231732

  ■  80386 System Software Writer's Guide, order number 231499

  ■  80386 High Performance 32-bit Microprocessor with Integrated Memory
     Management (Data Sheet), order number 231630


1.3  Notational Conventions

This manual uses special notations for data-structure formats, for symbolic
representation of instructions, for hexadecimal numbers, and for super- and
sub-scripts. Subscript characters are surrounded by {curly brackets}, for
example 10{2} = 10 base 2. Superscript characters are preceeded by a caret
and enclosed within (parentheses), for example 10^(3) = 10 to the third
power. A review of these notations will make it easier to read the
manual.

1.3.1  Data-Structure Formats

In illustrations of data structures in memory, smaller addresses appear at
the lower-right part of the figure; addresses increase toward the left and
upwards. Bit positions are numbered from right to left. Figure 1-1
illustrates this convention.


1.3.2  Undefined Bits and Software Compatibility

In many register and memory layout descriptions, certain bits are marked as
undefined. When bits are marked as undefined (as illustrated in Figure
1-1), it is essential for compatibility with future processors that
software treat these bits as undefined. Software should follow these
guidelines in dealing with undefined bits:

  ■  Do not depend on the states of any undefined bits when testing the
     values of registers that contain such bits. Mask out the undefined bits
     before testing.

  ■  Do not depend on the states of any undefined bits when storing them in
     memory or in another register.

  ■  Do not depend on the ability to retain information written into any
     undefined bits.

  ■  When loading a register, always load the undefined bits as zeros or
     reload them with values previously stored from the same register.

────────────────────────────────────────────────────────────────────────────
NOTE
  Depending upon the values of undefined register bits will make software
  dependent upon the unspecified manner in which the 80386 handles these
  bits. Depending upon undefined values risks making software incompatible
  with future processors that define usages for these bits. AVOID ANY
  SOFTWARE DEPENDENCE UPON THE STATE OF UNDEFINED 80386 REGISTER BITS.
────────────────────────────────────────────────────────────────────────────


Figure 1-1.  Example Data Structure

 GREATEST                 DATA STRUCTURE
 ADDRESS
  31              23              15              7             0 ──BIT
 ╔═══════════════╪═══════════════╪═══════════════╪═══════════════╗   OFFSET
 ║                                                               ║28
 ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣
 ║                                                               ║24
 ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣
 ║                                                               ║20
 ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣
 ║                                                               ║16
 ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣
 ║                                                               ║12
 ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣
 ║                                                               ║8
 ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣
 ║                           UNDEFINED                           ║4
 ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣  SMALLEST
 ║    BYTE 3          BYTE 2          BYTE 1          BYTE 0     ║0 ADDRESS
 ╚═══════════════╪═══════════════╪═══════════════╪═══════════════╝
                                                    BYTE OFFSET───┘


1.3.3  Instruction Operands

When instructions are represented symbolically, a subset of the 80386
Assembly Language is used. In this subset, an instruction has the following
format:

label: prefix mnemonic argument1, argument2, argument3

where:

  ■  A label is an identifier that is followed by a colon.

  ■  A prefix is an optional reserved name for one of the instruction
     prefixes.

  ■  A mnemonic is a reserved name for a class of instruction opcodes that
     have the same function.

  ■  The operands argument1, argument2, and argument3 are optional. There
     may be from zero to three operands, depending on the opcode.  When
     present, they take the form of either literals or identifiers for data
     items.  Operand identifiers are either reserved names of registers or
     are assumed to be assigned to data items declared in another part of
     the program (which may not be shown in the example). When two operands
     are present in an instruction that modifies data, the right operand is
     the source and the left operand is the destination.

For example:

LOADREG: MOV EAX, SUBTOTAL

In this example LOADREG is a label, MOV is the mnemonic identifier of an
opcode, EAX is the destination operand, and SUBTOTAL is the source operand.

1.3.4  Hexadecimal Numbers

Base 16 numbers are represented by a string of hexadecimal digits followed
by the character H.  A hexadecimal digit is a character from the set (0, 1,
2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F).  In some cases, especially in
examples of program syntax, a leading zero is added if the number would
otherwise begin with one of the digits A-F.  For example, 0FH is equivalent
to the decimal number 15.

1.3.5  Sub- and Super-Scripts

This manual uses special notation to represent sub- and super-script
characters. Sub-script characters are surrounded by {curly brackets}, for
example 10{2} = 10 base 2. Super-script characters are preceeded by a
caret and enclosed within (parentheses), for example 10^(3) = 10 to the
third power.


                     PART I    APPLICATIONS PROGRAMMING


Chapter 2  Basic Programming Model

────────────────────────────────────────────────────────────────────────────

This chapter describes the 80386 application programming environment as
seen by assembly language programmers when the processor is executing in
protected mode. The chapter introduces programmers to those features of the
80386 architecture that directly affect the design and implementation of
80386 applications programs. Other chapters discuss 80386 features that
relate to systems programming or to compatibility with other processors of
the 8086 family.

The basic programming model consists of these aspects:

  ■  Memory organization and segmentation
  ■  Data types
  ■  Registers
  ■  Instruction format
  ■  Operand selection
  ■  Interrupts and exceptions

Note that input/output is not included as part of the basic programming
model. Systems designers may choose to make I/O instructions available to
applications or may choose to reserve these functions for the operating
system. For this reason, the I/O features of the 80386 are discussed in Part
II.

This chapter contains a section for each aspect of the architecture that is
normally visible to applications.


2.1  Memory Organization and Segmentation

The physical memory of an 80386 system is organized as a sequence of 8-bit
bytes. Each byte is assigned a unique address that ranges from zero to a
maximum of 2^(32) -1 (4 gigabytes).

80386 programs, however, are independent of the physical address space.
This means that programs can be written without knowledge of how much
physical memory is available and without knowledge of exactly where in
physical memory the instructions and data are located.

The model of memory organization seen by applications programmers is
determined by systems-software designers. The architecture of the 80386
gives designers the freedom to choose a model for each task. The model of
memory organization can range between the following extremes:

  ■  A "flat" address space consisting of a single array of up to 4
     gigabytes.

  ■  A segmented address space consisting of a collection of up to 16,383
     linear address spaces of up to 4 gigabytes each.

Both models can provide memory protection. Different tasks may employ
different models of memory organization. The criteria that designers use to
determine a memory organization model and the means that systems programmers
use to implement that model are covered in Part II──Systems Programming.


2.1.1  The "Flat" Model

In a "flat" model of memory organization, the applications programmer sees
a single array of up to 2^(32) bytes (4 gigabytes). While the physical
memory can contain up to 4 gigabytes, it is usually much smaller; the
processor maps the 4 gigabyte flat space onto the physical address space by
the address translation mechanisms described in Chapter 5. Applications
programmers do not need to know the details of the mapping.

A pointer into this flat address space is a 32-bit ordinal number that may
range from 0 to 2^(32) -1. Relocation of separately-compiled modules in this
space must be performed by systems software (e.g., linkers, locators,
binders, loaders).


2.1.2  The Segmented Model

In a segmented model of memory organization, the address space as viewed by
an applications program (called the logical address space) is a much larger
space of up to 2^(46) bytes (64 terabytes). The processor maps the 64
terabyte logical address space onto the physical address space (up to 4
gigabytes) by the address translation mechanisms described in Chapter 5.
Applications programmers do not need to know the details of this mapping.

Applications programmers view the logical address space of the 80386 as a
collection of up to 16,383 one-dimensional subspaces, each with a specified
length. Each of these linear subspaces is called a segment. A segment is a
unit of contiguous address space. Segment sizes may range from one byte up
to a maximum of 2^(32) bytes (4 gigabytes).

A complete pointer in this address space consists of two parts (see Figure
2-1):

  1.  A segment selector, which is a 16-bit field that identifies a
      segment.

  2.  An offset, which is a 32-bit ordinal that addresses to the byte level
      within a segment.

During execution of a program, the processor associates with a segment
selector the physical address of the beginning of the segment. Separately
compiled modules can be relocated at run time by changing the base address
of their segments. The size of a segment is variable; therefore, a segment
can be exactly the size of the module it contains.


2.2  Data Types

Bytes, words, and doublewords are the fundamental data types (refer to
Figure 2-2). A byte is eight contiguous bits starting at any logical
address. The bits are numbered 0 through 7; bit zero is the least
significant bit.

A word is two contiguous bytes starting at any byte address. A word thus
contains 16 bits. The bits of a word are numbered from 0 through 15; bit 0
is the least significant bit. The byte containing bit 0 of the word is
called the low byte; the byte containing bit 15 is called the high byte.

Each byte within a word has its own address, and the smaller of the
addresses is the address of the word. The byte at this lower address
contains the eight least significant bits of the word, while the byte at the
higher address contains the eight most significant bits.

A doubleword is two contiguous words starting at any byte address. A
doubleword thus contains 32 bits. The bits of a doubleword are numbered from
0 through 31; bit 0 is the least significant bit. The word containing bit 0
of the doubleword is called the low word; the word containing bit 31 is
called the high word.

Each byte within a doubleword has its own address, and the smallest of the
addresses is the address of the doubleword. The byte at this lowest address
contains the eight least significant bits of the doubleword, while the byte
at the highest address contains the eight most significant bits. Figure 2-3
illustrates the arrangement of bytes within words anddoublewords.

Note that words need not be aligned at even-numbered addresses and
doublewords need not be aligned at addresses evenly divisible by four. This
allows maximum flexibility in data structures (e.g., records containing
mixed byte, word, and doubleword items) and efficiency in memory
utilization. When used in a configuration with a 32-bit bus, actual
transfers of data between processor and memory take place in units of
doublewords beginning at addresses evenly divisible by four; however, the
processor converts requests for misaligned words or doublewords into the
appropriate sequences of requests acceptable to the memory interface. Such
misaligned data transfers reduce performance by requiring extra memory
cycles. For maximum performance, data structures (including stacks) should
be designed in such a way that, whenever possible, word operands are aligned
at even addresses and doubleword operands are aligned at addresses evenly
divisible by four. Due to instruction prefetching and queuing within the
CPU, there is no requirement for instructions to be aligned on word or
doubleword boundaries. (However, a slight increase in speed results if the
target addresses of control transfers are evenly divisible by four.)

Although bytes, words, and doublewords are the fundamental types of
operands, the processor also supports additional interpretations of these
operands. Depending on the instruction referring to the operand, the
following additional data types are recognized:

Integer:
A signed binary numeric value contained in a 32-bit doubleword,16-bit word,
or 8-bit byte. All operations assume a 2's complement representation. The
sign bit is located in bit 7 in a byte, bit 15 in a word, and bit 31 in a
doubleword. The sign bit has the value zero for positive integers and one
for negative. Since the high-order bit is used for a sign, the range of an
8-bit integer is -128 through +127; 16-bit integers may range from -32,768
through +32,767; 32-bit integers may range from -2^(31) through +2^(31) -1.
The value zero has a positive sign.

Ordinal:
An unsigned binary numeric value contained in a 32-bit doubleword,
16-bit word, or 8-bit byte. All bits are considered in determining
magnitude of the number. The value range of an 8-bit ordinal number
is 0-255; 16 bits can represent values from 0 through 65,535; 32 bits
can represent values from 0 through 2^(32) -1.

Near Pointer:
A 32-bit logical address. A near pointer is an offset within a segment.
Near pointers are used in either a flat or a segmented model of memory
organization.

Far Pointer:
A 48-bit logical address of two components: a 16-bit segment selector
component and a 32-bit offset component. Far pointers are used by
applications programmers only when systems designers choose a
segmented memory organization.

String:
A contiguous sequence of bytes, words, or doublewords. A string may
contain from zero bytes to 2^(32) -1 bytes (4 gigabytes).

Bit field:
A contiguous sequence of bits. A bit field may begin at any bit position
of any byte and may contain up to 32 bits.

Bit string:
A contiguous sequence of bits. A bit string may begin at any bit position
of any byte and may contain up to 2^(32) -1 bits.

BCD:
A byte (unpacked) representation of a decimal digit in the range0 through
9. Unpacked decimal numbers are stored as unsigned byte quantities. One
digit is stored in each byte. The magnitude of the number is determined from
the low-order half-byte; hexadecimal values 0-9 are valid and are
interpreted as decimal numbers. The high-order half-byte must be zero for
multiplication and division; it may contain any value for addition and
subtraction.

Packed BCD:
A byte (packed) representation of two decimal digits, each in the range
0 through 9. One digit is stored in each half-byte. The digit in the
high-order half-byte is the most significant. Values 0-9 are valid in each
half-byte. The range of a packed decimal byte is 0-99.

Figure 2-4 graphically summarizes the data types supported by the 80386.


Figure 2-1.  Two-Component Pointer

                                                  
                                   ║               ║
                                   ╠═══════════════╣─┐
       32            0             ║               ║ │
      ╔═══════╪═══════╗   ╔═══╗    ╠═══════════════╣ │
      ║    OFFSET     ╟───╢ + ╟───║    OPERAND    ║ │
      ╚═══════╪═══════╝   ╚═══╝    ╠═══════════════╣ ├─ SELECTED SEGMENT
                                  ║               ║ │
           16    0          │      ║               ║ │
          ╔═══════╗         │      ║               ║ │
          ║SEGMENT╟─────────∙─────╠═══════════════╣─┘
          ╚═══════╝                ║               ║
                                   ║               ║
                                   ║               ║
                                                  


Figure 2-2.  Fundamental Data Types

  7              0
 ╔═══════════════╗
 ║     BYTE      ║  BYTE
 ╚═══════════════╝

  15              7              0
 ╔═══════════════╤═══════════════╗
 ║   HIGH BYTE   │   LOW BYTE    ║  WORD
 ╚═══════════════╧═══════════════╝
    address n+1      address n

  31              23              15              7             0
 ╔═══════════════╪═══════════════╪═══════════════╪══════════════╗
 ║           HIGH WORD           │            LOW WORD          ║ DOUBLEWORD
 ╚═══════════════╪═══════════════╪═══════════════╪══════════════╝
    address n+3     address n+2     address n+1      address n


Figure 2-3.  Bytes, Words, and Doublewords in Memory

                  MEMORY
       BYTE       VALUES
All values in hexadecimal
     ADDRESS   ╔══════════╗
              E║          ║
               ╠══════════╣──┐
              D║    7A    ║  ├─ DOUBLE WORD AT ADDRESS A
               ╠══════════╣─┐│  CONTAINS 7AFE0636
              C║    FE    ║ ││
               ╠══════════╣ ├─ WORD AT ADDRESS B
              B║    06    ║ ││ CONTAINS FE06
               ╠══════════╣─┘│
              A║    36    ║  │
               ╠══════════╣══╡
              9║    1F    ║  ├─ WORD AT ADDRESS 9
               ╠══════════╣──┘  CONTAINS IF
              8║          ║
               ╠══════════╣──┐
              7║    23    ║  │
               ╠══════════╣  ├─ WORD AT ADDRESS 6
              6║    OB    ║  │  CONTAINS 23OB
               ╠══════════╣──┘
              5║          ║
               ╠══════════╣
              4║          ║
               ╠══════════╣──┐
              3║    74    ║  │
               ╠══════════╣─┐├─ WORD AT ADDRESS 2
              2║    CB    ║ ││  CONTAINS 74CB
               ╠══════════╣──┘
              1║    31    ║ ├── WORD AT ADDRESS 1
               ╠══════════╣─┘   CONTAINS CB31
              0║          ║
               ╚══════════╝


Figure 2-4.  80386 Data Types

                                                           +1       0
          7       0              7       0            15 14    8 7      0
     BYTE ╔╤╤╤╤╤╤╤╗         BYTE ╔╤╤╤╤╤╤╤╗         WORD ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
  INTEGER ║│  │   ║      ORDINAL ║   │   ║      INTEGER ║│  │   │   │   ║
          ╚╧══════╝              ╚═══════╝              ╚╧══════╧═══════╝
  SIGN BIT┘└──────┘              └───────┘      SIGN BIT┘└MSB           │
           MAGNITUDE             MAGNITUDE              └───────────────┘
                                                            MAGNITUDE


            +1       0                       +3     +2      +1       0
         15              0               31            16 15             0
    WORD ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗    DOUBLEWORD ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
 ORDINAL ║│  │   │   │   ║       INTEGER ║│  │   │   │   │   │   │   │   ║
         ╚╧══════╧═══════╝               ╚╧══════╧═══════╧═══════╧═══════╝
         │               │       SIGN BIT┘└MSB                           │
         └───────────────┘               └───────────────────────────────┘
             MAGNITUDE                              MAGNITUDE


                           +3      +2      +1       0
                        31                              0
             DOUBLEWORD ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
                ORDINAL ║   │   │   │   │   │   │   │   ║
                        ╚═══════╧═══════╧═══════╧═══════╝
                        └───────────────────────────────┘
                                    MAGNITUDE

                           +N              +1       0
                        7       0       7      0 7      0
           BINARY CODED ╔╤╤╤╤╤╤╤╗       ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
          DECIMAL (BCD) ║   │   ║    ║   │   │   │   ║
                        ╚═══════╝       ╚═══════╧═══════╝
                           BCD             BCD     BCD
                         DIGIT N         DIGIT 1  DIGIT 0

                           +N              +1       0
                        7       0       7      0 7      0
                 PACKED ╔╤╤╤╤╤╤╤╗       ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
                    BCD ║   │   ║    ║   │   │   │   ║
                        ╚═══════╝       ╚═══════╧═══════╝
                        └───┘                       └───┘
                        MOST                        LEAST
                        SIGNIFICANT           SIGNIFICANT
                        DIGIT                       DIGIT

                           +N              +1       0
                        7       0       7      0 7      0
                   BYTE ╔╤╤╤╤╤╤╤╗       ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
                 STRING ║   │   ║    ║   │   │   │   ║
                        ╚═══════╝       ╚═══════╧═══════╝

                                              -2 GIGABYTES
                +2 GIGABYTES                           210
     BIT ╔╤╤╤╤════════════╤╤═══════  ════════════════╤╤╤╤╗
  STRING ║││││            ││                         ││││║
         ╚╧╧╧╧════════════╧╧════════  ═══════════════╧╧╧╧╝
                         BIT 0

               +3      +2      +1       0
            31                              0
NEAR 32-BIT ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
    POINTER ║   │   │   │   │   │   │   │   ║
            ╚═══════╧═══════╧═══════╧═══════╝
            └───────────────────────────────┘
                             OFFSET

              +5      +4      +3      +2      +1       0
           48                                              0
FAR 48-BIT ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
   POINTER ║   │   │   │   │   │   │   │   │   │   │   │   ║
           ╚═══════╧═══════╧═══════╧═══════╧═══════╧═══════╝
           └───────────────┴───────────────────────────────┘
                SELECTOR                 OFFSET

                  +5      +4      +3      +2      +1       0
    32-BIT ╔╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╤╗
 BIT FIELD ║   │   │   │   │   │   │   │   │   │   │   │   ║
           ╚═══════╧═══════╧═══════╧═══════╧═══════╧═══════╝
                   │───────── BIT FIELD ─────────│
                             1 TO 32 BITS


2.3  Registers

The 80386 contains a total of sixteen registers that are of interest to the
applications programmer. As Figure 2-5 shows, these registers may be
grouped into these basic categories:

  1.  General registers. These eight 32-bit general-purpose registers are
      used primarily to contain operands for arithmetic and logical
      operations.

  2.  Segment registers. These special-purpose registers permit systems
      software designers to choose either a flat or segmented model of
      memory organization. These six registers determine, at any given time,
      which segments of memory are currently addressable.

  3.  Status and instruction registers. These special-purpose registers are
      used to record and alter certain aspects of the 80386 processor state.


2.3.1  General Registers

The general registers of the 80386 are the 32-bit registers EAX, EBX, ECX,
EDX, EBP, ESP, ESI, and EDI. These registers are used interchangeably to
contain the operands of logical and arithmetic operations. They may also be
used interchangeably for operands of address computations (except that ESP
cannot be used as an index operand).

As Figure 2-5 shows, the low-order word of each of these eight registers
has a separate name and can be treated as a unit. This feature is useful for
handling 16-bit data items and for compatibility with the 8086 and 80286
processors. The word registers are named AX, BX, CX, DX, BP, SP, SI, and DI.

Figure 2-5 also illustrates that each byte of the 16-bit registers AX, BX,
CX, and DX has a separate name and can be treated as a unit. This feature is
useful for handling characters and other 8-bit data items. The byte
registers are named AH, BH, CH, and DH (high bytes); and AL, BL, CL, and DL
(low bytes).

All of the general-purpose registers are available for addressing
calculations and for the results of most arithmetic and logical
calculations; however, a few functions are dedicated to certain registers.
By implicitly choosing registers for these functions, the 80386 architecture
can encode instructions more compactly. The instructions that use specific
registers include: double-precision multiply and divide, I/O, string
instructions, translate, loop, variable shift and rotate, and stack
operations.


2.3.2  Segment Registers

The segment registers of the 80386 give systems software designers the
flexibility to choose among various models of memory organization.
Implementation of memory models is the subject of Part II ── Systems
Programming. Designers may choose a model in which applications programs do
not need to modify segment registers, in which case applications programmers
may skip this section.

Complete programs generally consist of many different modules, each
consisting of instructions and data. However, at any given time during
program execution, only a small subset of a program's modules are actually
in use. The 80386 architecture takes advantage of this by providing
mechanisms to support direct access to the instructions and data of the
current module's environment, with access to additional segments on demand.

At any given instant, six segments of memory may be immediately accessible
to an executing 80386 program. The segment registers CS, DS, SS, ES, FS, and
GS are used to identify these six current segments. Each of these registers
specifies a particular kind of segment, as characterized by the associated
mnemonics ("code," "data," or "stack") shown in Figure 2-6. Each register
uniquely determines one particular segment, from among the segments that
make up the program, that is to be immediately accessible at highest speed.

The segment containing the currently executing sequence of instructions is
known as the current code segment; it is specified by means of the CS
register. The 80386 fetches all instructions from this code segment, using
as an offset the contents of the instruction pointer. CS is changed
implicitly as the result of intersegment control-transfer instructions (for
example, CALL and JMP), interrupts, and exceptions.

Subroutine calls, parameters, and procedure activation records usually
require that a region of memory be allocated for a stack. All stack
operations use the SS register to locate the stack. Unlike CS, the SS
register can be loaded explicitly, thereby permitting programmers to define
stacks dynamically.

The DS, ES, FS, and GS registers allow the specification of four data
segments, each addressable by the currently executing program. Accessibility
to four separate data areas helps programs efficiently access different
types of data structures; for example, one data segment register can point
to the data structures of the current module, another to the exported data
of a higher-level module, another to a dynamically created data structure,
and another to data shared with another task. An operand within a data
segment is addressed by specifying its offset either directly in an
instruction or indirectly via general registers.

Depending on the structure of data (e.g., the way data is parceled into one
or more segments), a program may require access to more than four data
segments. To access additional segments, the DS, ES, FS, and GS registers
can be changed under program control during the course of a program's
execution. This simply requires that the program execute an instruction to
load the appropriate segment register prior to executing instructions that
access the data.

The processor associates a base address with each segment selected by a
segment register. To address an element within a segment, a 32-bit offset is
added to the segment's base address. Once a segment is selected (by loading
the segment selector into a segment register), a data manipulation
instruction only needs to specify the offset. Simple rules define which
segment register is used to form an address when only an offset is
specified.


Figure 2-5.  80386 Applications Register Set

                              GENERAL REGISTERS

  31                23                15                7               0
 ╔═════════════════╪═════════════════╬═════════════════╧═════════════════╗
 ║                                  EAX       AH       AX      AL        ║
 ╠═════════════════╪═════════════════╬═════════════════╩═════════════════╣
 ║                                  EDX       DH       DX      DL        ║
 ╠═════════════════╪═════════════════╬═════════════════╩═════════════════╣
 ║                                  ECX       CH       CX      CL        ║
 ╠═════════════════╪═════════════════╬═════════════════╩═════════════════╣
 ║                                  EBX       BH       BX      BL        ║
 ╠═════════════════╪═════════════════╬═════════════════╩═════════════════╣
 ║                                  EBP                BP                ║
 ╠═════════════════╪═════════════════╬═════════════════╪═════════════════╣
 ║                                  ESI                SI                ║
 ╠═════════════════╪═════════════════╬═════════════════╪═════════════════╣
 ║                                  EDI                DI                ║
 ╠═════════════════╪═════════════════╬═════════════════╪═════════════════╣
 ║                                  ESP                SP                ║
 ╚═════════════════╪═════════════════╬═════════════════╪═════════════════╝


                      15                7               0
                     ╔═════════════════╪═════════════════╗
                     ║         CS (CODE SEGMENT)         ║
                     ╟─────────────────┼─────────────────╢
                     ║         SS (STACK SEGMENT)        ║
    SEGMENT          ╟─────────────────┼─────────────────╢
    REGISTERS        ║         DS (DATA SEGMENT)         ║
                     ╟─────────────────┼─────────────────╢
                     ║         ES (DATA SEGMENT)         ║
                     ╟─────────────────┼─────────────────╢
                     ║         FS (DATA SEGMENT)         ║
                     ╟─────────────────┼─────────────────╢
                     ║         GS (DATA SEGMENT)         ║
                     ╚═════════════════╪═════════════════╝


                       STATUS AND INSTRUCTION REGISTERS

     31               23                15                7              0
   ╔═════════════════╪═════════════════╪═════════════════╪═════════════════╗
   ║                                EFLAGS                                 ║
   ╟───────────────────────────────────────────────────────────────────────╢
   ║                        EIP (INSTRUCTION POINTER)                      ║
   ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝


Figure 2-6.  Use of Memory Segmentation

     ╔════════════════╗                                ╔════════════════╗
     ║     MODULE     ║                                ║     MODULE     ║
     ║       A        ║──┐                        ┌──║       A        ║
     ║      CODE      ║   │                        │   ║      DATA      ║
     ╚════════════════╝   │  ╔══════════════════╗  │   ╚════════════════╝
                          └──╢    CS (CODE)     ║  │
                             ╠══════════════════╣  │
     ╔════════════════╗   ┌──╢    SS (STACK)    ║  │   ╔════════════════╗
     ║                ║   │  ╠══════════════════╣  │   ║      DATA      ║
     ║     STACK      ║──┘  ║    DS (DATA)     ╟──┘┌─║   STRUCTURE    ║
     ║                ║      ╠══════════════════╣   │  ║       1        ║
     ╚════════════════╝      ║    ES (DATA)     ╟───┘  ╚════════════════╝
                             ╠══════════════════╣
                          ┌──╢    FS (DATA)     ║
     ╔════════════════╗   │  ╠══════════════════╣      ╔════════════════╗
     ║      DATA      ║   │  ║    GS (DATA)     ╟──┐   ║      DATA      ║
     ║   STRUCTURE    ║──┘  ╚══════════════════╝  └──║   STRUCTURE    ║
     ║       2        ║                                ║       3        ║
     ╚════════════════╝                                ╚════════════════╝


2.3.3  Stack Implementation

Stack operations are facilitated by three registers:

  1.  The stack segment (SS) register. Stacks are implemented in memory. A
      system may have a number of stacks that is limited only by the maximum
      number of segments. A stack may be up to 4 gigabytes long, the maximum
      length of a segment. One stack is directly addressable at a time──the
      one located by SS. This is the current stack, often referred to simply
      as "the" stack. SS is used automatically by the processor for all
      stack operations.

  2.  The stack pointer (ESP) register. ESP points to the top of the
      push-down stack (TOS). It is referenced implicitly by PUSH and POP
      operations, subroutine calls and returns, and interrupt operations.
      When an item is pushed onto the stack (see Figure 2-7), the processor
      decrements ESP, then writes the item at the new TOS. When an item is
      popped off the stack, the processor copies it from TOS, then
      increments ESP. In other words, the stack grows down in memory toward
      lesser addresses.

  3.  The stack-frame base pointer (EBP) register. The EBP is the best
      choice of register for accessing data structures, variables and
      dynamically allocated work space within the stack. EBP is often used
      to access elements on the stack relative to a fixed point on the stack
      rather than relative to the current TOS. It typically identifies the
      base address of the current stack frame established for the current
      procedure. When EBP is used as the base register in an offset
      calculation, the offset is calculated automatically in the current
      stack segment (i.e., the segment currently selected by SS). Because
      SS does not have to be explicitly specified, instruction encoding in
      such cases is more efficient. EBP can also be used to index into
      segments addressable via other segment registers.


Figure 2-7.  80386 Stack

     31                         0
     ╔══════╪══════╪══════╪══════╗ ───────BOTTOM OF STACK
     ║                           ║       (INITIAL ESP VALUE)
     ╟══════╪══════╪══════╪══════╣
     ║                           ║
     ╠══════╪══════╪══════╪══════╣
     ║                           ║        │POP
     ╠══════╪══════╪══════╪══════╣        │
     ║                           ║        │
     ╠══════╪══════╪══════╪══════╣        │      TOP OF     ╔═════════════╗
     ║                           ║ ──────┼─────────────────╢     ESP     ║
     ╠══════╪══════╪══════╪══════╣        │      STACK      ╚═════════════╝
     ║                           ║        │
     ║                           ║        │
     ║                           ║        │PUSH
     ║                           ║


2.3.4  Flags Register

The flags register is a 32-bit register named EFLAGS. Figure 2-8 defines
the bits within this register. The flags control certain operations and
indicate the status of the 80386.

The low-order 16 bits of EFLAGS is named FLAGS and can be treated as a
unit. This feature is useful when executing 8086 and 80286 code, because
this part of EFLAGS is identical to the FLAGS register of the 8086 and the
80286.

The flags may be considered in three groups: the status flags, the control
flags, and the systems flags. Discussion of the systems flags is delayed
until Part II.


Figure 2-8.  EFLAGS Register

                                              16-BIT FLAGS REGISTER
                                                         A
                                     ┌───────────────────┴───────────────┐
  31                  23                  15               7            0
 ╔═══════════════════╪═══════════════╤═╤═╤═╤═╤═╪═╤═╤═╤═╤═╤═╤═╤═╤═╤═╤═╤═╤═╗
 ║                                   │V│R│ │N│ IO│O│D│I│T│S│Z│ │A│ │P│ │C║
 ║ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 │ │ │0│ │   │ │ │ │ │ │ │0│ │0│ │1│ ║
 ║                                   │M│F│ │T│ PL│F│F│F│F│F│F│ │F│ │F│ │F║
 ╚═══════════════════╪═══════════════╧╤╧╤╧═╧╤╧═╪═╧╤╧╤╧╤╧╤╧╤╧╤╧═╧╤╧═╧╤╧═╧╤╝
                                      │ │   │  │  │ │ │ │ │ │   │   │   │
       VIRTUAL 8086 MODE───X──────────┘ │   │  │  │ │ │ │ │ │   │   │   │
             RESUME FLAG───X────────────┘   │  │  │ │ │ │ │ │   │   │   │
        NESTED TASK FLAG───X────────────────┘  │  │ │ │ │ │ │   │   │   │
     I/O PRIVILEGE LEVEL───X───────────────────┘  │ │ │ │ │ │   │   │   │
                OVERFLOW───S──────────────────────┘ │ │ │ │ │   │   │   │
          DIRECTION FLAG───C────────────────────────┘ │ │ │ │   │   │   │
        INTERRUPT ENABLE───X──────────────────────────┘ │ │ │   │   │   │
               TRAP FLAG───S────────────────────────────┘ │ │   │   │   │
               SIGN FLAG───S──────────────────────────────┘ │   │   │   │
               ZERO FLAG───S────────────────────────────────┘   │   │   │
         AUXILIARY CARRY───S────────────────────────────────────┘   │   │
             PARITY FLAG───S────────────────────────────────────────┘   │
              CARRY FLAG───S────────────────────────────────────────────┘

          S = STATUS FLAG, C = CONTROL FLAG, X = SYSTEM FLAG

          NOTE: 0 OR 1 INDICATES INTEL RESERVED. DO NOT DEFINE


2.3.4.1  Status Flags

The status flags of the EFLAGS register allow the results of one
instruction to influence later instructions. The arithmetic instructions use
OF, SF, ZF, AF, PF, and CF. The SCAS (Scan String), CMPS (Compare String),
and LOOP instructions use ZF to signal that their operations are complete.
There are instructions to set, clear, and complement CF before execution of
an arithmetic instruction. Refer to Appendix C for definition of each
status flag.


2.3.4.2  Control Flag

The control flag DF of the EFLAGS register controls string instructions.

DF (Direction Flag, bit 10)

  Setting DF causes string instructions to auto-decrement; that is, to
  process strings from high addresses to low addresses. Clearing DF causes
  string instructions to auto-increment, or to process strings from low
  addresses to high addresses.


2.3.4.3  Instruction Pointer

The instruction pointer register (EIP) contains the offset address,
relative to the start of the current code segment, of the next sequential
instruction to be executed. The instruction pointer is not directly visible
to the programmer; it is controlled implicitly by control-transfer
instructions, interrupts, and exceptions.

As Figure 2-9 shows, the low-order 16 bits of EIP is named IP and can be
used by the processor as a unit. This feature is useful when executing
instructions designed for the 8086 and 80286 processors.


Figure 2-9.  Instruction Pointer Register

                                                 16-BIT IP REGISTER
                                       ┌──────────────────┴────────────────┐
    31                23                15                7               0
   ╔═════════════════╪═════════════════╪═════════════════╪═════════════════╗
   ║                       EIP (INSTRUCTION POINTER)                       ║
   ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝


2.4  Instruction Format

The information encoded in an 80386 instruction includes a specification of
the operation to be performed, the type of the operands to be manipulated,
and the location of these operands. If an operand is located in memory, the
instruction must also select, explicitly or implicitly, which of the
currently addressable segments contains the operand.

80386 instructions are composed of various elements and have various
formats. The exact format of instructions is shown in Appendix B; the
elements of instructions are described below. Of these instruction elements,
only one, the opcode, is always present. The other elements may or may not
be present, depending on the particular operation involved and on the
location and type of the operands. The elements of an instruction, in order
of occurrence are as follows:

  ■  Prefixes ── one or more bytes preceding an instruction that modify the
     operation of the instruction. The following types of prefixes can be
     used by applications programs:

     1.  Segment override ── explicitly specifies which segment register an
         instruction should use, thereby overriding the default
         segment-register selection used by the 80386 for that instruction.

     2.  Address size ── switches between 32-bit and 16-bit address
         generation.

     3.  Operand size ── switches between 32-bit and 16-bit operands.

     4.  Repeat ── used with a string instruction to cause the instruction
         to act on each element of the string.

  ■  Opcode ── specifies the operation performed by the instruction. Some
     operations have several different opcodes, each specifying a different
     variant of the operation.

  ■  Register specifier ── an instruction may specify one or two register
     operands. Register specifiers may occur either in the same byte as the
     opcode or in the same byte as the addressing-mode specifier.

  ■  Addressing-mode specifier ── when present, specifies whether an operand
     is a register or memory location; if in memory, specifies whether a
     displacement, a base register, an index register, and scaling are to be
     used.

  ■  SIB (scale, index, base) byte ── when the addressing-mode specifier
     indicates that an index register will be used to compute the address of
     an operand, an SIB byte is included in the instruction to encode the
     base register, the index register, and a scaling factor.

  ■  Displacement ── when the addressing-mode specifier indicates that a
     displacement will be used to compute the address of an operand, the
     displacement is encoded in the instruction. A displacement is a signed
     integer of 32, 16, or eight bits. The eight-bit form is used in the
     common case when the displacement is sufficiently small. The processor
     extends an eight-bit displacement to 16 or 32 bits, taking into
     account the sign.

  ■  Immediate operand ── when present, directly provides the value of an
     operand of the instruction. Immediate operands may be 8, 16, or 32 bits
     wide. In cases where an eight-bit immediate operand is combined in some
     way with a 16- or 32-bit operand, the processor automatically extends
     the size of the eight-bit operand, taking into account the sign.


2.5  Operand Selection

An instruction can act on zero or more operands, which are the data
manipulated by the instruction. An example of a zero-operand instruction is
NOP (no operation). An operand can be in any of these locations:

  ■  In the instruction itself (an immediate operand)

  ■  In a register (EAX, EBX, ECX, EDX, ESI, EDI, ESP, or EBP in the case
     of 32-bit operands; AX, BX, CX, DX, SI, DI, SP, or BP in the case of
     16-bit operands; AH, AL, BH, BL, CH, CL, DH, or DL in the case of 8-bit
     operands; the segment registers; or the EFLAGS register for flag
     operations)

  ■  In memory

  ■  At an I/O port

Immediate operands and operands in registers can be accessed more rapidly
than operands in memory since memory operands must be fetched from memory.
Register operands are available in the CPU. Immediate operands are also
available in the CPU, because they are prefetched as part of the
instruction.

Of the instructions that have operands, some specify operands implicitly;
others specify operands explicitly; still others use a combination of
implicit and explicit specification; for example:

Implicit operand: AAM

  By definition, AAM (ASCII adjust for multiplication) operates on the
  contents of the AX register.

Explicit operand: XCHG EAX, EBX

  The operands to be exchanged are encoded in the instruction after the
  opcode.

Implicit and explicit operands: PUSH COUNTER

  The memory variable COUNTER (the explicit operand) is copied to the top of
  the stack (the implicit operand).

Note that most instructions have implicit operands. All arithmetic
instructions, for example, update the EFLAGS register.

An 80386 instruction can explicitly reference one or two operands.
Two-operand instructions, such as MOV, ADD, XOR, etc., generally overwrite
one of the two participating operands with the result. A distinction can
thus be made between the source operand (the one unaffected by the
operation) and the destination operand (the one overwritten by the result).

For most instructions, one of the two explicitly specified operands──either
the source or the destination──can be either in a register or in memory.
The other operand must be in a register or be an immediate source operand.
Thus, the explicit two-operand instructions of the 80386 permit operations
of the following kinds:

  ■  Register-to-register
  ■  Register-to-memory
  ■  Memory-to-register
  ■  Immediate-to-register
  ■  Immediate-to-memory

Certain string instructions and stack manipulation instructions, however,
transfer data from memory to memory. Both operands of some string
instructions are in memory and are implicitly specified. Push and pop stack
operations allow transfer between memory operands and the memory-based
stack.


2.5.1  Immediate Operands

Certain instructions use data from the instruction itself as one (and
sometimes two) of the operands. Such an operand is called an immediate
operand. The operand may be 32-, 16-, or 8-bits long. For example:

SHR PATTERN, 2

One byte of the instruction holds the value 2, the number of bits by which
to shift the variable PATTERN.

TEST PATTERN, 0FFFF00FFH

A doubleword of the instruction holds the mask that is used to test the
variable PATTERN.


2.5.2  Register Operands

Operands may be located in one of the 32-bit general registers (EAX, EBX,
ECX, EDX, ESI, EDI, ESP, or EBP), in one of the 16-bit general registers
(AX, BX, CX, DX, SI, DI, SP, or BP), or in one of the 8-bit general
registers (AH, BH, CH, DH, AL, BL, CL,or DL).

The 80386 has instructions for referencing the segment registers (CS, DS,
ES, SS, FS, GS). These instructions are used by applications programs only
if systems designers have chosen a segmented memory model.

The 80386 also has instructions for referring to the flag register. The
flags may be stored on the stack and restored from the stack. Certain
instructions change the commonly modified flags directly in the EFLAGS
register. Other flags that are seldom modified can be modified indirectly
via the flags image in the stack.


2.5.3  Memory Operands

Data-manipulation instructions that address operands in memory must specify
(either directly or indirectly) the segment that contains the operand and
the offset of the operand within the segment. However, for speed and compact
instruction encoding, segment selectors are stored in the high speed segment
registers. Therefore, data-manipulation instructions need to specify only
the desired segment register and an offset in order to address a memory
operand.

An 80386 data-manipulation instruction that accesses memory uses one of the
following methods for specifying the offset of a memory operand within its
segment:

  1.  Most data-manipulation instructions that access memory contain a byte
      that explicitly specifies the addressing method for the operand. A
      byte, known as the modR/M byte, follows the opcode and specifies
      whether the operand is in a register or in memory. If the operand is
      in memory, the address is computed from a segment register and any of
      the following values: a base register, an index register, a scaling
      factor, a displacement. When an index register is used, the modR/M
      byte is also followed by another byte that identifies the index
      register and scaling factor. This addressing method is the
      mostflexible.

  2.  A few data-manipulation instructions implicitly use specialized
      addressing methods:

      ■   For a few short forms of MOV that implicitly use the EAX register,
          the offset of the operand is coded as a doubleword in the
          instruction. No base register, index register, or scaling factor
          are used.

      ■   String operations implicitly address memory via DS:ESI, (MOVS,
          CMPS, OUTS, LODS, SCAS) or via ES:EDI (MOVS, CMPS, INS, STOS).

      ■   Stack operations implicitly address operands via SS:ESP
          registers; e.g., PUSH, POP, PUSHA, PUSHAD, POPA, POPAD, PUSHF,
          PUSHFD, POPF, POPFD, CALL, RET, IRET, IRETD, exceptions, and
          interrupts.


2.5.3.1  Segment Selection

Data-manipulation instructions need not explicitly specify which segment
register is used. For all of these instructions, specification of a segment
register is optional. For all memory accesses, if a segment is not
explicitly specified by the instruction, the processor automatically chooses
a segment register according to the rules of Table 2-1. (If systems
designers have chosen a flat model of memory organization, the segment
registers and the rules that the processor uses in choosing them are not
apparent to applications programs.)

There is a close connection between the kind of memory reference and the
segment in which that operand resides. As a rule, a memory reference implies
the current data segment (i.e., the implicit segment selector is in DS).
However, ESP and EBP are used to access items on the stack; therefore, when
the ESP or EBP register is used as a base register, the current stack
segment is implied (i.e., SS contains the selector).

Special instruction prefix elements may be used to override the default
segment selection. Segment-override prefixes allow an explicit segment
selection. The 80386 has a segment-override prefix for each of the segment
registers. Only in the following special cases is there an implied segment
selection that a segment prefix cannot override:

  ■  The use of ES for destination strings in string instructions.
  ■  The use of SS in stack instructions.
  ■  The use of CS for instruction fetches.


Table 2-1. Default Segment Register Selection Rules

Memory Reference Needed  Segment     Implicit Segment Selection Rule
                         Register
                         Used

Instructions             Code (CS)   Automatic with instruction prefetch
Stack                    Stack (SS)  All stack pushes and pops. Any
                                     memory reference that uses ESP or
                                     EBP as a base register.
Local Data               Data (DS)   All data references except when
                                     relative to stack or string
                                     destination.
Destination Strings      Extra (ES)  Destination of string instructions.


2.5.3.2  Effective-Address Computation

The modR/M byte provides the most flexible of the addressing methods, and
instructions that require a modR/M byte as the second byte of the
instruction are the most common in the 80386 instruction set. For memory
operands defined by modR/M, the offset within the desired segment is
calculated by taking the sum of up to three components:

  ■  A displacement element in the instruction.

  ■  A base register.

  ■  An index register. The index register may be automatically multiplied
     by a scaling factor of 2, 4, or 8.

The offset that results from adding these components is called an effective
address. Each of these components of an effective address may have either a
positive or negative value. If the sum of all the components exceeds 2^(32),
the effective address is truncated to 32 bits.Figure 2-10 illustrates the
full set of possibilities for modR/M addressing.

The displacement component, because it is encoded in the instruction, is
useful for fixed aspects of addressing; for example:

  ■  Location of simple scalar operands.
  ■  Beginning of a statically allocated array.
  ■  Offset of an item within a record.

The base and index components have similar functions. Both utilize the same
set of general registers. Both can be used for aspects of addressing that
are determined dynamically; for example:

  ■  Location of procedure parameters and local variables in stack.

  ■  The beginning of one record among several occurrences of the same
     record type or in an array of records.

  ■  The beginning of one dimension of multiple dimension array.

  ■  The beginning of a dynamically allocated array.

The uses of general registers as base or index components differ in the
following respects:

  ■  ESP cannot be used as an index register.

  ■  When ESP or EBP is used as the base register, the default segment is
     the one selected by SS. In all other cases the default segment is DS.

The scaling factor permits efficient indexing into an array in the common
cases when array elements are 2, 4, or 8 bytes wide. The shifting of the
index register is done by the processor at the time the address is evaluated
with no performance loss. This eliminates the need for a separate shift or
multiply instruction.

The base, index, and displacement components may be used in any
combination; any of these components may be null. A scale factor can be used
only when an index is also used. Each possible combination is useful for
data structures commonly used by programmers in high-level languages and
assembly languages. Following are possible uses for some of the various
combinations of address components.

DISPLACEMENT

   The displacement alone indicates the offset of the operand. This
   combination is used to directly address a statically allocated scalar
   operand. An 8-bit, 16-bit, or 32-bit displacement can be used.

BASE

   The offset of the operand is specified indirectly in one of the general
   registers, as for "based" variables.

BASE + DISPLACEMENT

   A register and a displacement can be used together for two distinct
   purposes:

   1.  Index into static array when element size is not 2, 4, or 8 bytes.
       The displacement component encodes the offset of the beginning of
       the array. The register holds the results of a calculation to
       determine the offset of a specific element within the array.

   2.  Access item of a record. The displacement component locates an
       item within record. The base register selects one of several
       occurrences of record, thereby providing a compact encoding for
       this common function.

   An important special case of this combination, is to access parameters
   in the procedure activation record in the stack.  In this case, EBP is
   the best choice for the base register, because when EBP is used as a
   base register, the processor automatically uses the stack segment
   register (SS) to locate the operand, thereby providing a compact
   encoding for this common function.

(INDEX * SCALE) + DISPLACEMENT

   This combination provides efficient indexing into a static array when
   the element size is 2, 4, or 8 bytes. The displacement addresses the
   beginning of the array, the index register holds the subscript of the
   desired array element, and the processor automatically converts the
   subscript into an index by applying the scaling factor.

BASE + INDEX + DISPLACEMENT

   Two registers used together support either a two-dimensional array (the
   displacement determining the beginning of the array) or one of several
   instances of an array of records (the displacement indicating an item
   in the record).

BASE + (INDEX * SCALE) + DISPLACEMENT

   This combination provides efficient indexing of a two-dimensional array
   when the elements of the array are 2, 4, or 8 bytes wide.


Figure 2-10.  Effective Address Computation

      SEGMENT +    BASE   +    (INDEX * SCALE)  +     DISPLACEMENT

                 ┌     ┐
                 │ --- │     ┌     ┐     ┌   ┐
      ┌    ┐     │ EAX │     │ EAX │     │ 1 │
      │ CS │     │ ECX │     │ ECX │     │   │     ┌                     ┐
      │ SS │     │ EDX │     │ EDX │     │ 2 │     │     NO DISPLACEMENT │
     ─┤ DS ├─ + ─┤ EBX ├─ + ─┤ EBX ├─ * ─┤   ├─ + ─┤  8-BIT DISPLACEMENT ├─
      │ ES │     │ ESP │     │ --- │     │ 4 │     │ 32-BIT DISPLACEMENT │
      │ FS │     │ EBP │     │ EBP │     │   │     └                     ┘
      │ GS │     │ ESI │     │ ESI │     │ 6 │
      └    ┘     │ EDI │     │ EDI │     └   ┘
                 └     ┘     └     ┘


2.6  Interrupts and Exceptions

The 80386 has two mechanisms for interrupting program execution:

  1.  Exceptions are synchronous events that are the responses of the CPU
      to certain conditions detected during the execution of an instruction.

  2.  Interrupts are asynchronous events typically triggered by external
      devices needing attention.

Interrupts and exceptions are alike in that both cause the processor to
temporarily suspend its present program execution in order to execute a
program of higher priority. The major distinction between these two kinds of
interrupts is their origin. An exception is always reproducible by
re-executing with the program and data that caused the exception, whereas an
interrupt is generally independent of the currently executing program.

Application programmers are not normally concerned with servicing
interrupts. More information on interrupts for systems programmers may be
found in Chapter 9. Certain exceptions, however, are of interest to
applications programmers, and many operating systems give applications
programs the opportunity to service these exceptions. However, the operating
system itself defines the interface between the applications programs and
the exception mechanism of the 80386.

Table 2-2 highlights the exceptions that may be of interest to applications
programmers.

  ■  A divide error exception results when the instruction DIV or IDIV is
     executed with a zero denominator or when the quotient is too large for
     the destination operand. (Refer to Chapter 3 for a discussion of DIV
     and IDIV.)

  ■  The debug exception may be reflected back to an applications program
     if it results from the trap flag (TF).

  ■  A breakpoint exception results when the instruction INT 3 is executed.
     This instruction is used by some debuggers to stop program execution at
     specific points.

  ■  An overflow exception results when the INTO instruction is executed
     and the OF (overflow) flag is set (after an arithmetic operation that
     set the OF flag). (Refer to Chapter 3 for a discussion of INTO).

  ■  A bounds check exception results when the BOUND instruction is
     executed and the array index it checks falls outside the bounds of the
     array. (Refer to Chapter 3 for a discussion of the BOUND instruction.)

  ■  Invalid opcodes may be used by some applications to extend the
     instruction set. In such a case, the invalid opcode exception presents
     an opportunity to emulate the opcode.

  ■  The "coprocessor not available" exception occurs if the program
     contains instructions for a coprocessor, but no coprocessor is present
     in the system.

  ■  A coprocessor error is generated when a coprocessor detects an illegal
     operation.

The instruction INT generates an interrupt whenever it is executed; the
processor treats this interrupt as an exception. The effects of this
interrupt (and the effects of all other exceptions) are determined by
exception handler routines provided by the application program or as part of
the systems software (provided by systems programmers). The INT instruction
itself is discussed in Chapter 3. Refer to Chapter 9 for a more complete
description of exceptions.


Table 2-2. 80386 Reserved Exceptions and Interrupts

Vector Number      Description

0                  Divide Error
1                  Debug Exceptions
2                  NMI Interrupt
3                  Breakpoint
4                  INTO Detected Overflow
5                  BOUND Range Exceeded
6                  Invalid Opcode
7                  Coprocessor Not Available
8                  Double Exception
9                  Coprocessor Segment Overrun
10                 Invalid Task State Segment
11                 Segment Not Present
12                 Stack Fault
13                 General Protection
14                 Page Fault
15                 (reserved)
16                 Coprocessor Error
17-32              (reserved)


Chapter 3  Applications Instruction Set

────────────────────────────────────────────────────────────────────────────

This chapter presents an overview of the instructions which programmers can
use to write application software for the 80386 executing in protected
virtual-address mode. The instructions are grouped by categories of related
functions.

The instructions not discussed in this chapter are those that are normally
used only by operating-system programmers. Part II describes the operation
of these instructions.

The descriptions in this chapter assume that the 80386 is operating in
protected mode with 32-bit addressing in effect; however, all instructions
discussed are also available when 16-bit addressing is in effect in
protected mode, real mode, or virtual 8086 mode. For any differences of
operation that exist in the various modes, refer to Chapter 13,
Chapter 14, or Chapter 15.

The instruction dictionary in Chapter 17 contains more detailed
descriptions of all instructions, including encoding, operation, timing,
effect on flags, and exceptions.


3.1  Data Movement Instructions

These instructions provide convenient methods for moving bytes, words, or
doublewords of data between memory and the registers of the base
architecture. They fall into the following classes:

  1.  General-purpose data movement instructions.
  2.  Stack manipulation instructions.
  3.  Type-conversion instructions.


3.1.1  General-Purpose Data Movement Instructions

MOV (Move) transfers a byte, word, or doubleword from the source operand to
the destination operand. The MOV instruction is useful for transferring data
along any of these paths
There are also variants of MOV that operate on segment registers.  These
are covered in a later section of this chapter.:

  ■  To a register from memory
  ■  To memory from a register
  ■  Between general registers
  ■  Immediate data to a register
  ■  Immediate data to a memory

The MOV instruction cannot move from memory to memory or from segment
register to segment register are not allowed. Memory-to-memory moves can be
performed, however, by the string move instruction MOVS.

XCHG (Exchange) swaps the contents of two operands. This instruction takes
the place of three MOV instructions. It does not require a temporary
location to save the contents of one operand while load the other is being
loaded. XCHG is especially useful for implementing semaphores or similar
data structures for process synchronization.

The XCHG instruction can swap two byte operands, two word operands, or two
doubleword operands. The operands for the XCHG instruction may be two
register operands, or a register operand with a memory operand. When used
with a memory operand, XCHG automatically activates the LOCK signal. (Refer
to Chapter 11 for more information on the bus lock.)


3.1.2  Stack Manipulation Instructions

PUSH (Push) decrements the stack pointer (ESP), then transfers the source
operand to the top of stack indicated by ESP (see Figure 3-1). PUSH is
often used to place parameters on the stack before calling a procedure; it
is also the basic means of storing temporary variables on the stack. The
PUSH instruction operates on memory operands, immediate operands, and
register operands (including segment registers).

PUSHA (Push All Registers) saves the contents of the eight general
registers on the stack (see Figure 3-2). This instruction simplifies
procedure calls by reducing the number of instructions required to retain
the contents of the general registers for use in a procedure. The processor
pushes the general registers on the stack in the following order: EAX, ECX,
EDX, EBX, the initial value of ESP before EAX was pushed, EBP, ESI, and
EDI. PUSHA is complemented by the POPA instruction.

POP (Pop) transfers the word or doubleword at the current top of stack
(indicated by ESP) to the destination operand, and then increments ESP to
point to the new top of stack. See Figure 3-3. POP moves information from
the stack to a general register, or to memory
There are also a variant of POP that operates on segment registers. This
is covered in a later section of this chapter..

POPA (Pop All Registers) restores the registers saved on the stack by
PUSHA, except that it ignores the saved value of ESP. See Figure 3-4.


Figure 3-1.  PUSH

       D  O        BEFORE PUSH                  AFTER PUSH
       I  F      31          0             31          0 
       R        ║               ║           ║               ║
       E  E     ╠═══════╪═══════╣           ╠═══════╪═══════╣
       C  X     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
       T  P     ╠═══════╪═══════╣           ╠═══════╪═══════╣
       I  A     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
       O  N     ╠═══════╪═══════╣──ESP     ╠═══════╪═══════╣
       N  S     ║               ║           ║    OPERAND    ║
          I     ╠═══════╪═══════╣           ╠═══════╪═══════╣──ESP
        │ O     ║               ║           ║               ║
        │ N     ╠═══════╪═══════╣           ╠═══════╪═══════╣
        │       ║               ║           ║               ║
               ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║               ║
                                                         


Figure 3-2.  PUSHA

                   BEFORE PUSHA                AFTER PUSHA
                 31          0             31          0 
         D  O   ║               ║           ║               ║
         I  F   ╠═══════╪═══════╣           ╠═══════╪═══════╣
         R      ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
         E  E   ╠═══════╪═══════╣           ╠═══════╪═══════╣
         C  X   ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
         T  P   ╠═══════╪═══════╣──ESP     ╠═══════╪═══════╣
         I  A   ║               ║           ║      EAX      ║
         O  N   ╠═══════╪═══════╣           ╠═══════╪═══════╣
         N  S   ║               ║           ║      ECX      ║
            I   ╠═══════╪═══════╣           ╠═══════╪═══════╣
          │ O   ║               ║           ║      EDX      ║
          │ N   ╠═══════╪═══════╣           ╠═══════╪═══════╣
          │     ║               ║           ║      EBX      ║
               ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║    OLD ESP    ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║      EBP      ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║      ESI      ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║      EDI      ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣──ESP
                ║               ║           ║               ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║               ║
                                                         


3.1.3  Type Conversion Instructions

The type conversion instructions convert bytes into words, words into
doublewords, and doublewords into 64-bit items (quad-words). These
instructions are especially useful for converting signed integers, because
they automatically fill the extra bits of the larger item with the value of
the sign bit of the smaller item. This kind of conversion, illustrated by
Figure 3-5, is called sign extension.

There are two classes of type conversion instructions:

  1.  The forms CWD, CDQ, CBW, and CWDE which operate only on data in the
      EAX register.

  2.  The forms MOVSX and MOVZX, which permit one operand to be in any
      general register while permitting the other operand to be in memory or
      in a register.

CWD (Convert Word to Doubleword) and CDQ (Convert Doubleword to Quad-Word)
double the size of the source operand. CWD extends the sign of the
word in register AX throughout register DX. CDQ extends the sign of the
doubleword in EAX throughout EDX. CWD can be used to produce a doubleword
dividend from a word before a word division, and CDQ can be used to produce
a quad-word dividend from a doubleword before doubleword division.

CBW (Convert Byte to Word) extends the sign of the byte in register AL
throughout AX.

CWDE (Convert Word to Doubleword Extended) extends the sign of the word in
register AX throughout EAX.

MOVSX (Move with Sign Extension) sign-extends an 8-bit value to a 16-bit
value and a 8- or 16-bit value to 32-bit value.

MOVZX (Move with Zero Extension) extends an 8-bit value to a 16-bit value
and an 8- or 16-bit value to 32-bit value by inserting high-order zeros.


Figure 3-3.  POP

       D  O         BEFORE POP                  AFTER POP
       I  F      31          0             31          0 
       R        ║               ║           ║               ║
       E  E     ╠═══════╪═══════╣           ╠═══════╪═══════╣
       C  X     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
       T  P     ╠═══════╪═══════╣           ╠═══════╪═══════╣
       I  A     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
       O  N     ╠═══════╪═══════╣           ╠═══════╪═══════╣──ESP
       N  S     ║    OPERAND    ║           ║               ║
          I     ╠═══════╪═══════╣──ESP     ╠═══════╪═══════╣
        │ O     ║               ║           ║               ║
        │ N     ╠═══════╪═══════╣           ╠═══════╪═══════╣
        │       ║               ║           ║               ║
               ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║               ║
                                                         


Figure 3-4.  POPA

                   BEFORE POPA                  AFTER POPA
                 31          0             31          0 
         D  O   ║               ║           ║               ║
         I  F   ╠═══════╪═══════╣           ╠═══════╪═══════╣
         R      ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
         E  E   ╠═══════╪═══════╣           ╠═══════╪═══════╣
         C  X   ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║           ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
         T  P   ╠═══════╪═══════╣           ╠═══════╪═══════╣──ESP
         I  A   ║      EAX      ║           ║               ║
         O  N   ╠═══════╪═══════╣           ╠═══════╪═══════╣
         N  S   ║      ECX      ║           ║               ║
            I   ╠═══════╪═══════╣           ╠═══════╪═══════╣
          │ O   ║      EDX      ║           ║               ║
          │ N   ╠═══════╪═══════╣           ╠═══════╪═══════╣
          │     ║      EBX      ║           ║               ║
               ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║      ESP      ║           ║               ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║      EPB      ║           ║               ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║      ESI      ║           ║               ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║      EDI      ║           ║               ║
                ╠═══════╪═══════╣──ESP     ╠═══════╪═══════╣
                ║               ║           ║               ║
                ╠═══════╪═══════╣           ╠═══════╪═══════╣
                ║               ║           ║               ║
                                                         


Figure 3-5.  Sign Extension

                                    15                7               0
                                    ╔═╦══════════════╪════════════════╗
     BEFORE SIGN EXTENSION─────────║S║ N N N N N N N N N N N N N N N ║
                                    ╚═╩══════════════╪════════════════╝
     AFTER SIGN EXTENSION──────┐
                               │
    31               23             15              7              0
    ╔═╦═════════════╪═══════════════╪═══════════════╪═══════════════╗
    ║S║S S S S S S S S S S S S S S S S N N N N N N N N N N N N N N N║
    ╚═╩═════════════╪═══════════════╪═══════════════╪═══════════════╝


3.2  Binary Arithmetic Instructions

The arithmetic instructions of the 80386 processor simplify the
manipulation of numeric data that is encoded in binary. Operations include
the standard add, subtract, multiply, and divide as well as increment,
decrement, compare, and change sign. Both signed and unsigned binary
integers are supported. The binary arithmetic instructions may also be used
as one step in the process of performing arithmetic on decimal integers.

Many of the arithmetic instructions operate on both signed and unsigned
integers. These instructions update the flags ZF, CF, SF, and OF in such a
manner that subsequent instructions can interpret the results of the
arithmetic as either signed or unsigned. CF contains information relevant to
unsigned integers; SF and OF contain information relevant to signed
integers. ZF is relevant to both signed and unsigned integers; ZF is set
when all bits of the result are zero.

If the integer is unsigned, CF may be tested after one of these arithmetic
operations to determine whether the operation required a carry or borrow of
a one-bit in the high-order position of the destination operand. CF is set
if a one-bit was carried out of the high-order position (addition
instructions ADD, ADC, AAA, and DAA) or if a one-bit was carried (i.e.
borrowed) into the high-order bit (subtraction instructions SUB, SBB, AAS,
DAS, CMP, and NEG).

If the integer is signed, both SF and OF should be tested. SF always has
the same value as the sign bit of the result. The most significant bit (MSB)
of a signed integer is the bit next to the sign──bit 6 of a byte, bit 14 of
a word, or bit 30 of a doubleword. OF is set in either of these cases:

  ■  A one-bit was carried out of the MSB into the sign bit but no one bit
     was carried out of the sign bit (addition instructions ADD, ADC, INC,
     AAA, and DAA). In other words, the result was greater than the greatest
     positive number that could be contained in the destination operand.

  ■  A one-bit was carried from the sign bit into the MSB but no one bit
     was carried into the sign bit (subtraction instructions SUB, SBB, DEC,
     AAS, DAS, CMP, and NEG). In other words, the result was smaller that
     the smallest negative number that could be contained in the destination
     operand.

These status flags are tested by executing one of the two families of
conditional instructions: Jcc (jump on condition cc) or SETcc (byte set on
condition).


3.2.1  Addition and Subtraction Instructions

ADD (Add Integers) replaces the destination operand with the sum of the
source and destination operands. Sets CF if overflow.

ADC (Add Integers with Carry) sums the operands, adds one if CF is set, and
replaces the destination operand with the result. If CF is cleared, ADC
performs the same operation as the ADD instruction. An ADD followed by
multiple ADC instructions can be used to add numbers longer than 32 bits.

INC (Increment) adds one to the destination operand. INC does not affect
CF. Use ADD with an immediate value of 1 if an increment that updates carry
(CF) is needed.

SUB (Subtract Integers) subtracts the source operand from the destination
operand and replaces the destination operand with the result. If a borrow is
required, the CF is set. The operands may be signed or unsigned bytes,
words, or doublewords.

SBB (Subtract Integers with Borrow) subtracts the source operand from the
destination operand, subtracts 1 if CF is set, and returns the result to the
destination operand. If CF is cleared, SBB performs the same operation as
SUB. SUB followed by multiple SBB instructions may be used to subtract
numbers longer than 32 bits. If CF is cleared, SBB performs the same
operation as SUB.

DEC (Decrement) subtracts 1 from the destination operand. DEC does not
update CF. Use SUB with an immediate value of 1 to perform a decrement that
affects carry.


3.2.2  Comparison and Sign Change Instruction

CMP (Compare) subtracts the source operand from the destination operand. It
updates OF, SF, ZF, AF, PF, and CF but does not alter the source and
destination operands. A subsequent Jcc or SETcc instruction can test the
appropriate flags.

NEG (Negate) subtracts a signed integer operand from zero. The effect of
NEG is to reverse the sign of the operand from positive to negative or from
negative to positive.


3.2.3  Multiplication Instructions

The 80386 has separate multiply instructions for unsigned and signed
operands. MUL operates on unsigned numbers, while IMUL operates on signed
integers as well as unsigned.

MUL (Unsigned Integer Multiply) performs an unsigned multiplication of the
source operand and the accumulator. If the source is a byte, the processor
multiplies it by the contents of AL and returns the double-length result to
AH and AL. If the source operand is a word, the processor multiplies it by
the contents of AX and returns the double-length result to DX and AX. If the
source operand is a doubleword, the processor multiplies it by the contents
of EAX and returns the 64-bit result in EDX and EAX. MUL sets CF and OF
when the upper half of the result is nonzero; otherwise, they are cleared.

IMUL (Signed Integer Multiply) performs a signed multiplication operation.
IMUL has three variations:

  1.  A one-operand form. The operand may be a byte, word, or doubleword
      located in memory or in a general register. This instruction uses EAX
      and EDX as implicit operands in the same way as the MUL instruction.

  2.  A two-operand form. One of the source operands may be in any general
      register while the other may be either in memory or in a general
      register. The product replaces the general-register operand.

  3.  A three-operand form; two are source and one is the destination
      operand. One of the source operands is an immediate value stored in
      the instruction; the second may be in memory or in any general
      register. The product may be stored in any general register. The
      immediate operand is treated as signed. If the immediate operand is a
      byte, the processor automatically sign-extends it to the size of the
      second operand before performing the multiplication.

The three forms are similar in most respects:

  ■  The length of the product is calculated to twice the length of the
     operands.

  ■  The CF and OF flags are set when significant bits are carried into the
     high-order half of the result. CF and OF are cleared when the
     high-order half of the result is the sign-extension of the low-order
     half.

However, forms 2 and 3 differ in that the product is truncated to the
length of the operands before it is stored in the destination register.
Because of this truncation, OF should be tested to ensure that no
significant bits are lost. (For ways to test OF, refer to the INTO and PUSHF
instructions.)

Forms 2 and 3 of IMUL may also be used with unsigned operands because,
whether the operands are signed or unsigned, the low-order half of the
product is the same.


3.2.4  Division Instructions

The 80386 has separate division instructions for unsigned and signed
operands. DIV operates on unsigned numbers, while IDIV operates on signed
integers as well as unsigned. In either case, an exception (interrupt zero)
occurs if the divisor is zero or if the quotient is too large for AL, AX, or
EAX.

DIV (Unsigned Integer Divide) performs an unsigned division of the
accumulator by the source operand. The dividend (the accumulator) is twice
the size of the divisor (the source operand); the quotient and remainder
have the same size as the divisor, as the following table shows.

Size of Source Operand
      (divisor)             Dividend       Quotient      Remainder

Byte                        AX             AL            AH
Word                        DX:AX          AX            DX
Doubleword                  EDX:EAX        EAX           EDX

Non-integral quotients are truncated to integers toward 0. The remainder is
always less than the divisor. For unsigned byte division, the largest
quotient is 255. For unsigned word division, the largest quotient is 65,535.
For unsigned doubleword division the largest quotient is 2^(32) -1.

IDIV (Signed Integer Divide) performs a signed division of the accumulator
by the source operand. IDIV uses the same registers as the DIV instruction.

For signed byte division, the maximum positive quotient is +127, and the
minimum negative quotient is -128. For signed word division, the maximum
positive quotient is +32,767, and the minimum negative quotient is -32,768.
For signed doubleword division the maximum positive quotient is 2^(31) -1,
the minimum negative quotient is -2^(31). Non-integral results are truncated
towards 0. The remainder always has the same sign as the dividend and is
less than the divisor in magnitude.


3.3  Decimal Arithmetic Instructions

Decimal arithmetic is performed by combining the binary arithmetic
instructions (already discussed in the prior section) with the decimal
arithmetic instructions. The decimal arithmetic instructions are used in one
of the following ways:

  ■  To adjust the results of a previous binary arithmetic operation to
     produce a valid packed or unpacked decimal result.

  ■  To adjust the inputs to a subsequent binary arithmetic operation so
     that the operation will produce a valid packed or unpacked decimal
     result.

These instructions operate only on the AL or AH registers. Most utilize the
AF flag.


3.3.1  Packed BCD Adjustment Instructions

DAA (Decimal Adjust after Addition) adjusts the result of adding two valid
packed decimal operands in AL. DAA must always follow the addition of two
pairs of packed decimal numbers (one digit in each half-byte) to obtain a
pair of valid packed decimal digits as results. The carry flag is set if
carry was needed.

DAS (Decimal Adjust after Subtraction) adjusts the result of subtracting
two valid packed decimal operands in AL. DAS must always follow the
subtraction of one pair of packed decimal numbers (one digit in each half-
byte) from another to obtain a pair of valid packed decimal digits as
results. The carry flag is set if a borrow was needed.


3.3.2  Unpacked BCD Adjustment Instructions

AAA (ASCII Adjust after Addition) changes the contents of register AL to a
valid unpacked decimal number, and zeros the top 4 bits. AAA must always
follow the addition of two unpacked decimal operands in AL. The carry flag
is set and AH is incremented if a carry is necessary.

AAS (ASCII Adjust after Subtraction) changes the contents of register AL to
a valid unpacked decimal number, and zeros the top 4 bits. AAS must always
follow the subtraction of one unpacked decimal operand from another in AL.
The carry flag is set and AH decremented if a borrow is necessary.

AAM (ASCII Adjust after Multiplication) corrects the result of a
multiplication of two valid unpacked decimal numbers. AAM must always follow
the multiplication of two decimal numbers to produce a valid decimal result.
The high order digit is left in AH, the low order digit in AL.

AAD (ASCII Adjust before Division) modifies the numerator in AH and AL to
prepare for the division of two valid unpacked decimal operands so that the
quotient produced by the division will be a valid unpacked decimal number.
AH should contain the high-order digit and AL the low-order digit. This
instruction adjusts the value and places the result in AL. AH will contain
zero.


3.4  Logical Instructions

The group of logical instructions includes:

  ■  The Boolean operation instructions
  ■  Bit test and modify instructions
  ■  Bit scan instructions
  ■  Rotate and shift instructions
  ■  Byte set on condition


3.4.1  Boolean Operation Instructions

The logical operations are AND, OR, XOR, and NOT.

NOT (Not) inverts the bits in the specified operand to form a one's
complement of the operand. The NOT instruction is a unary operation that
uses a single operand in a register or memory. NOT has no effect on the
flags.

The AND, OR, and XOR instructions perform the standard logical operations
"and", "(inclusive) or", and "exclusive or". These instructions can use the
following combinations of operands:

  ■  Two register operands

  ■  A general register operand with a memory operand

  ■  An immediate operand with either a general register operand or a
     memory operand.

AND, OR, and XOR clear OF and CF, leave AF undefined, and update SF, ZF,
and PF.


3.4.2  Bit Test and Modify Instructions

This group of instructions operates on a single bit which can be in memory
or in a general register. The location of the bit is specified as an offset
from the low-order end of the operand. The value of the offset either may be
given by an immediate byte in the instruction or may be contained in a
general register.

These instructions first assign the value of the selected bit to CF, the
carry flag. Then a new value is assigned to the selected bit, as determined
by the operation. OF, SF, ZF, AF, PF are left in an undefined state. Table
3-1 defines these instructions.


Table 3-1. Bit Test and Modify Instructions

Instruction                      Effect on CF            Effect on
                                                         Selected Bit

Bit (Bit Test)                   CF  BIT                (none)
BTS (Bit Test and Set)           CF  BIT                BIT  1
BTR (Bit Test and Reset)         CF  BIT                BIT  0
BTC (Bit Test and Complement)    CF  BIT                BIT  NOT(BIT)


3.4.3  Bit Scan Instructions

These instructions scan a word or doubleword for a one-bit and store the
index of the first set bit into a register.  The bit string being scanned
may be either in a register or in memory. The ZF flag is set if the entire
word is zero (no set bits are found); ZF is cleared if a one-bit is found.
If no set bit is found, the value of the destination register is undefined.

BSF (Bit Scan Forward) scans from low-order to high-order (starting from
bit index zero).

BSR (Bit Scan Reverse) scans from high-order to low-order (starting from
bit index 15 of a word or index 31 of a doubleword).


3.4.4  Shift and Rotate Instructions

The shift and rotate instructions reposition the bits within the specified
operand.

These instructions fall into the following classes:

  ■  Shift instructions
  ■  Double shift instructions
  ■  Rotate instructions


3.4.4.1  Shift Instructions

The bits in bytes, words, and doublewords may be shifted arithmetically or
logically. Depending on the value of a specified count, bits can be shifted
up to 31 places.

A shift instruction can specify the count in one of three ways. One form of
shift instruction implicitly specifies the count as a single shift. The
second form specifies the count as an immediate value. The third form
specifies the count as the value contained in CL. This last form allows the
shift count to be a variable that the program supplies during execution.
Only the low order 5 bits of CL are used.

CF always contains the value of the last bit shifted out of the destination
operand. In a single-bit shift, OF is set if the value of the high-order
(sign) bit was changed by the operation. Otherwise, OF is cleared. Following
a multibit shift, however, the content of OF is always undefined.

The shift instructions provide a convenient way to accomplish division or
multiplication by binary power. Note however that division of signed numbers
by shifting right is not the same kind of division performed by the IDIV
instruction.

SAL (Shift Arithmetic Left) shifts the destination byte, word, or
doubleword operand left by one or by the number of bits specified in the
count operand (an immediate value or the value contained in CL). The
processor shifts zeros in from the right (low-order) side of the operand as
bits exit from the left (high-order) side. See Figure 3-6.

SHL (Shift Logical Left) is a synonym for SAL (refer to SAL).

SHR (Shift Logical Right) shifts the destination byte, word, or doubleword
operand right by one or by the number of bits specified in the count operand
(an immediate value or the value contained in CL). The processor shifts
zeros in from the left side of the operand as bits exit from the right side.
See Figure 3-7.

SAR (Shift Arithmetic Right) shifts the destination byte, word, or
doubleword operand to the right by one or by the number of bits specified in
the count operand (an immediate value or the value contained in CL). The
processor preserves the sign of the operand by shifting in zeros on the left
(high-order) side if the value is positive or by shifting by ones if the
value is negative. See Figure 3-8.

Even though this instruction can be used to divide integers by a power of
two, the type of division is not the same as that produced by the IDIV
instruction. The quotient of IDIV is rounded toward zero, whereas the
"quotient" of SAR is rounded toward negative infinity. This difference is
apparent only for negative numbers. For example, when IDIV is used to divide
-9 by 4, the result is -2 with a remainder of -1. If SAR is used to shift
-9 right by two bits, the result is -3. The "remainder" of this kind of
division is +3; however, the SAR instruction stores only the high-order bit
of the remainder (in CF).

The code sequence in Figure 3-9 produces the same result as IDIV for any M
= 2^(N), where 0 < N < 32. This sequence takes about 12 to 18 clocks,
depending on whether the jump is taken; if ECX contains M, the corresponding
IDIV ECX instruction will take about 43 clocks.


Figure 3-6.  SAL and SHL

                     OF   CF                OPERAND

       BEFORE SHL    X    X     10001000100010001000100010001111
       OR SAL

       AFTER SHL     1    1 ── 00010001000100010001000100011110 ── 0
       OR SAL BY 1

       AFTER SHL     X    0 ── 00100010001000100011110000000000 ── 0
       OR SAL BY 10

SHL (WHICH HAS THE SYNONYM SAL) SHIFTS THE BITS IN THE REGISTER OR MEMORY
OPERAND TO THE LEFT BY THE SPECIFIED NUMBER OF BIT POSITIONS. CF RECEIVES
THE LAST BIT SHIFTED OUT OF THE LEFT OF THE OPERAND. SHL SHIFTS IN ZEROS
TO FILL THE VACATED BIT LOCATIONS. THESE INSTRUCTIONS OPERATE ON BYTE,
WORD, AND DOUBLEWORD OPERANDS.


Figure 3-7.  SHR

                                      OPERAND                  CF

       BEFORE SHR         10001000100010001000100010001111     X

       AFTER SHR    0────01000100010001000100010001000111────1
       BY 1

       AFTER SHR    0────00000000001000100010001000100010────O
       BY 10

SHR SHIFTS THE BITS OF THE REGISTER OR MEMORY OPERAND TO THE RIGHT BY THE
SPECIFIED NUMBER OF BIT POSITIONS. CF RECEIVES THE LAST BIT SHIFTED OUT OF
THE RIGHT OF THE OPERAND. SHR SHIFTS IN ZEROS TO FILL THE VACATED BIT
LOCATIONS.


Figure 3-8.  SAR

                                  POSITIVE OPERAND             CF

       BEFORE SAR         01000100010001000100010001000111     X

       AFTER SAR    0────00100010001000100010001000100011────1
       BY 1

                                  NEGATIVE OPERAND             CF

       BEFORE SAR         11000100010001000100010001000111     X

       AFTER SAR    0────11100010001000100010001000100011────1
       BY 1

SAR PRESERVES THE SIGN OF THE REGISTER OR MEMORY OPERAND AS IT SHIFTS THE
OPERAND TO THE RIGHT BY THE SPECIFIED NUMBER OF BIT POSITIONS. CF RECIEVES
THE LAST BIT SHIFTED OUT OF THE RIGHT OF THE OPERAND.


Figure 3-9.  Using SAR to Simulate IDIV

    ; assuming N is in ECX, and the dividend is in EAX
    ;                                               CLOCKS
    CMP     EAX, 0      ; to set sign flag          2
    JGE     NoAdjust    ; jump if sign is zero      3 or 9
    ADD     EAX, ECX    ;                           2
    DEC     EAX         ; EAX := EAX + (N-1)        2
NoAdjust:
    SAR     EAX, CL     ;                           3
    ;                       TOTAL CLOCKS           12 or 18]


3.4.4.2  Double-Shift Instructions

These instructions provide the basic operations needed to implement
operations on long unaligned bit strings. The double shifts operate either
on word or doubleword operands, as follows:

  1.  Taking two word operands as input and producing a one-word output.

  2.  Taking two doubleword operands as input and producing a doubleword
      output.

Of the two input operands, one may either be in a general register or in
memory, while the other may only be in a general register. The results
replace the memory or register operand. The number of bits to be shifted is
specified either in the CL register or in an immediate byte of the
instruction.

Bits are shifted from the register operand into the memory or register
operand. CF is set to the value of the last bit shifted out of the
destination operand. SF, ZF, and PF are set according to the value of the
result. OF and AF are left undefined.

SHLD (Shift Left Double) shifts bits of the R/M field to the left, while
shifting high-order bits from the Reg field into the R/M field on the right
(see Figure 3-10). The result is stored back into the R/M operand. The Reg
field is not modified.

SHRD (Shift Right Double) shifts bits of the R/M field to the right, while
shifting low-order bits from the Reg field into the R/M field on the left
(see Figure 3-11). The result is stored back into the R/M operand. The Reg
field is not modified.


3.4.4.3  Rotate Instructions

Rotate instructions allow bits in bytes, words, and doublewords to be
rotated. Bits rotated out of an operand are not lost as in a shift, but are
"circled" back into the other "end" of the operand.

Rotates affect only the carry and overflow flags. CF may act as an
extension of the operand in two of the rotate instructions, allowing a bit
to be isolated and then tested by a conditional jump instruction (JC or
JNC). CF always contains the value of the last bit rotated out, even if the
instruction does not use this bit as an extension of the rotated operand.

In single-bit rotates, OF is set if the operation changes the high-order
(sign) bit of the destination operand. If the sign bit retains its original
value, OF is cleared. On multibit rotates, the value of OF is always
undefined.

ROL (Rotate Left) rotates the byte, word, or doubleword destination operand
left by one or by the number of bits specified in the count operand (an
immediate value or the value contained in CL). For each rotation specified,
the high-order bit that exits from the left of the operand returns at the
right to become the new low-order bit of the operand. See Figure 3-12.

ROR (Rotate Right) rotates the byte, word, or doubleword destination
operand right by one or by the number of bits specified in the count operand
(an immediate value or the value contained in CL). For each rotation
specified, the low-order bit that exits from the right of the operand
returns at the left to become the new high-order bit of the operand.
See Figure 3-13.

RCL (Rotate Through Carry Left) rotates bits in the byte, word, or
doubleword destination operand left by one or by the number of bits
specified in the count operand (an immediate value or the value contained in
CL).

This instruction differs from ROL in that it treats CF as a high-order
one-bit extension of the destination operand. Each high-order bit that exits
from the left side of the operand moves to CF before it returns to the
operand as the low-order bit on the next rotation cycle. See Figure 3-14.

RCR (Rotate Through Carry Right) rotates bits in the byte, word, or
doubleword destination operand right by one or by the number of bits
specified in the count operand (an immediate value or the value contained in
CL).

This instruction differs from ROR in that it treats CF as a low-order
one-bit extension of the destination operand. Each low-order bit that exits
from the right side of the operand moves to CF before it returns to the
operand as the high-order bit on the next rotation cycle. See Figure 3-15.


Figure 3-10.  Shift Left Double

                  31                   DESTINATION                 0
    ╔════╗       ╔══════════════════════════════════════════════════╗
    ║ CF ║──────╢                 MEMORY OF REGISTER               ║───┐
    ╚════╝       ╚══════════════════════════════════════════════════╝    │
             ┌───────────────────────────────────────────────────────────┘
             │    31                     SOURCE                    0
             │   ╔══════════════════════════════════════════════════╗
             └───╢                      REGISTER                    ║
                 ╚══════════════════════════════════════════════════╝


Figure 3-11.  Shift Right Double

        31                     SOURCE                    0
       ╔══════════════════════════════════════════════════╗
       ║                      REGISTER                    ╟───┐
       ╚══════════════════════════════════════════════════╝   │
   ┌──────────────────────────────────────────────────────────┘
   │    31                   DESTINATION                 0
   │   ╔══════════════════════════════════════════════════╗        ╔════╗
   └──║                 MEMORY OF REGISTER               ╟───────║ CF ║
       ╚══════════════════════════════════════════════════╝        ╚════╝


Figure 3-12.  ROL

                  31                   DESTINATION                 0
    ╔════╗       ╔══════════════════════════════════════════════════╗
    ║ CF ║───┬──╢                 MEMORY OF REGISTER               ║──┐
    ╚════╝    │  ╚══════════════════════════════════════════════════╝   │
              └─────────────────────────────────────────────────────────┘


Figure 3-13.  ROR

     ┌──────────────────────────────────────────────────────────┐
     │    31                   DESTINATION                 0    │
     │   ╔══════════════════════════════════════════════════╗   │    ╔════╗
     └──║                 MEMORY OF REGISTER               ╟───┴───║ CF ║
         ╚══════════════════════════════════════════════════╝        ╚════╝


Figure 3-14.  RCL

                  31                   DESTINATION                 0
    ╔════╗       ╔══════════════════════════════════════════════════╗
  ┌─╢ CF ║──────╢                 MEMORY OF REGISTER               ║──┐
  │ ╚════╝       ╚══════════════════════════════════════════════════╝   │
  └─────────────────────────────────────────────────────────────────────┘


Figure 3-15.  RCR

   ┌──────────────────────────────────────────────────────────────────────┐
   │    31                   DESTINATION                 0                │
   │   ╔══════════════════════════════════════════════════╗        ╔════╗ │
   └──║                 MEMORY OF REGISTER               ╟───────║ CF ╟─┘
       ╚══════════════════════════════════════════════════╝        ╚════╝


3.4.4.4  Fast "BIT BLT" Using Double Shift Instructions

One purpose of the double shifts is to implement a bit string move, with
arbitrary misalignment of the bit strings.  This is called a "bit blt" (BIT
BLock Transfer.)  A simple example is to move a bit string from an arbitrary
offset into a doubleword-aligned byte string.  A left-to-right string is
moved 32 bits at a time if a double shift is used inside the move loop.

     MOV   ESI,ScrAddr
     MOV   EDI,DestAddr
     MOV   EBX,WordCnt
     MOV   CL,RelOffset      ; relative offset Dest-Src
     MOV   EDX,[ESI]         ; load first word of source
     ADD   ESI,4             ; bump source address
BltLoop:
     LODS                    ; new low order part
     SHLD  EDX,EAX,CL        ; EDX overwritten with aligned stuff
     XCHG  EDX,EAS           ; Swap high/low order parts
     STOS                    ; Write out next aligned chunk
     DEC   EBX
     JA    BltLoop

This loop is simple yet allows the data to be moved in 32-bit pieces for
the highest possible performance. Without a double shift, the best that can
be achieved is 16 bits per loop iteration by using a 32-bit shift and
replacing the XCHG with a ROR by 16 to swap high and low order parts of
registers. A more general loop than shown above would require some extra
masking on the first doubleword moved (before the main loop), and on the
last doubleword moved (after the main loop), but would have the same basic
32-bits per loop iteration as the code above.


3.4.4.5  Fast Bit-String Insert and Extract

The double shift instructions also enable:

  ■  Fast insertion of a bit string from a register into an arbitrary bit
     location in a larger bit string in memory without disturbing the bits
     on either side of the inserted bits.

  ■  Fast extraction of a bits string into a register from an arbitrary bit
     location in a larger bit string in memory without disturbing the bits
     on either side of the extracted bits.

The following coded examples illustrate bit insertion and extraction under
variousconditions:

  1.  Bit String Insert into Memory (when bit string is 1-25 bits long,
      i.e., spans four bytes or less):

      ; Insert a right-justified bit string from register into
      ; memory bit string.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate value
      ;    but the bit offset is held in a register.
      ;
      ; Register ESI holds the right-justified bit string
      ; to be inserted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX and ECX are also used by this
      ; "insert" operation.
      ;
      MOV   ECX,EDI      ; preserve original offset for later use
      SHR   EDI,3        ; signed divide offset by 8 (byte address)
      AND   CL,7H        ; isolate low three bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move string dword into EAX
      ROR   EAX,CL       ; right justify old bit field
      SHRD  EAX,ESI,length          ; bring in new bits
      ROL   EAX,length   ; right justify new bit field
      ROL   EAX,CL       ; bring to final position
      MOV   [EDI]strg_base,EAX      ; replace dword in memory

  2.  Bit String Insert into Memory (when bit string is 1-31 bits long, i.e.
      spans five bytes or less):

      ; Insert a right-justified bit string from register into
      ; memory bit string.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate value
      ;    but the bit offset is held in a register.
      ;
      ; Register ESI holds the right-justified bit string
      ; to be inserted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX, EBX, ECX, and EDI are also used by
      ; this "insert" operation.
      ;
      MOV   ECX,EDI     ; temp storage for offset
      SHR   EDI,5       ; signed divide offset by 32 (dword address)
      SHL   EDI,2       ; multiply by 4 (in byte address format)
      AND   CL,1FH      ; isolate low five bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move low string dword into EAX
      MOV   EDX,[EDI]strg_base+4    ; other string dword into EDX
      MOV   EBX,EAX     ; temp storage for part of string     ┐ rotate
      SHRD  EAX,EDX,CL  ; double shift by offset within dword ├ EDX:EAX
      SHRD  EAX,EBX,CL  ; double shift by offset within dword ┘ right
      SHRD  EAX,ESI,length          ; bring in new bits
      ROL   EAX,length  ; right justify new bit field
      MOV   EBX,EAX     ; temp storage for part of string         ┐ rotate
      SHLD  EAX,EDX,CL  ; double shift back by offset within word ├ EDX:EAX
      SHLD  EDX,EBX,CL  ; double shift back by offset within word ┘ left
      MOV   [EDI]strg_base,EAX      ; replace dword in memory
      MOV   [EDI]strg_base+4,EDX    ; replace dword in memory

  3.  Bit String Insert into Memory (when bit string is exactly 32 bits
      long, i.e., spans five or four types of memory):

      ; Insert right-justified bit string from register into
      ; memory bit string.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is 32
      ;    but the bit offset is held in a register.
      ;
      ; Register ESI holds the 32-bit string to be inserted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX, EBX, ECX, and EDI are also used by
      ; this "insert" operation.
      ;
      MOV   EDX,EDI     ; preserve original offset for later use
      SHR   EDI,5       ; signed divide offset by 32 (dword address)
      SHL   EDI,2       ; multiply by 4 (in byte address format)
      AND   CL,1FH      ; isolate low five bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move low string dword into EAX
      MOV   EDX,[EDI]strg_base+4    ; other string dword into EDX
      MOV   EBX,EAX     ; temp storage for part of string     ┐ rotate
      SHRD  EAX,EDX     ; double shift by offset within dword ├ EDX:EAX
      SHRD  EDX,EBX     ; double shift by offset within dword ┘ right
      MOV   EAX,ESI     ; move 32-bit bit field into position
      MOV   EBX,EAX     ; temp storage for part of string         ┐ rotate
      SHLD  EAX,EDX     ; double shift back by offset within word ├ EDX:EAX
      SHLD  EDX,EBX     ; double shift back by offset within word ┘ left
      MOV   [EDI]strg_base,EAX      ; replace dword in memory
      MOV   [EDI]strg_base,+4,EDX   ; replace dword in memory

  4.  Bit String Extract from Memory (when bit string is 1-25 bits long,
      i.e., spans four bytes or less):

      ; Extract a right-justified bit string from memory bit
      ; string into register
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate value
      ;    but the bit offset is held in a register.
      ;
      ; Register EAX holds the right-justified, zero-padded
      ; bit string that was extracted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EDI, and ECX are also used by this "extract."
      ;
      MOV  ECX,EDI      ; temp storage for offset
      SHR  EDI,3        ; signed divide offset by 8 (byte address)
      AND  CL,7H        ; isolate low three bits of offset
      MOV  EAX,[EDI]strg_base       ; move string dword into EAX
      SHR  EAX,CL       ; shift by offset within dword
      AND  EAX,mask     ; extracted bit field in EAX

  5.  Bit String Extract from Memory (when bit string is 1-32 bits long,
      i.e., spans five bytes or less):

      ; Extract a right-justified bit string from memory bit
      ; string into register.
      ;
      ; Assumptions:
      ; 1) The base of the string array is dword aligned, and
      ; 2) the length of the bit string is an immediate
      ;    value but the bit offset is held in a register.
      ;
      ; Register EAX holds the right-justified, zero-padded
      ; bit string that was extracted.
      ; Register EDI holds the bit offset of the start of the
      ; substring.
      ; Registers EAX, EBX, and ECX are also used by this "extract."
      MOV   ECX,EDI     ; temp storage for offset
      SHR   EDI,5       ; signed divide offset by 32 (dword address)
      SHL   EDI,2       ; multiply by 4 (in byte address format)
      AND   CL,1FH      ; isolate low five bits of offset in CL
      MOV   EAX,[EDI]strg_base      ; move low string dword into EAX
      MOV   EDX,[EDI]strg_base+4    ; other string dword into EDX
      SHRD  EAX,EDX,CL  ; double shift right by offset within dword
      AND   EAX,mask    ; extracted bit field in EAX


3.4.5  Byte-Set-On-Condition Instructions

This group of instructions sets a byte to zero or one depending on any of
the 16 conditions defined by the status flags. The byte may be in memory or
may be a one-byte general register. These instructions are especially useful
for implementing Boolean expressions in high-level languages such as Pascal.

SETcc (Set Byte on Condition cc) set a byte to one if condition cc is true;
sets the byte to zero otherwise. Refer to Appendix D for a definition of
the possible conditions.


3.4.6  Test Instruction

TEST (Test) performs the logical "and" of the two operands, clears OF and
CF, leaves AF undefined, and updates SF, ZF, and PF. The flags can be tested
by conditional control transfer instructions or by the byte-set-on-condition
instructions. The operands may be doublewords, words, or bytes.

The difference between TEST and AND is that TEST does not alter the
destination operand. TEST differs from BT in that TEST is useful for testing
the value of multiple bits in one operations, whereas BT tests a single bit.


3.5  Control Transfer Instructions

The 80386 provides both conditional and unconditional control transfer
instructions to direct the flow of execution. Conditional control transfers
depend on the results of operations that affect the flag register.
Unconditional control transfers are always executed.


3.5.1  Unconditional Transfer Instructions

JMP, CALL, RET, INT and IRET instructions transfer control from one code
segment location to another. These locations can be within the same code
segment (near control transfers) or in different code segments (far control
transfers). The variants of these instructions that transfer control to
other segments are discussed in a later section of this chapter. If the
model of memory organization used in a particular 80386 application does
not make segments visible to applications programmers, intersegment control
transfers will not be used.


3.5.1.1  Jump Instruction

JMP (Jump) unconditionally transfers control to the target location. JMP is
a one-way transfer of execution; it does not save a return address on the
stack.

The JMP instruction always performs the same basic function of transferring
control from the current location to a new location. Its implementation
varies depending on whether the address is specified directly within the
instruction or indirectly through a register or memory.

A direct JMP instruction includes the destination address as part of the
instruction. An indirect JMP instruction obtains the destination address
indirectly through a register or a pointer variable.

Direct near JMP. A direct JMP uses a relative displacement value contained
in the instruction. The displacement is signed and the size of the
displacement may be a byte, word, or doubleword. The processor forms an
effective address by adding this relative displacement to the address
contained in EIP. When the additions have been performed, EIP refers to the
next instruction to be executed.

Indirect near JMP. Indirect JMP instructions specify an absolute address in
one of several ways:

  1.  The program can JMP to a location specified by a general register
      (any of EAX, EDX, ECX, EBX, EBP, ESI, or EDI). The processor moves
      this 32-bit value into EIP and resumes execution.

  2.  The processor can obtain the destination address from a memory
      operand specified in the instruction.

  3.  A register can modify the address of the memory pointer to select a
      destination address.


3.5.1.2  Call Instruction

CALL (Call Procedure) activates an out-of-line procedure, saving on the
stack the address of the instruction following the CALL for later use by a
RET (Return) instruction. CALL places the current value of EIP on the stack.
The RET instruction in the called procedure uses this address to transfer
control back to the calling program.

CALL instructions, like JMP instructions have relative, direct, and
indirect versions.

Indirect CALL instructions specify an absolute address in one of these
ways:

  1.  The program can CALL a location specified by a general register (any
      of EAX, EDX, ECX, EBX, EBP, ESI, or EDI). The processor moves this
      32-bit value into EIP.

  2.  The processor can obtain the destination address from a memory
      operand specified in the instruction.


3.5.1.3  Return and Return-From-Interrupt Instruction

RET (Return From Procedure) terminates the execution of a procedure and
transfers control through a back-link on the stack to the program that
originally invoked the procedure. RET restores the value of EIP that was
saved on the stack by the previous CALL instruction.

RET instructions may optionally specify an immediate operand. By adding
this constant to the new top-of-stack pointer, RET effectively removes any
arguments that the calling program pushed on the stack before the execution
of the CALL instruction.

IRET (Return From Interrupt) returns control to an interrupted procedure.
IRET differs from RET in that it also pops the flags from the stack into the
flags register. The flags are stored on the stack by the interrupt
mechanism.


3.5.2  Conditional Transfer Instructions

The conditional transfer instructions are jumps that may or may not
transfer control, depending on the state of the CPU flags when the
instruction executes.


3.5.2.1  Conditional Jump Instructions

Table 3-2 shows the conditional transfer mnemonics and their
interpretations. The conditional jumps that are listed as pairs are actually
the same instruction. The assembler provides the alternate mnemonics for
greater clarity within a program listing.

Conditional jump instructions contain a displacement which is added to the
EIP register if the condition is true. The displacement may be a byte, a
word, or a doubleword. The displacement is signed; therefore, it can be used
to jump forward or backward.


Table 3-2. Interpretation of Conditional Transfers

Unsigned Conditional Transfers

Mnemonic         Condition Tested          "Jump If..."

JA/JNBE           (CF or ZF) = 0           above/not below nor equal
JAE/JNB           CF = 0                   above or equal/not below
JB/JNAE           CF = 1                   below/not above nor equal
JBE/JNA           (CF or ZF) = 1           below or equal/not above
JC                CF = 1                   carry
JE/JZ             ZF = 1                   equal/zero
JNC               CF = 0                   not carry
JNE/JNZ           ZF = 0                   not equal/not zero
JNP/JPO           PF = 0                   not parity/parity odd
JP/JPE            PF = 1                   parity/parity even

Signed Conditional Transfers

Mnemonic         Condition Tested          "Jump If..."
JG/JNLE          ((SF xor OF) or ZF) = 0   greater/not less nor equal
JGE/JNL          (SF xor OF) = 0           greater or equal/not less
JL/JNGE          (SF xor OF) = 1           less/not greater nor equal
JLE/JNG          ((SF xor OF) or ZF) = 1   less or equal/not greater
JNO              OF = 0                    not overflow
JNS              SF = 0                    not sign (positive, including 0)
JO               OF = 1                    overflow
JS               SF = 1                    sign (negative)


3.5.2.2  Loop Instructions

The loop instructions are conditional jumps that use a value placed in ECX
to specify the number of repetitions of a software loop. All loop
instructions automatically decrement ECX and terminate the loop when ECX=0.
Four of the five loop instructions specify a condition involving ZF that
terminates the loop before ECX reaches zero.

LOOP (Loop While ECX Not Zero) is a conditional transfer that automatically
decrements the ECX register before testing ECX for the branch condition. If
ECX is non-zero, the program branches to the target label specified in the
instruction. The LOOP instruction causes the repetition of a code section
until the operation of the LOOP instruction decrements ECX to a value of
zero. If LOOP finds ECX=0, control transfers to the instruction immediately
following the LOOP instruction. If the value of ECX is initially zero, then
the LOOP executes 2^(32) times.

LOOPE (Loop While Equal) and LOOPZ (Loop While Zero) are synonyms for the
same instruction. These instructions automatically decrement the ECX
register before testing ECX and ZF for the branch conditions. If ECX is
non-zero and ZF=1, the program branches to the target label specified in the
instruction. If LOOPE or LOOPZ finds that ECX=0 or ZF=0, control transfers
to the instruction immediately following the LOOPE or LOOPZ instruction.

LOOPNE (Loop While Not Equal) and LOOPNZ (Loop While Not Zero) are synonyms
for the same instruction. These instructions automatically decrement the ECX
register before testing ECX and ZF for the branch conditions. If ECX is
non-zero and ZF=0, the program branches to the target label specified in the
instruction. If LOOPNE or LOOPNZ finds that ECX=0 or ZF=1, control transfers
to the instruction immediately following the LOOPNE or LOOPNZ instruction.


3.5.2.3  Executing a Loop or Repeat Zero Times

JCXZ (Jump if ECX Zero) branches to the label specified in the instruction
if it finds a value of zero in ECX. JCXZ is useful in combination with the
LOOP instruction and with the string scan and compare instructions, all of
which decrement ECX. Sometimes, it is desirable to design a loop that
executes zero times if the count variable in ECX is initialized to zero.
Because the LOOP instructions (and repeat prefixes) decrement ECX before
they test it, a loop will execute 2^(32) times if the program enters the
loop with a zero value in ECX. A programmer may conveniently overcome this
problem with JCXZ, which enables the program to branch around the code
within the loop if ECX is zero when JCXZ executes. When used with repeated
string scan and compare instructions, JCXZ can determine whether the
repetitions terminated due to zero in ECX or due to satisfaction of the
scan or compare conditions.


3.5.3  Software-Generated Interrupts

The INT n, INTO, and BOUND instructions allow the programmer to specify a
transfer to an interrupt service routine from within a program.

INT n (Software Interrupt) activates the interrupt service routine that
corresponds to the number coded within the instruction. The INT instruction
may specify any interrupt type. Programmers may use this flexibility to
implement multiple types of internal interrupts or to test the operation of
interrupt service routines. (Interrupts 0-31 are reserved by Intel.) The
interrupt service routine terminates with an IRET instruction that returns
control to the instruction that follows INT.

INTO (Interrupt on Overflow) invokes interrupt 4 if OF is set. Interrupt 4
is reserved for this purpose. OF is set by several arithmetic, logical, and
string instructions.

BOUND (Detect Value Out of Range) verifies that the signed value contained
in the specified register lies within specified limits. An interrupt (INT 5)
occurs if the value contained in the register is less than the lower bound
or greater than the upper bound.

The BOUND instruction includes two operands. The first operand specifies
the register being tested. The second operand contains the effective
relative address of the two signed BOUND limit values. The BOUND instruction
assumes that the upper limit and lower limit are in adjacent memory
locations. These limit values cannot be register operands; if they are, an
invalid opcode exception occurs.

BOUND is useful for checking array bounds before using a new index value to
access an element within the array. BOUND provides a simple way to check the
value of an index register before the program overwrites information in a
location beyond the limit of the array.

The block of memory that specifies the lower and upper limits of an array
might typically reside just before the array itself. This makes the array
bounds accessible at a constant offset from the beginning of the array.
Because the address of the array will already be present in a register, this
practice avoids extra calculations to obtain the effective address of the
array bounds.

The upper and lower limit values may each be a word or a doubleword.


3.6  String and Character Translation Instructions

The instructions in this category operate on strings rather than on logical
or numeric values. Refer also to the section on I/O for information about
the string I/O instructions (also known as block I/O).

The power of 80386 string operations derives from the following features of
the architecture:

1.  A set of primitive string operations

   MOVS   ── Move String
   CMPS   ── Compare string
   SCAS   ── Scan string
   LODS   ── Load string
   STOS   ── Store string

2.  Indirect, indexed addressing, with automatic incrementing or
    decrementing of the indexes.

   Indexes:

      ESI    ── Source index register
      EDI    ── Destination index register

   Control flag:

      DF     ── Direction flag

   Control flag instructions:

      CLD    ── Clear direction flag instruction
      STD    ── Set direction flag instruction

3.  Repeat prefixes

   REP          ── Repeat while ECX not xero
   REPE/REPZ    ── Repeat while equal or zero
   REPNE/REPNZ  ── Repeat while not equal or not zero

The primitive string operations operate on one element of a string. A
string element may be a byte, a word, or a doubleword. The string elements
are addressed by the registers ESI and EDI. After every primitive operation
ESI and/or EDI are automatically updated to point to the next element of the
string. If the direction flag is zero, the index registers are incremented;
if one, they are decremented. The amount of the increment or decrement is
1, 2, or 4 depending on the size of the string element.


3.6.1  Repeat Prefixes

The repeat prefixes REP (Repeat While ECX Not Zero), REPE/REPZ (Repeat
While Equal/Zero), and REPNE/REPNZ (Repeat While Not Equal/Not Zero) specify
repeated operation of a string primitive. This form of iteration allows the
CPU to process strings much faster than would be possible with a regular
software loop.

When a primitive string operation has a repeat prefix, the operation is
executed repeatedly, each time using a different element of the string. The
repetition terminates when one of the conditions specified by the prefix is
satisfied.

At each repetition of the primitive instruction, the string operation may
be suspended temporarily in order to handle an exception or external
interrupt. After the interruption, the string operation can be restarted
again where it left off. This method of handling strings allows operations
on strings of arbitrary length, without affecting interrupt response.

All three prefixes causes the hardware to automatically repeat the
associated string primitive until ECX=0. The differences among the repeat
prefixes have to do with the second termination condition. REPE/REPZ and
REPNE/REPNZ are used exclusively with the SCAS (Scan String) and CMPS
(Compare String) primitives. When these prefixes are used, repetition of the
next instruction depends on the zero flag (ZF) as well as the ECX register.
ZF does not require initialization before execution of a repeated string
instruction, because both SCAS and CMPS set ZF according to the results of
the comparisons they make. The differences are summarized in the
accompanying table.

Prefix                      Termination         Termination
                            Condition 1         Condition 2

REP                           ECX = 0             (none)
REPE/REPZ                     ECX = 0             ZF = 0
REPNE/REPNZ                   ECX = 0             ZF = 1


3.6.2  Indexing and Direction Flag Control

The addresses of the operands of string primitives are determined by the
ESI and EDI registers. ESI points to source operands. By default, ESI refers
to a location in the segment indicated by the DS segment register. A
segment-override prefix may be used, however, to cause ESI to refer to CS,
SS, ES, FS, or GS. EDI points to destination operands in the segment
indicated by ES; no segment override is possible. The use of two different
segment registers in one instruction allows movement of strings between
different segments.

This use of ESI and DSI has led to the descriptive names source index and
destination index for the ESI and EDI registers, respectively. In all
cases other than string instructions, however, the ESI and EDI registers may
be used as general-purpose registers.

When ESI and EDI are used in string primitives, they are automatically
incremented or decremented after to operation. The direction flag determines
whether they are incremented or decremented. The instruction CLD puts zero
in DF, causing the index registers to be incremented; the instruction STD
puts one in DF, causing the index registers to be decremented. Programmers
should always put a known value in DF before using string instructions in a
procedure.


3.6.3  String Instructions

MOVS (Move String) moves the string element pointed to by ESI to the
location pointed to by EDI. MOVSB operates on byte elements, MOVSW operates
on word elements, and MOVSD operates on doublewords. The destination segment
register cannot be overridden by a segment override prefix, but the source
segment register can be overridden.

The MOVS instruction, when accompanied by the REP prefix, operates as a
memory-to-memory block transfer. To set up for this operation, the program
must initialize ECX and the register pairs ESI and EDI. ECX specifies the
number of bytes, words, or doublewords in the block.

If DF=0, the program must point ESI to the first element of the source
string and point EDI to the destination address for the first element. If
DF=1, the program must point these two registers to the last element of the
source string and to the destination address for the last element,
respectively.

CMPS (Compare Strings) subtracts the destination string element (at ES:EDI)
from the source string element (at ESI) and updates the flags AF, SF, PF, CF
and OF. If the string elements are equal, ZF=1; otherwise, ZF=0. If DF=0,
the processor increments the memory pointers (ESI and EDI) for the two
strings. CMPSB compares bytes, CMPSW compares words, and CMPSD compares
doublewords. The segment register used for the source address can be changed
with a segment override prefix while the destination segment register
cannot be overridden.

SCAS (Scan String) subtracts the destination string element at ES:EDI from
EAX, AX, or AL and updates the flags AF, SF, ZF, PF, CF and OF. If the
values are equal, ZF=1; otherwise, ZF=0. If DF=0, the processor increments
the memory pointer (EDI) for the string. SCASB scans bytes; SCASW scans
words; SCASD scans doublewords. The destination segment register (ES) cannot
be overridden.

When either the REPE or REPNE prefix modifies either the SCAS or CMPS
primitives, the processor compares the value of the current string element
with the value in EAX for doubleword elements, in AX for word elements, or
in AL for byte elements. Termination of the repeated operation depends on
the resulting state of ZF as well as on the value in ECX.

LODS (Load String) places the source string element at ESI into EAX for
doubleword strings, into AX for word strings, or into AL for byte strings.
LODS increments or decrements ESI according to DF.

STOS (Store String) places the source string element from EAX, AX, or AL
into the string at ES:DSI. STOS increments or decrements EDI according to
DF.


3.7  Instructions for Block-Structured Languages

The instructions in this section provide machine-language support for
functions normally found in high-level languages. These instructions include
ENTER and LEAVE, which simplify the programming of procedures.

ENTER (Enter Procedure) creates a stack frame that may be used to implement
the scope rules of block-structured high-level languages. A LEAVE
instruction at the end of a procedure complements an ENTER at the beginning
of the procedure to simplify stack management and to control access to
variables for nested procedures.

The ENTER instruction includes two parameters. The first parameter
specifies the number of bytes of dynamic storage to be allocated on the
stack for the routine being entered. The second parameter corresponds to the
lexical nesting level (0-31) of the routine. (Note that the lexical level
has no relationship to either the protection privilege levels or to the I/O
privilege level.)

The specified lexical level determines how many sets of stack frame
pointers the CPU copies into the new stack frame from the preceding frame.
This list of stack frame pointers is sometimes called the display. The first
word of the display is a pointer to the last stack frame. This pointer
enables a LEAVE instruction to reverse the action of the previous ENTER
instruction by effectively discarding the last stack frame.

   Example: ENTER 2048,3

   Allocates 2048 bytes of dynamic storage on the stack and sets up pointers
   to two previous stack frames in the stack frame that ENTER creates for
   this procedure.

After ENTER creates the new display for a procedure, it allocates the
dynamic storage space for that procedure by decrementing ESP by the number
of bytes specified in the first parameter. This new value of ESP serves as a
starting point for all PUSH and POP operations within that procedure.

To enable a procedure to address its display, ENTER leaves EBP pointing to
the beginning of the new stack frame. Data manipulation instructions that
specify EBP as a base register implicitly address locations within the stack
segment instead of the data segment.

The ENTER instruction can be used in two ways: nested and non-nested. If
the lexical level is 0, the non-nested form is used. Since the second
operand is 0, ENTER pushes EBP, copies ESP to EBP and then subtracts the
first operand from ESP. The nested form of ENTER occurs when the second
parameter (lexical level) is not 0.

Figure 3-16 gives the formal definition of ENTER.

The main procedure (with other procedures nested within) operates at the
highest lexical level, level 1. The first procedure it calls operates at the
next deeper lexical level, level 2. A level 2 procedure can access the
variables of the main program which are at fixed locations specified by the
compiler. In the case of level 1, ENTER allocates only the requested
dynamic storage on the stack because there is no previous display to copy.

A program operating at a higher lexical level calling a program at a lower
lexical level requires that the called procedure should have access to the
variables of the calling program. ENTER provides this access through a
display that provides addressability to the calling program's stack frame.

A procedure calling another procedure at the same lexical level implies
that they are parallel procedures and that the called procedure should not
have access to the variables of the calling procedure. In this case, ENTER
copies only that portion of the display from the calling procedure which
refers to previously nested procedures operating at higher lexical levels.
The new stack frame does not include the pointer for addressing the calling
procedure's stack frame.

ENTER treats a reentrant procedure as a procedure calling another procedure
at the same lexical level. In this case, each succeeding iteration of the
reentrant procedure can address only its own variables and the variables of
the calling procedures at higher lexical levels. A reentrant procedure can
always address its own variables; it does not require pointers to the stack
frames of previous iterations.

By copying only the stack frame pointers of procedures at higher lexical
levels, ENTER makes sure that procedures access only those variables of
higher lexical levels, not those at parallel lexical levels (see Figure
3-17). Figures 3-18 through 3-21 demonstrate the actions of the ENTER
instruction if the modules shown in Figure 3-17 were to call one another in
alphabetic order.

Block-structured high-level languages can use the lexical levels defined by
ENTER to control access to the variables of previously nested procedures.
Referring to Figure 3-17 for example, if PROCEDURE A calls PROCEDURE B
which, in turn, calls PROCEDURE C, then PROCEDURE C will have access to the
variables of MAIN and PROCEDURE A, but not PROCEDURE B because they operate
at the same lexical level. Following is the complete definition of access to
variables for Figure 3-17.

  1.  MAIN PROGRAM has variables at fixed locations.

  2.  PROCEDURE A can access only the fixed variables of MAIN.

  3.  PROCEDURE B can access only the variables of PROCEDURE A and MAIN.
      PROCEDURE B cannot access the variables of PROCEDURE C or PROCEDURE D.

  4.  PROCEDURE C can access only the variables of PROCEDURE A and MAIN.
      PROCEDURE C cannot access the variables of PROCEDURE B or PROCEDURE D.

  5.  PROCEDURE D can access the variables of PROCEDURE C, PROCEDURE A, and
      MAIN. PROCEDURE D cannot access the variables of PROCEDURE B.

ENTER at the beginning of the MAIN PROGRAM creates dynamic storage space
for MAIN but copies no pointers. The first and only word in the display
points to itself because there is no previous value for LEAVE to return to
EBP. See Figure 3-18.

After MAIN calls PROCEDURE A, ENTER creates a new display for PROCEDURE A
with the first word pointing to the previous value of EBP (BPM for LEAVE to
return to the MAIN stack frame) and the second word pointing to the current
value of EBP. Procedure A can access variables in MAIN since MAIN is at
level 1. Therefore the base for the dynamic storage for MAIN is at [EBP-2].
All dynamic variables for MAIN are at a fixed offset from this value. See
Figure 3-19.

After PROCEDURE A calls PROCEDURE B, ENTER creates a new display for
PROCEDURE B with the first word pointing to the previous value of EBP, the
second word pointing to the value of EBP for MAIN, and the third word
pointing to the value of EBP for A and the last word pointing to the current
EBP. B can access variables in A and MAIN by fetching from the display the
base addresses of the respective dynamic storage areas. See Figure 3-20.

After PROCEDURE B calls PROCEDURE C, ENTER creates a new display for
PROCEDURE C with the first word pointing to the previous value of EBP, the
second word pointing to the value of EBP for MAIN, and the third word
pointing to the EBP value for A and the third word pointing to the current
value of EBP. Because PROCEDURE B and PROCEDURE C have the same lexical
level, PROCEDURE C is not allowed access to variables in B and therefore
does not receive a pointer to the beginning of PROCEDURE B's stack frame.
See Figure 3-21.

LEAVE (Leave Procedure) reverses the action of the previous ENTER
instruction. The LEAVE instruction does not include any operands. LEAVE
copies EBP to ESP to release all stack space allocated to the procedure by
the most recent ENTER instruction. Then LEAVE pops the old value of EBP from
the stack. A subsequent RET instruction can then remove any arguments that
were pushed on the stack by the calling program for use by the called
procedure.


Figure 3-16.  Formal Definition of the ENTER Instruction

The formal definition of the ENTER instruction for all cases is given by the
following listing. LEVEL denotes the value of the second operand.

Push EBP
Set a temporary value FRAME_PTR := ESP
If LEVEL > 0 then
      Repeat (LEVEL-1) times:
          EBP :=EBP - 4
          Push the doubleword pointed to by EBP
      End repeat
      Push FRAME_PTR
End if
EBP := FRAME_PTR
ESP := ESP - first operand.


Figure 3-17.  Variable Access in Nested Procedures

      ╔════════════════════════════════════════════════════════════════╗
      ║                MAIN PROCEDURE (LEXICAL LEVEL 1)                ║
      ║   ╔════════════════════════════════════════════════════════╗   ║
      ║   ║              PROCEDURE A (LEXICAL LEVEL 2)             ║   ║
      ║   ║  ╔══════════════════════════════════════════════════╗  ║   ║
      ║   ║  ║           PROCEDURE B (LEXICAL LEVEL 3)          ║  ║   ║
      ║   ║  ╚══════════════════════════════════════════════════╝  ║   ║
      ║   ║                                                        ║   ║
      ║   ║  ╔══════════════════════════════════════════════════╗  ║   ║
      ║   ║  ║           PROCEDURE C (LEXICAL LEVEL 3)          ║  ║   ║
      ║   ║  ║  ╔════════════════════════════════════════════╗  ║  ║   ║
      ║   ║  ║  ║        PROCEDURE D (LEXICAL LEVEL 4)       ║  ║  ║   ║
      ║   ║  ║  ╚════════════════════════════════════════════╝  ║  ║   ║
      ║   ║  ║                                                  ║  ║   ║
      ║   ║  ╚══════════════════════════════════════════════════╝  ║   ║
      ║   ║                                                        ║   ║
      ║   ╚════════════════════════════════════════════════════════╝   ║
      ║                                                                ║
      ╚════════════════════════════════════════════════════════════════╝


Figure 3-18.  Stack Frame for MAIN at Level 1

                                      31          0 
                D  O                 ║               ║
                I  F              ┌─ ╠═══════╪═══════╣
                R                 │  ║    OLD ESP    ║
                E  E     DISPLAY ─┤  ╠═══════╪═══════╣──EBP FOR
                C  X              │  ║      EBPM
EBPM = EBP VALUE FOR MAIN    ║    MAIN
                T  P              ╞═ ╠═══════╪═══════╣
                I  A              │  ║               ║
                O  N              │  ╠═══════╪═══════╣
                N  S     DYNAMIC ─┤  ║               ║
                   I     STORAGE  │  ╠═══════╪═══════╣
                 │ O              │  ║               ║
                 │ N              └─ ╠═══════╪═══════╣──ESP
                 │                   ║               ║
                                                   


Figure 3-19.  Stack Frame for Procedure A

                                      31          0 
                D  O                 ║               ║
                I  F                 ╠═══════╪═══════╣
                R                    ║    OLD ESP    ║
                E  E                 ╠═══════╪═══════╣
                C  X                 ║      EBPM
EBPM = EBP VALUE FOR MAIN    ║
                T  P                 ╠═══════╪═══════╣
                I  A                 ║               ║
                O  N                 ╠═══════╪═══════╣
                N  S                 ║               ║
                   I                 ╠═══════╪═══════╣
                 │ O                 ║               ║
                 │ N              ┌─ ╠═══════╪═══════╣
                 │                │  ║      EBPM     ║
                                 │  ╠═══════╪═══════╣──EBP FOR A
                         DISPLAY ─┤  ║      EBPM     ║
                                  │  ╠═══════╪═══════╣
                                  │  ║      EBPA
EBPA = EBP VALUE FOR PROCEDURE A    ║
                                  ╞═ ╠═══════╪═══════╣
                                  │  ║               ║
                                  │  ╠═══════╪═══════╣
                         DYNAMIC ─┤  ║               ║
                         STORAGE  │  ╠═══════╪═══════╣
                                  │  ║               ║
                                  └─ ╠═══════╪═══════╣──ESP
                                     ║               ║
                                                    


Figure 3-20.  Stack Frame for Procedure B at Level 3 Called from A

                                      31          0 
                D  O                 ║               ║
                I  F                 ╠═══════╪═══════╣
                R                    ║    OLD ESP    ║
                E  E                 ╠═══════╪═══════╣
                C  X                 ║      EBPM
EBPM = EBP VALUE FOR MAIN    ║
                T  P                 ╠═══════╪═══════╣
                I  A                 ║               ║
                O  N                 ╠═══════╪═══════╣
                N  S                 ║               ║
                   I                 ╠═══════╪═══════╣
                 │ O                 ║               ║
                 │ N                 ╠═══════╪═══════╣
                 │                   ║      EBPM     ║
                                    ╠═══════╪═══════╣
                                     ║      EBPM     ║
                                     ╠═══════╪═══════╣
                                     ║      EBPA     ║
                                     ╠═══════╪═══════╣
                                     ║               ║
                                     ╠═══════╪═══════╣
                                     ║               ║
                                     ╠═══════╪═══════╣
                                     ║               ║
                                  ┌─ ╠═══════╪═══════╣
                                  │  ║      EBPA     ║
                                  │  ╠═══════╪═══════╣──EBP
                                  │  ║      EBPM     ║
                         DISPLAY ─┤  ╠═══════╪═══════╣
                                  │  ║      EBPA     ║
                                  │  ╠═══════╪═══════╣
                                  │  ║      EBPB
EBPB = EBP VALUE FOR PROCEDURE B    ║
                                  ╞═ ╠═══════╪═══════╣
                                  │  ║               ║
                                  │  ╠═══════╪═══════╣
                         DYNAMIC ─┤  ║               ║
                         STORAGE  │  ╠═══════╪═══════╣
                                  │  ║               ║
                                  └─ ╠═══════╪═══════╣──ESP
                                     ║               ║
                                                    


Figure 3-21.  Stack Frame for Procedure C at Level 3 Called from B

                                      31          0 
                D  O                 ║               ║
                I  F                 ╠═══════╪═══════╣
                R                    ║    OLD ESP    ║
                E  E                 ╠═══════╪═══════╣
                C  X                 ║      EBPM
EBPM = EBP VALUE FOR MAIN    ║
                T  P                 ╠═══════╪═══════╣
                I  A                 ║               ║
                O  N                 ╠═══════╪═══════╣
                N  S                 ║               ║
                   I                 ╠═══════╪═══════╣
                 │ O                 ║               ║
                 │ N                 ╠═══════╪═══════╣
                 │                   ║      EBPM     ║
                                    ╠═══════╪═══════╣
                                     ║      EBPM     ║
                                     ╠═══════╪═══════╣
                                     ║      EBPA
EBPA = EBP VALUE FOR PROCEDURE A    ║
                                     ╠═══════╪═══════╣
                                     ║               ║
                                     ╠═══════╪═══════╣
                                     ║               ║
                                     ╠═══════╪═══════╣
                                     ║               ║
                                  ┌─ ╠═══════╪═══════╣
                                  │  ║      EBPA     ║
                                  │  ╠═══════╪═══════╣──EBP
                                  │  ║      EBPM     ║
                         DISPLAY ─┤  ╠═══════╪═══════╣
                                  │  ║      EBPA     ║
                                  │  ╠═══════╪═══════╣
                                  │  ║      EBPB
EBPB = EBP VALUE FOR PROCEDURE B    ║
                                  ╞═ ╠═══════╪═══════╣
                                  │  ║               ║
                                  │  ╠═══════╪═══════╣
                         DYNAMIC ─┤  ║               ║
                         STORAGE  │  ╠═══════╪═══════╣
                                  │  ║               ║
                                  └─ ╠═══════╪═══════╣──ESP
                                     ║               ║
                                                    


3.8  Flag Control Instructions

The flag control instructions provide a method for directly changing the
state of bits in the flag register.


3.8.1  Carry and Direction Flag Control Instructions

The carry flag instructions are useful in conjunction with
rotate-with-carry instructions RCL and RCR. They can initialize the carry
flag, CF, to a known state before execution of a rotate that moves the carry
bit into one end of the rotated operand.

The direction flag control instructions are specifically included to set or
clear the direction flag, DF, which controls the left-to-right or
right-to-left direction of string processing. If DF=0, the processor
automatically increments the string index registers, ESI and EDI, after each
execution of a string primitive. If DF=1, the processor decrements these
index registers. Programmers should use one of these instructions before any
procedure that uses string instructions to insure that DF is set properly.

Flag Control Instruction                  Effect

STC (Set Carry Flag)                      CF  1
CLC (Clear Carry Flag)                    CF  0
CMC (Complement Carry Flag)               CF  NOT (CF)
CLD (Clear Direction Flag)                DF  0
STD (Set Direction Flag)                  DF  1


3.8.2  Flag Transfer Instructions

Though specific instructions exist to alter CF and DF, there is no direct
method of altering the other applications-oriented flags. The flag transfer
instructions allow a program to alter the other flag bits with the bit
manipulation instructions after transferring these flags to the stack or the
AH register.

The instructions LAHF and SAHF deal with five of the status flags, which
are used primarily by the arithmetic and logical instructions.

LAHF (Load AH from Flags) copies SF, ZF, AF, PF, and CF to AH bits 7, 6, 4,
2, and 0, respectively (see Figure 3-22). The contents of the remaining bits
(5, 3, and 1) are undefined. The flags remain unaffected.

SAHF (Store AH into Flags) transfers bits 7, 6, 4, 2, and 0 from AH into
SF, ZF, AF, PF, and CF, respectively (see Figure 3-22).

The PUSHF and POPF instructions are not only useful for storing the flags
in memory where they can be examined and modified but are also useful for
preserving the state of the flags register while executing a procedure.

PUSHF (Push Flags) decrements ESP by two and then transfers the low-order
word of the flags register to the word at the top of stack pointed to by ESP
(see Figure 3-23). The variant PUSHFD decrements ESP by four, then
transfers both words of the extended flags register to the top of the stack
pointed to by ESP (the VM and RF flags are not moved, however).

POPF (Pop Flags) transfers specific bits from the word at the top of stack
into the low-order byte of the flag register (see Figure 3-23), then
increments ESP by two. The variant POPFD transfers specific bits from the
doubleword at the top of the stack into the extended flags register (the RF
and VM flags are not changed, however), then increments ESP by four.


Figure 3-22.  LAHF and SAHF

                     7    6    5    4    3    2    1    0
                   ╔════╦════╦════╦════╦════╦════╦════╦════╗
                   ║ SF ║ ZF ║ UU ║ AF ║ UU ║ PF ║ UU ║ CF ║
                   ╚════╩════╩════╩════╩════╩════╩════╩════╝

     LAHF LOADS FIVE FLAGS FROM THE FLAG REGISTER INTO REGISTER AH. SAHF
     STORES THESE SAME FIVE FLAGS FROM AH INTO THE FLAG REGISTER. THE BIT
     POSITION OF EACH FLAG IS THE SAME IN AH AS IT IS IN THE FLAG REGISTER.
     THE REMAINING BITS (MARKED UU) ARE RESERVED; DO NOT DEFINE.


3.9  Coprocessor Interface Instructions

A numerics coprocessor (e.g., the 80387 or 80287) provides an extension to
the instruction set of the base architecture. The coprocessor extends the
instruction set of the base architecture to support high-precision integer
and floating-point calculations. This extended instruction set includes
arithmetic, comparison, transcendental, and data transfer instructions. The
coprocessor also contains a set of useful constants to enhance the speed of
numeric calculations.

A program contains instructions for the coprocessor in line with the
instructions for the CPU. The system executes these instructions in the same
order as they appear in the instruction stream. The coprocessor operates
concurrently with the CPU to provide maximum throughput for numeric
calculations.

The 80386 also has features to support emulation of the numerics
coprocessor when the coprocessor is absent. The software emulation of the
coprocessor is transparent to application software but requires more time
for execution. Refer to Chapter 11 for more information on coprocessor
emulation.

ESC (Escape) is a 5-bit sequence that begins the opcodes that identify
floating point numeric instructions. The ESC pattern tells the 80386 to send
the opcode and addresses of operands to the numerics coprocessor. The
numerics coprocessor uses the escape instructions to perform
high-performance, high-precision floating point arithmetic that conforms to
the IEEE floating point standard 754.

WAIT (Wait) is an 80386 instruction that suspends program execution until
the 80386 CPU detects that the BUSY pin is inactive. This condition
indicates that the coprocessor has completed its processing task and that
the CPU may obtain the results.


Figure 3-23.  Flag Format for PUSHF and POPF

                                PUSHFD/POPFD
     ┌───────────────────────────────┴────────────────────────────────┐
                                                  PUSHF/POPF
                                     ┌────────────────┴───────────────┐
      31              23               15                7           0
     ╔═══════════════╪═══════════╤═╤═╤═╤═╤════╤═╤═╤═╤═╤═╤═╤═╤═╤═╤═╤═╤═╗
     ║                           │V│R│ │N│ID  │O│D│I│T│S│Z│ │A│ │P│ │C║
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0│ │ │0│ │    │ │ │ │ │ │ │0│ │0│ │1│ ║
     ║                           │M│F│ │T│  PL│F│F│F│F│F│F│ │F│ │F│ │F║
     ╚═══════════════╪═══════════╧═╧═╧═╧═╧════╧═╧═╧═╧═╧═╧═╧═╧═╧═╧═╧═╧═╝

     BITS MARKED 0 AND 1 ARE RESERVED BY INTEL. DO NOT DEFINE.

   SYSTEMS FLAGS (INCLUDING THE IOPL FIELD, AND THE VM, RF, AND IF FLAGS)
   ARE PUSHED AND ARE VISIBLE TO APPLICATIONS PROGRAMS. HOWEVER, WHEN AN
   APPLICATIONS PROGRAM POPS THE FLAGS, THESE ITEMS ARE NOT CHANGED,
   REGARDLESS OF THE VALUES POPPED INTO THEM.


3.10  Segment Register Instructions

This category actually includes several distinct types of instructions.
These various types are grouped together here because, if systems designers
choose an unsegmented model of memory organization, none of these
instructions is used by applications programmers. The instructions that deal
with segment registers are:

1.  Segment-register transfer instructions.

   MOV SegReg, ...
   MOV ..., SegReg
   PUSH SegReg
   POP SegReg

2.  Control transfers to another executable segment.

   JMP far    ; direct and indirect
   CALL far
   RET far

3.  Data pointer instructions.

   LDS
   LES
   LFS
   LGS
   LSS

Note that the following interrupt-related instructions are different; all
are capable of transferring control to another segment, but the use of
segmentation is not apparent to the applications programmer.

INT n
INTO
BOUND
IRET


3.10.1  Segment-Register Transfer Instructions

The MOV, POP, and PUSH instructions also serve to load and store segment
registers. These variants operate similarly to their general-register
counterparts except that one operand can be a segment register. MOV cannot
move segment register to a segment register. Neither POP nor MOV can place a
value in the code-segment register CS; only the far control-transfer
instructions can change CS.


3.10.2  Far Control Transfer Instructions

The far control-transfer instructions transfer control to a location in
another segment by changing the content of the CS register.

Direct far JMP. Direct JMP instructions that specify a target location
outside the current code segment contain a far pointer. This pointer
consists of a selector for the new code segment and an offset within the new
segment.

Indirect far JMP. Indirect JMP instructions that specify a target location
outside the current code segment use a 48-bit variable to specify the far
pointer.

Far CALL. An intersegment CALL places both the value of EIP and CS on the
stack.

Far RET. An intersegment RET restores the values of both CS and EIP which
were saved on the stack by the previous intersegment CALL instruction.


3.10.3  Data Pointer Instructions

The data pointer instructions load a pointer (consisting of a segment
selector and an offset) to a segment register and a general register.

LDS (Load Pointer Using DS) transfers a pointer variable from the source
operand to DS and the destination register. The source operand must be a
memory operand, and the destination operand must be a general register. DS
receives the segment-selector of the pointer. The destination register
receives the offset part of the pointer, which points to a specific location
within the segment.

Example: LDS ESI, STRING_X

Loads DS with the selector identifying the segment pointed to by a
STRING_X, and loads the offset of STRING_X into ESI.  Specifying ESI as the
destination operand is a convenient way to prepare for a string operation on
a source string that is not in the current data segment.

LES (Load Pointer Using ES) operates identically to LDS except that ES
receives the segment selector rather than DS.

Example: LES EDI, DESTINATION_X

Loads ES with the selector identifying the segment pointed to by
DESTINATION_X, and loads the offset of DESTINATION_X into EDI. This
instruction provides a convenient way to select a destination for a string
operation if the desired location is not in the current extra segment.

LFS (Load Pointer Using FS) operates identically to LDS except that FS
receives the segment selector rather than DS.

LGS (Load Pointer Using GS) operates identically to LDS except that GS
receives the segment selector rather than DS.

LSS (Load Pointer Using SS) operates identically to LDS except that SS
receives the segment selector rather than DS.  This instruction is
especially important, because it allows the two registers that identify the
stack (SS:ESP) to be changed in one uninterruptible operation.  Unlike the
other instructions which load SS, interrupts are not inhibited at the end
of the LSS instruction.  The other instructions (e.g., POP SS) inhibit
interrupts to permit the following instruction to load ESP, thereby forming
an indivisible load of SS:ESP.  Since both SS and ESP can be loaded by LSS,
there is no need to inhibit interrupts.


3.11  Miscellaneous Instructions

The following instructions do not fit in any of the previous categories,
but are nonetheless useful.


3.11.1  Address Calculation Instruction

LEA (Load Effective Address) transfers the offset of the source operand
(rather than its value) to the destination operand.  The source operand must
be a memory operand, and the destination operand must be a general register.
This instruction is especially useful for initializing registers before the
execution of the string primitives (ESI, EDI) or the XLAT instruction (EBX).
The LEA can perform any indexing or scaling that may be needed.

Example: LEA EBX, EBCDIC_TABLE

Causes the processor to place the address of the starting location of the
table labeled EBCDIC_TABLE into EBX.


3.11.2  No-Operation Instruction

NOP (No Operation) occupies a byte of storage but affects nothing but the
instruction pointer, EIP.


3.11.3  Translate Instruction

XLAT (Translate) replaced a byte in the AL register with a byte from a
user-coded translation table. When XLAT is executed, AL should have the
unsigned index to the table addressed by EBX. XLAT changes the contents of
AL from table index to table entry. EBX is unchanged. The XLAT instruction
is useful for translating from one coding system to another such as from
ASCII to EBCDIC.  The translate table may be up to 256 bytes long.  The
value placed in the AL register serves as an index to the location of the
corresponding translation value.


                        PART II  SYSTEMS PROGRAMMING


Chapter 4  Systems Architecture

────────────────────────────────────────────────────────────────────────────

Many of the architectural features of the 80386 are used only by systems
programmers. This chapter presents an overview of these aspects of the
architecture.

The systems-level features of the 80386 architecture include:

  Memory Management
  Protection
  Multitasking
  Input/Output
  Exceptions and Interrupts
  Initialization
  Coprocessing and Multiprocessing
  Debugging

These features are implemented by registers and instructions, all of which
are introduced in the following sections. The purpose of this chapter is not
to explain each feature in detail, but rather to place the remaining
chapters of Part II in perspective. Each mention in this chapter of a
register or instruction is either accompanied by an explanation or a
reference to a following chapter where detailed information can be obtained.


4.1  Systems Registers

The registers designed for use by systems programmers fall into these
classes:

  EFLAGS
  Memory-Management Registers
  Control Registers
  Debug Registers
  Test Registers


4.1.1  Systems Flags

The systems flags of the EFLAGS register control I/O, maskable interrupts,
debugging, task switching, and enabling of virtual 8086 execution in a
protected, multitasking environment. These flags are highlighted in Figure
4-1.

IF (Interrupt-Enable Flag, bit 9)

   Setting IF allows the CPU to recognize external (maskable) interrupt
   requests. Clearing IF disables these interrupts. IF has no effect on
   either exceptions or nonmaskable external interrupts. Refer to Chapter
   9 for more details about interrupts.

NT (Nested Task, bit 14)

   The processor uses the nested task flag to control chaining of
   interrupted and called tasks. NT influences the operation of the IRET
   instruction. Refer to Chapter 7 and Chapter 9 for more information on
   nested tasks.

RF (Resume Flag, bit 16)

   The RF flag temporarily disables debug exceptions so that an instruction
   can be restarted after a debug exception without immediately causing
   another debug exception. Refer to Chapter 12 for details.

TF (Trap Flag, bit 8)

   Setting TF puts the processor into single-step mode for debugging. In
   this mode, the CPU automatically generates an exception after each
   instruction, allowing a program to be inspected as it executes each
   instruction. Single-stepping is just one of several debugging features of
   the 80386. Refer to Chapter 12 for additional information.

VM (Virtual 8086 Mode, bit 17)

   When set, the VM flag indicates that the task is executing an 8086
   program. Refer to Chapter 14 for a detailed discussion of how the 80386
   executes 8086 tasks in a protected, multitasking environment.


Figure 4-1.  System Flags of EFLAGS Register

      31              23               15                7           0
     ╔═══════════════╪═══════════╤═╤═╤╪╤═╤════╤═╤═╤═╤═╤╪╤═╤═╤═╤═╤═╤═╤═╗
     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│V│R│▒│N│ID  │O│D│I│T│S│Z│▒│A│▒│P│▒│C║
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0│ │ │0│ │    │▒│▒│ │▒│▒│▒│0│▒│0│▒│1│▒║
     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│M│F│▒│T│  PL│F│F│F│F│F│F│▒│F│▒│F│▒│F║
     ╚═══════════════╪═══════════╧╤╧╤╧╪╧╤╧═╤══╧═╧═╧╤╧═╧╪╧═╧═╧═╧═╧═╧═╧═╝
                                  │ │   │  │       │
             VIRTUAL 8086 MODE────┘ │   │  │       │
                   RESUME FLAG──────┘   │  │       │
              NESTED TASK FLAG──────────┘  │       │
           I/O PRIVILEGE LEVEL─────────────┘       │
              INTERRUPT ENABLE─────────────────────┘

────────────────────────────────────────────────────────────────────────────
NOTE
      0 OR 1 INDICATES INTEL RESERVED. DO NOT DEFINE.
────────────────────────────────────────────────────────────────────────────


4.1.2  Memory-Management Registers

Four registers of the 80386 locate the data structures that control
segmented memory management:

GDTR    Global Descriptor Table Register
LDTR    Local Descriptor Table Register

   These registers point to the segment descriptor tables GDT and LDT.
   Refer to Chapter 5 for an explanation of addressing via descriptor
   tables.

IDTR    Interrupt Descriptor Table Register

   This register points to a table of entry points for interrupt handlers
   (the IDT). Refer to Chapter 9 for details of the interrupt mechanism.

TR      Task Register

   This register points to the information needed by the processor to define
   the current task. Refer to Chapter 7 for a description of the
   multitasking features of the 80386.


4.1.3  Control Registers

Figure 4-2 shows the format of the 80386 control registers CR0, CR2, and
CR3. These registers are accessible to systems programmers only via variants
of the MOV instruction, which allow them to be loaded from or stored in
general registers; for example:

MOV EAX, CR0
MOV CR3, EBX

CR0 contains system control flags, which control or indicate conditions
that apply to the system as a whole, not to an individual task.

EM (Emulation, bit 2)

   EM indicates whether coprocessor functions are to be emulated. Refer to
   Chapter 11 for details.

ET (Extension Type, bit 4)

   ET indicates the type of coprocessor present in the system (80287 or
   80387). Refer to Chapter 11 and Chapter 10 for details.

MP (Math Present, bit 1)

   MP controls the function of the WAIT instruction, which is used to
   coordinate a coprocessor. Refer to Chapter 11 for details.

PE (Protection Enable, bit 0)

   Setting PE causes the processor to begin executing in protected mode.
   Resetting PE returns to real-address mode. Refer to Chapter 14 and
   Chapter 10 for more information on changing processor modes.

PG (Paging, bit 31)

   PG indicates whether the processor uses page tables to translate linear
   addresses into physical addresses. Refer to Chapter 5 for a description
   of page translation; refer to Chapter 10 for a discussion of how to set
   PG.

TS (Task Switched, bit 3)

   The processor sets TS with every task switch and tests TS when
   interpreting coprocessor instructions. Refer to Chapter 11 for details.

CR2 is used for handling page faults when PG is set. The processor stores
in CR2 the linear address that triggers the fault. Refer to Chapter 9 for a
description of page-fault handling.

CR3 is used when PG is set. CR3 enables the processor to locate the page
table directory for the current task. Refer to Chapter 5 for a description
of page tables and page translation.


Figure 4-2.  Control Registers

 31                23                15                7               0
╔═════════════════╪═════════════════╪════════╦════════╪═════════════════╗
║                                            ║                          ║
║    PAGE DIRECTORY BASE REGISTER (PDBR)     ║         RESERVED         ║CR3
╟────────────────────────────────────────────╨──────────────────────────╢
║                                                                       ║
║                       PAGE FAULT LINEAR ADDRESS                       ║CR2
╟───────────────────────────────────────────────────────────────────────╢
║                                                                       ║
║                                RESERVED                               ║CR1
╟─┬───────────────────────────────────────────────────────────┬─┬─┬─┬─┬─╢
║P│                                                           │E│T│E│M│P║
║G│                              RESERVED                     │T│S│M│P│E║CR0
╚═╧═══════════════╪═════════════════╪═════════════════╪═══════╧═╧═╧═╧═╧═╝


4.1.4  Debug Register

The debug registers bring advanced debugging abilities to the 80386,
including data breakpoints and the ability to set instruction breakpoints
without modifying code segments. Refer to Chapter 12 for a complete
description of formats and usage.


4.1.5  Test Registers

The test registers are not a standard part of the 80386 architecture. They
are provided solely to enable confidence testing of the translation
lookaside buffer (TLB), the cache used for storing information from page
tables. Chapter 12 explains how to use these registers.


4.2  Systems Instructions

Systems instructions deal with such functions as:

  1.  Verification of pointer parameters (refer to Chapter 6):

    ARPL             ── Adjust RPL
    LAR              ── Load Access Rights
    LSL              ── Load Segment Limit
    VERR             ── Verify for Reading
    VERW             ── Verify for Writing

  2.  Addressing descriptor tables (refer to Chaper 5):

    LLDT             ── Load LDT Register
    SLDT             ── Store LDT Register
    LGDT             ── Load GDT Register
    SGDT             ── Store GDT Register

  3.  Multitasking (refer to Chapter 7):

    LTR              ── Load Task Register
    STR              ── Store Task Register

  4. Coprocessing and Multiprocessing (refer to Chapter 11):

    CLTS             ── Clear Task-Switched Flag
    ESC              ── Escape instructions
    WAIT             ── Wait until Coprocessor not Busy
    LOCK             ── Assert Bus-Lock Signal

  5.  Input and Output (refer to Chapter 8):

    IN               ── Input
    OUT              ── Output
    INS              ── Input String
    OUTS             ── Output String

  6.  Interrupt control (refer to Chapter 9):

    CLI              ── Clear Interrupt-Enable Flag
    STI              ── Set Interrupt-Enable Flag
    LIDT             ── Load IDT Register
    SIDT             ── Store IDT Register

  7.  Debugging (refer to Chapter 12):

    MOV              ── Move to and from debug registers

  8.  TLB testing (refer to Chapter 10):

    MOV              ── Move to and from test registers

  9.  System Control:

    SMSW             ── Set MSW
    LMSW             ── Load MSW
    HLT              ── Halt Processor
    MOV              ── Move to and from control registers

The instructions SMSW and LMSW are provided for compatibility with the
80286 processor.  80386 programs access the MSW in CR0 via variants of the
MOV instruction.  HLT stops the processor until receipt of an INTR or RESET
signal.

In addition to the chapters cited above, detailed information about each of
these instructions can be found in the instruction reference chapter,
Chapter 17.


Chapter 5  Memory Management

────────────────────────────────────────────────────────────────────────────

The 80386 transforms logical addresses (i.e., addresses as viewed by
programmers) into physical address (i.e., actual addresses in physical
memory) in two steps:

  ■  Segment translation, in which a logical address (consisting of a
     segment selector and segment offset) are converted to a linear address.

  ■  Page translation, in which a linear address is converted to a physical
     address. This step is optional, at the discretion of systems-software
     designers.

These translations are performed in a way that is not visible to
applications programmers. Figure 5-1 illustrates the two translations at a
high level of abstraction.

Figure 5-1 and the remainder of this chapter present a simplified view of
the 80386 addressing mechanism. In reality, the addressing mechanism also
includes memory protection features. For the sake of simplicity, however,
the subject of protection is taken up in another chapter, Chapter 6.


Figure 5-1.  Address Translation Overview

             15           0      31                           0
    LOGICAL ╔═══════════════╗   ╔══════════════════════════════╗
    ADDRESS ║    SELECTOR   ║   ║            OFFSET            ║
            ╚═══════════════╝   ╚═══╤══════════════════════════╝

                     ╔══════════════════════════════╗
                     ║     SEGMENT TRANSLATION      ║
                     ╚══════════════╤═══════════════╝
                                 ╔══╧═╗       PAGING ENABLED
                                 ║PG ?╟────────────────────┐
                                 ╚══╤═╝                    │
                   31        PAGING  DISABLED       0     │
          LINEAR  ╔═══════════╦═══════════╦═══════════╗    │
          ADDRESS ║    DIR    ║   PAGE    ║  OFFSET   ║    │
                  ╚═══════════╩═════╤═════╩═══════════╝    │
                                                          │
                     ╔══════════════════════════════╗      │
                     ║       PAGE TRANSLATION       ║      │
                     ╚══════════════╤═══════════════╝      │
                                    │─────────────────────┘
                      31                          0
            PHYSICAL ╔══════════════════════════════╗
            ADDRESS  ║                              ║
                     ╚══════════════════════════════╝


5.1  Segment Translation

Figure 5-2 shows in more detail how the processor converts a logical
address into a linear address.

To perform this translation, the processor uses the following data
structures:

  ■  Descriptors
  ■  Descriptor tables
  ■  Selectors
  ■  Segment Registers


5.1.1  Descriptors

The segment descriptor provides the processor with the data it needs to map
a logical address into a linear address. Descriptors are created by
compilers, linkers, loaders, or the operating system, not by applications
programmers. Figure 5-3 illustrates the two general descriptor formats. All
types of segment descriptors take one of these formats. Segment-descriptor
fields are:

BASE: Defines the location of the segment within the 4 gigabyte linear
address space. The processor concatenates the three fragments of the base
address to form a single 32-bit value.

LIMIT: Defines the size of the segment. When the processor concatenates the
two parts of the limit field, a 20-bit value results. The processor
interprets the limit field in one of two ways, depending on the setting of
the granularity bit:

  1.  In units of one byte, to define a limit of up to 1 megabyte.

  2.  In units of 4 Kilobytes, to define a limit of up to 4 gigabytes. The
      limit is shifted left by 12 bits when loaded, and low-order one-bits
      are inserted.

Granularity bit: Specifies the units with which the LIMIT field is
interpreted. When thebit is clear, the limit is interpreted in units of one
byte; when set, the limit is interpreted in units of 4 Kilobytes.

TYPE: Distinguishes between various kinds of descriptors.

DPL (Descriptor Privilege Level): Used by the protection mechanism (refer
to Chapter 6).

Segment-Present bit: If this bit is zero, the descriptor is not valid for
use in address transformation; the processor will signal an exception when a
selector for the descriptor is loaded into a segment register. Figure 5-4
shows the format of a descriptor when the present-bit is zero. The operating
system is free to use the locations marked AVAILABLE. Operating systems that
implement segment-based virtual memory clear the present bit in either of
these cases:

  ■  When the linear space spanned by the segment is not mapped by the
     paging mechanism.

  ■  When the segment is not present in memory.

Accessed bit: The processor sets this bit when the segment is accessed;
i.e., a selector for the descriptor is loaded into a segment register or
used by a selector test instruction. Operating systems that implement
virtual memory at the segment level may, by periodically testing and
clearing this bit, monitor frequency of segment usage.

Creation and maintenance of descriptors is the responsibility of systems
software, usually requiring the cooperation of compilers, program loaders or
system builders, and therating system.


Figure 5-2.  Segment Translation

           15              0    31                                   0
  LOGICAL ╔════════════════╗   ╔═════════════════════════════════════╗
  ADDRESS ║    SELECTOR    ║   ║                OFFSET               ║
          ╚═══╤═════════╤══╝   ╚═══════════════════╤═════════════════╝
       ┌──────┘                                   │
       │ DESCRIPTOR TABLE                          │
       │  ╔════════════╗                           │
       │  ║            ║                           │
       │  ║            ║                           │
       │  ║            ║                           │
       │  ║            ║                           │
       │  ╠════════════╣                           │
       │  ║  SEGMENT   ║ BASE          ╔═══╗       │
       └─║ DESCRIPTOR ╟──────────────║ + ║──────┘
          ╠════════════╣ ADDRESS       ╚═╤═╝
          ║            ║                 │
          ╚════════════╝                 │

              LINEAR  ╔════════════╦═══════════╦══════════════╗
              ADDRESS ║    DIR     ║   PAGE    ║    OFFSET    ║
                      ╚════════════╩═══════════╩══════════════╝


Figure 5-3.  General Segment-Descriptor Format

         DESCRIPTORS USED FOR APPLICATIONS CODE AND DATA SEGMENTS

  31                23                15                7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤═════╤═╤═════╤═╪═════════════════╗
 ║                 │ │ │ │A│         │ │     │ │     │ │                 ║
 ║   BASE 31..24   │G│X│O│V│ LIMIT   │P│ DPL │1│ TYPE│A│  BASE 23..16    ║ 4
 ║                 │ │ │ │L│ 19..16  │ │     │ │     │ │                 ║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴─────┴─┴─────────────────╢
 ║                                   │                                   ║
 ║        SEGMENT BASE 15..0         │       SEGMENT LIMIT 15..0         ║ 0
 ║                                   │                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝

                DESCRIPTORS USED FOR SPECIAL SYSTEM SEGMENTS

  31                23                15                7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤═════╤═╤═══════╪═════════════════╗
 ║                 │ │ │ │A│         │ │     │ │       │                 ║
 ║   BASE 31..24   │G│X│O│V│ LIMIT   │P│ DPL │0│  TYPE │  BASE 23..16    ║ 4
 ║                 │ │ │ │L│ 19..16  │ │     │ │       │                 ║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴───────┴─────────────────╢
 ║                                   │                                   ║
 ║        SEGMENT BASE 15..0         │       SEGMENT LIMIT 15..0         ║ 0
 ║                                   │                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝

           A      - ACCESSED
           AVL    - AVAILABLE FOR USE BY SYSTEMS PROGRAMMERS
           DPL    - DESCRIPTOR PRIVILEGE LEVEL
           G      - GRANULARITY
           P      - SEGMENT PRESENT


5.1.2  Descriptor Tables

Segment descriptors are stored in either of two kinds of descriptor table:

  ■  The global descriptor table (GDT)
  ■  A local descriptor table (LDT)

A descriptor table is simply a memory array of 8-byte entries that contain
descriptors, as Figure 5-5 shows. A descriptor table is variable in length
and may contain up to 8192 (2^(13)) descriptors. The first entry of the GDT
(INDEX=0) is not used by the processor, however.

The processor locates the GDT and the current LDT in memory by means of the
GDTR and LDTR registers. These registers store the base addresses of the
tables in the linear address space and store the segment limits. The
instructions LGDT and SGDT give access to the GDTR; the instructions LLDT
and SLDT give access to the LDTR.


Figure 5-4.  Format of Not-Present Descriptor

  31                23                15                7               0
 ╔═════════════════╪═════════════════╪═╤═════╤═╤═══════╪═════════════════╗
 ║                                   │ │     │ │       │                 ║
 ║            AVAILABLE              │O│ DPL │S│ TYPE  │   AVAILABLE     ║ 4
 ║                                   │ │     │ │       │                 ║
 ╟───────────────────────────────────┴─┴─────┴─┴───────┴─────────────────╢
 ║                                                                       ║
 ║                               AVAILABLE                               ║ 0
 ║                                                                       ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝


Figure 5-5.  Descriptor Tables

      GLOBAL DESCRIPTOR TABLE                 LOCAL DESCRIPTOR TABLE
    ╔══════╤═════╤═════╤══════╗            ╔══════╤═════╤═════╤══════╗
    ║      │     │     │      ║            ║      │     │     │      ║
    ╟──────┴─────┼─────┴──────╢            ╟──────┴─────┼─────┴──────╢
    ║            │            ║ M          ║            │            ║ M
    ╚════════════╧════════════╝            ╚════════════╧════════════╝
    |                         |            |                         |
    |                         |            |                         |
    ╔══════╤═════╤═════╤══════╗            ╔══════╤═════╤═════╤══════╗
    ║      │     │     │      ║            ║      │     │     │      ║
    ╟──────┴─────┼─────┴──────╢            ╟──────┴─────┼─────┴──────╢
    ║            │            ║ N + 3      ║            │            ║ N + 3
    ╠══════╤═════╪═════╤══════╣            ╠══════╤═════╪═════╤══════╣
    ║      │     │     │      ║            ║      │     │     │      ║
    ╟──────┴─────┼─────┴──────╢            ╟──────┴─────┼─────┴──────╢
    ║            │            ║ N + 2      ║            │            ║ N + 2
    ╠══════╤═════╪═════╤══════╣            ╠══════╤═════╪═════╤══════╣
    ║      │     │     │      ║            ║      │     │     │      ║
    ╟──────┴─────┼─────┴──────╢            ╟──────┴─────┼─────┴──────╢
    ║            │            ║ N + 1      ║            │            ║ N + 1
    ╠══════╤═════╪═════╤══════╣            ╠══════╤═════╪═════╤══════╣
    ║      │     │     │      ║            ║      │     │     │      ║
    ╟──────┴─────┼─────┴──────╢            ╟──────┴─────┼─────┴──────╢
    ║            │            ║ N          ║            │            ║ N
    ╚════════════╧════════════╝            ╚════════════╧════════════╝
    |                         |            |                         |
    |                         |            |                         |
    ╔══════╤═════╤═════╤══════╗            ╔══════╤═════╤═════╤══════╗
    ║      │     │     │      ║            ║      │     │     │      ║
    ╟──────┴──(UNUSED)─┴──────╢            ╟──────┴─────┼─────┴──────╢
    ║            │            ║            ║            │            ║
    ╚════════════╧════════════╝            ╚════════════╧════════════╝

     ╔═════════════════════╗  │             ╔═════════════════════╗  │
     ║         GDTR        ╟──┘             ║         LDTR        ╟──┘
     ╚═════════════════════╝                ╚═════════════════════╝


5.1.3  Selectors

The selector portion of a logical address identifies a descriptor by
specifying a descriptor table and indexing a descriptor within that table.
Selectors may be visible to applications programs as a field within a
pointer variable, but the values of selectors are usually assigned (fixed
up) by linkers or linking loaders. Figure 5-6 shows the format of a
selector.

Index: Selects one of 8192 descriptors in a descriptor table. The processor
simply multiplies this index value by 8 (the length of a descriptor), and
adds the result to the base address of the descriptor table in order to
access the appropriate segment descriptor in the table.

Table Indicator: Specifies to which descriptor table the selector refers. A
zero indicates the GDT; a one indicates the current LDT.

Requested Privilege Level: Used by the protection mechanism. (Refer to
Chapter 6.)

Because the first entry of the GDT is not used by the processor, a selector
that has an index of zero and a table indicator of zero (i.e., a selector
that points to the first entry of the GDT), can be used as a null selector.
The processor does not cause an exception when a segment register (other
than CS or SS) is loaded with a null selector. It will, however, cause an
exception when the segment register is used to access memory.  This feature
is useful for initializing unused segment registers so as to trap accidental
references.


Figure 5-6.  Format of a Selector

                         15                      4 3   0
                        ╔═════════════════════════╤═╤═══╗
                        ║                         │T│   ║
                        ║           INDEX         │ │RPL║
                        ║                         │I│   ║
                        ╚═════════════════════════╧═╧═══╝

                         TI  - TABLE INDICATOR
                         RPL - REQUESTOR'S PRIVILEGE LEVEL


Figure 5-7.  Segment Registers

             16-BIT VISIBLE
                SELECTOR                 HIDDEN DESCRIPTOR
           ╔════════════════╦════════════════════════════════════════╗
        CS ║                ║                                        ║
           ╟────────────────╫────────────────────────────────────────╢
        SS ║                ║                                        ║
           ╟────────────────╫────────────────────────────────────────╢
        DS ║                ║                                        ║
           ╟────────────────╫────────────────────────────────────────╢
        ES ║                ║                                        ║
           ╟────────────────╫────────────────────────────────────────╢
        FS ║                ║                                        ║
           ╟────────────────╫────────────────────────────────────────╢
        GS ║                ║                                        ║
           ╚════════════════╩════════════════════════════════════════╝


5.1.4  Segment Registers

The 80386 stores information from descriptors in segment registers, thereby
avoiding the need to consult a descriptor table every time it accesses
memory.

Every segment register has a "visible" portion and an "invisible" portion,
as Figure 5-7 illustrates. The visible portions of these segment address
registers are manipulated by programs as if they were simply 16-bit
registers. The invisible portions are manipulated by the processor.

The operations that load these registers are normal program instructions
(previously described in Chapter 3). These instructions are of two classes:

  1.  Direct load instructions; for example, MOV, POP, LDS, LSS, LGS, LFS.
      These instructions explicitly reference the segment registers.

  2.  Implied load instructions; for example, far CALL and JMP. These
      instructions implicitly reference the CS register, and load it with a
      new value.

Using these instructions, a program loads the visible part of the segment
register with a 16-bit selector. The processor automatically fetches the
base address, limit, type, and other information from a descriptor table and
loads them into the invisible part of the segment register.

Because most instructions refer to data in segments whose selectors have
already been loaded into segment registers, the processor can add the
segment-relative offset supplied by the instruction to the segment base
address with no additional overhead.


5.2  Page Translation

In the second phase of address transformation, the 80386 transforms a
linear address into a physical address. This phase of address transformation
implements the basic features needed for page-oriented virtual-memory
systems and page-level protection.

The page-translation step is optional. Page translation is in effect only
when the PG bit of CR0 is set. This bit is typically set by the operating
system during software initialization. The PG bit must be set if the
operating system is to implement multiple virtual 8086 tasks, page-oriented
protection, or page-oriented virtual memory.


5.2.1  Page Frame

A page frame is a 4K-byte unit of contiguous addresses of physical memory.
Pages begin onbyte boundaries and are fixed in size.


5.2.2  Linear Address

A linear address refers indirectly to a physical address by specifying a
page table, a page within that table, and an offset within that page. Figure
5-8 shows the format of a linear address.

Figure 5-9 shows how the processor converts the DIR, PAGE, and OFFSET
fields of a linear address into the physical address by consulting two
levels of page tables. The addressing mechanism uses the DIR field as an
index into a page directory, uses the PAGE field as an index into the page
table determined by the page directory, and uses the OFFSET field to address
a byte within the page determined by the page table.


Figure 5-8.  Format of a Linear Address

      31                 22 21                 12 11                 0
     ╔═════════════════════╦═════════════════════╦════════════════════╗
     ║                     ║                     ║                    ║
     ║         DIR         ║        PAGE         ║       OFFSET       ║
     ║                     ║                     ║                    ║
     ╚═════════════════════╩═════════════════════╩════════════════════╝


Figure 5-9.  Page Translation

                                                              PAGE FRAME
              ╔═══════════╦═══════════╦══════════╗         ╔═══════════════╗
              ║    DIR    ║   PAGE    ║  OFFSET  ║         ║               ║
              ╚═════╤═════╩═════╤═════╩═════╤════╝         ║               ║
                    │           │           │              ║               ║
      ┌─────────────┘           │           └─────────────║    PHYSICAL   ║
      │                         │                          ║    ADDRESS    ║
      │   PAGE DIRECTORY        │      PAGE TABLE          ║               ║
      │  ╔═══════════════╗      │   ╔═══════════════╗      ║               ║
      │  ║               ║      │   ║               ║      ╚═══════════════╝
      │  ║               ║      │   ╠═══════════════╣
      │  ║               ║      └──║ PG TBL ENTRY  ╟──────────────┘
      │  ╠═══════════════╣          ╠═══════════════╣
      └─║   DIR ENTRY   ╟──┐       ║               ║
         ╠═══════════════╣  │       ║               ║
         ║               ║  │       ║               ║
         ╚═══════════════╝  │       ╚═══════════════╝
                           │
╔═══════╗        │          └───────────────┘
║  CR3  ╟────────┘
╚═══════╝


5.2.3  Page Tables

A page table is simply an array of 32-bit page specifiers. A page table is
itself a page, and therefore contains 4 Kilobytes of memory or at most 1K
32-bit entries.

Two levels of tables are used to address a page of memory. At the higher
level is a page directory. The page directory addresses up to 1K page tables
of the second level. A page table of the second level addresses up to 1K
pages. All the tables addressed by one page directory, therefore, can
address 1M pages (2^(20)). Because each page contains 4K bytes 2^(12)
bytes), the tables of one page directory can span the entire physical
address space of the 80386 (2^(20) times 2^(12) = 2^(32)).

The physical address of the current page directory is stored in the CPU
register CR3, also called the page directory base register (PDBR). Memory
management software has the option of using one page directory for all
tasks, one page directory for each task, or some combination of the two.
Refer to Chapter 10 for information on initialization of CR3. Refer to
Chapter 7 to see how CR3 can change for each task.


5.2.4  Page-Table Entries

Entries in either level of page tables have the same format. Figure 5-10
illustrates this format.


5.2.4.1  Page Frame Address

The page frame address specifies the physical starting address of a page.
Because pages are located on 4K boundaries, the low-order 12 bits are always
zero. In a page directory, the page frame address is the address of a page
table. In a second-level page table, the page frame address is the address
of the page frame that contains the desired memory operand.


5.2.4.2  Present Bit

The Present bit indicates whether a page table entry can be used in address
translation. P=1 indicates that the entry can be used.

When P=0 in either level of page tables, the entry is not valid for address
translation, and the rest of the entry is available for software use; none
of the other bits in the entry is tested by the hardware. Figure 5-11
illustrates the format of a page-table entry when P=0.

If P=0 in either level of page tables when an attempt is made to use a
page-table entry for address translation, the processor signals a page
exception. In software systems that support paged virtual memory, the
page-not-present exception handler can bring the required page into physical
memory. The instruction that caused the exception can then be reexecuted.
Refer to Chapter 9 for more information on exception handlers.

Note that there is no present bit for the page directory itself. The page
directory may be not-present while the associated task is suspended, but the
operating system must ensure that the page directory indicated by the CR3
image in the TSS is present in physical memory before the task is
dispatched. Refer to Chapter 7 for an explanation of the TSS and task
dispatching.


Figure 5-10.  Format of a Page Table Entry

       31                                  12 11                      0
      ╔══════════════════════════════════════╤═══════╤═══╤═╤═╤═══╤═╤═╤═╗
      ║                                      │       │   │ │ │   │U│R│ ║
      ║      PAGE FRAME ADDRESS 31..12       │ AVAIL │0 0│D│A│0 0│/│/│P║
      ║                                      │       │   │ │ │   │S│W│ ║
      ╚══════════════════════════════════════╧═══════╧═══╧═╧═╧═══╧═╧═╧═╝

                P      - PRESENT
                R/W    - READ/WRITE
                U/S    - USER/SUPERVISOR
                D      - DIRTY
                AVAIL  - AVAILABLE FOR SYSTEMS PROGRAMMER USE

                NOTE: 0 INDICATES INTEL RESERVED. DO NOT DEFINE.


Figure 5-11.  Invalid Page Table Entry

       31                                                           1 0
      ╔══════════════════════════════════════════════════════════════╤═╗
      ║                                                              │ ║
      ║                            AVAILABLE                         │0║
      ║                                                              │ ║
      ╚══════════════════════════════════════════════════════════════╧═╝


5.2.4.3  Accessed and Dirty Bits

These bits provide data about page usage in both levels of the page tables.
With the exception of the dirty bit in a page directory entry, these bits
are set by the hardware; however, the processor does not clear any of these
bits.

The processor sets the corresponding accessed bits in both levels of page
tables to one before a read or write operation to a page.

The processor sets the dirty bit in the second-level page table to one
before a write to an address covered by that page table entry. The dirty bit
in directory entries is undefined.

An operating system that supports paged virtual memory can use these bits
to determine what pages to eliminate from physical memory when the demand
for memory exceeds the physical memory available. The operating system is
responsible for testing and clearing these bits.

Refer to Chapter 11 for how the 80386 coordinates updates to the accessed
and dirty bits in multiprocessor systems.


5.2.4.4  Read/Write and User/Supervisor Bits

These bits are not used for address translation, but are used for
page-level protection, which the processor performs at the same time as
address translation. Refer to Chapter 6 where protection is discussed in
detail.


5.2.5  Page Translation Cache

For greatest efficiency in address translation, the processor stores the
most recently used page-table data in an on-chip cache. Only if the
necessary paging information is not in the cache must both levels of page
tables be referenced.

The existence of the page-translation cache is invisible to applications
programmers but not to systems programmers; operating-system programmers
must flush the cache whenever the page tables are changed. The
page-translation cache can be flushed by either of two methods:

  1.  By reloading CR3 with a MOV instruction; for example:

      MOV CR3, EAX

  2.  By performing a task switch to a TSS that has a different CR3 image
      than the current TSS. (Refer to Chapter 7 for more information on
      task switching.)


5.3  Combining Segment and Page Translation

Figure 5-12 combines Figure 5-2 and Figure 5-9 to summarize both phases
of the transformation from a logical address to a physical address when
paging is enabled. By appropriate choice of options and parameters to both
phases, memory-management software can implement several different styles of
memory management.


5.3.1  "Flat" Architecture

When the 80386 is used to execute software designed for architectures that
don't have segments, it may be expedient to effectively "turn off" the
segmentation features of the 80386. The 80386 does not have a mode that
disables segmentation, but the same effect can be achieved by initially
loading the segment registers with selectors for descriptors that encompass
the entire 32-bit linear address space. Once loaded, the segment registers
don't need to be changed. The 32-bit offsets used by 80386 instructions are
adequate to address the entire linear-address space.


5.3.2  Segments Spanning Several Pages

The architecture of the 80386 permits segments to be larger or smaller than
the size of a page (4 Kilobytes). For example, suppose a segment is used to
address and protect a large data structure that spans 132 Kilobytes. In a
software system that supports paged virtual memory, it is not necessary for
the entire structure to be in physical memory at once. The structure is
divided into 33 pages, any number of which may not be present. The
applications programmer does not need to be aware that the virtual memory
subsystem is paging the structure in this manner.


Figure 5-12.  80306 Addressing Machanism

      16                0 32                                  0
    ╔════════════════════╦═════════════════════════════════════╗ LOGICAL
    ║      SELECTOR      ║                 OFFSET              ║ ADDRESS
    ╚════╤══════════╤════╩════════════════════╤════════════════╝
 ┌───────┘                                   │
 │   DESCRIPTOR TABLE                         │
 │  ╔═══════════════╗                         │
 │  ║               ║                         │
 │  ║               ║                         │
 │  ║               ║                         │
 │  ║               ║                         │
 │  ╠═══════════════╣                         │
 │  ║   SEGMENT     ║         ╔═══╗           │
 └─║  DESCRIPTOR   ╟────────║ + ║──────────┘
    ╠═══════════════╣         ╚═╤═╝
    ║               ║           │
    ╚═══════════════╝           │
                                                              PAGE FRAME
      LINEAR  ╔═══════════╦═══════════╦══════════╗         ╔═══════════════╗
      ADDRESS ║    DIR    ║   PAGE    ║  OFFSET  ║         ║               ║
              ╚═════╤═════╩═════╤═════╩═════╤════╝         ║               ║
                    │           │           │              ║               ║
      ┌─────────────┘           │           └─────────────║    PHYSICAL   ║
      │                         │                          ║    ADDRESS    ║
      │   PAGE DIRECTORY        │      PAGE TABLE          ║               ║
      │  ╔═══════════════╗      │   ╔═══════════════╗      ║               ║
      │  ║               ║      │   ║               ║      ║               ║
      │  ║               ║      │   ║               ║      ╚═══════════════╝
      │  ║               ║      │   ╠═══════════════╣
      │  ║               ║      └──║ PG TBL ENTRY  ╟──────────────┘
      │  ╠═══════════════╣          ╠═══════════════╣
      └─║   DIR ENTRY   ╟──┐       ║               ║
         ╠═══════════════╣  │       ║               ║
         ║               ║  │       ║               ║
         ╚═══════════════╝  │       ╚═══════════════╝
                           │
╔═══════╗        │          └───────────────┘
║  CR3  ╟────────┘
╚═══════╝


5.3.3  Pages Spanning Several Segments

On the other hand, segments may be smaller than the size of a page. For
example, consider a small data structure such as a semaphore. Because of the
protection and sharing provided by segments (refer to Chapter 6), it may be
useful to create a separate segment for each semaphore. But, because a
system may need many semaphores, it is not efficient to allocate a page for
each. Therefore, it may be useful to cluster many related segments within a
page.


5.3.4  Non-Aligned Page and Segment Boundaries

The architecture of the 80386 does not enforce any correspondence between
the boundaries of pages and segments. It is perfectly permissible for a page
to contain the end of one segment and the beginning of another. Likewise, a
segment may contain the end of one page and the beginning of another.


5.3.5  Aligned Page and Segment Boundaries

Memory-management software may be simpler, however, if it enforces some
correspondence between page and segment boundaries. For example, if segments
are allocated only in units of one page, the logic for segment and page
allocation can be combined. There is no need for logic to account for
partially used pages.


5.3.6  Page-Table per Segment

An approach to space management that provides even further simplification
of space-management software is to maintain a one-to-one correspondence
between segment descriptors and page-directory entries, as Figure 5-13
illustrates. Each descriptor has a base address in which the low-order 22
bits are zero; in other words, the base address is mapped by the first entry
of a page table. A segment may have any limit from 1 to 4 megabytes.
Depending on the limit, the segment is contained in from 1 to 1K page
frames. A task is thus limited to 1K segments (a sufficient number for many
applications), each containing up to 4 Mbytes. The descriptor, the
corresponding page-directory entry, and the corresponding page table can be
allocated and deallocated simultaneously.


Figure 5-13.  Descriptor per Page Table

                                                              PAGE FRAMES
                                                             ╔═══════════╗
         LDT          PAGE DIRECTORY       PAGE TABLES       ║           ║
    ╔══════════╗       ╔══════════╗        ╔══════════╗      ║           ║
    ║          ║       ║          ║        ║          ║   ┌─╚═══════════╝
    ╠══════════╣       ╠══════════╣        ╠══════════╣   │
    ║          ║       ║          ║        ║   PTE    ╟───┘  ╔═══════════╗
    ╠══════════╣       ╠══════════╣        ╠══════════╣      ║           ║
    ║          ║       ║          ║        ║   PTE    ╟───┐  ║           ║
    ╠══════════╣       ╠══════════╣        ╠══════════╣   └─╚═══════════╝
    ║          ║       ║          ║        ║   PTE    ╟───┐
    ╠══════════╣       ╠══════════╣   ┌───╚══════════╝   │  ╔═══════════╗
    ║DESCRIPTOR╟──────║   PDE    ╟───┘                   │  ║           ║
    ╠══════════╣       ╠══════════╣                       │  ║           ║
    ║DESCRIPTOR╟──────║   PDE    ╟───┐                   └─╚═══════════╝
    ╠══════════╣       ╠══════════╣   │    ╔══════════╗
    ║          ║       ║          ║   │    ║          ║      ╔═══════════╗
    ╠══════════╣       ╠══════════╣   │    ╠══════════╣      ║           ║
    ║          ║       ║          ║   │    ║          ║      ║           ║
    ╠══════════╣       ╠══════════╣   │    ╠══════════╣   ┌─╚═══════════╝
    ║          ║       ║          ║   │    ║   PTE    ╟───┘
    ╠══════════╣       ╠══════════╣   │    ╠══════════╣      ╔═══════════╗
    ║          ║       ║          ║   │    ║   PTE    ╟───┐  ║           ║
    ╚══════════╝       ╚══════════╝   └───╚══════════╝   │  ║           ║
        LDT           PAGE DIRECTORY       PAGE TABLES    └─╚═══════════╝
                                                              PAGE FRAMES


Chapter 6  Protection

────────────────────────────────────────────────────────────────────────────

6.1  Why Protection?

The purpose of the protection features of the 80386 is to help detect and
identify bugs. The 80386 supports sophisticated applications that may
consist of hundreds or thousands of program modules. In such applications,
the question is how bugs can be found and eliminated as quickly as possible
and how their damage can be tightly confined. To help debug applications
faster and make them more robust in production, the 80386 contains
mechanisms to verify memory accesses and instruction execution for
conformance to protection criteria. These mechanisms may be used or ignored,
according to system design objectives.


6.2  Overview of 80386 Protection Mechanisms

Protection in the 80386 has five aspects:

  1.  Type checking
  2.  Limit checking
  3.  Restriction of addressable domain
  4.  Restriction of procedure entry points
  5.  Restriction of instruction set

The protection hardware of the 80386 is an integral part of the memory
management hardware. Protection applies both to segment translation and to
page translation.

Each reference to memory is checked by the hardware to verify that it
satisfies the protection criteria. All these checks are made before the
memory cycle is started; any violation prevents that cycle from starting and
results in an exception. Since the checks are performed concurrently with
address formation, there is no performance penalty.

Invalid attempts to access memory result in an exception. Refer to
Chapter 9 for an explanation of the exception mechanism. The present
chapter defines the protection violations that lead to exceptions.

The concept of "privilege" is central to several aspects of protection
(numbers 3, 4, and 5 in the preceeding list). Applied to procedures,
privilege is the degree to which the procedure can be trusted not to make a
mistake that might affect other procedures or data. Applied to data,
privilege is the degree of protection that a data structure should have
from less trusted procedures.

The concept of privilege applies both to segment protection and to page
protection.


6.3  Segment-Level Protection

All five aspects of protection apply to segment translation:

  1.  Type checking
  2.  Limit checking
  3.  Restriction of addressable domain
  4.  Restriction of procedure entry points
  5.  Restriction of instruction set

The segment is the unit of protection, and segment descriptors store
protection parameters. Protection checks are performed automatically by the
CPU when the selector of a segment descriptor is loaded into a segment
register and with every segment access. Segment registers hold the
protection parameters of the currently addressable segments.


6.3.1  Descriptors Store Protection Parameters

Figure 6-1 highlights the protection-related fields of segment descriptors.

The protection parameters are placed in the descriptor by systems software
at the time a descriptor is created. In general, applications programmers do
not need to be concerned about protection parameters.

When a program loads a selector into a segment register, the processor
loads not only the base address of the segment but also protection
information. Each segment register has bits in the invisible portion for
storing base, limit, type, and privilege level; therefore, subsequent
protection checks on the same segment do not consume additional clock
cycles.


Figure 6-1.  Protection Fields of Segment Descriptors

                           DATA SEGMENT DESCRIPTOR

  31                23                15                7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤═════╤═════════╪═════════════════╗
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒│▒│▒│A│ LIMIT   │▒│     │  TYPE   │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
 ║▒▒▒BASE 31..24▒▒▒│G│B│0│V│ 19..16  │P│ DPL │         │▒▒▒BASE 23..16▒▒▒║ 4
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒│▒│▒│L│         │▒│     │1│0│E│W│A│▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴─┴─┴─┴─┴─────────────────╢
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                                   ║
 ║▒▒▒▒▒▒▒▒SEGMENT BASE 15..0▒▒▒▒▒▒▒▒▒│        SEGMENT LIMIT 15..0        ║ 0
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝

                        EXECUTABLE SEGMENT DESCRIPTOR

  31                23                15                7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤═════╤═════════╪═════════════════╗
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒│▒│▒│A│ LIMIT   │▒│     │  TYPE   │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
 ║▒▒▒BASE 31..24▒▒▒│G│D│0│V│ 19..16  │P│ DPL │         │▒▒▒BASE 23..16▒▒▒║ 4
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒│▒│▒│L│         │▒│     │1│0│C│R│A│▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴─┴─┴─┴─┴─────────────────╢
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                                   ║
 ║▒▒▒▒▒▒▒▒SEGMENT BASE 15..0▒▒▒▒▒▒▒▒▒│        SEGMENT LIMIT 15..0        ║ 0
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝

                         SYSTEM SEGMENT DESCRIPTOR

  31                23                15                7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤═════╤═╤═══════╪═════════════════╗
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒│▒│▒│A│ LIMIT   │▒│     │ │       │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
 ║▒▒▒BASE 31..24▒▒▒│G│X│0│V│ 19..16  │P│ DPL │0│ TYPE  │▒▒▒BASE 23..16▒▒▒║ 4
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒│▒│▒│L│         │▒│     │ │       │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴───────┴─────────────────╢
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                                   ║
 ║▒▒▒▒▒▒▒▒SEGMENT BASE 15..0▒▒▒▒▒▒▒▒▒│       SEGMENT LIMIT 15..0         ║ 0
 ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝


        A   - ACCESSED                              E   - EXPAND-DOWN
        AVL - AVAILABLE FOR PROGRAMMERS USE         G   - GRANULARITY
        B   - BIG                                   P   - SEGMENT PRESENT
        C   - CONFORMING                            R   - READABLE
        D   - DEFAULT                               W   - WRITABLE
        DPL - DESCRIPTOR PRIVILEGE LEVEL


6.3.1.1  Type Checking

The TYPE field of a descriptor has two functions:

  1.  It distinguishes among different descriptor formats.
  2.  It specifies the intended usage of a segment.

Besides the descriptors for data and executable segments commonly used by
applications programs, the 80386 has descriptors for special segments used
by the operating system and for gates. Table 6-1 lists all the types defined
for system segments and gates. Note that not all descriptors define
segments; gate descriptors have a different purpose that is discussed later
in this chapter.

The type fields of data and executable segment descriptors include bits
which further define the purpose of the segment (refer to Figure 6-1):

  ■  The writable bit in a data-segment descriptor specifies whether
     instructions can write into the segment.

  ■  The readable bit in an executable-segment descriptor specifies
     whether instructions are allowed to read from the segment (for example,
     to access constants that are stored with instructions). A readable,
     executable segment may be read in two ways:

  1.  Via the CS register, by using a CS override prefix.

  2.  By loading a selector of the descriptor into a data-segment register
      (DS, ES, FS,or GS).

Type checking can be used to detect programming errors that would attempt
to use segments in ways not intended by the programmer. The processor
examines type information on two kinds of occasions:

  1.  When a selector of a descriptor is loaded into a segment register.
      Certain segment registers can contain only certain descriptor types;
      for example:

  ■  The CS register can be loaded only with a selector of an executable
     segment.

  ■  Selectors of executable segments that are not readable cannot be
     loaded into data-segment registers.

  ■  Only selectors of writable data segments can be loaded into SS.

  2.  When an instruction refers (implicitly or explicitly) to a segment
      register. Certain segments can be used by instructions only in certain
      predefined ways; for example:

  ■  No instruction may write into an executable segment.

  ■  No instruction may write into a data segment if the writable bit is
     not set.

  ■  No instruction may read an executable segment unless the readable bit
     is set.


Table 6-1. System and Gate Descriptor Types

Code      Type of Segment or Gate

  0       -reserved
  1       Available 286 TSS
  2       LDT
  3       Busy 286 TSS
  4       Call Gate
  5       Task Gate
  6       286 Interrupt Gate
  7       286 Trap Gate
  8       -reserved
  9       Available 386 TSS
  A       -reserved
  B       Busy 386 TSS
  C       386 Call Gate
  D       -reserved
  E       386 Interrupt Gate
  F       386 Trap Gate


6.3.1.2  Limit Checking

The limit field of a segment descriptor is used by the processor to prevent
programs from addressing outside the segment. The processor's interpretation
of the limit depends on the setting of the G (granularity) bit. For data
segments, the processor's interpretation of the limit depends also on the
E-bit (expansion-direction bit) and the B-bit (big bit) (refer to Table
6-2).

When G=0, the actual limit is the value of the 20-bit limit field as it
appears in the descriptor. In this case, the limit may range from 0 to
0FFFFFH (2^(20) - 1 or 1 megabyte). When G=1, the processor appends 12
low-order one-bits to the value in the limit field. In this case the actual
limit may range from 0FFFH (2^(12) - 1 or 4 kilobytes) to 0FFFFFFFFH(2^(32)
- 1 or 4 gigabytes).

For all types of segments except expand-down data segments, the value of
the limit is one less than the size (expressed in bytes) of the segment. The
processor causes a general-protection exception in any of these cases:

  ■  Attempt to access a memory byte at an address > limit.
  ■  Attempt to access a memory word at an address ≥limit.
  ■  Attempt to access a memory doubleword at an address ≥(limit-2).

For expand-down data segments, the limit has the same function but is
interpreted differently. In these cases the range of valid addresses is from
limit + 1 to either 64K or 2^(32) - 1 (4 Gbytes) depending on the B-bit. An
expand-down segment has maximum size when the limit is zero.

The expand-down feature makes it possible to expand the size of a stack by
copying it to a larger segment without needing also to update intrastack
pointers.

The limit field of descriptors for descriptor tables is used by the
processor to prevent programs from selecting a table entry outside the
descriptor table. The limit of a descriptor table identifies the last valid
byte of the last descriptor in the table. Since each descriptor is eight
bytes long, the limit value is N * 8 - 1 for a table that can contain up to
N descriptors.

Limit checking catches programming errors such as runaway subscripts and
invalid pointer calculations. Such errors are detected when they occur, so
that identification of the cause is easier. Without limit checking, such
errors could corrupt other modules; the existence of such errors would not
be discovered until later, when the corrupted module behaves incorrectly,
and when identification of the cause is difficult.


Table 6-2. Useful Combinations of E, G, and B Bits


Case:                    1         2         3         4

Expansion Direction      U         U         D         D
G-bit                    0         1         0         1
B-bit                    X         X         0         1

Lower bound is:
     0                   X         X
     LIMIT+1                                 X
shl(LIMIT,12,1)+1                                      X

Upper bound is:
     LIMIT               X
     shl(LIMIT,12,1)               X
     64K-1                                   X
     4G-1                                              X

Max seg size is:
     64K                 X
     64K-1                         X
     4G-4K                                   X
     4G                                                X

Min seg size is:
     0                   X         X
     4K                                      X         X

shl (X, 12, 1) = shift X left by 12 bits inserting one-bits on the right


6.3.1.3  Privilege Levels

The concept of privilege is implemented by assigning a value from zero to
three to key objects recognized by the processor. This value is called the
privilege level. The value zero represents the greatest privilege, the
value three represents the least privilege. The following
processor-recognized objects contain privilege levels:

  ■  Descriptors contain a field called the descriptor privilege level
     (DPL).

  ■  Selectors contain a field called the requestor's privilege level
     (RPL). The RPL is intended to represent the privilege level of
     the procedure that originates a selector.

  ■  An internal processor register records the current privilege level
     (CPL). Normally the CPL is equal to the DPL of the segment that
     the processor is currently executing. CPL changes as control is
     transferred to segments with differing DPLs.

The processor automatically evaluates the right of a procedure to access
another segment by comparing the CPL to one or more other privilege levels.
The evaluation is performed at the time the selector of a descriptor is
loaded into a segment register. The criteria used for evaluating access to
data differs from that for evaluating transfers of control to executable
segments; therefore, the two types of access are considered separately in
the following sections.

Figure 6-2 shows how these levels of privilege can be interpreted as rings
of protection. The center is for the segments containing the most critical
software, usually the kernel of the operating system. Outer rings are for
the segments of less critical software.

It is not necessary to use all four privilege levels. Existing software
that was designed to use only one or two levels of privilege can simply
ignore the other levels offered by the 80386. A one-level system should use
privilege level zero; a two-level system should use privilege levels zero
and three.


Figure 6-2.  Levels of Privilege

                                     TASK C
              ┌───────────────────────────────────────────────────┐
              │ ╔═══════════════════════════════════════════════╗ │
              │ ║                 APPLICATIONS                  ║ │
              │ ║     ╔═══════════════════════════════════╗     ║ │
              │ ║     ║        CUSTOM EXTENSIONS          ║     ║ │
              │ ║     ║     ╔═══════════════════════╗     ║     ║ │
              │ ║     ║     ║    SYSTEM SERVICES    ║     ║     ║ │
              │ ║     ║     ║     ╔═══════════╗     ║     ║     ║ │
              │ ║     ║     ║     ║  KERNAL   ║     ║     ║     ║ │
              ╞═╟─────╫─────╫─────╫─────┬─────╫─────╫─────╫─────╢═╡
              │ ║     ║     ║     ║     │LEVEL║LEVEL║LEVEL║LEVEL║ │
              │ ║     ║     ║     ║     │  0  ║  1  ║  2  ║  3  ║ │
              │ ║     ║     ║     ╚═════╪═════╝     ║     ║     ║ │
              │ ║     ║     ║           │           ║     ║     ║ │
              │ ║     ║     ╚═══════════╪═══════════╝     ║     ║ │
              │ ║     ║                 │                 ║     ║ │
              │ ║     ╚═════════════════╪═════════════════╝     ║ │
              │ ║                       │                       ║ │
        TASK B┤ ╚═══════════════════════╧═══════════════════════╝ ├TASK A
              └────────────────────────┘ └────────────────────────┘


6.3.2  Restricting Access to Data

To address operands in memory, an 80386 program must load the selector of a
data segment into a data-segment register (DS, ES, FS, GS, SS). The
processor automatically evaluates access to a data segment by comparing
privilege levels. The evaluation is performed at the time a selector for the
descriptor of the target segment is loaded into the data-segment register.
As Figure 6-3 shows, three different privilege levels enter into this type
of privilege check:

  1.  The CPL (current privilege level).

  2.  The RPL (requestor's privilege level) of the selector used to specify
      the target segment.

  3.  The DPL of the descriptor of the target segment.

Instructions may load a data-segment register (and subsequently use the
target segment) only if the DPL of the target segment is numerically greater
than or equal to the maximum of the CPL and the selector's RPL. In other
words, a procedure can only access data that is at the same or less
privileged level.

The addressable domain of a task varies as CPL changes. When CPL is zero,
data segments at all privilege levels are accessible; when CPL is one, only
data segments at privilege levels one through three are accessible; when CPL
is three, only data segments at privilege level three are accessible. This
property of the 80386 can be used, for example, to prevent applications
procedures from reading or changing tables of the operating system.


Figure 6-3.  Privilege Check for Data Access

          16-BIT VISIBLE
             SELECTOR            INVISIBLE DESCRIPTOR
        ╔═══════════════╦═══════════════════╦═══╦═══════════╗
     CS ║               ║                   ║CPL║           ║
        ╚═══════════════╩═══════════════════╩═╤═╩═══════════╝
                                              │
    TARGET SEGMENT SELECTOR                   │        ╔═══════════╗
 ╔═══════════════════════╦═╦═══╗              └───────║ PRIVILEGE ║
 ║         INDEX         ║ ║RPL╟──────────────────────║ CHECK     ║
 ╚═══════════════════════╩═╩═══╝              ┌───────║ BY CPU    ║
                                              │        ╚═══════════╝
     DATA SEGMENT DESCRIPTOR              ┌───┘
                                          │
  31                23                15  │             7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤══╧══╤═════════╪═════════════════╗
 ║                 │ │ │ │A│ LIMIT   │ │     │  TYPE   │                 ║
 ║   BASE 31..24   │G│B│0│V│         │P│ DPL │         │   BASE 23..16   ║ 4
 ║                 │ │ │ │L│  19..16 │ │     │1│0│E│W│A│                 ║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴─┴─┴─┴─┴─────────────────╢
 ║                                   │                                   ║
 ║        SEGMENT BASE 15..0         │        SEGMENT LIMIT 15..0        ║ 0
 ║                                   │                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝


    CPL - CURRENT PRIVILEGE LEVEL
    RPL - REQUESTOR'S PRIVILEGE LEVEL
    DPL - DESCRIPTOR PRIVILEGE LEVEL


6.3.2.1  Accessing Data in Code Segments

Less common than the use of data segments is the use of code segments to
store data. Code segments may legitimately hold constants; it is not
possible to write to a segment described as a code segment. The following
methods of accessing data in code segments are possible:

  1.  Load a data-segment register with a selector of a nonconforming,
      readable, executable segment.

  2.  Load a data-segment register with a selector of a conforming,
      readable, executable segment.

  3.  Use a CS override prefix to read a readable, executable segment whose
      selector is already loaded in the CS register.

The same rules as for access to data segments apply to case 1. Case 2 is
always valid because the privilege level of a segment whose conforming bit
is set is effectively the same as CPL regardless of its DPL. Case 3 always
valid because the DPL of the code segment in CS is, by definition, equal to
CPL.


6.3.3  Restricting Control Transfers

With the 80386, control transfers are accomplished by the instructions JMP,
CALL, RET, INT, and IRET, as well as by the exception and interrupt
mechanisms. Exceptions and interrupts are special cases that Chapter 9
covers. This chapter discusses only JMP, CALL, and RET instructions.

The "near" forms of JMP, CALL, and RET transfer within the current code
segment, and therefore are subject only to limit checking. The processor
ensures that the destination of the JMP, CALL, or RET instruction does not
exceed the limit of the current executable segment. This limit is cached in
the CS register; therefore, protection checks for near transfers require no
extra clock cycles.

The operands of the "far" forms of JMP and CALL refer to other segments;
therefore, the processor performs privilege checking. There are two ways a
JMP or CALL can refer to another segment:

  1.  The operand selects the descriptor of another executable segment.

  2.  The operand selects a call gate descriptor. This gated form of
      transfer is discussed in a later section on call gates.

As Figure 6-4 shows, two different privilege levels enter into a privilege
check for a control transfer that does not use a call gate:

  1.  The CPL (current privilege level).
  2.  The DPL of the descriptor of the target segment.

Normally the CPL is equal to the DPL of the segment that the processor is
currently executing. CPL may, however, be greater than DPL if the conforming
bit is set in the descriptor of the current executable segment. The
processor keeps a record of the CPL cached in the CS register; this value
can be different from the DPL in the descriptor of the code segment.

The processor permits a JMP or CALL directly to another segment only if one
of the following privilege rules is satisfied:

  ■  DPL of the target is equal to CPL.

  ■  The conforming bit of the target code-segment descriptor is set, and
     the DPL of the target is less than or equal to CPL.

An executable segment whose descriptor has the conforming bit set is called
a conforming segment. The conforming-segment mechanism permits sharing of
procedures that may be called from various privilege levels but should
execute at the privilege level of the calling procedure. Examples of such
procedures include math libraries and some exception handlers. When control
is transferred to a conforming segment, the CPL does not change. This is
the only case when CPL may be unequal to the DPL of the current executable
segment.

Most code segments are not conforming. The basic rules of privilege above
mean that, for nonconforming segments, control can be transferred without a
gate only to executable segments at the same level of privilege. There is a
need, however, to transfer control to (numerically) smaller privilege
levels; this need is met by the CALL instruction when used with call-gate
descriptors, which are explained in the next section. The JMP instruction
may never transfer control to a nonconforming segment whose DPL does not
equal CPL.


Figure 6-4.  Privilege Check for Control Transfer without Gate

          16-BIT VISIBLE
             SELECTOR               INVISIBLE PART
        ╔═══════════════╦═══════════════════╦═══╦═══════════╗
        ║               ║                   ║CPL║           ║ CS
        ╚═══════════════╩═══════════════════╩═╤═╩═══════════╝
                                              │
                                              │        ╔═══════════╗
                                              └───────║ PRIVILEGE ║
                                          ┌───────────║ CHECK     ║
                                          │       ┌───║ BY CPU    ║
     CODE-SEGMENT DESCRIPTOR              │       │    ╚═══════════╝
                                          │       │
  31                23                15  │       │     7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤══╧══╤════╪════╪═════════════════╗
 ║                 │ │ │ │A│ LIMIT   │ │     │    │    │                 ║
 ║   BASE 31..24   │G│D│0│V│         │P│ DPL │    │    │   BASE 23..16   ║ 4
 ║                 │ │ │ │L│  19..16 │ │     │1│1│C│R│A│                 ║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴─┴─┴─┴─┴─────────────────╢
 ║                                   │                                   ║
 ║        SEGMENT BASE 15..0         │        SEGMENT LIMIT 15..0        ║ 0
 ║                                   │                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝

    CPL - CURRENT PRIVILEGE LEVEL
    DPL - DESCRIPTOR PRIVILEGE LEVEL
    C   - CONFORMING BIT


6.3.4  Gate Descriptors Guard Procedure Entry Points

To provide protection for control transfers among executable segments
at different privilege levels, the 80386 uses gate descriptors. There are
four kinds of gate descriptors:

  ■  Call gates
  ■  Trap gates
  ■  Interrupt gates
  ■  Task gates

This chapter is concerned only with call gates. Task gates are used for
task switching, and therefore are discussed in Chapter 7. Chapter 9
explains how trap gates and interrupt gates are used by exceptions and
interrupts. Figure 6-5 illustrates the format of a call gate. A call gate
descriptor may reside in the GDT or in an LDT, but not in the IDT.

A call gate has two primary functions:

  1.  To define an entry point of a procedure.
  2.  To specify the privilege level of the entry point.

Call gate descriptors are used by call and jump instructions in the same
manner as code segment descriptors. When the hardware recognizes that the
destination selector refers to a gate descriptor, the operation of the
instruction is expanded as determined by the contents of the call gate.

The selector and offset fields of a gate form a pointer to the entry point
of a procedure. A call gate guarantees that all transitions to another
segment go to a valid entry point, rather than possibly into the middle of a
procedure (or worse, into the middle of an instruction). The far pointer
operand of the control transfer instruction does not point to the segment
and offset of the target instruction; rather, the selector part of the
pointer selects a gate, and the offset is not used. Figure 6-6 illustrates
this style of addressing.

As Figure 6-7 shows, four different privilege levels are used to check the
validity of a control transfer via a call gate:

  1.  The CPL (current privilege level).

  2.  The RPL (requestor's privilege level) of the selector used to specify
      the call gate.

  3.  The DPL of the gate descriptor.

  4.  The DPL of the descriptor of the target executable segment.

The DPL field of the gate descriptor determines what privilege levels can
use the gate. One code segment can have several procedures that are intended
for use by different privilege levels. For example, an operating system may
have some services that are intended to be used by applications, whereas
others may be intended only for use by other systems software.

Gates can be used for control transfers to numerically smaller privilege
levels or to the same privilege level (though they are not necessary for
transfers to the same level). Only CALL instructions can use gates to
transfer to smaller privilege levels. A gate may be used by a JMP
instruction only to transfer to an executable segment with the same
privilege level or to a conforming segment.

For a JMP instruction to a nonconforming segment, both of the following
privilege rules must be satisfied; otherwise, a general protection exception
results.

   MAX (CPL,RPL) ≤ gate DPL
   target segment DPL = CPL

For a CALL instruction (or for a JMP instruction to a conforming segment),
both of the following privilege rules must be satisfied; otherwise, a
general protection exception results.

   MAX (CPL,RPL) ≤ gate DPL
   target segment DPL ≤ CPL


Figure 6-5.  Format of 80386 Call Gate

   31                23                15               7              0
  ╔═════════════════╪═════════════════╪═╤═════╤═════════╪═════╤═════════╗
  ║                                   │ │     │  TYPE   │     │  DWORD  ║
  ║           OFFSET 31..16           │P│ DPL │         │0 0 0│         ║ 4
  ║                                   │ │     │0 1 1 0 0│     │  COUNT  ║
  ╟───────────────────────────────────┼─┴─────┴─────────┴─────┴─────────╢
  ║                                   │                                 ║
  ║              SELECTOR             │           OFFSET 15..0          ║ 0
  ║                                   │                                 ║
  ╚═════════════════╪═════════════════╪═════════════════╪═══════════════╝


Figure 6-6.  Indirect Transfer via Call Gate

         OPCODE                 OFFSET                   SELECTOR
       ╔═════════╦════════════════════════════════════╦═══════╦═╦═══╗
       ║  CALL   ║            (NOT USED)              ║ INDEX ║ ║RPL║
       ╚═════════╩════════════════════════════════════╩═══╤═══╩═╩═══╝
                                                          │
                      DESCRIPTOR TABLE                    │
                 ╔══════╤═════╤═════╤══════╗              │
                 ║      │     │     │      ║              │
                 ╟──────┴─────┼─────┴──────╢              │
                 ║            │            ║              │
                 ╚════════════╧════════════╝              │
                                                        │
                                                        │
                                                        │
                 ╔════════════╤═════╤══════╗              │
         GATE    ║   OFFSET   │ DPL │COUNT ║─────────────┘    EXECUTABLE
      DESCRIPTOR ╟────────────┼─────┴──────╢                     SEGMENT
          ┌──────╢  SELECTOR  │   OFFSET   ╟─────┐          ╔══════════════╗
          │      ╠══════╤═════╪═════╤══════╣     │          ║              ║
          │      ║      │     │     │      ║     │          ║              ║
          │      ╟──────┴─────┼─────┴──────╢     │          ║              ║
          │      ║            │            ║     │          ║              ║
          │      ╠══════╤═════╪═════╤══════╣     │          ║              ║
          │      ║      │     │     │      ║     └─────────║  PROCEDURE   ║
          │      ╟──────┴─────┼─────┴──────╢                ║              ║
          │      ║            │            ║                ║              ║
                ╠══════╤═════╪═════╤══════╣                ║              ║
      EXECUTABLE ║ BASE │     │ DPL │ BASE ║                ║              ║
       SEGMENT   ╟──────┴─────┼─────┴──────╢     ┌─────────╚══════════════╝
      DESCRIPTOR ║    BASE    │            ╟─────┘
                 ╚════════════╧════════════╝
                                          
                                          
                                          
                 ╔══════╤═════╤═════╤══════╗
                 ║      │     │     │      ║
                 ╟──────┴─────┼─────┴──────╢
                 ║            │            ║
                 ╚════════════╧════════════╝


Figure 6-7.  Privilege Check via Call Gate

               16-BIT VISIBLE
                  SELECTOR            INVISIBLE DESCRIPTOR
             ╔═══════════════╦═══════════════════╦═══╦═══════════╗
          CS ║               ║                   ║CPL║           ║
             ╚═══════════════╩═══════════════════╩═╤═╩═══════════╝
                                                   │
              TARGET SELECTOR                      │        ╔═══════════╗
      ╔═══════════════════════╦═╦═══╗              └───────║ PRIVILEGE ║
      ║         INDEX         ║ ║RPL╟──────────────────────║ CHECK     ║
      ╚═══════════════════════╩═╩═══╝   ┌──────────────────║ BY        ║
                                        │               ┌──║ CPU       ║
                                 ┌──────┘               │   ╚═══════════╝
                                 │                      │
         GATE DESCRIPTOR                               │
    ╔═══════════════════════╦══════════╦═══════════╗    │
    ║        OFFSET         ║   DPL    ║   COUNT   ║    │
    ╠═══════════════════════╬══════════╩═══════════╣    │
    ║       SELECTOR        ║        OFFSET        ║    │
    ╚═══════════════════════╩══════════════════════╝    │
                                                        │
                                                        │
                          ╔═══════════╦═══════════╦═════╧════╦═══════════╗
               EXECUTABLE ║   BASE    ║   LIMIT   ║    DPL   ║   BASE    ║
                SEGMENT   ╠═══════════╩═══════════╬══════════╩═══════════╣
               DESCRIPTOR ║          BASE         ║        LIMIT         ║
                          ╚═══════════════════════╩══════════════════════╝

            CPL     - CURRENT PRIVILEGE LEVEL
            RPL     - REQUESTOR'S PRIVILEGE LEVEL
            DPL     - DESCRIPTOR PRIVILEGE LEVEL


6.3.4.1  Stack Switching

If the destination code segment of the call gate is at a different
privilege level than the CPL, an interlevel transfer is being requested.

To maintain system integrity, each privilege level has a separate stack.
These stacks assure sufficient stack space to process calls from less
privileged levels. Without them, a trusted procedure would not work
correctly if the calling procedure did not provide sufficient space on the
caller's stack.

The processor locates these stacks via the task state segment (see Figure
6-8). Each task has a separate TSS, thereby permitting tasks to have
separate stacks. Systems software is responsible for creating TSSs and
placing correct stack pointers in them. The initial stack pointers in the
TSS are strictly read-only values. The processor never changes them during
the course of execution.

When a call gate is used to change privilege levels, a new stack is
selected by loading a pointer value from the Task State Segment (TSS). The
processor uses the DPL of the target code segment (the new CPL) to index the
initial stack pointer for PL 0, PL 1, or PL 2.

The DPL of the new stack data segment must equal the new CPL; if it does
not, a stack exception occurs. It is the responsibility of systems software
to create stacks and stack-segment descriptors for all privilege levels that
are used. Each stack must contain enough space to hold the old SS:ESP, the
return address, and all parameters and local variables that may be required
to process a call.

As with intralevel calls, parameters for the subroutine are placed on the
stack. To make privilege transitions transparent to the called procedure,
the processor copies the parameters to the new stack. The count field of a
call gate tells the processor how many doublewords (up to 31) to copy from
the caller's stack to the new stack. If the count is zero, no parameters are
copied.

The processor performs the following stack-related steps in executing an
interlevel CALL.

  1.  The new stack is checked to assure that it is large enough to hold
      the parameters and linkages; if it is not, a stack fault occurs with
      an error code of 0.

  2.  The old value of the stack registers SS:ESP is pushed onto the new
      stack as two doublewords.

  3.  The parameters are copied.

  4.  A pointer to the instruction after the CALL instruction (the former
      value of CS:EIP) is pushed onto the new stack. The final value of
      SS:ESP points to this return pointer on the new stack.

Figure 6-9 illustrates the stack contents after a successful interlevel
call.

The TSS does not have a stack pointer for a privilege level 3 stack,
because privilege level 3 cannot be called by any procedure at any other
privilege level.

Procedures that may be called from another privilege level and that require
more than the 31 doublewords for parameters must use the saved SS:ESP link
to access all parameters beyond the last doubleword copied.

A call via a call gate does not check the values of the words copied onto
the new stack. The called procedure should check each parameter for
validity. A later section discusses how the ARPL, VERR, VERW, LSL, and LAR
instructions can be used to check pointer values.


Figure 6-8.  Initial Stack Pointers of TSS

                 31       23       15       7      0
                ╔════════╪════════╪════════╪════════╗64
                                                   
                                                   
                                                   
                ║                                   ║
                ╠════════╪════════╪════════╪════════╣
                ║              EFLAGS               ║24
                ╠════════╪════════╪════════╪════════╣
                ║     INSTRUCTION POINTER (EIP)     ║20
                ╠════════╪════════╪════════╪════════╣
                ║            CR3 (PDBR)             ║1C
                ╠════════╪════════╦════════╪═════╦══╣  ─┐
                ║00000000 00000000║       SS2    ║10║18 │
                ╠════════╪════════╩════════╪═════╩══╣   │
                ║               ESP2                ║14 │
                ╠════════╪════════╦════════╪═════╦══╣   │
                ║00000000 00000000║       SS1    ║01║10 │  INITIAL
                ╠════════╪════════╩════════╪═════╩══╣   ├─ STACK
                ║               ESP1                ║0C │  POINTERS
                ╠════════╪════════╦════════╪═════╦══╣   │
                ║00000000 00000000║       SS0    ║00║8  │
                ╠════════╪════════╩════════╪═════╩══╣   │
                ║               ESP0                ║4  │
                ╠════════╪════════╦════════╪════════╣  ─┘
                ║00000000 00000000║  TSS BACK LINK  ║0
                ╚════════╪════════╩════════╪════════╝


Figure 6-9.  Stack Contents after an Interlevel Call

                                            31            0     SS:ESP
                                           ╔═══════╪═══════╗──FROM TSS
                 31            0           ║▒▒▒▒▒▒▒│OLD SS ║
                ╔═══════╪═══════╗          ╠═══════╪═══════╣
       D  O     ║               ║          ║    OLD ESP    ║
       I  F     ║               ║          ╠═══════╪═══════╣
       R        ║               ║          ║    PARM 3     ║
       E  E     ║               ║          ╠═══════╪═══════╣
       C  X     ║               ║          ║    PARM 2     ║
       T  P     ╠═══════╪═══════╣          ╠═══════╪═══════╣
       I  A     ║    PARM 3     ║          ║    PARM 1     ║
       O  N     ╠═══════╪═══════╣          ╠═══════╪═══════╣
       N  S     ║    PARM 2     ║          ║▒▒▒▒▒▒▒│OLD CS ║     NEW
          I     ╠═══════╪═══════╣  OLD     ╠═══════╪═══════╣    SS:ESP
        │ O     ║    PARM 1     ║  SS:ESP  ║    OLD EIP    ║      │
        │ N     ╠═══════╪═══════╣───┘     ╠═══════╪═══════╣─────┘
        │       ║               ║          ║               ║
               ║               ║          ║               ║
                ╚═══════╪═══════╝          ╚═══════╪═══════╝
                    OLD STACK                  NEW STACK


6.3.4.2  Returning from a Procedure

The "near" forms of the RET instruction transfer control within the current
code segment and therefore are subject only to limit checking. The offset of
the instruction following the corresponding CALL, is popped from the stack.
The processor ensures that this offset does not exceed the limit of the
current executable segment.

The "far" form of the RET instruction pops the return pointer that was
pushed onto the stack by a prior far CALL instruction. Under normal
conditions, the return pointer is valid, because of its relation to the
prior CALL or INT. Nevertheless, the processor performs privilege checking
because of the possibility that the current procedure altered the pointer or
failed to properly maintain the stack. The RPL of the CS selector popped
off the stack by the return instruction identifies the privilege level of
the calling procedure.

An intersegment return instruction can change privilege levels, but only
toward procedures of lesser privilege. When the RET instruction encounters a
saved CS value whose RPL is numerically greater than the CPL, an interlevel
return occurs. Such a return follows these steps:

  1.  The checks shown in Table 6-3 are made, and CS:EIP and SS:ESP are
      loaded with their former values that were saved on the stack.

  2.  The old SS:ESP (from the top of the current stack) value is adjusted
      by the number of bytes indicated in the RET instruction. The resulting
      ESP value is not compared to the limit of the stack segment. If ESP is
      beyond the limit, that fact is not recognized until the next stack
      operation. (The SS:ESP value of the returning procedure is not
      preserved; normally, this value is the same as that contained in the
      TSS.)

  3.  The contents of the DS, ES, FS, and GS segment registers are checked.
      If any of these registers refer to segments whose DPL is greater than
      the new CPL (excluding conforming code segments), the segment register
      is loaded with the null selector (INDEX = 0, TI = 0). The RET
      instruction itself does not signal exceptions in these cases;
      however, any subsequent memory reference that attempts to use a
      segment register that contains the null selector will cause a general
      protection exception. This prevents less privileged code from
      accessing more privileged segments using selectors left in the
      segment registers by the more privileged procedure.


6.3.5  Some Instructions are Reserved for Operating System

Instructions that have the power to affect the protection mechanism or to
influence general system performance can only be executed by trusted
procedures. The 80386 has two classes of such instructions:

  1.  Privileged instructions ── those used for system control.

  2.  Sensitive instructions ── those used for I/O and I/O related
      activities.


Table 6-3. Interlevel Return Checks


Type of Check                              Exception
SF  Stack Fault
GP  General Protection Exception
NP  Segment-Not-Present Exception  Error Code

ESP is within current SS segment           SF          0
ESP + 7 is within current SS segment       SF          0
RPL of return CS is greater than CPL       GP          Return CS
Return CS selector is not null             GP          Return CS
Return CS segment is within descriptor
table limit                                GP          Return CS
Return CS descriptor is a code segment     GP          Return CS
Return CS segment is present               NP          Return CS
DPL of return nonconforming code
segment = RPL of return CS, or DPL of
return conforming code segment ≤ RPL
of return CS                               GP          Return CS
ESP + N + 15 is within SS segment
N   Immediate Operand of RET N Instruction         SF          Return SS
SS selector at ESP + N + 12 is not null    GP          Return SS
SS selector at ESP + N + 12 is within
descriptor table limit                     GP          Return SS
SS descriptor is writable data segment     GP          Return SS
SS segment is present                      SF          Return SS
Saved SS segment DPL = RPL of saved
CS                                         GP          Return SS
Saved SS selector RPL = Saved SS
segment DPL                                GP          Return SS


6.3.5.1  Privileged Instructions

The instructions that affect system data structures can only be executed
when CPL is zero. If the CPU encounters one of these instructions when CPL
is greater than zero, it signals a general protection exception. These
instructions include:

   CLTS                 ── Clear Task─Switched Flag
   HLT                  ── Halt Processor
   LGDT                 ── Load GDL Register
   LIDT                 ── Load IDT Register
   LLDT                 ── Load LDT Register
   LMSW                 ── Load Machine Status Word
   LTR                  ── Load Task Register
   MOV to/from CRn      ── Move to Control Register n
   MOV to /from DRn     ── Move to Debug Register n
   MOV to/from TRn      ── Move to Test Register n


6.3.5.2  Sensitive Instructions

Instructions that deal with I/O need to be restricted but also need to be
executed by procedures executing at privilege levels other than zero. The
mechanisms for restriction of I/O operations are covered in detail in
Chapter 8, "Input/Output".


6.3.6  Instructions for Pointer Validation

Pointer validation is an important part of locating programming errors.
Pointer validation is necessary for maintaining isolation between the
privilege levels. Pointer validation consists of the following steps:

  1.  Check if the supplier of the pointer is entitled to access the
      segment.

  2.  Check if the segment type is appropriate to its intended use.

  3.  Check if the pointer violates the segment limit.

Although the 80386 processor automatically performs checks 2 and 3 during
instruction execution, software must assist in performing the first check.
The unprivileged instruction ARPL is provided for this purpose. Software can
also explicitly perform steps 2 and 3 to check for potential violations
(rather than waiting for an exception). The unprivileged instructions LAR,
LSL, VERR, and VERW are provided for this purpose.

LAR (Load Access Rights) is used to verify that a pointer refers to a
segment of the proper privilege level and type. LAR has one operand──a
selector for a descriptor whose access rights are to be examined. The
descriptor must be visible at the privilege level which is the maximum of
the CPL and the selector's RPL. If the descriptor is visible, LAR obtains a
masked form of the second doubleword of the descriptor, masks this value
with 00FxFF00H, stores the result into the specified 32-bit destination
register, and sets the zero flag. (The x indicates that the corresponding
four bits of the stored value are undefined.) Once loaded, the access-rights
bits can be tested. All valid descriptor types can be tested by the LAR
instruction. If the RPL or CPL is greater than DPL, or if the selector is
outside the table limit, no access-rights value is returned, and the zero
flag is cleared. Conforming code segments may be accessed from any privilege
level.

LSL (Load Segment Limit) allows software to test the limit of a descriptor.
If the descriptor denoted by the given selector (in memory or a register) is
visible at the CPL, LSL loads the specified 32-bit register with a 32-bit,
byte granular, unscrambled limit that is calculated from fragmented limit
fields and the G-bit of that descriptor. This can only be done for segments
(data, code, task state, and local descriptor tables); gate descriptors are
inaccessible. (Table 6-4 lists in detail which types are valid and which
are not.) Interpreting the limit is a function of the segment type. For
example, downward expandable data segments treat the limit differently than
code segments do. For both LAR and LSL, the zero flag (ZF) is set if the
loading was performed; otherwise, the ZF is cleared.


Table 6-4. Valid Descriptor Types for LSL

    Type   Descriptor Type             Valid?
    Code

    0      (invalid)                   NO
    1      Available 286 TSS           YES
    2      LDT                         YES
    3      Busy 286 TSS                YES
    4      286 Call Gate               NO
    5      Task Gate                   NO
    6      286 Trap Gate               NO
    7      286 Interrupt Gate          NO
    8      (invalid)                   NO
    9      Available 386 TSS           YES
    A      (invalid)                   NO
    B      Busy 386 TSS                YES
    C      386 Call Gate               NO
    D      (invalid)                   NO
    E      386 Trap Gate               NO
    F      386 Interrupt Gate          NO


6.3.6.1  Descriptor Validation

The 80386 has two instructions, VERR and VERW, which determine whether a
selector points to a segment that can be read or written at the current
privilege level. Neither instruction causes a protection fault if the result
is negative.

VERR (Verify for Reading) verifies a segment for reading and loads ZF with
1 if that segment is readable from the current privilege level. VERR checks
that:

  ■  The selector points to a descriptor within the bounds of the GDT or
     LDT.

  ■  It denotes a code or data segment descriptor.

  ■  The segment is readable and of appropriate privilege level.

The privilege check for data segments and nonconforming code segments is
that the DPL must be numerically greater than or equal to both the CPL and
the selector's RPL. Conforming segments are not checked for privilege level.

VERW (Verify for Writing) provides the same capability as VERR for
verifying writability. Like the VERR instruction, VERW loads ZF if the
result of the writability check is positive. The instruction checks that the
descriptor is within bounds, is a segment descriptor, is writable, and that
its DPL is numerically greater or equal to both the CPL and the selector's
RPL. Code segments are never writable, conforming or not.


6.3.6.2  Pointer Integrity and RPL

The Requestor's Privilege Level (RPL) feature can prevent inappropriate use
of pointers that could corrupt the operation of more privileged code or data
from a less privileged level.

A common example is a file system procedure, FREAD (file_id, n_bytes,
buffer_ptr). This hypothetical procedure reads data from a file into a
buffer, overwriting whatever is there. Normally, FREAD would be available at
the user level, supplying only pointers to the file system procedures and
data located and operating at a privileged level. Normally, such a procedure
prevents user-level procedures from directly changing the file tables.
However, in the absence of a standard protocol for checking pointer
validity, a user-level procedure could supply a pointer into the file tables
in place of its buffer pointer, causing the FREAD procedure to corrupt them
unwittingly.

Use of RPL can avoid such problems. The RPL field allows a privilege
attribute to be assigned to a selector. This privilege attribute would
normally indicate the privilege level of the code which generated the
selector. The 80386 processor automatically checks the RPL of any selector
loaded into a segment register to determine whether the RPL allows access.

To take advantage of the processor's checking of RPL, the called procedure
need only ensure that all selectors passed to it have an RPL at least as
high (numerically) as the original caller's CPL. This action guarantees that
selectors are not more trusted than their supplier. If one of the selectors
is used to access a segment that the caller would not be able to access
directly, i.e., the RPL is numerically greater than the DPL, then a
protection fault will result when that selector is loaded into a segment
register.

ARPL (Adjust Requestor's Privilege Level) adjusts the RPL field of a
selector to become the larger of its original value and the value of the RPL
field in a specified register. The latter is normally loaded from the image
of the caller's CS register which is on the stack. If the adjustment changes
the selector's RPL, ZF (the zero flag) is set; otherwise, ZF is cleared.


6.4  Page-Level Protection

Two kinds of protection are related to pages:

  1.  Restriction of addressable domain.
  2.  Type checking.


6.4.1  Page-Table Entries Hold Protection Parameters

Figure 6-10 highlights the fields of PDEs and PTEs that control access to
pages.


Figure 6-10.  Protection Fields of Page Table Entries

      31                                  12 11          7           0
     ╔══════════════════════════════════════╪═══════╤═══╤═╤═╤═══╤═╤═╤═╗
     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒▒▒▒▒▒▒│▒▒▒│▒│▒│▒▒▒│U│R│▒║
     ║▒▒▒▒▒▒PAGE FRAME ADDRESS 31..12▒▒▒▒▒▒▒│▒AVAIL▒│0▒0│D│A│0▒0│/│/│P║
     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│▒▒▒▒▒▒▒│▒▒▒│▒│▒│▒▒▒│S│W│▒║
     ╚══════════════════════════════════════╪═══════╧═══╧═╧═╧═══╧═╧═╧═╝
      R/W     - READ/WRITE
      U/S     - USER/SUPERVISOR


6.4.1.1  Restricting Addressable Domain

The concept of privilege for pages is implemented by assigning each page to
one of two levels:

  1.  Supervisor level (U/S=0) ── for the operating system and other systems
      software and related data.

  2.  User level (U/S=1) ── for applications procedures and data.

The current level (U or S) is related to CPL.  If CPL is 0, 1, or 2, the
processor is executing at supervisor level. If CPL is 3, the processor is
executing at user level.

When the processor is executing at supervisor level, all pages are
addressable, but, when the processor is executing at user level, only pages
that belong to the user level are addressable.


6.4.1.2  Type Checking

At the level of page addressing, two types are defined:

  1.  Read-only access (R/W=0)
  2.  Read/write access (R/W=1)

When the processor is executing at supervisor level, all pages are both
readable and writable. When the processor is executing at user level, only
pages that belong to user level and are marked for read/write access are
writable; pages that belong to supervisor level are neither readable nor
writable from user level.


6.4.2  Combining Protection of Both Levels of Page Tables

For any one page, the protection attributes of its page directory entry may
differ from those of its page table entry. The 80386 computes the effective
protection attributes for a page by examining the protection attributes in
both the directory and the page table.  Table 6-5 shows the effective
protection provided by the possible combinations of protection attributes.


6.4.3  Overrides to Page Protection

Certain accesses are checked as if they are privilege-level 0 references,
even if CPL = 3:

  ■  LDT, GDT, TSS, IDT references.
  ■  Access to inner stack during ring-crossing CALL/INT.


6.5  Combining Page and Segment Protection

When paging is enabled, the 80386 first evaluates segment protection, then
evaluates page protection. If the processor detects a protection violation
at either the segment or the page level, the requested operation cannot
proceed; a protection exception occurs instead.

For example, it is possible to define a large data segment which has some
subunits that are read-only and other subunits that are read-write.  In this
case, the page directory (or page table) entries for the read-only subunits
would have the U/S and R/W bits set to x0, indicating no write rights for
all the pages described by that directory entry (or for individual pages).
This technique might be used, for example, in a UNIX-like system to define
a large data segment, part of which is read only (for shared data or ROMmed
constants). This enables UNIX-like systems to define a "flat" data space as
one large segment, use "flat" pointers to address within this "flat" space,
yet be able to protect shared data, shared files mapped into the virtual
space, and supervisor areas.


Table 6-5. Combining Directory and Page Protection

Page Directory Entry     Page Table Entry      Combined Protection
  U/S          R/W         U/S      R/W          U/S         R/W

  S-0          R-0         S-0      R-0           S           x
  S-0          R-0         S-0      W-1           S           x
  S-0          R-0         U-1      R-0           S           x
  S-0          R-0         U-1      W-1           S           x
  S-0          W-1         S-0      R-0           S           x
  S-0          W-1         S-0      W-1           S           x
  S-0          W-1         U-1      R-0           S           x
  S-0          W-1         U-1      W-1           S           x
  U-1          R-0         S-0      R-0           S           x
  U-1          R-0         S-0      W-1           S           x
  U-1          R-0         U-1      R-0           U           R
  U-1          R-0         U-1      W-1           U           R
  U-1          W-1         S-0      R-0           S           x
  U-1          W-1         S-0      W-1           S           x
  U-1          W-1         U-1      R-0           U           R
  U-1          W-1         U-1      W-1           U           W

───────────────────────────────────────────────────────────────────────────
NOTE
  S ── Supervisor
  R ── Read only
  U ── User
  W ── Read and Write
  x indicates that when the combined U/S attribute is S, the R/W attribute
  is not checked.
───────────────────────────────────────────────────────────────────────────


Chapter 7  Multitasking

────────────────────────────────────────────────────────────────────────────

To provide efficient, protected multitasking, the 80386 employs several
special data structures. It does not, however, use special instructions to
control multitasking; instead, it interprets ordinary control-transfer
instructions differently when they refer to the special data structures. The
registers and data structures that support multitasking are:

  ■  Task state segment
  ■  Task state segment descriptor
  ■  Task register
  ■  Task gate descriptor

With these structures the 80386 can rapidly switch execution from one task
to another, saving the context of the original task so that the task can be
restarted later. In addition to the simple task switch, the 80386 offers two
other task-management features:

  1.  Interrupts and exceptions can cause task switches (if needed in the
      system design). The processor not only switches automatically to the
      task that handles the interrupt or exception, but it automatically
      switches back to the interrupted task when the interrupt or exception
      has been serviced. Interrupt tasks may interrupt lower-priority
      interrupt tasks to any depth.

  2.  With each switch to another task, the 80386 can also switch to
      another LDT and to another page directory. Thus each task can have a
      different logical-to-linear mapping and a different linear-to-physical
      mapping. This is yet another protection feature, because tasks can be
      isolated and prevented from interfering with one another.


7.1  Task State Segment

All the information the processor needs in order to manage a task is stored
in a special type of segment, a task state segment (TSS). Figure 7-1 shows
the format of a TSS for executing 80386 tasks. (Another format is used for
executing 80286 tasks; refer to Chapter 13.)

The fields of a TSS belong to two classes:

  1.  A dynamic set that the processor updates with each switch from the
      task. This set includes the fields that store:

      ■  The general registers (EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI).

      ■  The segment registers (ES, CS, SS, DS, FS, GS).

      ■  The flags register (EFLAGS).

      ■  The instruction pointer (EIP).

      ■  The selector of the TSS of the previously executing task (updated
         only when a return is expected).

  2.  A static set that the processor reads but does not change. This set
      includes the fields that store:

      ■  The selector of the task's LDT.

      ■  The register (PDBR) that contains the base address of the task's
         page directory (read only when paging is enabled).

      ■  Pointers to the stacks for privilege levels 0-2.

      ■  The T-bit (debug trap bit) which causes the processor to raise a
         debug exception when a task switch occurs. (Refer to Chapter 12
         for more information on debugging.)

      ■  The I/O map base (refer to Chapter 8 for more information on the
         use of the I/O map).

Task state segments may reside anywhere in the linear space. The only case
that requires caution is when the TSS spans a page boundary and the
higher-addressed page is not present. In this case, the processor raises an
exception if it encounters the not-present page while reading the TSS during
a task switch. Such an exception can be avoided by either of two strategies:

  1.  By allocating the TSS so that it does not cross a page boundary.

  2.  By ensuring that both pages are either both present or both
      not-present at the time of a task switch. If both pages are
      not-present, then the page-fault handler must make both pages present
      before restarting the instruction that caused the task switch.


Figure 7-1.  80386 32-Bit Task State Segment

      31              23              15              7             0
     ╔═══════════════╪═══════════════╬═══════════════╪═════════════╦═╗
     ║          I/O MAP BASE         ║ 0 0 0 0 0 0 0   0 0 0 0 0 0 ║T║64
     ╟───────────────┼───────────────╫───────────────┼─────────────╨─╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              LDT              ║60
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              GS               ║5C
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              FS               ║58
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              DS               ║54
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              SS               ║50
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              CS               ║4C
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              ES               ║48
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║                              EDI                              ║44
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                              ESI                              ║40
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                              EBP                              ║3C
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                              ESP                              ║38
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                              EBX                              ║34
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                              EDX                              ║30
     ╠═══════════════╪═══════════════╪═══════════════╪═══════════════╣
     ║                              ECX                              ║2C
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                              EAX                              ║28
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                            EFLAGS                             ║24
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                    INSTRUCTION POINTER (EIP)                  ║20
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                          CR3  (PDPR)                          ║1C
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              SS2              ║18
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║                             ESP2                              ║14
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              SS1              ║10
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║                             ESP1                              ║0C
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║              SS0              ║8
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║                             ESP0                              ║4
     ╟───────────────┼───────────────╫───────────────┼───────────────╢
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0║   BACK LINK TO PREVIOUS TSS   ║0
     ╚═══════════════╪═══════════════╬═══════════════╪═══════════════╝

────────────────────────────────────────────────────────────────────────────
NOTE
      0 MEANS INTEL RESERVED. DO NOT DEFINE.
────────────────────────────────────────────────────────────────────────────


7.2  TSS Descriptor

The task state segment, like all other segments, is defined by a
descriptor. Figure 7-2 shows the format of a TSS descriptor.

The B-bit in the type field indicates whether the task is busy. A type code
of 9 indicates a non-busy task; a type code of 11 indicates a busy task.
Tasks are not reentrant. The B-bit allows the processor to detect an attempt
to switch to a task that is already busy.

The BASE, LIMIT, and DPL fields and the G-bit and P-bit have functions
similar to their counterparts in data-segment descriptors. The LIMIT field,
however, must have a value equal to or greater than 103. An attempt to
switch to a task whose TSS descriptor has a limit less that 103 causes an
exception. A larger limit is permissible, and a larger limit is required if
an I/O permission map is present. A larger limit may also be convenient for
systems software if additional data is stored in the same segment as the
TSS.

A procedure that has access to a TSS descriptor can cause a task switch. In
most systems the DPL fields of TSS descriptors should be set to zero, so
that only trusted software has the right to perform task switching.

Having access to a TSS-descriptor does not give a procedure the right to
read or modify a TSS. Reading and modification can be accomplished only with
another descriptor that redefines the TSS as a data segment. An attempt to
load a TSS descriptor into any of the segment registers (CS, SS, DS, ES, FS,
GS) causes an exception.

TSS descriptors may reside only in the GDT. An attempt to identify a TSS
with a selector that has TI=1 (indicating the current LDT) results in an
exception.


Figure 7-2.  TSS Descriptor for 32-bit TSS

  31                23                15                7               0
 ╔═════════════════╪═╤═╤═╤═╤═════════╪═╤═════╤═════════╪═════════════════╗
 ║                 │ │ │ │A│ LIMIT   │ │     │  TYPE   │                 ║
 ║   BASE 31..24   │G│0│0│V│         │P│ DPL │         │   BASE 23..16   ║ 4
 ║                 │ │ │ │L│  19..16 │ │     │0│1│0│B│1│                 ║
 ╟─────────────────┴─┴─┴─┴─┴─────────┼─┴─────┴─┴─┴─┴─┴─┴─────────────────╢
 ║                                   │                                   ║
 ║             BASE 15..0            │             LIMIT 15..0           ║ 0
 ║                                   │                                   ║
 ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝


7.3  Task Register

The task register (TR) identifies the currently executing task by pointing
to the TSS. Figure 7-3 shows the path by which the processor accesses the
current TSS.

The task register has both a "visible" portion (i.e., can be read and
changed by instructions) and an "invisible" portion (maintained by the
processor to correspond to the visible portion; cannot be read by any
instruction). The selector in the visible portion selects a TSS descriptor
in the GDT. The processor uses the invisible portion to cache the base and
limit values from the TSS descriptor. Holding the base and limit in a
register makes execution of the task more efficient, because the processor
does not need to repeatedly fetch these values from memory when it
references the TSS of the current task.

The instructions LTR and STR are used to modify and read the visible
portion of the task register. Both instructions take one operand, a 16-bit
selector located in memory or in a general register.

LTR (Load task register) loads the visible portion of the task register
with the selector operand, which must select a TSS descriptor in the GDT.
LTR also loads the invisible portion with information from the TSS
descriptor selected by the operand. LTR is a privileged instruction; it may
be executed only when CPL is zero. LTR is generally used during system
initialization to give an initial value to the task register; thereafter,
the contents of TR are changed by task switch operations.

STR (Store task register) stores the visible portion of the task register
in a general register or memory word. STR is not privileged.


Figure 7-3.  Task Register

                          ╔═════════════════════════╗
                          ║                         ║
                          ║                         ║
                          ║       TASK STATE        ║
                          ║        SEGMENT          ║─────────┐
                          ║                         ║          │
                          ║                         ║          │
                          ╚═════════════════════════╝          │
           16-BIT VISIBLE                                     │
              REGISTER                │   HIDDEN REGISTER      │
       ╔════════════════════╦═════════╧══════════╦═════════════╧══════╗
    TR ║      SELECTOR      ║      (BASE)        ║       (LIMT)       ║
       ╚═════════╤══════════╩════════════════════╩════════════════════╝
                 │
                 │                    └─────────────────┐   │
                 │          GLOBAL DESCRIPTOR TABLE     │   │
                 │        ╒═════════════════════════╕   │   │
                 │        |     TSS DESCRIPTOR      |   │   │
                 │        ╔══════╦═════╦═════╦══════╗   │   │
                 │        ║      ║     ║     ║      ╟───┘   │
                 │        ╠══════╩═════╬═════╩══════╣       │
                 └───────║            ║            ╟───────┘
                          ╚════════════╩════════════╝
                          |                         |
                          ╘═════════════════════════╛


7.4  Task Gate Descriptor

A task gate descriptor provides an indirect, protected reference to a TSS.
Figure 7-4 illustrates the format of a task gate.

The SELECTOR field of a task gate must refer to a TSS descriptor. The value
of the RPL in this selector is not used by the processor.

The DPL field of a task gate controls the right to use the descriptor to
cause a task switch. A procedure may not select a task gate descriptor
unless the maximum of the selector's RPL and the CPL of the procedure is
numerically less than or equal to the DPL of the descriptor. This constraint
prevents untrusted procedures from causing a task switch. (Note that when a
task gate is used, the DPL of the target TSS descriptor is not used for
privilege checking.)

A procedure that has access to a task gate has the power to cause a task
switch, just as a procedure that has access to a TSS descriptor. The 80386
has task gates in addition to TSS descriptors to satisfy three needs:

  1.  The need for a task to have a single busy bit. Because the busy-bit
      is stored in the TSS descriptor, each task should have only one such
      descriptor. There may, however, be several task gates that select the
      single TSS descriptor.

  2.  The need to provide selective access to tasks. Task gates fulfill
      this need, because they can reside in LDTs and can have a DPL that is
      different from the TSS descriptor's DPL. A procedure that does not
      have sufficient privilege to use the TSS descriptor in the GDT (which
      usually has a DPL of 0) can still switch to another task if it has
      access to a task gate for that task in its LDT. With task gates,
      systems software can limit the right to cause task switches to
      specific tasks.

  3.  The need for an interrupt or exception to cause a task switch. Task
      gates may also reside in the IDT, making it possible for interrupts
      and exceptions to cause task switching. When interrupt or exception
      vectors to an IDT entry that contains a task gate, the 80386 switches
      to the indicated task. Thus, all tasks in the system can benefit from
      the protection afforded by isolation from interrupt tasks.

Figure 7-5 illustrates how both a task gate in an LDT and a task gate in
the IDT can identify the same task.


Figure 7-4.  Task Gate Descriptor

   31                23               15                7             0
  ╔═════════════════╪════════════════╪═╤═════╤═════════╪═════════════════╗
  ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ │     │         │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
  ║▒▒▒▒▒▒▒▒▒▒▒▒(NOT USED)▒▒▒▒▒▒▒▒▒▒▒▒│P│ DPL │0 0 1 0 1│▒▒▒(NOT USED)▒▒▒▒║ 4
  ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ │     │         │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
  ╟──────────────────────────────────┼─┴─────┴─────────┴─────────────────╢
  ║                                  │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
  ║              SELECTOR            │▒▒▒▒▒▒▒▒▒▒▒▒(NOT USED)▒▒▒▒▒▒▒▒▒▒▒▒▒║ 0
  ║                                  │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
  ╚═════════════════╪════════════════╪═════════════════╪═════════════════╝


Figure 7-5.  Task Gate Indirectly Identifies Task

         LOCAL DESCRIPTOR TABLE              INTERRUPT DESCRIPTOR TABLE
       ╒═════════════════════════╕           ╒═════════════════════════╕
       |                         |           |                         |
       |        TASK GATE        |           |        TASK GATE        |
       ╔══════╤═════╤═════╤══════╗           ╔══════╤═════╤═════╤══════╗
       ║      │     │     │      ║           ║      │     │     │      ║
       ╟──────┴─────┼─────┴──────╢           ╟──────┴─────┼─────┴──────╢
    ┌──╢            │            ║        ┌──╢            │            ║
    │  ╚════════════╧════════════╝        │  ╚════════════╧════════════╝
    │  |                         |        │  |                         |
    │  |                         |        │  |                         |
    │  ╘═════════════════════════╛        │  ╘═════════════════════════╛
    └────────────────┐  ┌─────────────────┘
                     │  │    GLOBAL DESCRIPTOR TABLE
                     │  │  ╒═════════════════════════╕
                     │  │  |                         |
                     │  │  |     TASK DESCRIPTOR     |
                     │  │  ╔══════╤═════╤═════╤══════╗
                     │  │  ║      │     │     │      ║
                     │  └─╟──────┴─────┼─────┴──────╢
                     └────║            │            ╟──┐
                           ╚════════════╧════════════╝  │
                           |                         |  │
                           |                         |  │
                           ╘═════════════════════════╛  │
                                                        │
                           ╔═════════════════════════╗  │
                           ║                         ║  │
                           ║                         ║  │
                           ║                         ║  │
                           ║       TASK STATE        ║  │
                           ║         SEGMENT         ║  │
                           ║                         ║  │
                           ║                         ║  │
                           ║                         ║  │
                           ╚═════════════════════════╝─┘


7.5  Task Switching

The 80386 switches execution to another task in any of four cases:

  1.  The current task executes a JMP or CALL that refers to a TSS
      descriptor.

  2.  The current task executes a JMP or CALL that refers to a task gate.

  3.  An interrupt or exception vectors to a task gate in the IDT.

  4.  The current task executes an IRET when the NT flag is set.

JMP, CALL, IRET, interrupts, and exceptions are all ordinary mechanisms of
the 80386 that can be used in circumstances that do not require a task
switch. Either the type of descriptor referenced or the NT (nested task) bit
in the flag word distinguishes between the standard mechanism and the
variant that causes a task switch.

To cause a task switch, a JMP or CALL instruction can refer either to a TSS
descriptor or to a task gate. The effect is the same in either case: the
80386 switches to the indicated task.

An exception or interrupt causes a task switch when it vectors to a task
gate in the IDT. If it vectors to an interrupt or trap gate in the IDT, a
task switch does not occur. Refer to Chapter 9 for more information on the
interrupt mechanism.

Whether invoked as a task or as a procedure of the interrupted task, an
interrupt handler always returns control to the interrupted procedure in the
interrupted task. If the NT flag is set, however, the handler is an
interrupt task, and the IRET switches back to the interrupted task.

A task switching operation involves these steps:

  1.  Checking that the current task is allowed to switch to the designated
      task. Data-access privilege rules apply in the case of JMP or CALL
      instructions. The DPL of the TSS descriptor or task gate must be less
      than or equal to the maximum of CPL and the RPL of the gate selector.
      Exceptions, interrupts, and IRETs are permitted to switch tasks
      regardless of the DPL of the target task gate or TSS descriptor.

  2.  Checking that the TSS descriptor of the new task is marked present
      and has a valid limit. Any errors up to this point occur in the
      context of the outgoing task. Errors are restartable and can be
      handled in a way that is transparent to applications procedures.

  3.  Saving the state of the current task. The processor finds the base
      address of the current TSS cached in the task register. It copies the
      registers into the current TSS (EAX, ECX, EDX, EBX, ESP, EBP, ESI,
      EDI, ES, CS, SS, DS, FS, GS, and the flag register). The EIP field of
      the TSS points to the instruction after the one that caused the task
      switch.

  4.  Loading the task register with the selector of the incoming task's
      TSS descriptor, marking the incoming task's TSS descriptor as busy,
      and setting the TS (task switched) bit of the MSW. The selector is
      either the operand of a control transfer instruction or is taken from
      a task gate.

  5.  Loading the incoming task's state from its TSS and resuming
      execution. The registers loaded are the LDT register; the flag
      register; the general registers EIP, EAX, ECX, EDX, EBX, ESP, EBP,
      ESI, EDI; the segment registers ES, CS, SS, DS, FS, and GS; and PDBR.
      Any errors detected in this step occur in the context of the incoming
      task. To an exception handler, it appears that the first instruction
      of the new task has not yet executed.

Note that the state of the outgoing task is always saved when a task switch
occurs. If execution of that task is resumed, it starts after the
instruction that caused the task switch. The registers are restored to the
values they held when the task stopped executing.

Every task switch sets the TS (task switched) bit in the MSW (machine
status word). The TS flag is useful to systems software when a coprocessor
(such as a numerics coprocessor) is present. The TS bit signals that the
context of the coprocessor may not correspond to the current 80386 task.
Chapter 11 discusses the TS bit and coprocessors in more detail.

Exception handlers that field task-switch exceptions in the incoming task
(exceptions due to tests 4 thru 16 of Table 7-1) should be cautious about
taking any action that might load the selector that caused the exception.
Such an action will probably cause another exception, unless the exception
handler first examines the selector and fixes any potential problem.

The privilege level at which execution resumes in the incoming task is
neither restricted nor affected by the privilege level at which the outgoing
task was executing. Because the tasks are isolated by their separate address
spaces and TSSs and because privilege rules can be used to prevent improper
access to a TSS, no privilege rules are needed to constrain the relation
between the CPLs of the tasks. The new task begins executing at the
privilege level indicated by the RPL of the CS selector value that is loaded
from the TSS.


Table 7-1. Checks Made during a Task Switch


Test     Test Description                   Exception
NP = Segment-not-present exception, GP = General protection fault, TS =
Invalid TSS, SF = Stack fault   Error Code Selects

  1      Incoming TSS descriptor is         NP           Incoming TSS
         present
  2      Incoming TSS descriptor is         GP           Incoming TSS
         marked not-busy
  3      Limit of incoming TSS is           TS           Incoming TSS
         greater than or equal to 103

             ── All register and selector values are loaded ──

  4      LDT selector of incoming           TS           Incoming TSS
         task is valid
  5      LDT of incoming task is            TS           Incoming TSS
         present
  6      CS selector is valid
Validity tests of a selector check that the selector is in the proper
table (eg., the LDT selector refers to the GDT), lies within the bounds of
the table, and refers to the proper type of descriptor (e.g., the LDT
selector refers to an LDT descriptor).              TS           Code segment
  7      Code segment is present            NP           Code segment
  8      Code segment DPL matches           TS           Code segment
         CS RPL
  9      Stack segment is valid
Validity tests of a selector check that the selector is in the proper
table (eg., the LDT selector refers to the GDT), lies within the bounds of
the table, and refers to the proper type of descriptor (e.g., the LDT
selector refers to an LDT descriptor).            GP           Stack segment
 10      Stack segment is present           SF           Stack segment
 11      Stack segment DPL = CPL            SF           Stack segment
 12      Stack-selector RPL = CPL           GP           Stack segment
 13      DS, ES, FS, GS selectors are       GP           Segment
         valid
Validity tests of a selector check that the selector is in the proper
table (eg., the LDT selector refers to the GDT), lies within the bounds of
the table, and refers to the proper type of descriptor (e.g., the LDT
selector refers to an LDT descriptor).


 14      DS, ES, FS, GS segments            GP           Segment
         are readable
 15      DS, ES, FS, GS segments            NP           Segment
         are present
 16      DS, ES, FS, GS segment DPL         GP           Segment
         ≥ CPL (unless these are
         conforming segments)


7.6  Task Linking

The back-link field of the TSS and the NT (nested task) bit of the flag
word together allow the 80386 to automatically return to a task that CALLed
another task or was interrupted by another task. When a CALL instruction, an
interrupt instruction, an external interrupt, or an exception causes a
switch to a new task, the 80386 automatically fills the back-link of the new
TSS with the selector of the outgoing task's TSS and, at the same time,
sets the NT bit in the new task's flag register. The NT flag indicates
whether the back-link field is valid. The new task releases control by
executing an IRET instruction. When interpreting an IRET, the 80386 examines
the NT flag. If NT is set, the 80386 switches back to the task selected by
the back-link field. Table 7-2 summarizes the uses of these fields.


Table 7-2. Effect of Task Switch on BUSY, NT, and Back-Link

Affected Field      Effect of JMP      Effect of            Effect of
                    Instruction        CALL Instruction     IRET Instruction

Busy bit of         Set, must be       Set, must be 0       Unchanged,
incoming task       0 before           before               must be set

Busy bit of         Cleared            Unchanged            Cleared
outgoing task                          (already set)

NT bit of           Cleared            Set                  Unchanged
incoming task

NT bit of           Unchanged          Unchanged            Cleared
outgoing task

Back-link of        Unchanged          Set to outgoing      Unchanged
incoming task                          TSS selector

Back-link of        Unchanged          Unchanged            Unchanged
outgoing task


7.6.1  Busy Bit Prevents Loops

The B-bit (busy bit) of the TSS descriptor ensures the integrity of the
back-link. A chain of back-links may grow to any length as interrupt tasks
interrupt other interrupt tasks or as called tasks call other tasks. The
busy bit ensures that the CPU can detect any attempt to create a loop. A
loop would indicate an attempt to reenter a task that is already busy;
however, the TSS is not a reentrable resource.

The processor uses the busy bit as follows:

  1.  When switching to a task, the processor automatically sets the busy
      bit of the new task.

  2.  When switching from a task, the processor automatically clears the
      busy bit of the old task if that task is not to be placed on the
      back-link chain (i.e., the instruction causing the task switch is JMP
      or IRET). If the task is placed on the back-link chain, its busy bit
      remains set.

  3.  When switching to a task, the processor signals an exception if the
      busy bit of the new task is already set.

By these actions, the processor prevents a task from switching to itself or
to any task that is on a back-link chain, thereby preventing invalid reentry
into a task.

The busy bit is effective even in multiprocessor configurations, because
the processor automatically asserts a bus lock when it sets or clears the
busy bit. This action ensures that two processors do not invoke the same
task at the same time. (Refer to Chapter 11 for more on multiprocessing.)


7.6.2  Modifying Task Linkages

Any modification of the linkage order of tasks should be accomplished only
by software that can be trusted to correctly update the back-link and the
busy-bit. Such changes may be needed to resume an interrupted task before
the task that interrupted it. Trusted software that removes a task from the
back-link chain must follow one of the following policies:

  1.  First change the back-link field in the TSS of the interrupting task,
      then clear the busy-bit in the TSS descriptor of the task removed from
      the list.

  2.  Ensure that no interrupts occur between updating the back-link chain
      and the busy bit.


7.7  Task Address Space

The LDT selector and PDBR fields of the TSS give software systems designers
flexibility in utilization of segment and page mapping features of the
80386. By appropriate choice of the segment and page mappings for each task,
tasks may share address spaces, may have address spaces that are largely
distinct from one another, or may have any degree of sharing between these
two extremes.

The ability for tasks to have distinct address spaces is an important
aspect of 80386 protection. A module in one task cannot interfere with a
module in another task if the modules do not have access to the same address
spaces. The flexible memory management features of the 80386 allow systems
designers to assign areas of shared address space to those modules of
different tasks that are designed to cooperate with each other.


7.7.1  Task Linear-to-Physical Space Mapping

The choices for arranging the linear-to-physical mappings of tasks fall
into two general classes:

  1.  One linear-to-physical mapping shared among all tasks.

      When paging is not enabled, this is the only possibility. Without page
      tables, all linear addresses map to the same physical addresses.

      When paging is enabled, this style of linear-to-physical mapping
      results from using one page directory for all tasks. The linear space
      utilized may exceed the physical space available if the operating
      system also implements page-level virtual memory.

  2.  Several partially overlapping linear-to-physical mappings.

      This style is implemented by using a different page directory for each
      task. Because the PDBR (page directory base register) is loaded from
      the TSS with each task switch, each task may have a different page
      directory.

In theory, the linear address spaces of different tasks may map to
completely distinct physical addresses. If the entries of different page
directories point to different page tables and the page tables point to
different pages of physical memory, then the tasks do not share any physical
addresses.

In practice, some portion of the linear address spaces of all tasks must
map to the same physical addresses. The task state segments must lie in a
common space so that the mapping of TSS addresses does not change while the
processor is reading and updating the TSSs during a task switch. The linear
space mapped by the GDT should also be mapped to a common physical space;
otherwise, the purpose of the GDT is defeated. Figure 7-6 shows how the
linear spaces of two tasks can overlap in the physical space by sharing
page tables.


7.7.2  Task Logical Address Space

By itself, a common linear-to-physical space mapping does not enable
sharing of data among tasks. To share data, tasks must also have a common
logical-to-linear space mapping; i.e., they must also have access to
descriptors that point into a shared linear address space. There are three
ways to create common logical-to-physical address-space mappings:

  1.  Via the GDT. All tasks have access to the descriptors in the GDT. If
      those descriptors point into a linear-address space that is mapped to
      a common physical-address space for all tasks, then the tasks can
      share data and instructions.

  2.  By sharing LDTs. Two or more tasks can use the same LDT if the LDT
      selectors in their TSSs select the same LDT segment. Those
      LDT-resident descriptors that point into a linear space that is mapped
      to a common physical space permit the tasks to share physical memory.
      This method of sharing is more selective than sharing by the GDT; the
      sharing can be limited to specific tasks. Other tasks in the system
      may have different LDTs that do not give them access to the shared
      areas.

  3.  By descriptor aliases in LDTs. It is possible for certain descriptors
      of different LDTs to point to the same linear address space. If that
      linear address space is mapped to the same physical space by the page
      mapping of the tasks involved, these descriptors permit the tasks to
      share the common space. Such descriptors are commonly called
      "aliases". This method of sharing is even more selective than the
      prior two; other descriptors in the LDTs may point to distinct linear
      addresses or to linear addresses that are not shared.


Figure 7-6. Partially-Overlapping Linear Spaces

         TSSs                                              PAGE FRAMES
                                                          ╔══════════╗
      TASK A TSS    PAGE DIRECTORIES     PAGE TABLES      ║  TASK A  ║
     ╔══════════╗     ╔═══════════╗     ╔═══════════╗  ┌─║   PAGE   ║
     ║          ║     ║           ║     ║           ║  │  ╚══════════╝
     ║          ║     ╠═══════════╣     ╠═══════════╣  │  ╔══════════╗
     ║          ║     ║           ║     ║    PTE    ╟──┘  ║  TASK A  ║
     ║          ║     ╠═══════════╣     ╠═══════════╣  ┌─║   PAGE   ║
     ║          ║     ║           ║     ║    PTE    ╟──┘  ╚══════════╝
     ╠══════════╣     ╠═══════════╣     ╠═══════════╣     ╔══════════╗
     ║   PDBR   ╟────║    PDE    ╟────║    PTE    ╟──┐  ║  TASK A  ║
     ╠══════════╣     ╠═══════════╣     ╚═══════════╝  └─║   PAGE   ║
     ║          ║     ║    PDE    ╟──┐    SHARED PT       ╚══════════╝
     ╚══════════╝     ╚═══════════╝  │  ╔═══════════╗     ╔══════════╗
                                     │  ║           ║     ║  SHARED  ║
                                     │  ╠═══════════╣  ┌─║   PAGE   ║
                                     │  ║           ║  │  ╚══════════╝
                                     │  ╠═══════════╣  │  ╔══════════╗
                                     │  ║    PTE    ╟──┘  ║  SHARED  ║
                                     │  ╠═══════════╣  ┌─║   PAGE   ║
                                     ├─║    PTE    ╟──┘  ╚══════════╝
      TASK B TSS                     │  ╚═══════════╝     ╔══════════╗
     ╔══════════╗     ╔═══════════╗  │                    ║  TASK B  ║
     ║          ║     ║           ║  │                ┌──║   PAGE   ║
     ║          ║     ╠═══════════╣  │  ╔═══════════╗ │   ╚══════════╝
     ║          ║     ║           ║  │  ║           ║ │   ╔══════════╗
     ║          ║     ╠═══════════╣  │  ╠═══════════╣ │   ║  TASK B  ║
     ║          ║     ║           ║  │  ║           ║ │ ┌║   PAGE   ║
     ╠══════════╣     ╠═══════════╣  │  ╠═══════════╣ │ │ ╚══════════╝
     ║   PDBR   ╟────║    PDE    ╟──┘  ║    PTE    ╟─┘ │  PAGE FRAMES
     ╠══════════╣     ╠═══════════╣     ╠═══════════╣   │
     ║          ║     ║    PDE    ╟────║    PTE    ╟───┘
     ╚══════════╝     ╚═══════════╝     ╚═══════════╝
         TSSs       PAGE DIRECTORIES     PAGE TABLES


Chapter 8  Input/Output

────────────────────────────────────────────────────────────────────────────

This chapter presents the I/O features of the 80386 from the following
perspectives:

  ■  Methods of addressing I/O ports

  ■  Instructions that cause I/O operations

  ■  Protection as it applies to the use of I/O instructions and I/O port
     addresses.


8.1  I/O Addressing

The 80386 allows input/output to be performed in either of two ways:

  ■  By means of a separate I/O address space (using specific I/O
     instructions)

  ■  By means of memory-mapped I/O (using general-purpose operand
     manipulationinstructions).


8.1.1  I/O Address Space

The 80386 provides a separate I/O address space, distinct from physical
memory, that can be used to address the input/output ports that are used for
external 16 devices. The I/O address space consists of 2^(16) (64K)
individually addressable 8-bit ports; any two consecutive 8-bit ports can be
treated as a 16-bit port; and four consecutive 8-bit ports can be treated
as a 32-bit port. Thus, the I/O address space can accommodate up to 64K
8-bit ports, up to 32K 16-bit ports, or up to 16K 32-bit ports.

The program can specify the address of the port in two ways. Using an
immediate byte constant, the program can specify:

  ■  256 8-bit ports numbered 0 through 255.
  ■  128 16-bit ports numbered 0, 2, 4, . . . , 252, 254.
  ■  64 32-bit ports numbered 0, 4, 8, . . . , 248, 252.

Using a value in DX, the program can specify:

  ■  8-bit ports numbered 0 through 65535
  ■  16-bit ports numbered 0, 2, 4, . . . , 65532, 65534
  ■  32-bit ports numbered 0, 4, 8, . . . , 65528, 65532

The 80386 can transfer 32, 16, or 8 bits at a time to a device located in
the I/O space. Like doublewords in memory, 32-bit ports should be aligned at
addresses evenly divisible by four so that the 32 bits can be transferred in
a single bus access. Like words in memory, 16-bit ports should be aligned at
even-numbered addresses so that the 16 bits can be transferred in a single
bus access. An 8-bit port may be located at either an even or odd address.

The instructions IN and OUT move data between a register and a port in the
I/O address space. The instructions INS and OUTS move strings of data
between the memory address space and ports in the I/O address space.


8.1.2  Memory-Mapped I/O

I/O devices also may be placed in the 80386 memory address space. As long
as the devices respond like memory components, they are indistinguishable to
the processor.

Memory-mapped I/O provides additional programming flexibility. Any
instruction that references memory may be used to access an I/O port located
in the memory space. For example, the MOV instruction can transfer data
between any register and a port; and the AND, OR, and TEST instructions may
be used to manipulate bits in the internal registers of a device (see Figure
8-1). Memory-mapped I/O performed via the full instruction set maintains
the full complement of addressing modes for selecting the desired I/O
device (e.g., direct address, indirect address, base register, index
register, scaling).

Memory-mapped I/O, like any other memory reference, is subject to access
protection and control when executing in protected mode. Refer to Chapter 6
for a discussion of memory protection.


8.2  I/O Instructions

The I/O instructions of the 80386 provide access to the processor's I/O
ports for the transfer of data to and from peripheral devices. These
instructions have as one operand the address of a port in the I/O address
space. There are two classes of I/O instruction:

  1.  Those that transfer a single item (byte, word, or doubleword) located
      in a register.

  2.  Those that transfer strings of items (strings of bytes, words, or
      doublewords) located in memory. These are known as "string I/O
      instructions" or "block I/O instructions".


8.2.1  Register I/O Instructions

The I/O instructions IN and OUT are provided to move data between I/O ports
and the EAX (32-bit I/O), the AX (16-bit I/O), or AL (8-bit I/O) general
registers. IN and OUT instructions address I/O ports either directly, with
the address of one of up to 256 port addresses coded in the instruction, or
indirectly via the DX register to one of up to 64K port addresses.

IN (Input from Port) transfers a byte, word, or doubleword from an input
port to AL, AX, or EAX. If a program specifies AL with the IN instruction,
the processor transfers 8 bits from the selected port to AL. If a program
specifies AX with the IN instruction, the processor transfers 16 bits from
the port to AX. If a program specifies EAX with the IN instruction, the
processor transfers 32 bits from the port to EAX.

OUT (Output to Port) transfers a byte, word, or doubleword to an output
port from AL, AX, or EAX. The program can specify the number of the port
using the same methods as the IN instruction.


Figure 8-1.  Memory-Mapped I/O

                MEMORY
             ADDRESS SPACE                        I/O DEVICE 1
           ╔═══════════════╗                  ╔═══════════════════╗
           ║               ║                  ║ INTERNAL REGISTER ║
           ╟───────────────╢ ─ ─ ─ ─ ─ ─ ─ ─ ─║─╔═══════════════╗ ║
           ║               ║                  ║ ║               ║ ║
           ╟───────────────╢ ─ ─ ─ ─ ─ ─ ─ ─ ─║─╚═══════════════╝ ║
           ║               ║                  ╚═══════════════════╝
           ║               ║
           ║               ║
           ║               ║
           ║               ║                      I/O DEVICE 2
           ║               ║                  ╔═══════════════════╗
           ║               ║                  ║ INTERNAL REGISTER ║
           ╟───────────────╢ ─ ─ ─ ─ ─ ─ ─ ─ ─║─╔═══════════════╗ ║
           ║               ║                  ║ ║               ║ ║
           ╟───────────────╢ ─ ─ ─ ─ ─ ─ ─ ─ ─║─╚═══════════════╝ ║
           ║               ║                  ╚═══════════════════╝
           ╚═══════════════╝


8.2.2  Block I/O Instructions

The block (or string) I/O instructions INS and OUTS move blocks of data
between I/O ports and memory space. Block I/O instructions use the DX
register to specify the address of a port in the I/O address space. INS and
OUTS use DX to specify:

  ■  8-bit ports numbered 0 through 65535
  ■  16-bit ports numbered 0, 2, 4, . . . , 65532, 65534
  ■  32-bit ports numbered 0, 4, 8, . . . , 65528, 65532

Block I/O instructions use either SI or DI to designate the source or
destination memory address. For each transfer, SI or DI are automatically
either incremented or decremented as specified by the direction bit in the
flags register.

INS and OUTS, when used with repeat prefixes, cause block input or output
operations. REP, the repeat prefix, modifies INS and OUTS to provide a means
of transferring blocks of data between an I/O port and memory. These block
I/O instructions are string primitives (refer also to Chapter 3 for more on
string primitives). They simplify programming and increase the speed of data
transfer by eliminating the need to use a separate LOOP instruction or an
intermediate register to hold the data.

The string I/O primitives can operate on byte strings, word strings, or
doubleword strings. After each transfer, the memory address in ESI or EDI is
updated by 1 for byte operands, by 2 for word operands, or by 4 for
doubleword operands. The value in the direction flag (DF) determines whether
the processor automatically increments ESI or EDI (DF=0) or whether it
automatically decrements these registers (DF=1).

INS (Input String from Port) transfers a byte or a word string element from
an input port to memory. The mnemonics INSB, INSW, and INSD are variants
that explicitly specify the size of the operand. If a program specifies
INSB, the processor transfers 8 bits from the selected port to the memory
location indicated by ES:EDI. If a program specifies INSW, the processor
transfers 16 bits from the port to the memory location indicated by ES:EDI.
If a program specifies INSD, the processor transfers 32 bits from the port
to the memory location indicated by ES:EDI. The destination segment register
choice (ES) cannot be changed for the INS instruction. Combined with the REP
prefix, INS moves a block of information from an input port to a series of
consecutive memory locations.

OUTS (Output String to Port) transfers a byte, word, or doubleword string
element to an output port from memory. The mnemonics OUTSB, OUTSW, and OUTSD
are variants that explicitly specify the size of the operand. If a program
specifies OUTSB, the processor transfers 8 bits from the memory location
indicated by ES:EDI to the the selected port. If a program specifies OUTSW,
the processor transfers 16 bits from the memory location indicated by ES:EDI
to the the selected port. If a program specifies OUTSD, the processor
transfers 32 bits from the memory location indicated by ES:EDI to the the
selected port. Combined with the REP prefix, OUTS moves a block of
information from a series of consecutive memory locations indicated by
DS:ESI to an output port.


8.3  Protection and I/O

Two mechanisms provide protection for I/O functions:

  1.  The IOPL field in the EFLAGS register defines the right to use
      I/O-related instructions.

  2.  The I/O permission bit map of a 80386 TSS segment defines the right
      to use ports in the I/O address space.

These mechanisms operate only in protected mode, including virtual 8086
mode; they do not operate in real mode. In real mode, there is no protection
of the I/O space; any procedure can execute I/O instructions, and any I/O
port can be addressed by the I/O instructions.


8.3.1  I/O Privilege Level

Instructions that deal with I/O need to be restricted but also need to be
executed by procedures executing at privilege levels other than zero. For
this reason, the processor uses two bits of the flags register to store the
I/O privilege level (IOPL). The IOPL defines the privilege level
needed to execute I/O-related instructions.

The following instructions can be executed only if CPL ≤ IOPL:

IN      ── Input
INS     ── Input String
OUT     ── Output
OUTS    ── Output String
CLI     ── Clear Interrupt-Enable Flag
STI     ── Set Interrupt-Enable

These instructions are called "sensitive" instructions, because they are
sensitive to IOPL.

To use sensitive instructions, a procedure must execute at a privilege
level at least as privileged as that specified by the IOPL (CPL ≤ IOPL). Any
attempt by a less privileged procedure to use a sensitive instruction
results in a general protection exception.

Because each task has its own unique copy of the flags register, each task
can have a different IOPL. A task whose primary function is to perform I/O
(a device driver) can benefit from having an IOPL of three, thereby
permitting all procedures of the task to performI/O. Other tasks typically
have IOPL set to zero or one, reserving the right to perform I/O
instructions for the most privileged procedures.

A task can change IOPL only with the POPF instruction; however, such
changes are privileged. No procedure may alter IOPL (the I/O privilege level
in the flag register) unless the procedure is executing at privilege level
0. An attempt by a less privileged procedure to alter IOPL does not result
in an exception; IOPL simply remains unaltered.

The POPF instruction may be used in addition to CLI and STI to alter the
interrupt-enable flag (IF); however, changes to IF by POPF are
IOPL-sensitive. A procedure may alter IF with a POPF instruction only when
executing at a level that is at least as privileged as IOPL. An attempt by a
less privileged procedure to alter IF in this manner does not result in an
exception; IF simply remains unaltered.


8.3.2  I/O Permission Bit Map

The I/O instructions that directly refer to addresses in the processor's
I/O space are IN, INS, OUT, OUTS. The 80386 has the ability to selectively
trap references to specific I/O addresses. The structure that enables
selective trapping is the I/O Permission Bit Map in the TSS segment (see
Figure 8-2). The I/O permission map is a bit vector. The size of the map
and its location in the TSS segment are variable. The processor locates the
I/O permission map by means of the I/O map base field in the fixed portion
of the TSS. The I/O map base field is 16 bits wide and contains the offset
of the beginning of the I/O permission map. The upper limit of the I/O
permission map is the same as the limit of the TSS segment.

In protected mode, when it encounters an I/O instruction (IN, INS, OUT, or
OUTS), the processor first checks whether CPL ≤ IOPL. If this condition is
true, the I/O operation may proceed. If not true, the processor checks the
I/O permission map. (In virtual 8086 mode, the processor consults the map
without regard for IOPL. Refer to Chapter 15.)

Each bit in the map corresponds to an I/O port byte address; for example,
the bit for port 41 is found at I/O map base + 5, bit offset 1. The
processor tests all the bits that correspond to the I/O addresses spanned by
an I/O operation; for example, a doubleword operation tests four bits
corresponding to four adjacent byte addresses. If any tested bit is set,
the processor signals a general protection exception. If all the tested bits
are zero, the I/O operation may proceed.

It is not necessary for the I/O permission map to represent all the I/O
addresses. I/O addresses not spanned by the map are treated as if they had
one bits in the map. For example, if TSS limit is equal to I/O map base +
31, the first 256 I/O ports are mapped; I/O operations on any port greater
than 255 cause an exception.

If I/O map base is greater than or equal to TSS limit, the TSS segment has
no I/O permission map, and all I/O instructions in the 80386 program cause
exceptions when CPL > IOPL.

Because the I/O permission map is in the TSS segment, different tasks can
have different maps. Thus, the operating system can allocate ports to a task
by changing the I/O permission map in the task's TSS.


Figure 8-2.  I/O Address Bit Map

                                    TSS SEGMEMT

                        31       23       15       7      0
                       ╔════════╪════════╪════════╪════════╗
              LIMIT───║                                   ║
                       ║ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ║
                                                          
                             I/O PERMISSION BIT MAP       
                                                          
                       ║ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ║
                 ┌────║                                   ║
                 │     ╟────────┼────────┼────────┼────────╢
                 │                                        
                 │                                        
                 │                                        
                 │     ╟────────┼────────┼────────┼────────╢
                 └─────╢  I/O MAP BASE   │uuuuuuuu uuuuuuuT║64
                       ╟────────┼────────┼────────┼────────╢
                       ║00000000 00000000│       LOT       ║60
                       ╟────────┼────────┼────────┼────────╢
                       ║00000000 00000000│       GS        ║5C
                       ╟────────┼────────┼────────┼────────╢
                       ║                                   ║58
                                                          
                                                          
                                                          
                       ║                                   ║4
                       ╟────────┼────────┼────────┼────────╢
                       ║00000000 00000000│  TSS BACK LINK  ║0
                       ╚════════╪════════╪════════╪════════╝


Chapter 9  Exceptions and Interrupts

────────────────────────────────────────────────────────────────────────────

Interrupts and exceptions are special kinds of control transfer; they work
somewhat like unprogrammed CALLs. They alter the normal program flow to
handle external events or to report errors or exceptional conditions. The
difference between interrupts and exceptions is that interrupts are used to
handle asynchronous events external to the processor, but exceptions handle
conditions detected by the processor itself in the course of executing
instructions.

There are two sources for external interrupts and two sources for
exceptions:

  1.  Interrupts

      ■  Maskable interrupts, which are signalled via the INTR pin.

      ■  Nonmaskable interrupts, which are signalled via the NMI
         (Non-Maskable Interrupt) pin.

  2.  Exceptions

      ■  Processor detected. These are further classified as faults, traps,
         and aborts.

      ■  Programmed. The instructions INTO, INT 3, INT n, and BOUND can
         trigger exceptions. These instructions are often called "software
         interrupts", but the processor handles them as exceptions.

This chapter explains the features that the 80386 offers for controlling
and responding to interrupts when it is executing in protected mode.


9.1  Identifying Interrupts

The processor associates an identifying number with each different type of
interrupt or exception.

The NMI and the exceptions recognized by the processor are assigned
predetermined identifiers in the range 0 through 31. Not all of these
numbers are currently used by the 80386; unassigned identifiers in this
range are reserved by Intel for possible future expansion.

The identifiers of the maskable interrupts are determined by external
interrupt controllers (such as Intel's 8259A Programmable Interrupt
Controller) and communicated to the processor during the processor's
interrupt-acknowledge sequence. The numbers assigned by an 8259A PIC can be
specified by software. Any numbers in the range 32 through 255 can be used.
Table 9-1 shows the assignment of interrupt and exception identifiers.

Exceptions are classified as faults, traps, or aborts depending on the way
they are reported and whether restart of the instruction that caused the
exception is supported.

Faults  Faults are exceptions that are reported "before" the
        instruction causingthe exception. Faults are either detected before
        the instruction begins to execute, or during execution of the
        instruction. If detected during the instruction, the fault is
        reported with the machine restored to a state that permits the
        instruction to be restarted.

Traps   A trap is an exception that is reported at the instruction
        boundary immediately after the instruction in which the
        exception was detected.

Aborts  An abort is an exception that permits neither precise location
        of the instruction causing the exception nor restart of the program
        that caused the exception. Aborts are used to report severe errors,
        such as hardware errors and inconsistent or illegal values in system
        tables.


Table 9-1. Interrupt and Exception ID Assignments

Identifier   Description

0            Divide error
1            Debug exceptions
2            Nonmaskable interrupt
3            Breakpoint (one-byte INT 3 instruction)
4            Overflow (INTO instruction)
5            Bounds check (BOUND instruction)
6            Invalid opcode
7            Coprocessor not available
8            Double fault
9            (reserved)
10           Invalid TSS
11           Segment not present
12           Stack exception
13           General protection
14           Page fault
15           (reserved)
16           Coprecessor error
17-31        (reserved)
32-255       Available for external interrupts via INTR pin


9.2  Enabling and Disabling Interrupts

The processor services interrupts and exceptions only between the end of
one instruction and the beginning of the next. When the repeat prefix is
used to repeat a string instruction, interrupts and exceptions may occur
between repetitions. Thus, operations on long strings do not delay interrupt
response.

Certain conditions and flag settings cause the processor to inhibit certain
interrupts and exceptions at instruction boundaries.


9.2.1  NMI Masks Further NMIs

While an NMI handler is executing, the processor ignores further interrupt
signals at the NMI pin until the next IRET instruction is executed.


9.2.2  IF Masks INTR

The IF (interrupt-enable flag) controls the acceptance of external
interrupts signalled via the INTR pin. When IF=0, INTR interrupts are
inhibited; when IF=1, INTR interrupts are enabled. As with the other flag
bits, the processor clears IF in response to a RESET signal. The
instructions CLI and STI alter the setting of IF.

CLI (Clear Interrupt-Enable Flag) and STI (Set Interrupt-Enable Flag)
explicitly alter IF (bit 9 in the flag register). These instructions may be
executed only if CPL ≤ IOPL. A protection exception occurs if they are
executed when CPL > IOPL.

The IF is also affected implicitly by the following operations:

  ■  The instruction PUSHF stores all flags, including IF, in the stack
     where they can be examined.

  ■  Task switches and the instructions POPF and IRET load the flags
     register; therefore, they can be used to modify IF.

  ■  Interrupts through interrupt gates automatically reset IF, disabling
     interrupts. (Interrupt gates are explained later in this chapter.)


9.2.3  RF Masks Debug Faults

The RF bit in EFLAGS controls the recognition of debug faults. This permits
debug faults to be raised for a given instruction at most once, no matter
how many times the instruction is restarted. (Refer to Chapter 12 for more
information on debugging.)


9.2.4  MOV or POP to SS Masks Some Interrupts and Exceptions

Software that needs to change stack segments often uses a pair of
instructions; for example:

  MOV SS, AX
  MOV ESP, StackTop

If an interrupt or exception is processed after SS has been changed but
before ESP has received the corresponding change, the two parts of the stack
pointer SS:ESP are inconsistent for the duration of the interrupt handler or
exception handler.

To prevent this situation, the 80386, after both a MOV to SS and a POP to
SS instruction, inhibits NMI, INTR, debug exceptions, and single-step traps
at the instruction boundary following the instruction that changes SS. Some
exceptions may still occur; namely, page fault and general protection fault.
Always use the 80386 LSS instruction, and the problem will not occur.


9.3  Priority Among Simultaneous Interrupts and Exceptions

If more than one interrupt or exception is pending at an instruction
boundary, the processor services one of them at a time. The priority among
classes of interrupt and exception sources is shown in Table 9-2. The
processor first services a pending interrupt or exception from the class
that has the highest priority, transferring control to the first
instruction of the interrupt handler. Lower priority exceptions are
discarded; lower priority interrupts are held pending. Discarded exceptions
will be rediscovered when the interrupt handler returns control to the point
of interruption.


9.4  Interrupt Descriptor Table

The interrupt descriptor table (IDT) associates each interrupt or exception
identifier with a descriptor for the instructions that service the
associated event. Like the GDT and LDTs, the IDT is an array of 8-byte
descriptors. Unlike the GDT and LDTs, the first entry of the IDT may contain
a descriptor. To form an index into the IDT, the processor multiplies the
interrupt or exception identifier by eight. Because there are only 256
identifiers, the IDT need not contain more than 256 descriptors. It can
contain fewer than 256 entries; entries are required only for interrupt
identifiers that are actually used.

The IDT may reside anywhere in physical memory. As Figure 9-1 shows, the
processor locates the IDT by means of the IDT register (IDTR). The
instructions LIDT and SIDT operate on the IDTR. Both instructions have one
explicit operand: the address in memory of a 6-byte area. Figure 9-2 shows
the format of this area.

LIDT (Load IDT register) loads the IDT register with the linear base
address and limit values contained in the memory operand.  This instruction
can be executed only when the CPL is zero. It is normally used by the
initialization logic of an operating system when creating an IDT.  An
operating system may also use it to change from one IDT to another.

SIDT (Store IDT register) copies the base and limit value stored in IDTR
to a memory location. This instruction can be executed at any privilege
level.


Table 9-2. Priority Among Simultaneous Interrupts and Exceptions

Priority   Class of Interrupt or Exception

HIGHEST    Faults except debug faults
           Trap instructions INTO, INT n, INT 3
           Debug traps for this instruction
           Debug faults for next instruction
           NMI interrupt
LOWEST     INTR interrupt


Figure 9-1.  IDT Register and Table

                                              INTERRUPT DESCRIPTOR TABLE
                                              ╔══════╤═════╤═════╤══════╗
                                        ┌────║      │     │     │      ║
                                        │     ╟─ GATE FOR INTERRUPT #N ─╢
                                        │     ║      │     │     │      ║
                                        │     ╚══════╧═════╧═════╧══════╝
                                        │                              
                                        │                              
                                        │                              
                                        │     ╔══════╤═════╤═════╤══════╗
                                        │     ║      │     │     │      ║
                                        │     ╟─ GATE FOR INTERRUPT #2 ─╢
                                        │     ║      │     │     │      ║
                                        │     ╠══════╤═════╤═════╤══════╣
            IDT REGISTER                │     ║      │     │     │      ║
                                        │     ╟─ GATE FOR INTERRUPT #1 ─╢
                    15            0     │     ║      │     │     │      ║
                   ╔═══════════════╗    │     ╠══════╤═════╤═════╤══════╣
                   ║   IDT LIMIT   ╟────┘     ║      │     │     │      ║
  ╔════════════════╩═══════════════╣          ╟─ GATE FOR INTERRUPT #0 ─╢
  ║            IDT BASE            ╟─────────║      │     │     │      ║
  ╚════════════════════════════════╝          ╚══════╧═════╧═════╧══════╝
   31                             0


Figure 9-2.  Pseudo-Descriptor Format for LIDT and SIDT

  31                23                15                7               0
 ╔═════════════════╪═════════════════╪═════════════════╪═════════════════╗
 ║                                 BASE                                  ║2
 ╚═════════════════╪═════════════════╦═════════════════╪═════════════════╣
                                     ║               LIMIT               ║0
                                     ╚═════════════════╪═════════════════╝


9.5  IDT Descriptors

The IDT may contain any of three kinds of descriptor:

   ■  Task gates
   ■  Interrupt gates
   ■  Trap gates

Figure 9-3 illustrates the format of task gates and 80386 interrupt gates
and trap gates. (The task gate in an IDT is the same as the task gate
already discussed in Chapter 7.)


Figure 9-3.  80306 IDT Gate Descriptors

                                80386 TASK GATE
   31                23                15                7                0
  ╔═════════════════╪═════════════════╪═══╤═══╤═════════╪═════════════════╗
  ║▒▒▒▒▒▒▒▒▒▒▒▒▒(NOT USED)▒▒▒▒▒▒▒▒▒▒▒▒│ P │DPL│0 0 1 0 1│▒▒▒(NOT USED)▒▒▒▒║4
  ╟───────────────────────────────────┼───┴───┴─────────┴─────────────────╢
  ║             SELECTOR              │▒▒▒▒▒▒▒▒▒▒▒▒▒(NOT USED)▒▒▒▒▒▒▒▒▒▒▒▒║0
  ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝

                                80386 INTERRUPT GATE
   31                23                15                7                0
  ╔═════════════════╪═════════════════╪═══╤═══╤═════════╪═════╪═══════════╗
  ║           OFFSET 31..16           │ P │DPL│0 1 1 1 0│0 0 0│(NOT USED) ║4
  ╟───────────────────────────────────┼───┴───┴─────────┴─────┴───────────╢
  ║             SELECTOR              │           OFFSET 15..0            ║0
  ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝

                                80386 TRAP GATE
   31                23                15                7                0
  ╔═════════════════╪═════════════════╪═══╤═══╤═════════╪═════╪═══════════╗
  ║          OFFSET 31..16            │ P │DPL│0 1 1 1 1│0 0 0│(NOT USED) ║4
  ╟───────────────────────────────────┼───┴───┴─────────┴─────┴───────────╢
  ║             SELECTOR              │           OFFSET 15..0            ║0
  ╚═════════════════╪═════════════════╪═════════════════╪═════════════════╝


9.6  Interrupt Tasks and Interrupt Procedures

Just as a CALL instruction can call either a procedure or a task, so an
interrupt or exception can "call" an interrupt handler that is either a
procedure or a task. When responding to an interrupt or exception, the
processor uses the interrupt or exception identifier to index a descriptor
in the IDT. If the processor indexes to an interrupt gate or trap gate, it
invokes the handler in a manner similar to a CALL to a call gate. If the
processor finds a task gate, it causes a task switch in a manner similar to
a CALL to a task gate.


9.6.1  Interrupt Procedures

An interrupt gate or trap gate points indirectly to a procedure which will
execute in the context of the currently executing task as illustrated by
Figure 9-4. The selector of the gate points to an executable-segment
descriptor in either the GDT or the current LDT. The offset field of the
gate points to the beginning of the interrupt or exception handling
procedure.

The 80386 invokes an interrupt or exception handling procedure in much the
same manner as it CALLs a procedure; the differences are explained in the
following sections.


Figure 9-4.  Interrupt Vectoring for Procedures

                  IDT                                    EXECUTABLE SEGMENT
           ╔═══════════════╗                             ╔═══════════════╗
           ║               ║                       OFFSET║               ║
           ╠═══════════════╣  ┌─────────────────────────║ ENTRY POINT   ║
           ║               ║  │      LDT OR GDT          ║               ║
           ╠═══════════════╣  │   ╔═══════════════╗      ║               ║
           ║               ║  │   ║               ║      ║               ║
INTERRUPT  ╠═══════════════╣  │   ╠═══════════════╣      ║               ║
   ID─────║ TRAP GATE OR  ╟──┘   ║               ║      ║               ║
           ║INTERRUPT GATE ╟──┐   ╠═══════════════╣      ║               ║
           ╠═══════════════╣  │   ║               ║      ║               ║
           ║               ║  │   ╠═══════════════╣      ║               ║
           ╠═══════════════╣  └──║   SEGMENT     ╟─┐    ║               ║
           ║               ║      ║  DESCRIPTOR   ║ │    ║               ║
           ╠═══════════════╣      ╠═══════════════╣ │    ║               ║
           ║               ║      ║               ║ │    ║               ║
           ╠═══════════════╣      ╠═══════════════╣ │    ║               ║
           ║               ║      ║               ║ │BASE║               ║
           ╚═══════════════╝      ╠═══════════════╣ └───╚═══════════════╝
                                  ║               ║
                                  ║               ║
                                  ║               ║
                                  ╚═══════════════╝


9.6.1.1  Stack of Interrupt Procedure

Just as with a control transfer due to a CALL instruction, a control
transfer to an interrupt or exception handling procedure uses the stack to
store the information needed for returning to the original procedure. As
Figure 9-5 shows, an interrupt pushes the EFLAGS register onto the stack
before the pointer to the interrupted instruction.

Certain types of exceptions also cause an error code to be pushed on the
stack. An exception handler can use the error code to help diagnose the
exception.


9.6.1.2  Returning from an Interrupt Procedure

An interrupt procedure also differs from a normal procedure in the method
of leaving the procedure. The IRET instruction is used to exit from an
interrupt procedure. IRET is similar to RET except that IRET increments EIP
by an extra four bytes (because of the flags on the stack) and moves the
saved flags into the EFLAGS register. The IOPL field of EFLAGS is changed
only if the CPL is zero. The IF flag is changed only if CPL ≤ IOPL.


Figure 9-5.  Stack Layout after Exception of Interrupt

                           WITHOUT PRIVILEGE TRANSITION

      D  O      31          0                     31          0
      I  F    ╠═══════╦═══════╣                 ╠═══════╦═══════╣
      R       ║▒▒▒▒▒▒▒║▒▒▒▒▒▒▒║    OLD          ║▒▒▒▒▒▒▒║▒▒▒▒▒▒▒║    OLD
      E  E    ╠═══════╬═══════╣   SS:ESP        ╠═══════╬═══════╣   SS:ESP
      C  X    ║▒▒▒▒▒▒▒║▒▒▒▒▒▒▒║     │           ║▒▒▒▒▒▒▒║▒▒▒▒▒▒▒║     │
      T  P    ╠═══════╩═══════╣────┘           ╠═══════╩═══════╣────┘
      I  A    ║  OLD EFLAGS   ║                 ║  OLD EFLAGS   ║
      O  N    ╠═══════╦═══════╣                 ╠═══════╦═══════╣
      N  S    ║▒▒▒▒▒▒▒║OLD CS ║    NEW          ║▒▒▒▒▒▒▒║OLD CS ║
         I    ╠═══════╩═══════╣   SS:ESP        ╠═══════╩═══════╣
       │ O    ║    OLD EIP    ║     │           ║    OLD EIP    ║    NEW
       │ N    ╠═══════════════╣────┘           ╠═══════════════╣   SS:ESP
       │      ║               ║                 ║  ERROR CODE   ║     │
                                             ╠═══════════════╣────┘
                                              ║               ║
                             
              WITHOUT ERROR CODE                 WITH ERROR CODE

                             WITH PRIVILEGE TRANSITION

      D  O     31            0                     31          0
      I  F    ╔═══════╦═══════╗────┐           ╔═══════╦═══════╗────┐
      R       ║▒▒▒▒▒▒▒║OLD SS ║     │           ║▒▒▒▒▒▒▒║OLD SS ║     │
      E  E    ╠═══════╩═══════╣   SS:ESP        ╠═══════╩═══════╣   SS:ESP
      C  X    ║    OLD ESP    ║  FROM TSS       ║    OLD ESP    ║  FROM TSS
      T  P    ╠═══════════════╣                 ╠═══════════════╣
      I  A    ║  OLD EFLAGS   ║                 ║  OLD EFLAGS   ║
      O  N    ╠═══════╦═══════╣                 ╠═══════╦═══════╣
      N  S    ║▒▒▒▒▒▒▒║OLD CS ║    NEW          ║▒▒▒▒▒▒▒║OLD CS ║
         I    ╠═══════╩═══════╣   SS:EIP        ╠═══════╩═══════╣
       │ O    ║    OLD EIP    ║     │           ║    OLD EIP    ║    NEW
       │ N    ╠═══════════════╣────┘           ╠═══════════════╣   SS:ESP
       │      ║               ║                 ║  ERROR CODE   ║     │
                                             ╠═══════════════╣────┘
                                              ║               ║
                             
              WITHOUT ERROR CODE                 WITH ERROR CODE


9.6.1.3  Flags Usage by Interrupt Procedure

Interrupts that vector through either interrupt gates or trap gates cause
TF (the trap flag) to be reset after the current value of TF is saved on the
stack as part of EFLAGS. By this action the processor prevents debugging
activity that uses single-stepping from affecting interrupt response. A
subsequent IRET instruction restores TF to the value in the EFLAGS image on
the stack.

The difference between an interrupt gate and a trap gate is in the effect
on IF (the interrupt-enable flag). An interrupt that vectors through an
interrupt gate resets IF, thereby preventing other interrupts from
interfering with the current interrupt handler. A subsequent IRET
instruction restores IF to the value in the EFLAGS image on the stack. An
interrupt through a trap gate does not change IF.


9.6.1.4  Protection in Interrupt Procedures

The privilege rule that governs interrupt procedures is similar to that for
procedure calls: the CPU does not permit an interrupt to transfer control to
a procedure in a segment of lesser privilege (numerically greater privilege
level) than the current privilege level. An attempt to violate this rule
results in a general protection exception.

Because occurrence of interrupts is not generally predictable, this
privilege rule effectively imposes restrictions on the privilege levels at
which interrupt and exception handling procedures can execute. Either of the
following strategies can be employed to ensure that the privilege rule is
never violated.

  ■  Place the handler in a conforming segment. This strategy suits the
     handlers for certain exceptions (divide error, for example). Such a
     handler must use only the data available to it from the stack. If it
     needed data from a data segment, the data segment would have to have
     privilege level three, thereby making it unprotected.

  ■  Place the handler procedure in a privilege level zero segment.


9.6.2  Interrupt Tasks

A task gate in the IDT points indirectly to a task, as Figure 9-6
illustrates. The selector of the gate points to a TSS descriptor in the GDT.

When an interrupt or exception vectors to a task gate in the IDT, a task
switch results. Handling an interrupt with a separate task offers two
advantages:

  ■  The entire context is saved automatically.

  ■  The interrupt handler can be isolated from other tasks by giving it a
     separate address space, either via its LDT or via its page directory.

The actions that the processor takes to perform a task switch are discussed
in Chapter 7. The interrupt task returns to the interrupted task by
executing an IRET instruction.

If the task switch is caused by an exception that has an error code, the
processor automatically pushes the error code onto the stack that
corresponds to the privilege level of the first instruction to be executed
in the interrupt task.

When interrupt tasks are used in an operating system for the 80386, there
are actually two schedulers: the software scheduler (part of the operating
system) and the hardware scheduler (part of the processor's interrupt
mechanism). The design of the software scheduler should account for the fact
that the hardware scheduler may dispatch an interrupt task whenever
interrupts are enabled.


Figure 9-6.  Interrupt Vectoring for Tasks

            IDT                       GDT
     ╔════════════════╗        ╔════════════════╗
     ║                ║        ║                ║              TSS
     ╟────────────────╢        ╟────────────────╢       ╔════════════════╗
     ║                ║        ║                ║       ║                ║
     ╟────────────────╢        ╟────────────────╢       ║                ║
     ║                ║        ║                ║       ║                ║
     ╟────────────────╢        ╟────────────────╢       ║                ║
 ┌──║   TASK GATE    ╟───┐    ║                ║       ║                ║
 │   ╟────────────────╢   │    ╟────────────────╢       ║                ║
 │   ║                ║   └───║ TSS DESCRIPTOR ╟───┐   ║                ║
 │   ╟────────────────╢        ╟────────────────╢   │   ║                ║
 │   ║                ║        ║                ║   │   ║                ║
 │   ╟────────────────╢        ╟────────────────╢   └──╚════════════════╝
 │   ║                ║        ║                ║
 │   ╟────────────────╢        ╟────────────────╢
 │   ║                ║        ║                ║
 │   ╚════════════════╝        ╚════════════════╝
 │
 └─INTERRUPT ID


9.7  Error Code

With exceptions that relate to a specific segment, the processor pushes an
error code onto the stack of the exception handler (whether procedure or
task). The error code has the format shown in Figure 9-7. The format of the
error code resembles that of a selector; however, instead of an RPL field,
the error code contains two one-bit items:

  1.  The processor sets the EXT bit if an event external to the program
      caused the exception.

  2.  The processor sets the I-bit (IDT-bit) if the index portion of the
      error code refers to a gate descriptor in the IDT.

If the I-bit is not set, the TI bit indicates whether the error code refers
to the GDT (value 0) or to the LDT (value 1). The remaining 14 bits are the
upper 14 bits of the segment selector involved. In some cases the error code
on the stack is null, i.e., all bits in the low-order word are zero.


Figure 9-7.  Error Code Format

       31              15                                         2 1 0
      ╔═══════════════╪════════════════╤═════════════════╪═══════╤═╤═╤═╗
      ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                         │T│ │E║
      ║▒▒▒▒▒▒▒▒▒▒▒UNDEFINED▒▒▒▒▒▒▒▒▒▒▒▒│     SELECTOR INDEX      │ │I│ ║
      ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│                         │I│ │X║
      ╚═══════════════╪════════════════╧═════════════════╪═══════╧═╧═╧═╝


9.8  Exception Conditions

The following sections describe each of the possible exception conditions
in detail. Each description classifies the exception as a fault, trap, or
abort. This classification provides information needed by systems
programmers for restarting the procedure in which the exception occurred:

Faults   The CS and EIP values saved when a fault is reported point to the
         instruction causing the fault.

Traps    The CS and EIP values stored when the trap is reported point to the
         instruction dynamically after the instruction causing the trap. If
         a trap is detected during an instruction that alters program flow,
         the reported values of CS and EIP reflect the alteration of program
         flow. For example, if a trap is detected in a JMP instruction, the
         CS and EIP values pushed onto the stack point to the target of the
         JMP, not to the instruction after the JMP.

Aborts   An abort is an exception that permits neither precise location of
         the instruction causing the exception nor restart of the program
         that caused the exception. Aborts are used to report severe errors,
         such as hardware errors and inconsistent or illegal values in
         system tables.


9.8.1  Interrupt 0 ── Divide Error

The divide-error fault occurs during a DIV or an IDIV instruction when the
divisor is zero.


9.8.2  Interrupt 1 ── Debug Exceptions

The processor triggers this interrupt for any of a number of conditions;
whether the exception is a fault or a trap depends on the condition:

   ■  Instruction address breakpoint fault.
   ■  Data address breakpoint trap.
   ■  General detect fault.
   ■  Single-step trap.
   ■  Task-switch breakpoint trap.

The processor does not push an error code for this exception. An exception
handler can examine the debug registers to determine which condition caused
the exception. Refer to Chapter 12 for more detailed information about
debugging and the debug registers.


9.8.3  Interrupt 3 ── Breakpoint

The INT 3 instruction causes this trap. The INT 3 instruction is one byte
long, which makes it easy to replace an opcode in an executable segment with
the breakpoint opcode. The operating system or a debugging subsystem can use
a data-segment alias for an executable segment to place an INT 3 anywhere it
is convenient to arrest normal execution so that some sort of special
processing can be performed. Debuggers typically use breakpoints as a way of
displaying registers, variables, etc., at crucial points in a task.

The saved CS:EIP value points to the byte following the breakpoint. If a
debugger replaces a planted breakpoint with a valid opcode, it must subtract
one from the saved EIP value before returning. Refer also to Chapter 12 for
more information on debugging.


9.8.4  Interrupt 4 ── Overflow

This trap occurs when the processor encounters an INTO instruction and the
OF (overflow) flag is set. Since signed arithmetic and unsigned arithmetic
both use the same arithmetic instructions, the processor cannot determine
which is intended and therefore does not cause overflow exceptions
automatically. Instead it merely sets OF when the results, if interpreted as
signed numbers, would be out of range. When doing arithmetic on signed
operands, careful programmers and compilers either test OF directly or use
the INTO instruction.


9.8.5  Interrupt 5 ── Bounds Check

This fault occurs when the processor, while executing a BOUND instruction,
finds that the operand exceeds the specified limits. A program can use the
BOUND instruction to check a signed array index against signed limits
defined in a block of memory.


9.8.6  Interrupt 6 ── Invalid Opcode

This fault occurs when an invalid opcode is detected by the execution unit.
(The exception is not detected until an attempt is made to execute the
invalid opcode; i.e., prefetching an invalid opcode does not cause this
exception.) No error code is pushed on the stack. The exception can be
handled within the same task.

This exception also occurs when the type of operand is invalid for the
given opcode. Examples include an intersegment JMP referencing a register
operand, or an LES instruction with a register source operand.


9.8.7  Interrupt 7 ── Coprocessor Not Available

This exception occurs in either of two conditions:

  ■  The processor encounters an ESC (escape) instruction, and the EM
     (emulate) bit ofCR0 (control register zero) is set.

  ■  The processor encounters either the WAIT instruction or an ESC
     instruction, and both the MP (monitor coprocessor) and TS (task
     switched) bits of CR0 are set.

Refer to Chapter 11 for information about the coprocessor interface.


9.8.8  Interrupt 8 ── Double Fault

Normally, when the processor detects an exception while trying to invoke
the handler for a prior exception, the two exceptions can be handled
serially. If, however, the processor cannot handle them serially, it signals
the double-fault exception instead. To determine when two faults are to be
signalled as a double fault, the 80386 divides the exceptions into three
classes: benign exceptions, contributory exceptions, and page faults. Table
9-3 shows this classification.

Table 9-4 shows which combinations of exceptions cause a double fault and
which do not.

The processor always pushes an error code onto the stack of the
double-fault handler; however, the error code is always zero. The faulting
instruction may not be restarted. If any other exception occurs while
attempting to invoke the double-fault handler, the processor shuts down.


Table 9-3. Double-Fault Detection Classes

Class           ID          Description

                 1          Debug exceptions
                 2          NMI
                 3          Breakpoint
Benign           4          Overflow
Exceptions       5          Bounds check
                 6          Invalid opcode
                 7          Coprocessor not available
                16          Coprocessor error

                 0          Divide error
                 9          Coprocessor Segment Overrun
Contributory    10          Invalid TSS
Exceptions      11          Segment not present
                12          Stack exception
                13          General protection

Page Faults     14          Page fault


Table 9-4. Double-Fault Definition

                                   SECOND EXCEPTION

                           Benign       Contributory    Page
                           Exception    Exception       Fault


           Benign          OK           OK              OK
           Exception

FIRST      Contributory    OK           DOUBLE          OK
EXCEPTION  Exception

           Page
           Fault           OK           DOUBLE          DOUBLE


9.8.9  Interrupt 9 ── Coprocessor Segment Overrun

This exception is raised in protected mode if the 80386 detects a page or
segment violation while transferring the middle portion of a coprocessor
operand to the NPX. This exception is avoidable. Refer to Chapter 11 for
more information about the coprocessor interface.


9.8.10  Interrupt 10 ── Invalid TSS

Interrupt 10 occurs if during a task switch the new TSS is invalid. A TSS
is considered invalid in the cases shown in Table 9-5. An error code is
pushed onto the stack to help identify the cause of the fault. The EXT bit
indicates whether the exception was caused by a condition outside the
control of the program; e.g., an external interrupt via a task gate
triggered a switch to an invalid TSS.

This fault can occur either in the context of the original task or in the
context of the new task. Until the processor has completely verified the
presence of the new TSS, the exception occurs in the context of the original
task. Once the existence of the new TSS is verified, the task switch is
considered complete; i.e., TR is updated and, if the switch is due to a
CALL or interrupt, the backlink of the new TSS is set to the old TSS. Any
errors discovered by the processor after this point are handled in the
context of the new task.

To insure a proper TSS to process it, the handler for exception 10 must be
a task invoked via a task gate.


Table 9-5. Conditions That Invalidate the TSS

Error Code              Condition

TSS id + EXT            The limit in the TSS descriptor is less than 103
LTD id + EXT            Invalid LDT selector or LDT not present
SS id + EXT             Stack segment selector is outside table limit
SS id + EXT             Stack segment is not a writable segment
SS id + EXT             Stack segment DPL does not match new CPL
SS id + EXT             Stack segment selector RPL < >  CPL
CS id + EXT             Code segment selector is outside table limit
CS id + EXT             Code segment selector does not refer to code
                        segment
CS id + EXT             DPL of non-conforming code segment < > new CPL
CS id + EXT             DPL of conforming code segment > new CPL
DS/ES/FS/GS id + EXT    DS, ES, FS, or GS segment selector is outside
                        table limits
DS/ES/FS/GS id + EXT    DS, ES, FS, or GS is not readable segment


9.8.11  Interrupt 11 ── Segment Not Present

Exception 11 occurs when the processor detects that the present bit of a
descriptor is zero. The processor can trigger this fault in any of these
cases:

  ■  While attempting to load the CS, DS, ES, FS, or GS registers; loading
     the SS register, however, causes a stack fault.

  ■  While attempting loading the LDT register with an LLDT instruction;
     loading the LDT register during a task switch operation, however,
     causes the "invalid TSS" exception.

  ■  While attempting to use a gate descriptor that is marked not-present.

This fault is restartable. If the exception handler makes the segment
present and returns, the interrupted program will resume execution.

If a not-present exception occurs during a task switch, not all the steps
of the task switch are complete. During a task switch, the processor first
loads all the segment registers, then checks their contents for validity. If
a not-present exception is discovered, the remaining segment registers have
not been checked and therefore may not be usable for referencing memory. The
not-present handler should not rely on being able to use the values found
in CS, SS, DS, ES, FS, and GS without causing another exception. The
exception handler should check all segment registers before trying to resume
the new task; otherwise, general protection faults may result later under
conditions that make diagnosis more difficult. There are three ways to
handle this case:

  1.  Handle the not-present fault with a task. The task switch back to the
      interrupted task will cause the processor to check the registers as it
      loads them from the TSS.

  2.  PUSH and POP all segment registers. Each POP causes the processor to
      check the new contents of the segment register.

  3.  Scrutinize the contents of each segment-register image in the TSS,
      simulating the test that the processor makes when it loads a segment
      register.

This exception pushes an error code onto the stack. The EXT bit of the
error code is set if an event external to the program caused an interrupt
that subsequently referenced a not-present segment. The I-bit is set if the
error code refers to an IDT entry, e.g., an INT instruction referencing a
not-present gate.

An operating system typically uses the "segment not present" exception to
implement virtual memory at the segment level. A not-present indication in a
gate descriptor, however, usually does not indicate that a segment is not
present (because gates do not necessarily correspond to segments).
Not-present gates may be used by an operating system to trigger exceptions
of special significance to the operating system.


9.8.12  Interrupt 12 ── Stack Exception

A stack fault occurs in either of two general conditions:

  ■  As a result of a limit violation in any operation that refers to the
     SS register. This includes stack-oriented instructions such as POP,
     PUSH, ENTER, and LEAVE, as well as other memory references that
     implicitly use SS (for example, MOV AX, [BP+6]). ENTER causes this
     exception when the stack is too small for the indicated local-variable
     space.

  ■  When attempting to load the SS register with a descriptor that is
     marked not-present but is otherwise valid. This can occur in a task
     switch, an interlevel CALL, an interlevel return, an LSS instruction,
     or a MOV or POP instruction to SS.

When the processor detects a stack exception, it pushes an error code onto
the stack of the exception handler. If the exception is due to a not-present
stack segment or to overflow of the new stack during an interlevel CALL, the
error code contains a selector to the segment in question (the exception
handler can test the present bit in the descriptor to determine which
exception occurred); otherwise the error code is zero.

An instruction that causes this fault is restartable in all cases. The
return pointer pushed onto the exception handler's stack points to the
instruction that needs to be restarted. This instruction is usually the one
that caused the exception; however, in the case of a stack exception due to
loading of a not-present stack-segment descriptor during a task switch, the
indicated instruction is the first instruction of the new task.

When a stack fault occurs during a task switch, the segment registers may
not be usable for referencing memory. During a task switch, the selector
values are loaded before the descriptors are checked. If a stack fault is
discovered, the remaining segment registers have not been checked and
therefore may not be usable for referencing memory. The stack fault handler
should not rely on being able to use the values found in CS, SS, DS, ES,
FS, and GS without causing another exception. The exception handler should
check all segment registers before trying to resume the new task; otherwise,
general protection faults may result later under conditions that make
diagnosis more difficult.


9.8.13  Interrupt 13 ── General Protection Exception

All protection violations that do not cause another exception cause a
general protection exception. This includes (but is not limited to):

  1.  Exceeding segment limit when using CS, DS, ES, FS, or GS

  2.  Exceeding segment limit when referencing a descriptor table

  3.  Transferring control to a segment that is not executable

  4.  Writing into a read-only data segment or into a code segment

  5.  Reading from an execute-only segment

  6.  Loading the SS register with a read-only descriptor (unless the
      selector comes from the TSS during a task switch, in which case a TSS
      exception occurs

  7.  Loading SS, DS, ES, FS, or GS with the descriptor of a system segment

  8.  Loading DS, ES, FS, or GS with the descriptor of an executable
      segment that is not also readable

  9.  Loading SS with the descriptor of an executable segment

  10. Accessing memory via DS, ES, FS, or GS when the segment register
      contains a null selector

  11. Switching to a busy task

  12. Violating privilege rules

  13. Loading CR0 with PG=1 and PE=0.

  14. Interrupt or exception via trap or interrupt gate from V86 mode to
      privilege level other than zero.

  15. Exceeding the instruction length limit of 15 bytes (this can occur
      only if redundant prefixes are placed before an instruction)

The general protection exception is a fault. In response to a general
protection exception, the processor pushes an error code onto the exception
handler's stack. If loading a descriptor causes the exception, the error
code contains a selector to the descriptor; otherwise, the error code is
null. The source of the selector in an error code may be any of the
following:

  1.  An operand of the instruction.
  2.  A selector from a gate that is the operand of the instruction.
  3.  A selector from a TSS involved in a task switch.


9.8.14  Interrupt 14 ── Page Fault

This exception occurs when paging is enabled (PG=1) and the processor
detects one of the following conditions while translating a linear address
to a physical address:

  ■  The page-directory or page-table entry needed for the address
     translation has zero in its present bit.

  ■  The current procedure does not have sufficient privilege to access the
     indicated page.

The processor makes available to the page fault handler two items of
information that aid in diagnosing the exception and recovering from it:

  ■  An error code on the stack. The error code for a page fault has a
     format different from that for other exceptions (see Figure 9-8). The
     error code tells the exception handler three things:

     1.  Whether the exception was due to a not present page or to an access
         rights violation.

     2.  Whether the processor was executing at user or supervisor level at
         the time of the exception.

     3.  Whether the memory access that caused the exception was a read or
         write.

  ■  CR2 (control register two). The processor stores in CR2 the linear
     address used in the access that caused the exception (see Figure 9-9).
     The exception handler can use this address to locate the corresponding
     page directory and page table entries. If another page fault can occur
     during execution of the page fault handler, the handler should push CR2
     onto the stack.


Figure 9-8.  Page-Fault Error Code Format

 ╔═════╤═════╤════════════════════════════════════════════════════════════╗
 ║Field│Value│                         Description                        ║
 ╟─────┼─────┼────────────────────────────────────────────────────────────╢
 ║ U/S │  0  │ The access causing the fault originated when the processor ║
 ║     │     │ was executing in supervisor mode.                          ║
 ║     │     │                                                            ║
 ║     │  1  │ The access causing the fault originated when the processor ║
 ║     │     │ was executing in user mode.                                ║
 ║     │     │                                                            ║
 ║ W/R │  0  │ The access causing the fault was a read.                   ║
 ║     │     │                                                            ║
 ║     │  1  │ The access causing the fault was a write.                  ║
 ║     │     │                                                            ║
 ║ P   │  0  │ The fault was caused by a not-present page.                ║
 ║     │     │                                                            ║
 ║     │  1  │ The fault was caused by a page-level protection violation. ║
 ╚═════╧═════╧════════════════════════════════════════════════════════════╝

       31                               15             7        3 2 1 0
      ╔════════════════════════════════╪═════════════════════════╤═╤═╤═╗
      ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│U│W│ ║
      ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒UNDEFINED▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│/│/│P║
      ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│S│R│ ║
      ╚════════════════════════════════╪═════════════════════════╧═╧═╧═╝


9.8.14.1  Page Fault During Task Switch

The processor may access any of four segments during a task switch:

  1.  Writes the state of the original task in the TSS of that task.

  2.  Reads the GDT to locate the TSS descriptor of the new task.

  3.  Reads the TSS of the new task to check the types of segment
      descriptors from the TSS.

  4.  May read the LDT of the new task in order to verify the segment
      registers stored in the new TSS.

A page fault can result from accessing any of these segments. In the latter
two cases the exception occurs in the context of the new task. The
instruction pointer refers to the next instruction of the new task, not to
the instruction that caused the task switch. If the design of the operating
system permits page faults to occur during task-switches, the page-fault
handler should be invoked via a task gate.


Figure 9-9.  CR2 Format

      31               23               15               7              0
     ╔════════════════╪════════════════╪════════════════╪════════════════╗
     ║                                                                   ║
     ║                      PAGE FAULT LINEAR ADDRESS                    ║
     ║                                                                   ║
     ╚════════════════╪════════════════╪════════════════╪════════════════╝


9.8.14.2  Page Fault with Inconsistent Stack Pointer

Special care should be taken to ensure that a page fault does not cause the
processor to use an invalid stack pointer (SS:ESP). Software written for
earlier processors in the 8086 family often uses a pair of instructions to
change to a new stack; for example:

MOV SS, AX
MOV SP, StackTop

With the 80386, because the second instruction accesses memory, it is
possible to get a page fault after SS has been changed but before SP has
received the corresponding change. At this point, the two parts of the stack
pointer SS:SP (or, for 32-bit programs, SS:ESP) are inconsistent.

The processor does not use the inconsistent stack pointer if the handling
of the page fault causes a stack switch to a well defined stack (i.e., the
handler is a task or a more privileged procedure). However, if the page
fault handler is invoked by a trap or interrupt gate and the page fault
occurs at the same privilege level as the page fault handler, the processor
will attempt to use the stack indicated by the current (invalid) stack
pointer.

In systems that implement paging and that handle page faults within the
faulting task (with trap or interrupt gates), software that executes at the
same privilege level as the page fault handler should initialize a new stack
by using the new LSS instruction rather than an instruction pair shown
above. When the page fault handler executes at privilege level zero (the
normal case), the scope of the problem is limited to privilege-level zero
code, typically the kernel of the operating system.


9.8.15  Interrupt 16 ── Coprocessor Error

The 80386 reports this exception when it detects a signal from the 80287 or
80387 on the 80386's ERROR# input pin. The 80386 tests this pin only at the
beginning of certain ESC instructions and when it encounters a WAIT
instruction while the EM bit of the MSW is zero (no emulation). Refer to
Chapter 11 for more information on the coprocessor interface.


9.9  Exception Summary


Table 9-6 summarizes the exceptions recognized by the 386.

Table 9-6. Exception Summary


Description               Interrupt   Return Address  Exception     Function That Can Generate
                          Number      Points to       Type          the Exception
                                      Faulting
                                      Instruction

Divide error               0          YES             FAULT         DIV, IDIV
Debug exceptions           1
Some debug exceptions are traps and some are faults.  The exception
handler can determine which has occurred by examining DR6.  (Refer to
Chapter 12.)
Some debug exceptions are traps and some are faults.  The exception
handler can determine which has occurred by examining DR6.  (Refer to
Chapter 12.)             Any instruction
Breakpoint                 3          NO              TRAP          One-byte INT 3
Overflow                   4          NO              TRAP          INTO
Bounds check               5          YES             FAULT         BOUND
Invalid opcode             6          YES             FAULT         Any illegal instruction
Coprocessor not available  7          YES             FAULT         ESC, WAIT
Double fault               8          YES             ABORT         Any instruction that can
                                                                    generate an exception
Coprocessor Segment
Overrun                    9          NO              ABORT         Any operand of an ESC
                                                                    instruction that wraps around
                                                                    the end of a segment.
Invalid TSS               10          YES             FAULT
An invalid-TSS fault is not restartable if it occurs during the
processing of an external interrupt.        JMP, CALL, IRET, any interrupt
Segment not present       11          YES             FAULT         Any segment-register modifier
Stack exception           12          YES             FAULT         Any memory reference thru SS
General Protection        13          YES             FAULT/ABORT
All GP faults are restartable. If the fault occurs while attempting to
vector to the handler for an external interrupt, the interrupted program is
restartable, but the interrupt may be lost.  Any memory reference or code
                                                                    fetch
Page fault                14          YES             FAULT         Any memory reference or code
                                                                    fetch
Coprocessor error         16          YES             FAULT
Coprocessor errors are reported as a fault on the first ESC or WAIT
instruction executed after the ESC instruction that caused the error.        ESC, WAIT
Two-byte SW Interrupt     0-255       NO              TRAP          INT n


9.10  Error Code Summary

Table 9-7 summarizes the error information that is available with each
exception.


Table 9-7. Error-Code Summary

Description                       Interrupt     Error Code
                                  Number

Divide error                       0            No
Debug exceptions                   1            No
Breakpoint                         3            No
Overflow                           4            No
Bounds check                       5            No
Invalid opcode                     6            No
Coprocessor not available          7            No
System error                       8            Yes (always 0)
Coprocessor Segment Overrun        9            No
Invalid TSS                       10            Yes
Segment not present               11            Yes
Stack exception                   12            Yes
General protection fault          13            Yes
Page fault                        14            Yes
Coprocessor error                 16            No
Two-byte SW interrupt             0-255         No


Chapter 10  Initialization

────────────────────────────────────────────────────────────────────────────

After a signal on the RESET pin, certain registers of the 80386 are set to
predefined values. These values are adequate to enable execution of a
bootstrap program, but additional initialization must be performed by
software before all the features of the processor can be utilized.


10.1  Processor State After Reset

The contents of EAX depend upon the results of the power-up self test. The
self-test may be requested externally by assertion of BUSY# at the end of
RESET. The EAX register holds zero if the 80386 passed the test. A nonzero
value in EAX after self-test indicates that the particular 80386 unit is
faulty. If the self-test is not requested, the contents of EAX after RESET
is undefined.

DX holds a component identifier and revision number after RESET as Figure
10-1 illustrates. DH contains 3, which indicates an 80386 component. DL
contains a unique identifier of the revision level.

Control register zero (CR0) contains the values shown in Figure 10-2. The
ET bit of CR0 is set if an 80387 is present in the configuration (according
to the state of the ERROR# pin after RESET). If ET is reset, the
configuration either contains an 80287 or does not contain a coprocessor. A
software test is required to distinguish between these latter two
possibilities.

The remaining registers and flags are set as follows:

   EFLAGS             =00000002H
   IP                 =0000FFF0H
   CS selector        =000H
   DS selector        =0000H
   ES selector        =0000H
   SS selector        =0000H
   FS selector        =0000H
   GS selector        =0000H
   IDTR:
              base    =0
              limit   =03FFH

All registers not mentioned above are undefined.

These settings imply that the processor begins in real-address mode with
interrupts disabled.


Figure 10-1.  Contents of EDX after RESET

                                 EDX REGISTER

      31               23               15               7              0
     ╔════════════════╪════════════════╪════════════════╪════════════════╗
     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│       DH       │      DL        ║
     ║▒▒▒▒▒▒▒▒▒▒▒▒UNDEFINED▒▒▒▒▒▒▒▒▒▒▒▒│   DEVICE ID    │  STEPPING ID   ║
     ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│       3        │   (UNIQUE)     ║
     ╚════════════════╪════════════════╪════════════════╪════════════════╝


Figure 10-2.  Initial Contents of CR0

                               CONTROL REGISTER ZERO

   31                23                15                  7     4 3   1  0
  ╔═╤═══════════════╪═════════════════╪═══════════════════╪═════╤═╤═╤═╤═╤═╗
  ║P│                                                           │E│T│E│M│P║
  ║ │                            UNDEFINED                      │ │ │ │ │ ║
  ║G│                                                           │T│S│M│P│E║
  ╚╤╧═══════════════╪═════════════════╪═══════════════════╪═════╧╤╧╤╧╤╧╤╧╤╝
   │                                                             │ │ │ │ │
   └─────────────0 - PAGING DISABLED                             │ │ │ │ │
                 * - INDICATES PRESENCE OF 80387─────────────────┘ │ │ │ │
                 0 - NO TASK SWITCH────────────────────────────────┘ │ │ │
                 0 - DO NOT MONITOR COPROCESSOR──────────────────────┘ │ │
                 0 - COPROCESSOR NOT PRESENT───────────────────────────┘ │
                 0 - PROTECTION NOT ENABLED (REAL ADDRESS MODE)──────────┘


10.2  Software Initialization for Real-Address Mode

In real-address mode a few structures must be initialized before a program
can take advantage of all the features available in this mode.


10.2.1  Stack

No instructions that use the stack can be used until the stack-segment
register (SS) has been loaded. SS must point to an area in RAM.


10.2.2  Interrupt Table

The initial state of the 80386 leaves interrupts disabled; however, the
processor will still attempt to access the interrupt table if an exception
or nonmaskable interrupt (NMI) occurs. Initialization software should take
one of the following actions:

  ■  Change the limit value in the IDTR to zero. This will cause a shutdown
     if an exception or nonmaskable interrupt occurs. (Refer to the 80386
     Hardware Reference Manual to see how shutdown is signalled externally.)

  ■  Put pointers to valid interrupt handlers in all positions of the
     interrupt table that might be used by exceptions or interrupts.

  ■  Change the IDTR to point to a valid interrupt table.


10.2.3  First Instructions

After RESET, address lines A{31-20} are automatically asserted for
instruction fetches. This fact, together with the initial values of CS:IP,
causes instruction execution to begin at physical address FFFFFFF0H. Near
(intrasegment) forms of control transfer instructions may be used to pass
control to other addresses in the upper 64K bytes of the address space. The
first far (intersegment) JMP or CALL instruction causes A{31-20} to drop
low, and the 80386 continues executing instructions in the lower one
megabyte of physical memory. This automatic assertion of address lines
A{31-20} allows systems designers to use a ROM at the high end of
the address space to initialize the system.


10.3  Switching to Protected Mode

Setting the PE bit of the MSW in CR0 causes the 80386 to begin executing in
protected mode. The current privilege level (CPL) starts at zero. The
segment registers continue to point to the same linear addresses as in real
address mode (in real address mode, linear addresses are the same physical
addresses).

Immediately after setting the PE flag, the initialization code must flush
the processor's instruction prefetch queue by executing a JMP instruction.
The 80386 fetches and decodes instructions and addresses before they are
used; however, after a change into protected mode, the prefetched
instruction information (which pertains to real-address mode) is no longer
valid. A JMP forces the processor to discard the invalid information.


10.4  Software Initialization for Protected Mode

Most of the initialization needed for protected mode can be done either
before or after switching to protected mode. If done in protected mode,
however, the initialization procedures must not use protected-mode features
that are not yet initialized.


10.4.1  Interrupt Descriptor Table

The IDTR may be loaded in either real-address or protected mode. However,
the format of the interrupt table for protected mode is different than that
for real-address mode. It is not possible to change to protected mode and
change interrupt table formats at the same time; therefore, it is inevitable
that, if IDTR selects an interrupt table, it will have the wrong format at
some time. An interrupt or exception that occurs at this time will have
unpredictable results. To avoid this unpredictability, interrupts should
remain disabled until interrupt handlers are in place and a valid IDT has
been created in protected mode.


10.4.2  Stack

The SS register may be loaded in either real-address mode or protected
mode. If loaded in real-address mode, SS continues to point to the same
linear base-address after the switch to protected mode.


10.4.3  Global Descriptor Table

Before any segment register is changed in protected mode, the GDT register
must point to a valid GDT. Initialization of the GDT and GDTR may be done in
real-address mode. The GDT (as well as LDTs) should reside in RAM, because
the processor modifies the accessed bit of descriptors.


10.4.4  Page Tables

Page tables and the PDBR in CR3 can be initialized in either real-address
mode or in protected mode; however, the paging enabled (PG) bit of CR0
cannot be set until the processor is in protected mode. PG may be set
simultaneously with PE, or later. When PG is set, the PDBR in CR3 should
already be initialized with a physical address that points to a valid page
directory. The initialization procedure should adopt one of the following
strategies to ensure consistent addressing before and after paging is
enabled:

  ■  The page that is currently being executed should map to the same
     physical addresses both before and after PG is set.

  ■  A JMP instruction should immediately follow the setting of PG.


10.4.5  First Task

The initialization procedure can run awhile in protected mode without
initializing the task register; however, before the first task switch, the
following conditions must prevail:

  ■  There must be a valid task state segment (TSS) for the new task. The
     stack pointers in the TSS for privilege levels numerically less than or
     equal to the initial CPL must point to valid stack segments.

  ■  The task register must point to an area in which to save the current
     task state. After the first task switch, the information dumped in this
     area is not needed, and the area can be used for other purposes.


10.5  Initialization Example

$TITLE ('Initial Task')

    NAME    INIT

init_stack  SEGMENT RW
            DW  20  DUP(?)
tos         LABEL   WORD
init_stack  ENDS

init_data   SEGMENT RW PUBLIC
            DW  20  DUP(?)
init_data   ENDS

init_code   SEGMENT ER PUBLIC

ASSUME      DS:init_data

    nop
    nop
    nop
init_start:
                                    ; set up stack
    mov ax, init_stack
    mov ss, ax
    mov esp, offset tos

    mov a1,1
blink:
    xor a1,1
    out 0e4h,a1
    mov cx,3FFFh
here:
    dec cx
    jnz here

    jmp SHORT blink

    hlt
init_code   ends

    END init_start, SS:init_stack, DS:init_data

$TITLE('Protected Mode Transition -- 386 initialization')
NAME  RESET

;*****************************************************************
; Upon reset the 386 starts executing at address 0FFFFFFF0H.  The
; upper 12 address bits remain high until a FAR call or jump is
; executed.
;
; Assume the following:
;
;
; -  a short jump at address 0FFFFFFF0H (placed there by the
;    system builder) causes execution to begin at START in segment
;    RESET_CODE.
;
;
; -  segment RESET_CODE is based at physical address 0FFFF0000H,
;    i.e.   at the start of the last  64K in the 4G address space.
;    Note that  this is the base of the CS register at reset.  If
;    you locate ROMcode above  this  address,  you  will  need  to
;    figure out an adjustment factor to address things within this
;    segment.
;
;*****************************************************************
$EJECT ;

; Define addresses to locate GDT and IDT in RAM.
; These addresses are also used in the BLD386 file that defines
; the GDT and IDT. If you change these addresses, make sure you
; change the base addresses specified in the build file.

GDTbase         EQU    00001000H   ; physical address for GDT base
IDTbase         EQU    00000400H   ; physical address for IDT base

PUBLIC     GDT_EPROM
PUBLIC     IDT_EPROM
PUBLIC     START

DUMMY      segment rw      ; ONLY for ASM386 main module stack init
           DW 0
DUMMY   ends

;*****************************************************************
;
; Note: RESET CODE must be USEl6 because the 386 initally executes
;       in real mode.
;

RESET_CODE segment er PUBLIC    USE16

ASSUME DS:nothing, ES:nothing

;
; 386 Descriptor template

DESC       STRUC
    lim_0_15    DW  0              ; limit bits (0..15)
    bas_0_15    DW  0              ; base bits (0..15)
    bas_16_23   DB  0              ; base bits (16..23)
    access      DB  0              ; access byte
    gran        DB  0              ; granularity byte
    bas_24_31   DB  0              ; base bits (24..31)
DESC       ENDS

; The following is the layout of the real GDT created by BLD386.
; It is located in EPROM and will be copied to RAM.
;
; GDT[O]      ...  NULL
; GDT[1]      ...  Alias for RAM GDT
; GDT[2]      ...  Alias for RAM IDT
; GDT[2]      ...  initial task TSS
; GDT[3]      ...  initial task TSS alias
; GDT[4]      ...  initial task LDT
; GDT[5]      ...  initial task LDT alias

;
; define entries in GDT and IDT.

GDT_ENTRIES    EQU    8
IDT_ENTRIES    EQU    32

; define some constants to index into the real GDT

GDT_ALIAS      EQU    1*SIZE DESC
IDT_ALIAS      EQU    2*SIZE DESC
INIT_TSS       EQU    3*SIZE DESC
INIT_TSS_A     EQU    4*SIZE DESC
INIT_LDT       EQU    5*SIZE DESC
INIT_LDT_A     EQU    6*SIZE DESC

;
; location of alias in INIT_LDT

INIT_LDT_ALIAS    EQU    1*SIZE DESC

;
; access rights byte for DATA and TSS descriptors

DS_ACCESS   EQU   010010010B
TSS_ACCESS  EQU   010001001B


;
; This temporary GDT will be used to set up the real GDT in RAM.

Temp_GDT    LABEL   BYTE        ; tag for begin of scratch GDT

NULL_DES    DESC <>             ; NULL descriptor

                                ; 32-Gigabyte data segment based at 0
FLAT_DES    DESC <0FFFFH,0,0,92h,0CFh,0>

GDT_eprom     DP    ?           ; Builder places GDT address and limit
                                ; in this 6 byte area.

IDT_eprom     DP    ?           ; Builder places IDT address and limit
                                ; in this 6 byte area.

;
; Prepare operand for loadings GDTR and LDTR.


TGDT_pword     LABEL  PWORD                 ; for temp GDT
        DW     end_Temp_GDT_Temp_GDT -1
        DD     0

GDT_pword      LABEL  PWORD                 ; for GDT in RAM
        DW     GDT_ENTRIES * SIZE DESC -1
        DD     GDTbase

IDT_pword      LABEL   PWORD                ; for IDT in RAM
        DW     IDT_ENTRIES * SIZE DESC -1
        DD     IDTbase


end_Temp_GDT   LABEL   BYTE

;
; Define equates for addressing convenience.

GDT_DES_FLAT        EQU DS:GDT_ALIAS +GDTbase
IDT_DES_FLAT        EQU DS:IDT_ALIAS +GDTbase

INIT_TSS_A_OFFSET   EQU DS:INIT_TSS_A
INIT_TSS_OFFSET     EQU DS:INIT_TSS

INIT_LDT_A_OFFSET   EQU DS:INIT_LDT_A
INIT_LDT_OFFSET     EQU DS:INIT_LDT


; define pointer for first task switch

ENTRY POINTER LABEL DWORD
             DW 0, INIT_TSS

;******************************************************************
;
;   Jump from reset vector to here.

START:

    CLI                ;disable interrupts
    CLD                ;clear direction flag

    LIDT    NULL_des   ;force shutdown on errors

;
;   move scratch GDT to RAM at physical 0

    XOR DI,DI
    MOV ES,DI           ;point ES:DI to physical location 0


    MOV SI,OFFSET Temp_GDT
    MOV CX,end_Temp_GDT-Temp_GDT        ;set byte count
    INC CX
;
;   move table

    REP MOVS BYTE PTR ES:[DI],BYTE PTR CS:[SI]

    LGDT    tGDT_pword                ;load GDTR for Temp. GDT
                                      ;(located at 0)

;   switch to protected mode

    MOV EAX,CR0                       ;get current CRO
    MOV EAX,1                         ;set PE bit
    MOV CRO,EAX                       ;begin protected mode
;
;   clear prefetch queue

    JMP SHORT flush
flush:

; set DS,ES,SS to address flat linear space (0 ... 4GB)

    MOV BX,FLAT_DES-Temp_GDT
    MOV US,BX
    MOV ES,BX
    MOV SS,BX
;
; initialize stack pointer to some (arbitrary) RAM location

    MOV ESP, OFFSET end_Temp_GDT

;
; copy eprom GDT to RAM

    MOV ESI,DWORD PTR GDT_eprom +2 ; get base of eprom GDT
                                   ; (put here by builder).

    MOV EDI,GDTbase                ; point ES:EDI to GDT base in RAM.

    MOV CX,WORD PTR gdt_eprom +0   ; limit of eprom GDT
    INC CX
    SHR CX,1                       ; easier to move words
    CLD
    REP MOVS   WORD PTR ES:[EDI],WORD PTR DS:[ESI]

;
; copy eprom IDT to RAM
;
    MOV ESI,DWORD PTR IDT_eprom +2 ; get base of eprom IDT
                                   ; (put here by builder)

    MOV EDI,IDTbase                ; point ES:EDI to IDT base in RAM.

    MOV CX,WORD PTR idt_eprom +0   ; limit of eprom IDT
    INC CX
    SHR CX,1
    CLD
    REP MOVS   WORD PTR ES:[EDI],WORD PTR DS:[ESI]

; switch to RAM GDT and IDT
;
    LIDT IDT_pword
    LGDT GDT_pword

;
    MOV BX,GDT_ALIAS               ; point DS to GDT alias
    MOV DS,BX
;
; copy eprom TSS to RAM
;
    MOV BX,INIT_TSS_A              ; INIT TSS A descriptor base
                                   ; has RAM location of INIT TSS.

    MOV ES,BX                      ; ES points to TSS in RAM

    MOV BX,INIT_TSS                ; get inital task selector
    LAR DX,BX                      ; save access byte
    MOV [BX].access,DS_ACCESS      ; set access as data segment
    MOV FS,BX                      ; FS points to eprom TSS

    XOR si,si                      ; FS:si points to eprom TSS
    XOR di,di                      ; ES:di points to RAM TSS

    MOV CX,[BX].lim_0_15           ; get count to move
    INC CX

;
; move INIT_TSS to RAM.

    REP MOVS BYTE PTR ES:[di],BYTE PTR FS:[si]

    MOV [BX].access,DH             ; restore access byte

;
; change base of INIT TSS descriptor to point to RAM.

    MOV AX,INIT_TSS_A_OFFSET.bas_0_15
    MOV INIT_TSS_OFFSET.bas_0_15,AX
    MOV AL,INIT_TSS_A_OFFSET.bas_16_23
    MOV INIT_TSS_OFFSET.bas_16_23,AL
    MOV AL,INIT_TSS_A_OFFSET.bas_24_31
    MOV INIT_TSS_OFFSET.bas_24_31,AL

;
; change INIT TSS A to form a save area for TSS on first task
; switch. Use RAM at location 0.

    MOV BX,INIT_TSS_A
    MOV WORD PTR [BX].bas_0_15,0
    MOV [BX].bas_16_23,0
    MOV [BX].bas_24_31,0
    MOV [BX].access,TSS_ACCESS
    MOV [BX].gran,O
    LTR BX                         ; defines save area for TSS

;
; copy eprom LDT to RAM

    MOV BX,INIT_LDT_A              ; INIT_LDT_A descriptor has
                                   ; base address in RAM for INIT_LDT.

    MOV ES,BX                      ; ES points LDT location in RAM.

    MOV AH,[BX].bas_24_31
    MOV AL,[BX].bas_16_23
    SHL EAX,16
    MOV AX,[BX].bas_0_15           ; save INIT_LDT base (ram) in EAX

    MOV BX,INIT_LDT                ; get inital LDT selector
    LAR DX,BX                      ; save access rights
    MOV [BX].access,DS_ACCESS      ; set access as data segment
    MOV FS,BX                      ; FS points to eprom LDT

    XOR si,si                      ; FS:SI points to eprom LDT
    XOR di,di                      ; ES:DI points to RAM LDT

    MOV CX,[BX].lim_0_15           ; get count to move
    INC CX
;
; move initial LDT to RAM

    REP MOVS BYTE PTR ES:[di],BYTE PTR FS:[si]

    MOV [BX].access,DH             ; restore access rights in
                                   ; INIT_LDT descriptor

;
; change base of alias (of INIT_LDT) to point to location in RAM.

    MOV ES:[INIT_LDT_ALIAS].bas_0_15,AX
    SHR EAX,16
    MOV ES:[INIT_LDT_ALIAS].bas_16_23,AL
    MOV ES:[INIT_LDT_ALIAS].bas_24_31,AH

;
; now set the base value in INIT_LDT descriptor

    MOV AX,INIT_LDT_A_OFFSET.bas_0_15
    MOV INIT_LDT_OFFSET.bas_0_15,AX
    MOV AL,INIT_LDT_A_OFFSET.bas_16_23
    MOV INIT_LDT_OFFSET.bas_16_23,AL
    MOV AL,INIT_LDT_A_OFFSET.bas_24_31
    MOV INIT_LDT_OFFSET.bas_24_31,AL

;
; Now GDT, IDT, initial TSS and initial LDT are all set up.
;
; Start the first task!
'
   JMP ENTRY_POINTER

RESET_CODE ends
   END START, SS:DUMMY,DS:DUMMY


10.6  TLB Testing

The 80386 provides a mechanism for testing the Translation Lookaside Buffer
(TLB), the cache used for translating linear addresses to physical
addresses. Although failure of the TLB hardware is extremely unlikely, users
may wish to include TLB confidence tests among other power-up confidence
tests for the 80386.

───────────────────────────────────────────────────────────────────────────
NOTE
  This TLB testing mechanism is unique to the 80386 and may not be
  continued in the same way in future processors. Sortware that uses
  this mechanism may be incompatible with future processors.
───────────────────────────────────────────────────────────────────────────

When testing the TLB it is recommended that paging be turned off (PG=0 in
CR0) to avoid interference with the test data being written to the TLB.


10.6.1  Structure of the TLB

The TLB is a four-way set-associative memory. Figure 10-3 illustrates the
structure of the TLB. There are four sets of eight entries each. Each entry
consists of a tag and data. Tags are 24-bits wide. They contain the
high-order 20 bits of the linear address, the valid bit, and three attribute
bits. The data portion of each entry contains the high-order 20 bits of the
physical address.


10.6.2  Test Registers

Two test registers, shown in Figure 10-4, are provided for the purpose of
testing. TR6 is the test command register, and TR7 is the test data
register. These registers are accessed by variants of the MOV
instruction. A test register may be either the source operand or destination
operand. The MOV instructions are defined in both real-address mode and
protected mode. The test registers are privileged resources; in protected
mode, the MOV instructions that access them can only be executed at
privilege level 0. An attempt to read or write the test registers when
executing at any other privilege level causes a general
protection exception.

The test command register (TR6) contains a command and an address tag to
use in performing the command:

C         This is the command bit. There are two TLB testing commands:
          write entries into the TLB, and perform TLB lookups. To cause an
          immediate write into the TLB entry, move a doubleword into TR6
          that contains a 0 in this bit. To cause an immediate TLB lookup,
          move a doubleword into TR6 that contains a 1 in this bit.

Linear    On a TLB write, a TLB entry is allocated to this linear address;
Address   the rest of that TLB entry is set per the value of TR7 and the
          value just written into TR6. On a TLB lookup, the TLB is
          interrogated per this value; if one and only one TLB entry
          matches, the rest of the fields of TR6 and TR7 are set from the
          matching TLB entry.

V         The valid bit for this TLB entry. The TLB uses the valid bit to
          identify entries that contain valid data. Entries of the TLB
          that have not been assigned values have zero in the valid bit.
          All valid bits can be cleared by writing to CR3.

D, D#     The dirty bit (and its complement) for/from the TLB entry.

U, U#     The U/S bit (and its complement) for/from the TLB entry.

W, W#     The R/W bit (and its complement) for/from the TLB entry.

          The meaning of these pairs of bits is given by Table 10-1,
          where X represents D, U, or W.

The test data register (TR7) holds data read from or data to be written to
the TLB.

Physical  This is the data field of the TLB. On a write to the TLB, the
Address   TLB entry allocated to the linear address in TR6 is set to this
          value. On a TLB lookup, if HT is set, the data field (physical
          address) from the TLB is read out to this field. If HT is not
          set, this field is undefined.

HT        For a TLB lookup, the HT bit indicates whether the lookup was a
          hit (HT  1) or a miss (HT  0). For a TLB write, HT must be set
          to 1.

REP       For a TLB write, selects which of four associative blocks of the
          TLB is to be written. For a TLB read, if HT is set, REP reports
          in which of the four associative blocks the tag was found; if HT
          is not set, REP is undefined.


Table 10-1. Meaning of D, U, and W Bit Pairs

X     X#      Effect during        Value of bit X
              TLB Lookup           after TLB Write

0     0       (undefined)          (undefined)
0     1       Match if X=0         Bit X becomes 0
1     0       Match if X=1         Bit X becomes 1
1     1       (undefined)          (undefined)


Figure 10-3.  TLB Structure

                                   ╔═════════════════╦════════════════╗
                                  7║       TAG       ║      DATA      ║
                                   ╠═════════════════╬════════════════╣
                                                                    
                 ┌───────                                           
                 │       SET 11                                     
                 │    ┌──          ╠═════════════════╬════════════════╣
                 │    │           1║       TAG       ║      DATA      ║
                 │    │            ╠═════════════════╬════════════════╣
                 │    │           0║       TAG       ║      DATA      ║
                 │    │            ╚═════════════════╩════════════════╝
                 │    │
                 │    │            ╔═════════════════╦════════════════╗
                 │    │           7║       TAG       ║      DATA      ║
                 │    │            ╠═════════════════╬════════════════╣
                 │    │                                             
                 │    └──                                           
                 │       SET 10                                     
                 │    ┌──          ╠═════════════════╬════════════════╣
                 │    │           1║       TAG       ║      DATA      ║
      │ D │      │    │            ╠═════════════════╬════════════════╣
      │ A │      │    │           0║       TAG       ║      DATA      ║
      │ T └──────┘    │            ╚═════════════════╩════════════════╝
      │ A             │
      │   ┌──────┐    │            ╔═════════════════╦════════════════╗
      │ B │      │    │           7║       TAG       ║      DATA      ║
      │ U │      │    │            ╠═════════════════╬════════════════╣
      │ S │      │    │                                             
                 │    └──                                           
                 │       SET 01                                     
                 │    ┌──          ╠═════════════════╬════════════════╣
                 │    │           1║       TAG       ║      DATA      ║
                 │    │            ╠═════════════════╬════════════════╣
                 │    │           0║       TAG       ║      DATA      ║
                 │    │            ╚═════════════════╩════════════════╝
                 │    │
                 │    │            ╔═════════════════╦════════════════╗
                 │    │           7║       TAG       ║      DATA      ║
                 │    │            ╠═════════════════╬════════════════╣
                 │    │                                             
                 │    └──                                           
                 │       SET 00                                     
                 └───────          ╠═════════════════╬════════════════╣
                                  1║       TAG       ║      DATA      ║
                                   ╠═════════════════╬════════════════╣
                                  0║       TAG       ║      DATA      ║
                                   ╚═════════════════╩════════════════╝


Figure 10-4.  Test Registers

      31                23              15   11      7             0
     ╔═════════════════╪═══════════════╪════╪═══════╪═════╤═╤═══╤═══╗
     ║                                      │             │H│   │   ║
     ║           PHYSICAL ADDRESS           │0 0 0 0 0 0 0│ │REP│0 0║ TR7
     ║                                      │             │T│   │   ║
     ╟──────────────────────────────────────┼─┬─┬─┬─┬─┬─┬─┼─┴───┴─┬─╢
     ║                                      │ │ │D│ │U│ │W│       │ ║
     ║            LINEAR ADDRESS            │V│D│ │U│ │ │ │0 0 0 0│C║ TR8
     ║                                      │ │ │#│ │#│ │#│       │ ║
     ╚═════════════════╪═══════════════╪════╪═╧═╧═╧═╪═╧═╧═╧═══════╧═╝

     NOTE: 0 INDICATES INTEL RESERVED. NO NOT DEFINE


10.6.3  Test Operations

To write a TLB entry:

  1.  Move a doubleword to TR7 that contains the desired physical address,
      HT, and REP values. HT must contain 1. REP must point to the
      associative block in which to place the entry.

  2.  Move a doubleword to TR6 that contains the appropriate linear
      address, and values for V, D, U, and W. Be sure C=0 for "write"
      command.

Be careful not to write duplicate tags; the results of doing so are
undefined.

To look up (read) a TLB entry:

  1.  Move a doubleword to TR6 that contains the appropriate linear address
      and attributes. Be sure C=1 for "lookup" command.

  2.  Store TR7. If the HT bit in TR7 indicates a hit, then the other
      values reveal the TLB contents. If HT indicates a miss, then the other
      values in TR7 are indeterminate.

For the purposes of testing, the V bit functions as another bit of
addresss.  The V bit for a lookup request should usually be set, so that
uninitialized tags do not match. Lookups with V=0 are unpredictable if any
tags are uninitialized.


Chapter 11  Coprocessing and Multiprocessing

────────────────────────────────────────────────────────────────────────────

The 80386 has two levels of support for multiple parallel processing units:

  ■  A highly specialized interface for very closely coupled processors of
     a type known as coprocessors.

  ■  A more general interface for more loosely coupled processors of
     unspecified type.


11.1  Coprocessing

The components of the coprocessor interface include:

  ■  ET bit of control register zero (CR0)
  ■  The EM, and MP bits of CR0
  ■  The ESC instructions
  ■  The WAIT instruction
  ■  The TS bit of CR0
  ■  Exceptions


11.1.1  Coprocessor Identification

The 80386 is designed to operate with either an 80287 or 80387 math
coprocessor. The ET bit of CR0 indicates which type of coprocessor is
present. ET is set automatically by the 80386 after RESET according to the
level detected on the ERROR# input. If desired, ET may also be set or reset
by loading CR0 with a MOV instruction. If ET is set, the 80386 uses the
32-bit protocol of the 80387; if reset, the 80386 uses the 16-bit protocol
of the 80287.


11.1.2  ESC and WAIT Instructions

The 80386 interprets the pattern 11011B in the first five bits of an
instruction as an opcode intended for a coprocessor. Instructions thus
marked are called ESCAPE or ESC instructions. The CPU performs the following
functions upon encountering an ESC instruction before sending the
instruction to the coprocessor:

  ■  Tests the emulation mode (EM) flag to determine whether coprocessor
     functions are being emulated by software.

  ■  Tests the TS flag to determine whether there has been a context change
     since the last ESC instruction.

  ■  For some ESC instructions, tests the ERROR# pin to determine whether
     the coprocessor detected an error in the previous ESC instruction.

The WAIT instruction is not an ESC instruction, but WAIT causes the CPU to
perform some of the same tests that it performs upon encountering an ESC
instruction. The processor performs the following actions for a WAIT
instruction:

  ■  Waits until the coprocessor no longer asserts the BUSY# pin.

  ■  Tests the ERROR# pin (after BUSY# goes inactive). If ERROR# is active,
     the 80386 signals exception 16, which indicates that the coprocessor
     encountered an error in the previous ESC instruction.

  ■  WAIT can therefore be used to cause exception 16 if an error is
     pending from a previous ESC instruction. Note that, if no coprocessor
     is present, the ERROR# and BUSY# pins should be tied inactive to
     prevent WAIT from waiting forever or causing spurious exceptions.


11.1.3  EM and MP Flags

The EM and MP flags of CR0 control how the processor reacts to coprocessor
instructions.

The EM bit indicates whether coprocessor functions are to be emulated. If
the processor finds EM set when executing an ESC instruction, it signals
exception 7, giving the exception handler an opportunity to emulate the ESC
instruction.

The MP (monitor coprocessor) bit indicates whether a coprocessor is
actually attached. The MP flag controls the function of the WAIT
instruction. If, when executing a WAIT instruction, the CPU finds MP set,
then it tests the TS flag; it does not otherwise test TS during a WAIT
instruction. If it finds TS set under these conditions, the CPU signals
exception 7.

The EM and MP flags can be changed with the aid of a MOV instruction using
CR0 as the destination operand and read with the aid of a MOV instruction
with CR0 as the source operand. These forms of the MOV instruction can be
executed only at privilege level zero.


11.1.4  The Task-Switched Flag

The TS bit of CR0 helps to determine when the context of the coprocessor
does not match that of the task being executed by the 80386 CPU. The 80386
sets TS each time it performs a task switch (whether triggered by software
or by hardware interrupt). If, when interpreting one of the ESC
instructions, the CPU finds TS already set, it causes exception 7. The WAIT
instruction also causes exception 7 if both TS and MP are set. Operating
systems can use this exception to switch the context of the coprocessor to
correspond to the current task. Refer to the 80386 System Software Writer's
Guide for an example.

The CLTS instruction (legal only at privilege level zero) resets the TS
flag.


11.1.5  Coprocessor Exceptions

Three exceptions aid in interfacing to a coprocessor: interrupt 7
(coprocessor not available), interrupt 9 (coprocessor segment overrun), and
interrupt 16 (coprocessor error).


11.1.5.1  Interrupt 7 ── Coprocessor Not Available

This exception occurs in either of two conditions:

  1.  The CPU encounters an ESC instruction and EM is set. In this case,
      the exception handler should emulate the instruction that caused the
      exception. TS may also be set.

  2.  The CPU encounters either the WAIT instruction or an ESC instruction
      when both MP and TS are set. In this case, the exception handler
      should update the state of the coprocessor, if necessary.


11.1.5.2  Interrupt 9 ── Coprocessor Segment Overrun

This exception occurs in protected mode under the following conditions:

  ■  An operand of a coprocessor instruction wraps around an addressing
     limit (0FFFFH for small segments, 0FFFFFFFFH for big segments, zero for
     expand-down segments). An operand may wrap around an addressing limit
     when the segment limit is near an addressing limit and the operand is
     near the largest valid address in the segment. Because of the
     wrap-around, the beginning and ending addresses of such an operand
     will be near opposite ends of the segment.

  ■  Both the first byte and the last byte of the operand (considering
     wrap-around) are at addresses located in the segment and in present and
     accessible pages.

  ■  The operand spans inaccessible addresses. There are two ways that such
     an operand may also span inaccessible addresses:

     1.  The segment limit is not equal to the addressing limit (e.g.,
         addressing limit is FFFFH and segment limit is FFFDH); therefore,
         the operand will span addresses that are not within the segment
         (e.g., an 8-byte operand that starts at valid offset FFFC will span
         addresses FFFC-FFFF and 0000-0003; however, addresses FFFE and FFFF
         are not valid, because they exceed the limit);

     2.  The operand begins and ends in present and accessible pages but
         intermediate bytes of the operand fall either in a not-present page
         or in a page to which the current procedure does not have access
         rights.

The address of the failing numerics instruction and data operand may be
lost; an FSTENV does not return reliable addresses. As with the 80286/80287,
the segment overrun exception should be handled by executing an FNINIT
instruction (i.e., an FINIT without a preceding WAIT). The return address on
the stack does not necessarily point to the failing instruction nor to the
following instruction. The failing numerics instruction is not restartable.

Case 2 can be avoided by either aligning all segments on page boundaries or
by not starting them within 108 bytes of the start or end of a page. (The
maximum size of a coprocessor operand is 108 bytes.) Case 1 can be avoided
by making sure that the gap between the last valid offset and the first
valid offset of a segment is either no less than 108 bytes or is zero (i.e.,
the segment is of full size). If neither software system design constraint
is acceptable, the exception handler should execute FNINIT and should
probably terminate the task.


11.1.5.3  Interrupt 16 ── Coprocessor Error

The numerics coprocessors can detect six different exception conditions
during instruction execution. If the detected exception is not masked by a
bit in the control word, the coprocessor communicates the fact that an error
occurred to the CPU by a signal at the ERROR# pin. The CPU causes interrupt
16 the next time it checks the ERROR# pin, which is only at the beginning of
a subsequent WAIT or certain ESC instructions. If the exception is masked,
the numerics coprocessor handles the exception according to on-board logic;
it does not assert the ERROR# pin in this case.


11.2  General Multiprocessing

The components of the general multiprocessing interface include:

  ■  The LOCK# signal

  ■  The LOCK instruction prefix, which gives programmed control of the
     LOCK# signal.

  ■  Automatic assertion of the LOCK# signal with implicit memory updates
     by the processor


11.2.1  LOCK and the LOCK# Signal

The LOCK instruction prefix and its corresponding output signal LOCK# can
be used to prevent other bus masters from interrupting a data movement
operation. LOCK may only be used with the following 80386 instructions when
they modify memory. An undefined-opcode exception results from using LOCK
before any instruction other than:

  ■  Bit test and change: BTS, BTR, BTC.
  ■  Exchange: XCHG.
  ■  Two-operand arithmetic and logical: ADD, ADC, SUB, SBB, AND, OR, XOR.
  ■  One-operand arithmetic and logical: INC, DEC, NOT, and NEG.

A locked instruction is only guaranteed to lock the area of memory defined
by the destination operand, but it may lock a larger memory area. For
example, typical 8086 and 80286 configurations lock the entire physical
memory space. The area of memory defined by the destination operand is
guaranteed to be locked against access by a processor executing a locked
instruction on exactly the same memory area, i.e., an operand with
identical starting address and identical length.

The integrity of the lock is not affected by the alignment of the memory
field. The LOCK signal is asserted for as many bus cycles as necessary to
update the entire operand.


11.2.2  Automatic Locking

In several instances, the processor itself initiates activity on the data
bus. To help ensure that such activities function correctly in
multiprocessor configurations, the processor automatically asserts the LOCK#
signal. These instances include:

  ■  Acknowledging interrupts.

     After an interrupt request, the interrupt controller uses the data bus
     to send the interrupt ID of the interrupt source to the CPU. The CPU
     asserts LOCK# to ensure that no other data appears on the data bus
     during this time.

  ■  Setting busy bit of TSS descriptor.

     The processor tests and sets the busy-bit in the type field of the TSS
     descriptor when switching to a task. To ensure that two different
     processors cannot simultaneously switch to the same task, the processor
     asserts LOCK# while testing and setting this bit.

  ■  Loading of descriptors.

     While copying the contents of a descriptor from a descriptor table into
     a segment register, the processor asserts LOCK# so that the descriptor
     cannot be modified by another processor while it is being loaded. For
     this action to be effective, operating-system procedures that update
     descriptors should adhere to the following steps:

     ──  Use a locked update to the access-rights byte to mark the
         descriptor not-present.

     ──  Update the fields of the descriptor.  (This may require several
         memory accesses; therefore, LOCK cannot be used.)

     ──  Use a locked update to the access-rights byte to mark the
         descriptor present again.

  ■  Updating page-table A and D bits.

     The processor exerts LOCK# while updating the A (accessed) and D
     (dirty) bits of page-table entries.  Also the processor bypasses the
     page-table cache and directly updates these bits in memory.

  ■  Executing XCHG instruction.

     The 80386 always asserts LOCK during an XCHG instruction that
     references memory (even if the LOCK prefix is not used).


11.2.3  Cache Considerations

Systems programmers must take care when updating shared data that may also
be stored in on-chip registers and caches.  With the 80386, such  shared
data includes:

  ■  Descriptors, which may be held in segment registers.

     A change to a descriptor that is shared among processors should be
     broadcast to all processors.  Segment registers are effectively
     "descriptor caches".  A change to a descriptor will not be utilized by
     another processor if that processor already has a copy of the old
     version of the descriptor in a segment register.

  ■  Page tables, which may be held in the page-table cache.

     A change to a page table that is shared among processors should be
     broadcast to all processors, so that others can flush their page-table
     caches and reload them with up-to-date page tables from memory.

Systems designers can employ an interprocessor interrupt to handle the
above cases. When one processor changes data that may be cached by other
processors, it can send an interrupt signal to all other processors that may
be affected by the change. If the interrupt is serviced by an interrupt
task, the task switch automatically flushes the segment registers. The task
switch also flushes the page-table cache if the PDBR (the contents of CR3)
of the interrupt task is different from the PDBR of every other task.

In multiprocessor systems that need a cacheability signal from the CPU, it
is recommended that physical address pin A31 be used to indicate
cacheability. Such a system can then possess up to 2 Gbytes of physical
memory. The virtual address range available to the programmer is not
affected by this convention.


Chapter 12  Debugging

────────────────────────────────────────────────────────────────────────────

The 80386 brings to Intel's line of microprocessors significant advances in
debugging power. The single-step exception and breakpoint exception of
previous processors are still available in the 80386, but the principal
debugging support takes the form of debug registers. The debug registers
support both instruction breakpoints and data breakpoints. Data breakpoints
are an important innovation that can save hours of debugging time by
pinpointing, for example, exactly when a data structure is being
overwritten. The breakpoint registers also eliminate the complexities
associated with writing a breakpoint instruction into a code segment
(requires a data-segment alias for the code segment) or a code segment
shared by multiple tasks (the breakpoint exception can occur in the context
of any of the tasks). Breakpoints can even be set in code contained in ROM.


12.1  Debugging Features of the Architecture

The features of the 80386 architecture that support debugging include:

Reserved debug interrupt vector

Permits processor to automatically invoke a debugger task or procedure when
an event occurs that is of interest to the debugger.

Four debug address registers

Permit programmers to specify up to four addresses that the CPU will
automatically monitor.

Debug control register

Allows programmers to selectively enable various debug conditions
associated with the four debug addresses.

Debug status register

Helps debugger identify condition that caused debug exception.

Trap bit of TSS (T-bit)

Permits monitoring of task switches.

Resume flag (RF) of flags register

Allows an instruction to be restarted after a debug exception without
immediately causing another debug exception due to the same condition.

Single-step flag (TF)

Allows complete monitoring of program flow by specifying whether the CPU
should cause a debug exception with the execution of every instruction.

Breakpoint instruction

Permits debugger intervention at any point in program execution and aids
debugging of debugger programs.

Reserved interrupt vector for breakpoint exception

Permits processor to automatically invoke a handler task or procedure upon
encountering a breakpoint instruction.

These features make it possible to invoke a debugger that is either a
separate task or a procedure in the context of the current task. The
debugger can be invoked under any of the following kinds of conditions:

  ■  Task switch to a specific task.
  ■  Execution of the breakpoint instruction.
  ■  Execution of every instruction.
  ■  Execution of any instruction at a given address.
  ■  Read or write of a byte, word, or doubleword at any specified address.
  ■  Write to a byte, word, or doubleword at any specified address.
  ■  Attempt to change a debug register.


12.2  Debug Registers

Six 80386 registers are used to control debug features. These registers are
accessed by variants of the MOV instruction. A debug register may be either
the source operand or destination operand. The debug registers are
privileged resources; the MOV instructions that access them can only be
executed at privilege level zero. An attempt to read or write the debug
registers when executing at any other privilege level causes a general
protection exception. Figure 12-1 shows the format of the debug registers.


Figure 12-1.  Debug Registers

      31              23              15              7             0
     ╔═══╤═══╤═══╤═══╪═══╤═══╤═══╤═══╪═══╤═╤═════╤═╤═╪═╤═╤═╤═╤═╤═╤═╤═╗
     ║LEN│R/W│LEN│R/W│LEN│R/W│LEN│R/W│   │ │     │G│L│G│L│G│L│G│L│G│L║
     ║   │   │   │   │   │   │   │   │0 0│0│0 0 0│ │ │ │ │ │ │ │ │ │ ║ DR7
     ║ 3 │ 3 │ 2 │ 2 │ 1 │ 1 │ 0 │ 0 │   │ │     │E│E│3│3│2│2│1│1│0│0║
     ╟───┴───┴───┴───┴───┴───┴───┴───┼─┬─┼─┼─────┴─┴─┴─┴─┴─┴─┼─┼─┼─┼─╢
     ║                               │B│B│B│                 │B│B│B│B║
     ║0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0│ │ │ │0 0 0 0 0 0 0 0 0│ │ │ │ ║ DR6
     ║                               │T│S│D│                 │3│2│1│0║
     ╟───────────────┼───────────────┼─┴─┴─┴─────────┼───────┴─┴─┴─┴─╢
     ║                                                               ║
     ║                            RESERVED                           ║ DR5
     ║                                                               ║
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                                                               ║
     ║                            RESERVED                           ║ DR4
     ║                                                               ║
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                                                               ║
     ║                 BREAKPOINT 3 LINEAR ADDRESS                   ║ DR3
     ║                                                               ║
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                                                               ║
     ║                 BREAKPOINT 2 LINEAR ADDRESS                   ║ DR2
     ║                                                               ║
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                                                               ║
     ║                 BREAKPOINT 1 LINEAR ADDRESS                   ║ DR1
     ║                                                               ║
     ╟───────────────┼───────────────┼───────────────┼───────────────╢
     ║                                                               ║
     ║                 BREAKPOINT 0 LINEAR ADDRESS                   ║ DR0
     ║                                                               ║
     ╚═══════════════╪═══════════════╪════════════════╪══════════════╝

────────────────────────────────────────────────────────────────────────────
NOTE
      0 MEANS INTEL RESERVED. DO NOT DEFINE.
────────────────────────────────────────────────────────────────────────────


12.2.1  Debug Address Registers (DR0-DR3)

Each of these registers contains the linear address associated with one of
four breakpoint conditions. Each breakpoint condition is further defined by
bits in DR7.

The debug address registers are effective whether or not paging is enabled.
The addresses in these registers are linear addresses. If paging is enabled,
the linear addresses are translated into physical addresses by the
processor's paging mechanism (as explained in Chapter 5). If paging is not
enabled, these linear addresses are the same as physical addresses.

Note that when paging is enabled, different tasks may have different
linear-to-physical address mappings. When this is the case, an address in a
debug address register may be relevant to one task but not to another. For
this reason the 80386  has both global and local enable bits in DR7. These
bits indicate whether a given debug address has a global (all tasks) or
local (current task only) relevance.


12.2.2  Debug Control Register (DR7)

The debug control register shown in Figure 12-1 both helps to define the
debug conditions and selectively enables and disables those conditions.

For each address in registers DR0-DR3, the corresponding fields R/W0
through R/W3 specify the type of action that should cause a breakpoint. The
processor interprets these bits as follows:

   00 ── Break on instruction execution only
   01 ── Break on data writes only
   10 ── undefined
   11 ── Break on data reads or writes but not instruction fetches

Fields LEN0 through LEN3 specify the length of data item to be monitored. A
length of 1, 2, or 4 bytes may be specified. The values of the length fields
are interpreted as follows:

   00 ── one-byte length
   01 ── two-byte length
   10 ── undefined
   11 ── four-byte length

If RWn is 00 (instruction execution), then LENn should also be 00. Any other
length is undefined.

The low-order eight bits of DR7 (L0 through L3 and G0 through G3)
selectively enable the four address breakpoint conditions. There are two
levels of enabling: the local (L0 through L3) and global (G0 through G3)
levels. The local enable bits are automatically reset by the processor at
every task switch to avoid unwanted breakpoint conditions in the new task.
The global enable bits are not reset by a task switch; therefore, they can
be used for conditions that are global to all tasks.

The LE and GE bits control the "exact data breakpoint match" feature of the
processor. If either LE or GE is set, the processor slows execution so that
data breakpoints are reported on the instruction that causes them. It is
recommended that one of these bits be set whenever data breakpoints are
armed. The processor clears LE at a task switch but does not clear GE.


12.2.3  Debug Status Register (DR6)

The debug status register shown in Figure 12-1 permits the debugger to
determine which debug conditions have occurred.

When the processor detects an enabled debug exception, it sets the
low-order bits of this register (B0 thru B3) before entering the debug
exception handler. Bn is set if the condition described by DRn, LENn, and
R/Wn occurs. (Note that the processor sets Bn regardless of whether Gn or
Ln is set. If more than one breakpoint condition occurs at one time and if
the breakpoint trap occurs due to an enabled condition other than n, Bn may
be set, even though neither Gn nor Ln is set.)

The BT bit is associated with the T-bit (debug trap bit) of the TSS (refer
to 7 for the location of the T-bit). The processor sets the BT bit before
entering the debug handler if a task switch has occurred and the T-bit of
the new TSS is set. There is no corresponding bit in DR7 that enables and
disables this trap; the T-bit of the TSS is the sole enabling bit.

The BS bit is associated with the TF (trap flag) bit of the EFLAGS
register. The BS bit is set if the debug handler is entered due to the
occurrence of a single-step exception. The single-step trap is the
highest-priority debug exception; therefore, when BS is set, any of the
other debug status bits may also be set.

The BD bit is set if the next instruction will read or write one of the
eight debug registers and ICE-386 is also using the debug registers at the
same time.

Note that the bits of DR6 are never cleared by the processor. To avoid any
confusion in identifying the next debug exception, the debug handler should
move zeros to DR6 immediately before returning.


12.2.4  Breakpoint Field Recognition

The linear address and LEN field for each of the four breakpoint conditions
define a range of sequential byte addresses for a data breakpoint. The LEN
field permits specification of a one-, two-, or four-byte field. Two-byte
fields must be aligned on word boundaries (addresses that are multiples of
two) and four-byte fields must be aligned on doubleword boundaries
(addresses that are multiples of four). These requirements are enforced by
the processor; it uses the LEN bits to mask the low-order bits of the
addresses in the debug address registers. Improperly aligned code or data
breakpoint addresses will not yield the expected results.

A data read or write breakpoint is triggered if any of the bytes
participating in a memory access is within the field defined by a breakpoint
address register and the corresponding LEN field. Table 12-1 gives some
examples of breakpoint fields with memory references that both do and do not
cause traps.

To set a data breakpoint for a misaligned field longer than one byte, it
may be desirable to put two sets of entries in the breakpoint register such
that each entry is properly aligned and the two entries together span the
length of the field.

Instruction breakpoint addresses must have a length specification of one
byte (LEN = 00); other values are undefined. The processor recognizes an
instruction breakpoint address only when it points to the first byte of an
instruction. If the instruction has any prefixes, the breakpoint address
must point to the first prefix.


Table 12-1. Breakpoint Field Recognition Examples

                                    Address (hex)          Length

                       DR0             0A0001          1 (LEN0 = 00)
Register Contents      DR1             0A0002          1 (LEN1 = 00)
                       DR2             0B0002          2 (LEN2 = 01)
                       DR3             0C0000          4 (LEN3 = 11)

Some Examples of Memory                0A0001          1
References That Cause Traps            0A0002          1
                                       0A0001          2
                                       0A0002          2
                                       0B0002          2
                                       0B0001          4
                                       0C0000          4
                                       0C0001          2
                                       0C0003          1

Some Examples of Memory                0A0000          1
References That Don't Cause Traps      0A0003          4
                                       0B0000          2
                                       0C0004          4


12.3  Debug Exceptions

Two of the interrupt vectors of the 80386 are reserved for exceptions that
relate to debugging. Interrupt 1 is the primary means of invoking debuggers
designed expressly for the 80386; interrupt 3 is intended for debugging
debuggers and for compatibility with prior processors in Intel's 8086
processor family.


12.3.1  Interrupt 1 ── Debug Exceptions

The handler for this exception is usually a debugger or part of a debugging
system. The processor causes interrupt 1 for any of several conditions. The
debugger can check flags in DR6 and DR7 to determine what condition caused
the exception and what other conditions might be in effect at the same time.
Table 12-2 associates with each breakpoint condition the combination of
bits that indicate when that condition has caused the debug exception.

Instruction address breakpoint conditions are faults, while other debug
conditions are traps. The debug exception may report either or both at one
time. The following paragraphs present details for each class of debug
exception.


Table 12-2. Debug Exception Conditions

Flags to Test              Condition

BS=1                       Single-step trap
B0=1 AND (GE0=1 OR LE0=1)  Breakpoint DR0, LEN0, R/W0
B1=1 AND (GE1=1 OR LE1=1)  Breakpoint DR1, LEN1, R/W1
B2=1 AND (GE2=1 OR LE2=1)  Breakpoint DR2, LEN2, R/W2
B3=1 AND (GE3=1 OR LE3=1)  Breakpoint DR3, LEN3, R/W3
BD=1                       Debug registers not available; in use by ICE-386.
BT=1                       Task switch


12.3.1.1  Instruction Addrees Breakpoint

The processor reports an instruction-address breakpoint before it executes
the instruction that begins at the given address; i.e., an instruction-
address breakpoint exception is a fault.

The RF (restart flag) permits the debug handler to retry instructions that
cause other kinds of faults in addition to debug faults. When it detects a
fault, the processor automatically sets RF in the flags image that it pushes
onto the stack. (It does not, however, set RF for traps and aborts.)

When RF is set, it causes any debug fault to be ignored during the next
instruction. (Note, however, that RF does not cause breakpoint traps to be
ignored, nor other kinds of faults.)

The processor automatically clears RF at the successful completion of every
instruction except after the IRET instruction, after the POPF instruction,
and after a JMP, CALL, or INT instruction that causes a task switch. These
instructions set RF to the value specified by the memory image of the EFLAGS
register.

The processor automatically sets RF in the EFLAGS image on the stack before
entry into any fault handler. Upon entry into the fault handler for
instruction address breakpoints, for example, RF is set in the EFLAGS image
on the stack; therefore, the IRET instruction at the end of the handler will
set RF in the EFLAGS register, and execution will resume at the breakpoint
address without generating another breakpoint fault at the same address.

If, after a debug fault, RF is set and the debug handler retries the
faulting instruction, it is possible that retrying the instruction will
raise other faults. The retry of the instruction after these faults will
also be done with RF=1, with the result that debug faults continue to be
ignored. The processor clears RF only after successful completion of the
instruction.

Real-mode debuggers can control the RF flag by using a 32-bit IRET. A
16-bit IRET instruction does not affect the RF bit (which is in the
high-order 16 bits of EFLAGS). To use a 32-bit IRET, the debugger must
rearrange the stack so that it holds appropriate values for the 32-bit EIP,
CS, and EFLAGS (with RF set in the EFLAGS image). Then executing an IRET
with an operand size prefix causes a 32-bit return, popping the RF flag
into EFLAGS.


12.3.1.2  Data Address Breakpoint

A data-address breakpoint exception is a trap; i.e., the processor reports
a data-address breakpoint after executing the instruction that accesses the
given memory item.

When using data breakpoints it is recommended that either the LE or GE bit
of DR7 be set also. If either LE or GE is set, any data breakpoint trap is
reported exactly after completion of the instruction that accessed the
specified memory item. This exact reporting is accomplished by forcing the
80386 execution unit to wait for completion of data operand transfers before
beginning execution of the next instruction. If neither GE nor LE is set,
data breakpoints may not be reported until one instruction after the data is
accessed or may not be reported at all. This is due to the fact that,
normally, instruction execution is overlapped with memory transfers to such
a degree that execution of the next instruction may begin before memory
transfers for the prior instruction are completed.

If a debugger needs to preserve the contents of a write breakpoint
location, it should save the original contents before setting a write
breakpoint. Because data breakpoints are traps, a write into a breakpoint
location will complete before the trap condition is reported. The handler
can report the saved value after the breakpoint is triggered. The data in
the debug registers can be used to address the new value stored by the
instruction that triggered the breakpoint.


12.3.1.3  General Detect Fault

This exception occurs when an attempt is made to use the debug registers at
the same time that ICE-386 is using them. This additional protection feature
is provided to guarantee that ICE-386 can have full control over the
debug-register resources when required. ICE-386 uses the debug-registers;
therefore, a software debugger that also uses these registers cannot run
while ICE-386 is in use. The exception handler can detect this condition by
examining the BD bit of DR6.


12.3.1.4  Single-Step Trap

This debug condition occurs at the end of an instruction if the trap flag
(TF) of the flags register held the value one at the beginning of that
instruction.  Note that the exception does not occur at the end of an
instruction that sets TF. For example, if POPF is used to set TF, a
single-step trap does not occur until after the instruction that follows
POPF.

The processor clears the TF bit before invoking the handler.  If TF=1 in
the flags image of a TSS at the time of a task switch, the exception occurs
after the first instruction is executed in the new task.

The single-step flag is normally not cleared by privilege changes inside a
task.  INT instructions, however, do clear TF.  Therefore, software
debuggers that single-step code must recognize and emulate INT n or INTO
rather than executing them directly.

To maintain protection, system software should check the current execution
privilege level after any single step interrupt to see whether single
stepping should continue at the current privilege level.

The interrupt priorities in hardware guarantee that if an external
interrupt occurs, single stepping stops. When both an external interrupt and
a single step interrupt occur together, the single step interrupt is
processed first. This clears the TF bit. After saving the return address or
switching tasks, the external interrupt input is examined before the first
instruction of the single step handler executes.  If the external interrupt
is still pending, it is then serviced. The external interrupt handler is not
single-stepped. To single step an interrupt handler, just single step an INT
n instruction that refers to the interrupt handler.


12.3.1.5  Task Switch Breakpoint

The debug exception also occurs after a switch to an 80386 task if the
T-bit of the new TSS is set.  The exception occurs after control has passed
to the new task, but before the first instruction of that task is executed.
The exception handler can detect this condition by examining the BT bit of
the debug status register DR6.

Note that if the debug exception handler is a task, the T-bit of its TSS
should not be set. Failure to observe this rule will cause the processor to
enter an infinite loop.


12.3.2  Interrupt 3 ── Breakpoint Exception

This exception is caused by execution of the breakpoint instruction INT 3.
Typically, a debugger prepares a breakpoint by substituting the opcode of
the one-byte breakpoint instruction in place of the first opcode byte of the
instruction to be trapped. When execution of the INT 3 instruction causes
the exception handler to be invoked, the saved value of ES:EIP points to the
byte following the INT 3 instruction.

With prior generations of processors, this feature is used extensively for
trapping execution of specific instructions. With the 80386, the needs
formerly filled by this feature are more conveniently solved via the debug
registers and interrupt 1.  However, the breakpoint exception is still
useful for debugging debuggers, because the breakpoint exception can vector
to a different exception handler than that used by the debugger. The
breakpoint exception can also be useful when it is necessary to set a
greater number of breakpoints than permitted by the debug registers.


                          PART III  COMPATIBILITY


Chapter 13  Executing 80286 Protected-Mode Code

────────────────────────────────────────────────────────────────────────────

13.1  80286 Code Executes as a Subset of the 80386

In general, programs designed for execution in protected mode on an 80286
execute without modification on the 80386, because the features of the 80286
are a subset of those of the 80386.

All the descriptors used by the 80286 are supported by the 80386 as long as
the Intel-reserved word (last word) of the 80286 descriptor is zero.

The descriptors for data segments, executable segments, local descriptor
tables, and task gates are common to both the 80286 and the 80386. Other
80286 descriptors──TSS segment, call gate, interrupt gate, and trap
gate──are supported by the 80386. The 80386 also has new versions of
descriptors for TSS segment, call gate, interrupt gate, and trap gate that
support the 32-bit nature of the 80386. Both sets of descriptors can be
used simultaneously in the same system.

For those descriptors that are common to both the 80286 and the 80386, the
presence of zeros in the final word causes the 80386 to interpret these
descriptors exactly as 80286 does; for example:

Base Address      The high-order eight bits of the 32-bit base address are
                  zero, limiting base addresses to 24 bits.

Limit             The high-order four bits of the limit field are zero,
                  restricting the value of the limit field to 64K.

Granularity bit   The granularity bit is zero, which implies that the value
                  of the 16-bit limit is interpreted in units of one byte.

B-bit             In a data-segment descriptor, the B-bit is zero, implying
                  that the segment is no larger than 64 Kbytes.

D-bit             In an executable-segment descriptor, the D-bit is zero,
                  implying that 16-bit addressing and operands are the
                  default.

For formats of these descriptors and documentation of their use refer to
the iAPX 286 Programmer's Reference Manual.


13.2  Two ways to Execute 80286 Tasks

When porting 80286 programs to the 80386, there are two cases to consider:

  1.  Porting an entire 80286 system to the 80386, complete with 80286
      operating system, loader, and system builder.

      In this case, all tasks will have 80286 TSSs. The 80386 is being used
      as a faster 286.

  2.  Porting selected 80286 applications to run in an 80386 environment
      with an 80386 operating system, loader, and system builder.

      In this case, the TSSs used to represent 80286 tasks should be
      changed to 80386 TSSs. It is theoretically possible to mix 80286 and
      80386 TSSs, but the benefits are slight and the problems are great. It
      is recommended that all tasks in a 80386 software system have 80386
      TSSs. It is not necessary to change the 80286 object modules
      themselves; TSSs are usually constructed by the operating system, by
      the loader, or by the system builder. Refer to Chapter 16 for further
      discussion of the interface between 16-bit and 32-bit code.


13.3  Differences From 80286

The few differences that do exist primarily affect operating system code.


13.3.1  Wraparound of 80286 24-Bit Physical Address Space

With the 80286, any base and offset combination that addresses beyond 16M
bytes wraps around to the first megabyte of the 80286 address space. With
the 80386, since it has a greater physical address space, any such address
falls into the 17th megabyte. In the unlikely event that any software
depends on this anomaly, the same effect can be simulated on the 80386 by
using paging to map the first 64K bytes of the 17th megabyte of logical
addresses to physical addresses in the first megabyte.


13.3.2  Reserved Word of Descriptor

Because the 80386 uses the contents of the reserved word (last word) of
every descriptor, 80286 programs that place values in this word may not
execute correctly on the 80386.


13.3.3  New Descriptor Type Codes

Operating-system code that manages space in descriptor tables often uses an
invalid value in the access-rights field of descriptor-table entries to
identify unused entries. Access rights values of 80H and 00H remain invalid
for both the 80286 and 80386. Other values that were invalid on for the
80286 may be valid for the 80386 because of the additional descriptor types
defined by the 80386.


13.3.4  Restricted Semantics of LOCK

The 80286 processor implements the bus lock function differently than the
80386. Programs that use forms of memory locking specific to the 80286 may
not execute properly when transported to a specific application of the
80386.

The LOCK prefix and its corresponding output signal should only be used to
prevent other bus masters from interrupting a data movement operation.  LOCK
may only be used with the following 80386 instructions when they modify
memory. An undefined-opcode exception results from using LOCK before any
other instruction.

  ■  Bit test and change:  BTS, BTR, BTC.
  ■  Exchange: XCHG.
  ■  One-operand arithmetic and logical: INC, DEC, NOT, and NEG.
  ■  Two-operand arithmetic and logical:  ADD, ADC, SUB, SBB, AND, OR, XOR.

A locked instruction is guaranteed to lock only the area of memory defined
by the destination operand, but may lock a larger memory area.  For example,
typical 8086 and 80286 configurations lock the entire physical memory space.
With the 80386, the defined area of memory is guaranteed to be locked
against access by a processor executing a locked instruction on exactly the
same memory area, i.e., an operand with identical starting address and
identical length.


13.3.5  Additional Exceptions

The 80386 defines new exceptions that can occur even in systems designed
for the 80286.

  ■  Exception #6 ── invalid opcode

     This exception can result from improper use of the LOCK instruction.

  ■  Exception #14 ── page fault

     This exception may occur in an 80286 program if the operating system
     enables paging. Paging can be used in a system with 80286 tasks as long
     as all tasks use the same page directory. Because there is no place in
     an 80286 TSS to store the PDBR, switching to an 80286 task does not
     change the value of PDBR. Tasks ported from the 80286 should be given
     80386 TSSs so they can take full advantage of paging.


Chapter 14  80386 Real-Address Mode

────────────────────────────────────────────────────────────────────────────

The real-address mode of the 80386 executes object code designed for
execution on 8086, 8088, 80186, or 80188 processors, or for execution in the
real-address mode of an 80286:

In effect, the architecture of the 80386 in this mode is almost identical
to that of the 8086, 8088, 80186, and 80188. To a programmer, an 80386 in
real-address mode appears as a high-speed 8086 with extensions to the
instruction set and registers. The principal features of this architecture
are defined in Chapters 2 and 3.

This chapter discusses certain additional topics that complete the system
programmer's view of the 80386 in real-address mode:

  ■  Address formation.
  ■  Extensions to registers and instructions.
  ■  Interrupt and exception handling.
  ■  Entering and leaving real-address mode.
  ■  Real-address-mode exceptions.
  ■  Differences from 8086.
  ■  Differences from 80286 real-address mode.


14.1  Physical Address Formation

The 80386 provides a one Mbyte + 64 Kbyte memory space for an 8086 program.
Segment relocation is performed as in the 8086: the 16-bit value in a
segment selector is shifted left by four bits to form the base address of a
segment. The effective address is extended with four high order zeros and
added to the base to form a linear address as Figure 14-1 illustrates. (The
linear address is equivalent to the physical address, because paging is not
used in real-address mode.) Unlike the 8086, the resulting linear address
may have up to 21 significant bits. There is a possibility of a carry when
the base address is added to the effective address. On the 8086, the carried
bit is truncated, whereas on the 80386 the carried bit is stored in bit
position 20 of the linear address.

Unlike the 8086 and 80286, 32-bit effective addresses can be generated (via
the address-size prefix); however, the value of a 32-bit address may not
exceed 65535 without causing an exception. For full compatibility with 80286
real-address mode, pseudo-protection faults (interrupt 12 or 13 with no
error code) occur if an effective address is generated outside the range 0
through 65535.


Figure 14-1.  Real-Address Mode Address Formation

                      19                                3       0
                     ╔═════════════════════════════════╪═════════╗
         BASE        ║     16-BIT SEGMENT SELECTOR     │ 0 0 0 0 ║
                     ╚═════════════════════════════════╪═════════╝

         +
                      19        15                              0
                     ╔═════════╪═════════════════════════════════╗
         OFFSET      ║ 0 0 0 0 │    16-BIT EFFECTIVE ADDRESS     ║
                     ╚═════════╪═════════════════════════════════╝

         =
                    20                                          0
         LINEAR    ╔═════════════════════════════════════════════╗
         ADDRESS   ║ X X X X X X X X X X X X X X X X X X X X X X ║
                   ╚═════════════════════════════════════════════╝


14.2  Registers and Instructions

The register set available in real-address mode includes all the registers
defined for the 8086 plus the new registers introduced by the 80386: FS, GS,
debug registers, control registers, and test registers. New instructions
that explicitly operate on the segment registers FS and GS are available,
and the new segment-override prefixes can be used to cause instructions to
utilize FS and GS for address calculations. Instructions can utilize 32-bit
operands through the use of the operand size prefix.

The instruction codes that cause undefined opcode traps (interrupt 6)
include instructions of the protected mode that manipulate or interrogate
80386 selectors and descriptors; namely, VERR, VERW, LAR, LSL, LTR, STR,
LLDT, and SLDT. Programs executing in real-address mode are able to take
advantage of the new applications-oriented instructions added to the
architecture by the introduction of the 80186/80188, 80286 and 80386:

■ New instructions introduced by 80186/80188 and 80286.

   ── PUSH immediate data
   ── Push all and pop all (PUSHA and POPA)
   ── Multiply immediate data
   ── Shift and rotate by immediate count
   ── String I/O
   ── ENTER and LEAVE
   ── BOUND

■ New instructions introduced by 80386.

   ── LSS, LFS, LGS instructions
   ── Long-displacement conditional jumps
   ── Single-bit instructions
   ── Bit scan
   ── Double-shift instructions
   ── Byte set on condition
   ── Move with sign/zero extension
   ── Generalized multiply
   ── MOV to and from control registers
   ── MOV to and from test registers
   ── MOV to and from debug registers


14.3  Interrupt and Exception Handling

Interrupts and exceptions in 80386 real-address mode work as much as they
do on an 8086. Interrupts and exceptions vector to interrupt procedures via
an interrupt table. The processor multiplies the interrupt or exception
identifier by four to obtain an index into the interrupt table. The entries
of the interrupt table are far pointers to the entry points of interrupt or
exception handler procedures. When an interrupt occurs, the processor
pushes the current values of CS:IP onto the stack, disables interrupts,
clears TF (the single-step flag), then transfers control to the location
specified in the interrupt table. An IRET instruction at the end of the
handler procedure reverses these steps before returning control to the
interrupted procedure.

The primary difference in the interrupt handling of the 80386 compared to
the 8086 is that the location and size of the interrupt table depend on the
contents of the IDTR (IDT register). Ordinarily, this fact is not apparent
to programmers, because, after RESET, the IDTR contains a base address of 0
and a limit of 3FFH, which is compatible with the 8086. However, the LIDT
instruction can be used in real-address mode to change the base and limit
values in the IDTR. Refer to Chapter 9 for details on the IDTR, and the
LIDT and SIDT instructions. If an interrupt occurs and the corresponding
entry of the interrupt table is beyond the limit stored in the IDTR, the
processor raises exception 8.


14.4  Entering and Leaving Real-Address Mode

Real-address mode is in effect after a signal on the RESET pin. Even if the
system is going to be used in protected mode, the start-up program will
execute in real-address mode temporarily while initializing for protected
mode.


14.4.1  Switching to Protected Mode

The only way to leave real-address mode is to switch to protected mode. The
processor enters protected mode when a MOV to CR0 instruction sets the PE
(protection enable) bit in CR0. (For compatibility with the 80286, the LMSW
instruction may also be used to set the PE bit.)

Refer to Chapter 10 "Initialization" for other aspects of switching to
protected mode.


14.5  Switching Back to Real-Address Mode

The processor reenters real-address mode if software clears the PE bit in
CR0 with a MOV to CR0 instruction. A procedure that attempts to do this,
however, should proceed as follows:

  1.  If paging is enabled, perform the following sequence:

      ■  Transfer control to linear addresses that have an identity mapping;
         i.e., linear addresses equal physical addresses.

      ■  Clear the PG bit in CR0.

      ■  Move zeros to CR3 to clear out the paging cache.

  2.  Transfer control to a segment that has a limit of 64K (FFFFH). This
      loads the CS register with the limit it needs to have in real mode.

  3.  Load segment registers SS, DS, ES, FS, and GS with a selector that
      points to a descriptor containing the following values, which are
      appropriate to real mode:

      ■  Limit = 64K   (FFFFH)
      ■  Byte granular (G = 0)
      ■  Expand up     (E = 0)
      ■  Writable      (W = 1)
      ■  Present       (P = 1)
      ■  Base = any value

  4.  Disable interrupts. A CLI instruction disables INTR interrupts. NMIs
      can be disabled with external circuitry.

  5.  Clear the PE bit.

  6.  Jump to the real mode code to be executed using a far JMP. This
      action flushes the instruction queue and puts appropriate values in
      the access rights of the CS register.

  7.  Use the LIDT instruction to load the base and limit of the real-mode
      interrupt vector table.

  8.  Enable interrupts.

  9.  Load the segment registers as needed by the real-mode code.


14.6  Real-Address Mode Exceptions

The 80386 reports some exceptions differently when executing in
real-address mode than when executing in protected mode. Table 14-1 details
the real-address-mode exceptions.


14.7  Differences From 8086

In general, the 80386 in real-address mode will correctly execute ROM-based
software designed for the 8086, 8088, 80186, and 80188. Following is a list
of the minor differences between 8086 execution on the 80386 and on an 8086.

  1.  Instruction clock counts.

      The 80386 takes fewer clocks for most instructions than the 8086/8088.
      The areas most likely to be affected are:

      ■  Delays required by I/O devices between I/O operations.

      ■  Assumed delays with 8086/8088 operating in parallel with an 8087.

  2.  Divide Exceptions Point to the DIV instruction.

      Divide exceptions on the 80386 always leave the saved CS:IP value
      pointing to the instruction that failed. On the 8086/8088, the CS:IP
      value points to the next instruction.

  3.  Undefined 8086/8088 opcodes.

      Opcodes that were not defined for the 8086/8088 will cause exception
      6 or will execute one of the new instructions defined for the 80386.

  4.  Value written by PUSH SP.

      The 80386 pushes a different value on the stack for PUSH SP than the
      8086/8088. The 80386 pushes the value of SP before SP is incremented
      as part of the push operation; the 8086/8088 pushes the value of SP
      after it is incremented. If the value pushed is important, replace
      PUSH SP instructions with the following three instructions:

      PUSH  BP
      MOV   BP, SP
      XCHG  BP, [BP]

      This code functions as the 8086/8088 PUSH SP instruction on the 80386.

  5.  Shift or rotate by more than 31 bits.

      The 80386 masks all shift and rotate counts to the low-order five
      bits. This MOD 32 operation limits the count to a maximum of 31 bits,
      thereby limiting the time that interrupt response is delayed while
      the instruction is executing.

  6.  Redundant prefixes.

      The 80386 sets a limit of 15 bytes on instruction length. The only
      way to violate this limit is by putting redundant prefixes before an
      instruction. Exception 13 occurs if the limit on instruction length
      is violated. The 8086/8088 has no instruction length limit.

  7.  Operand crossing offset 0 or 65,535.

      On the 8086, an attempt to access a memory operand that crosses
      offset 65,535 (e.g., MOV a word to offset 65,535) or offset 0 (e.g.,
      PUSH a word when SP = 1) causes the offset to wrap around modulo
      65,536. The 80386 raises an exception in these cases──exception 13 if
      the segment is a data segment (i.e., if CS, DS, ES, FS, or GS is being
      used to address the segment), exception 12 if the segment is a stack
      segment (i.e., if SS is being used).

  8.  Sequential execution across offset 65,535.

      On the 8086, if sequential execution of instructions proceeds past
      offset 65,535, the processor fetches the next instruction byte from
      offset 0 of the same segment. On the 80386, the processor raises
      exception 13 in such a case.

  9.  LOCK is restricted to certain instructions.

      The LOCK prefix and its corresponding output signal should only be
      used to prevent other bus masters from interrupting a data movement
      operation. The 80386 always asserts the LOCK signal during an XCHG
      instruction with memory (even if the LOCK prefix is not used). LOCK
      may only be used with the following 80386 instructions when they
      update memory: BTS, BTR, BTC, XCHG, ADD, ADC, SUB, SBB, INC, DEC,
      AND, OR, XOR, NOT, and NEG. An undefined-opcode exception
      (interrupt 6) results from using LOCK before any other instruction.

 10.  Single-stepping external interrupt handlers.

      The priority of the 80386 single-step exception is different from that
      of the 8086/8088. The change prevents an external interrupt handler
      from being single-stepped if the interrupt occurs while a program is
      being single-stepped. The 80386 single-step exception has higher
      priority that any external interrupt. The 80386 will still single-step
      through an interrupt handler invoked by the INT instructions or by an
      exception.

 11.  IDIV exceptions for quotients of 80H or 8000H.

      The 80386 can generate the largest negative number as a quotient for
      the IDIV instruction. The 8086/8088 causes exception zero instead.

 12.  Flags in stack.

      The setting of the flags stored by PUSHF, by interrupts, and by
      exceptions is different from that stored by the 8086 in bit positions
      12 through 15. On the 8086 these bits are stored as ones, but in
      80386 real-address mode bit 15 is always zero, and bits 14 through 12
      reflect the last value loaded into them.

 13.  NMI interrupting NMI handlers.

      After an NMI is recognized on the 80386, the NMI interrupt is masked
      until an IRET instruction is executed.

 14.  Coprocessor errors vector to interrupt 16.

      Any 80386 system with a coprocessor must use interrupt vector 16 for
      the coprocessor error exception. If an 8086/8088 system uses another
      vector for the 8087 interrupt, both vectors should point to the
      coprocessor-error exception handler.

 15.  Numeric exception handlers should allow prefixes.

      On the 80386, the value of CS:IP saved for coprocessor exceptions
      points at any prefixes before an ESC instruction. On 8086/8088
      systems, the saved CS:IP points to the ESC instruction.

 16.  Coprocessor does not use interrupt controller.

      The coprocessor error signal to the 80386 does not pass through an
      interrupt controller (an 8087 INT signal does). Some instructions in
      a coprocessor error handler may need to be deleted if they deal with
      the interrupt controller.

 17.  Six new interrupt vectors.

      The 80386 adds six exceptions that arise only if the 8086 program has
      a hidden bug. It is recommended that exception handlers be added that
      treat these exceptions as invalid operations. This additional
      software does not significantly affect the existing 8086 software
      because the interrupts do not normally occur. These interrupt
      identifiers should not already have been used by the 8086 software,
      because they are in the range reserved by Intel. Table 14-2 describes
      the new 80386 exceptions.

 18.  One megabyte wraparound.

      The 80386 does not wrap addresses at 1 megabyte in real-address mode.
      On members of the 8086 family, it possible to specify addresses
      greater than one megabyte.  For example, with a selector value 0FFFFH
      and an offset of 0FFFFH, the effective address would be 10FFEFH (1
      Mbyte + 65519).  The 8086, which can form adresses only up to 20 bits
      long, truncates the high-order bit, thereby "wrapping" this address
      to 0FFEFH.  However, the 80386, which can form addresses up to 32
      bits long does not truncate such an address.


Table 14-1. 80386 Real-Address Mode Exceptions


Description                      Interrupt  Function that Can                   Return Address
                                 Number     Generate the Exception              Points to Faulting
                                                                                Instruction
Divide error                     0          DIV, IDIV                           YES
Debug exceptions                 1          All
Some debug exceptions point to the faulting instruction, others to the
next instruction. The exception handler can determine which has occurred by
examining DR6.





Breakpoint                       3          INT                                 NO
Overflow                         4          INTO                                NO
Bounds check                     5          BOUND                               YES
Invalid opcode                   6          Any undefined opcode or LOCK        YES
                                            used with wrong instruction
Coprocessor not available        7          ESC or WAIT                         YES
Interrupt table limit too small  8          INT vector is not within IDTR       YES
                                            limit
Reserved                         9-12
Stack fault                      12         Memory operand crosses offset       YES
                                            0 or 0FFFFH
Pseudo-protection exception      13         Memory operand crosses offset       YES
                                            0FFFFH or attempt to execute
                                            past offset 0FFFFH or
                                            instruction longer than 15
                                            bytes
Reserved                         14,15
Coprocessor error                16         ESC or WAIT                         YES
Coprocessor errors are reported on the first ESC or WAIT instruction
after the ESC instruction that caused the error.





Two-byte SW interrupt            0-255      INT n                               NO


Table 14-2. New 80386 Exceptions

Interrupt   Function
Identifier

    5       A BOUND instruction was executed with a register value outside
            the limit values.

    6       An undefined opcode was encountered or LOCK was used improperly
            before an instruction to which it does not apply.

    7       The EM bit in the MSW is set when an ESC instruction was
            encountered. This exception also occurs on a WAIT instruction
            if TS is set.

    8       An exception or interrupt has vectored to an interrupt table
            entry beyond the interrupt table limit in IDTR. This can occur
            only if the LIDT instruction has changed the limit from the
            default value of 3FFH, which is enough for all 256 interrupt
            IDs.

   12       Operand crosses extremes of stack segment, e.g., MOV operation
            at offset 0FFFFH or push with SP=1 during PUSH, CALL, or INT.

   13       Operand crosses extremes of a segment other than a stack
            segment; or sequential instruction execution attempts to
            proceed beyond offset 0FFFFH; or an instruction is longer than
            15 bytes (including prefixes).


14.8  Differences From 80286 Real-Address Mode

The few differences that exist between 80386 real-address mode and 80286
real-address mode are not likely to affect any existing 80286 programs
except possibly the system initialization procedures.


14.8.1  Bus Lock

The 80286 processor implements the bus lock function differently than the
80386. Programs that use forms of memory locking specific to the 80286 may
not execute properly if transported to a specific application of the 80386.

The LOCK prefix and its corresponding output signal should only be used to
prevent other bus masters from interrupting a data movement operation.  LOCK
may only be used with the following 80386 instructions when they modify
memory.  An undefined-opcode exception results from using LOCK before any
other instruction.

  ■  Bit test and change:  BTS, BTR, BTC.
  ■  Exchange: XCHG.
  ■  One-operand arithmetic and logical: INC, DEC, NOT, and NEG.
  ■  Two-operand arithmetic and logical: ADD, ADC, SUB, SBB, AND, OR, XOR.

A locked instruction is guaranteed to lock only the area of memory defined
by the destination operand, but may lock a larger memory area.  For example,
typical 8086 and 80286 configurations lock the entire physical memory space.
With the 80386, the defined area of memory is guranteed to be locked against
access by a processor executing a locked instruction on exactly the same
memory area, i.e., an operand with identical starting address and identical
length.


14.8.2  Location of First Instruction

The starting location is 0FFFFFFF0H (sixteen bytes from end of 32-bit
address space) on the 80386 rather than 0FFFFF0H (sixteen bytes from end of
24-bit address space) as on the 80286.  Many 80286 ROM initialization
programs will work correctly in this new environment.  Others can be made to
work correctly with external hardware that redefines the signals on
A{31-20}.


14.8.3  Initial Values of General Registers

On the 80386, certain general registers may contain different values after
RESET than on the 80286. This should not cause compatibility problems,
because the content of 8086 registers after RESET is undefined.  If
self-test is requested during the reset sequence and errors are detected in
the 80386 unit, EAX will contain a nonzero value. EDX contains the component
and revision identifier. Refer to Chapter 10 for more information.


14.8.4  MSW Initialization

The 80286 initializes the MSW register to FFF0H, but the 80386 initializes
this register to 0000H. This difference should have no effect, because the
bits that are different are undefined on the 80286.  Programs that read the
value of the MSW will behave differently on the 80386 only if they depend on
the setting of the undefined, high-order bits.


Chapter 15  Virtual 8086 Mode

────────────────────────────────────────────────────────────────────────────

The 80386 supports execution of one or more 8086, 8088, 80186, or 80188
programs in an 80386 protected-mode environment. An 8086 program runs in
this environment as part of a V86 (virtual 8086) task. V86 tasks take
advantage of the hardware support of multitasking offered by the protected
mode. Not only can there be multiple V86 tasks, each one executing an 8086
program, but V86 tasks can be multiprogrammed with other 80386 tasks.

The purpose of a V86 task is to form a "virtual machine" with which to
execute an 8086 program. A complete virtual machine consists not only of
80386 hardware but also of systems software. Thus, the emulation of an 8086
is the result of cooperation between hardware and software:

  ■  The hardware provides a virtual set of registers (via the TSS), a
     virtual memory space (the first megabyte of the linear address space of
     the task), and directly executes all instructions that deal with these
     registers and with this address space.

  ■  The software controls the external interfaces of the virtual machine
     (I/O, interrupts, and exceptions) in a manner consistent with the
     larger environment in which it executes. In the case of I/O, software
     can choose either to emulate I/O instructions or to let the hardware
     execute them directly without software intervention.

Software that helps implement virtual 8086 machines is called a V86
monitor.


15.1  Executing 8086 Code

The processor executes in V86 mode when the VM (virtual machine) bit in the
EFLAGS register is set. The processor tests this flag under two general
conditions:

  1.  When loading segment registers to know whether to use 8086-style
      address formation.

  2.  When decoding instructions to determine which instructions are
      sensitive to IOPL.

Except for these two modifications to its normal operations, the 80386 in
V86 mode operated much as in protected mode.


15.1.1  Registers and Instructions

The register set available in V86 mode includes all the registers defined
for the 8086 plus the new registers introduced by the 80386: FS, GS, debug
registers, control registers, and test registers. New instructions that
explicitly operate on the segment registers FS and GS are available, and the
new segment-override prefixes can be used to cause instructions to utilize
FS and GS for address calculations. Instructions can utilize 32-bit
operands through the use of the operand size prefix.

8086 programs running as V86 tasks are able to take advantage of the new
applications-oriented instructions added to the architecture by the
introduction of the 80186/80188, 80286 and 80386:

  ■  New instructions introduced by 80186/80188 and 80286.
     ── PUSH immediate data
     ── Push all and pop all (PUSHA and POPA)
     ── Multiply immediate data
     ── Shift and rotate by immediate count
     ── String I/O
     ── ENTER and LEAVE
     ── BOUND

  ■  New instructions introduced by 80386.
     ── LSS, LFS, LGS instructions
     ── Long-displacement conditional jumps
     ── Single-bit instructions
     ── Bit scan
     ── Double-shift instructions
     ── Byte set on condition
     ── Move with sign/zero extension
     ── Generalized multiply


15.1.2  Linear Address Formation

In V86 mode, the 80386 processor does not interpret 8086 selectors by
referring to descriptors; instead, it forms linear addresses as an 8086
would. It shifts the selector left by four bits to form a 20-bit base
address. The effective address is extended with four high-order zeros and
added to the base address to create a linear address as Figure 15-1
illustrates.

Because of the possibility of a carry, the resulting linear address may
contain up to 21 significant bits. An 8086 program may generate linear
addresses anywhere in the range 0 to 10FFEFH (one megabyte plus
approximately 64 Kbytes) of the task's linear address space.

V86 tasks generate 32-bit linear addresses. While an 8086 program can only
utilize the low-order 21 bits of a linear address, the linear address can be
mapped via page tables to any 32-bit physical address.

Unlike the 8086 and 80286, 32-bit effective addresses can be generated (via
the address-size prefix); however, the value of a 32-bit address may not
exceed 65,535 without causing an exception. For full compatibility with
80286 real-address mode, pseudo-protection faults (interrupt 12 or 13 with
no error code) occur if an address is generated outside the range 0 through
65,535.


Figure 15-1.  V86 Mode Address Formation

                      19                                3       0
                     ╔═════════════════════════════════╪═════════╗
         BASE        ║     16-BIT SEGMENT SELECTOR     │ 0 0 0 0 ║
                     ╚═════════════════════════════════╪═════════╝

         +
                      19        15                              0
                     ╔═════════╪═════════════════════════════════╗
         OFFSET      ║ 0 0 0 0 │    16-BIT EFFECTIVE ADDRESS     ║
                     ╚═════════╪═════════════════════════════════╝

         =
                    20                                          0
         LINEAR    ╔═════════════════════════════════════════════╗
         ADDRESS   ║ X X X X X X X X X X X X X X X X X X X X X X ║
                   ╚═════════════════════════════════════════════╝


15.2  Structure of a V86 Task

A V86 task consists partly of the 8086 program to be executed and partly of
80386 "native mode" code that serves as the virtual-machine monitor. The
task must be represented by an 80386 TSS (not an 80286 TSS). The processor
enters V86 mode to execute the 8086 program and returns to protected mode to
execute the monitor or other 80386 tasks.

To run successfully in V86 mode, an existing 8086 program needs the
following:

  ■  A V86 monitor.
  ■  Operating-system services.

The V86 monitor is 80386 protected-mode code that executes at
privilege-level zero. The monitor consists primarily of initialization and
exception-handling procedures. As for any other 80386 program,
executable-segment descriptors for the monitor must exist in the GDT or in
the task's LDT. The linear addresses above 10FFEFH are available for the
V86 monitor, the operating system, and other systems software. The monitor
may also need data-segment descriptors so that it can examine the interrupt
vector table or other parts of the 8086 program in the first megabyte of the
address space.

In general, there are two options for implementing the 8086 operating
system:

  1.  The 8086 operating system may run as part of the 8086 code. This
      approach is desirable for any of the following reasons:

      ■  The 8086 applications code modifies the operating system.

      ■  There is not sufficient development time to reimplement the 8086
         operating system as 80386 code.

  2.  The 8086 operating system may be implemented or emulated in the V86
      monitor. This approach is desirable for any of the following reasons:

      ■  Operating system functions can be more easily coordinated among
         several V86 tasks.

      ■  The functions of the 8086 operating system can be easily emulated
         by calls to the 80386 operating system.

Note that, regardless of the approach chosen for implementing the 8086
operating system, different V86 tasks may use different 8086 operating
systems.


15.2.1  Using Paging for V86 Tasks

Paging is not necessary for a single V86 task, but paging is useful or
necessary for any of the following reasons:

  ■  To create multiple V86 tasks. Each task must map the lower megabyte of
     linear addresses to different physical locations.

  ■  To emulate the megabyte wrap. On members of the 8086 family, it is
     possible to specify addresses larger than one megabyte. For example,
     with a selector value of 0FFFFH and an offset of 0FFFFH, the effective
     address would be 10FFEFH (one megabyte + 65519). The 8086, which can
     form addresses only up to 20 bits long, truncates the high-order bit,
     thereby "wrapping" this address to 0FFEFH. The 80386, however, which
     can form addresses up to 32 bits long does not truncate such an
     address. If any 8086 programs depend on this addressing anomaly, the
     same effect can be achieved in a V86 task by mapping linear addresses
     between 100000H and 110000H and linear addresses between 0 and 10000H
     to the same physical addresses.

  ■  To create a virtual address space larger than the physical address
     space.

  ■  To share 8086 OS code or ROM code that is common to several 8086
     programs that are executing simultaneously.

  ■  To redirect or trap references to memory-mapped I/O devices.


15.2.2  Protection within a V86 Task

Because it does not refer to descriptors while executing 8086 programs, the
processor also does not utilize the protection mechanisms offered by
descriptors. To protect the systems software that runs in a V86 task from
the 8086 program, software designers may follow either of these approaches:

  ■  Reserve the first megabyte (plus 64 kilobytes) of each task's linear
     address space for the 8086 program. An 8086 task cannot generate
     addresses outside this range.

  ■  Use the U/S bit of page-table entries to protect the virtual-machine
     monitor and other systems software in each virtual 8086 task's space.
     When the processor is in V86 mode, CPL is 3. Therefore, an 8086 program
     has only user privileges. If the pages of the virtual-machine monitor
     have supervisor privilege, they cannot be accessed by the 8086 program.


15.3  Entering and Leaving V86 Mode

Figure 15-2 summarizes the ways that the processor can enter and leave an
8086 program. The processor can enter V86 by either of two means:

  1.  A task switch to an 80386 task loads the image of EFLAGS from the new
      TSS. The TSS of the new task must be an 80386 TSS, not an 80286 TSS,
      because the 80286 TSS does not store the high-order word of EFLAGS,
      which contains the VM flag. A value of one in the VM bit of the new
      EFLAGS indicates that the new task is executing 8086 instructions;
      therefore, while loading the segment registers from the TSS, the
      processor forms base addresses as the 8086 would.

  2.  An IRET from a procedure of an 80386 task loads the image of EFLAGS
      from the stack. A value of one in VM in this case indicates that the
      procedure to which control is being returned is an 8086 procedure. The
      CPL at the time the IRET is executed must be zero, else the processor
      does not change VM.

The processor leaves V86 mode when an interrupt or exception occurs. There
are two cases:

  1.  The interrupt or exception causes a task switch. A task switch from a
      V86 task to any other task loads EFLAGS from the TSS of the new task.
      If the new TSS is an 80386 TSS and the VM bit in the EFLAGS image is
      zero or if the new TSS is an 80286 TSS, then the processor clears the
      VM bit of EFLAGS, loads the segment registers from the new TSS using
      80386-style address formation, and begins executing the instructions
      of the new task according to 80386 protected-mode semantics.

  2.  The interrupt or exception vectors to a privilege-level zero
      procedure. The processor stores the current setting of EFLAGS on the
      stack, then clears the VM bit. The interrupt or exception handler,
      therefore, executes as "native" 80386 protected-mode code. If an
      interrupt or exception vectors to a conforming segment or to a
      privilege level other than three, the processor causes a
      general-protection exception; the error code is the selector of the
      executable segment to which transfer was attempted.

Systems software does not manipulate the VM flag directly, but rather
manipulates the image of the EFLAGS register that is stored on the stack or
in the TSS. The V86 monitor sets the VM flag in the EFLAGS image on the
stack or in the TSS when first creating a V86 task. Exception and interrupt
handlers can examine the VM flag on the stack. If the interrupted procedure
was executing in V86 mode, the handler may need to invoke the V86 monitor.


Figure 15-2.  Entering and Leaving the 8086 Program

                            MODE TRANSITION DIAGRAM

                                 ╔═══════════╗
                  TASK SWITCH    ║  INITIAL  ║
                ┌────────────────╢   ENTRY   ║
                │   OR IRET      ╚═══════════╝
                │

        ╔══════════════╗    INTERRUPT, EXCEPTION      ╔═════════════╗
        ║ 8086 PROGRAM ╟─────────────────────────────║ V86 MONITOR ║
        ║  (V86 MODE)  ║─────────────────────────────╢ (PROTECTED  ║
        ╚═══════╤══════╝            IRET              ║    MODE)    ║
               │                                     ╚═════╤═══════╝
              │ │                                           │
              │ │                                           │  │
              │ │                                           │  │
              │ │TASK SWITCH ╔═══════════════════╗ TASK SWITCH │
              │ └───────────║ OTHER 80386 TASKS ║─────────┘  │
              └──────────────╢ (PROTECTED MODE)  ╟─────────────┘
                 TASK SWITCH ╚═══════════════════╝ TASK SWITCH


15.3.1  Transitions Through Task Switches

A task switch to or from a V86 task may be due to any of three causes:

  1.  An interrupt that vectors to a task gate.
  2.  An action of the scheduler of the 80386 operating system.
  3.  An IRET when the NT flag is set.

In any of these cases, the processor changes the VM bit in EFLAGS according
to the image of EFLAGS in the new TSS. If the new TSS is an 80286 TSS, the
high-order word of EFLAGS is not in the TSS; the processor clears VM in this
case. The processor updates VM prior to loading the segment registers from
the images in the new TSS. The new setting of VM determines whether the
processor interprets the new segment-register images as 8086 selectors or
80386/80286 selectors.


15.3.2  Transitions Through Trap Gates and Interrupt Gates

The processor leaves V86 mode as the result of an exception or interrupt
that vectors via a trap or interrupt gate to a privilege-level zero
procedure. The exception or interrupt handler returns to the 8086 code by
executing an IRET.

Because it was designed for execution by an 8086 processor, an 8086 program
in a V86 task will have an 8086-style interrupt table starting at linear
address zero. However, the 80386 does not use this table directly. For all
exceptions and interrupts that occur in V86 mode, the processor vectors
through the IDT. The IDT entry for an interrupt or exception that occurs in
a V86 task must contain either:

  ■  A task gate.

  ■  An 80386 trap gate (type 14) or an 80386 interrupt gate (type 15),
     which must point to a nonconforming, privilege-level zero, code
     segment.

Interrupts and exceptions that have 80386 trap or interrupt gates in the
IDT vector to the appropriate handler procedure at privilege-level zero. The
contents of all the 8086 segment registers are stored on the PL 0 stack.
Figure 15-3 shows the format of the PL 0 stack after an exception or
interrupt that occurs while a V86 task is executing an 8086 program.

After the processor stores all the 8086 segment registers on the PL 0
stack, it loads all the segment registers with zeros before starting to
execute the handler procedure. This permits the interrupt handler to safely
save and restore the DS, ES, FS, and GS registers as 80386 selectors.
Interrupt handlers that may be invoked in the context of either a regular
task or a V86 task, can use the same prolog and epilog code for register
saving regardless of the kind of task. Restoring zeros to these registers
before execution of the IRET does not cause a trap in the interrupt handler.
Interrupt procedures that expect values in the segment registers or that
return values via segment registers have to use the register images stored
on the PL 0 stack. Interrupt handlers that need to know whether the
interrupt occurred in V86 mode can examine the VM bit in the stored EFLAGS
image.

An interrupt handler passes control to the V86 monitor if the VM bit is set
in the EFLAGS image stored on the stack and the interrupt or exception is
one that the monitor needs to handle. The V86 monitor may either:

  ■  Handle the interrupt completely within the V86 monitor.
  ■  Invoke the 8086 program's interrupt handler.

Reflecting an interrupt or exception back to the 8086 code involves the
following steps:

  1.  Refer to the 8086 interrupt vector to locate the appropriate handler
      procedure.

  2.  Store the state of the 8086 program on the privilege-level three
      stack.

  3.  Change the return link on the privilege-level zero stack to point to
      the privilege-level three handler procedure.

  4.  Execute an IRET so as to pass control to the handler.

  5.  When the IRET by the privilege-level three handler again traps to the
      V86 monitor, restore the return link on the privilege-level zero stack
      to point to the originally interrupted, privilege-level three
      procedure.

  6.  Execute an IRET so as to pass control back to the interrupted
      procedure.


Figure 15-3. PL 0 Stack after Interrupt in V86 Task


                WITHOUT ERROR CODE            WITH ERROR CODE
                 31            0               31            0
                ╔══════╦═══════╗────┐        ╔══════╦═══════╗────┐
                ║▒▒▒▒▒▒║OLD GS ║     │        ║▒▒▒▒▒▒║OLD GS ║     │
                ╠══════╬═══════╣   SS:ESP     ╠══════╬═══════╣   SS:ESP
      D  O      ║▒▒▒▒▒▒║OLD FS ║  FROM TSS    ║▒▒▒▒▒▒║OLD FS ║  FROM TSS
      I  F      ╠══════╬═══════╣              ╠══════╬═══════╣
      R         ║▒▒▒▒▒▒║OLD DS ║              ║▒▒▒▒▒▒║OLD DS ║
      E  E      ╠══════╬═══════╣              ╠══════╬═══════╣
      C  X      ║▒▒▒▒▒▒║OLD ES ║              ║▒▒▒▒▒▒║OLD ES ║
      T  P      ╠══════╬═══════╣              ╠══════╬═══════╣
      I  A      ║▒▒▒▒▒▒║OLD SS ║              ║▒▒▒▒▒▒║OLD SS ║
      O  N      ╠══════╩═══════╣              ╠══════╩═══════╣
      N  S      ║    OLD ESP   ║              ║    OLD ESP   ║
         I      ╠══════════════╣              ╠══════════════╣
       │ O      ║  OLD EFLAGS  ║              ║  OLD EFLAGS  ║
       │ N      ╠══════╦═══════╣              ╠══════╦═══════╣
       │        ║▒▒▒▒▒▒║OLD CS ║   NEW        ║▒▒▒▒▒▒║OLD CS ║
               ╠══════╩═══════╣  SS:EIP      ╠══════╩═══════╣
                ║    OLD EIP   ║    │         ║    OLD EIP   ║   NEW
                ╠══════════════╣───┘         ╠══════════════╣  SS:EIP
                ║              ║              ║  ERROR CODE  ║    │
                                            ╠══════════════╣───┘
                                            ║              ║
                                                          


15.4  Additional Sensitive Instructions

When the 80386 is executing in V86 mode, the instructions PUSHF, POPF,
INT n, and IRET are sensitive to IOPL. The instructions IN, INS, OUT, and
OUTS, which are ordinarily sensitive in protected mode, are not sensitive
in V86 mode. Following is a complete list of instructions that are sensitive
in V86 mode:

   CLI     ── Clear Interrupt-Enable Flag
   STI     ── Set Interrupt-Enable Flag
   LOCK    ── Assert Bus-Lock Signal
   PUSHF   ── Push Flags
   POPF    ── Pop Flags
   INT n   ── Software Interrupt
   RET     ── Interrupt Return

CPL is always three in V86 mode; therefore, if IOPL < 3, these instructions
will trigger a general-protection exceptions. These instructions are made
sensitive so that their functions can be simulated by the V86 monitor.


15.4.1  Emulating 8086 Operating System Calls

INT n is sensitive so that the V86 monitor can intercept calls to the
8086 OS. Many 8086 operating systems are called by pushing parameters onto
the stack, then executing an INT n instruction. If IOPL < 3, INT n
instructions will be intercepted by the V86 monitor. The V86 monitor can
then emulate the function of the 8086 operating system or reflect the
interrupt back to the 8086 operating system in V86 mode.


15.4.2  Virtualizing the Interrupt-Enable Flag

When the processor is executing 8086 code in a V86 task, the instructions
PUSHF, POPF, and IRET are sensitive to IOPL so that the V86 monitor can
control changes to the interrupt-enable flag (IF). Other instructions that
affect IF (STI and CLI) are IOPL sensitive both in 8086 code and in
80386/80386 code.

Many 8086 programs that were designed to execute on single-task systems set
and clear IF to control interrupts. However, when these same programs are
executed in a multitasking environment, such control of IF can be
disruptive. If IOPL is less than three, all instructions that change or
interrogate IF will trap to the V86 monitor. The V86 monitor can then
control IF in a manner that both suits the needs of the larger environment
and is transparent to the 8086 program.


15.5  Virtual I/O

Many 8086 programs that were designed to execute on single-task systems use
I/O devices directly. However, when these same programs are executed in a
multitasking environment, such use of devices can be disruptive. The 80386
provides sufficient flexibility to control I/O in a manner that both suits
the needs of the new environment and is transparent to the 8086 program.
Designers may take any of several possible approaches to controlling I/O:

  ■  Implement or emulate the 8086 operating system as an 80386 program and
     require the 8086 application to do I/O via software interrupts to the
     operating system, trapping all attempts to do I/O directly.

  ■  Let the 8086 program take complete control of all I/O.

  ■  Selectively trap and emulate references that a task makes to specific
     I/O ports.

  ■  Trap or redirect references to memory-mapped I/O addresses.

The method of controlling I/O depends upon whether I/O ports are I/O mapped
or memory mapped.


15.5.1  I/O-Mapped I/O

I/O-mapped I/O in V86 mode differs from protected mode only in that the
protection mechanism does not consult IOPL when executing the I/O
instructions IN, INS, OUT, OUTS. Only the I/O permission bit map controls
the right for V86 tasks to execute these I/O instructions.

The I/O permission map traps I/O instructions selectively depending on the
I/O addresses to which they refer. The I/O permission bit map of each V86
task determines which I/O addresses are trapped for that task. Because each
task may have a different I/O permission bit map, the addresses trapped for
one task may be different from those trapped for others. Refer to Chapter 8
for more information about the I/O permission map.


15.5.2  Memory-Mapped I/O

In hardware designs that utilize memory-mapped I/O, the paging facilities
of the 80386 can be used to trap or redirect I/O operations. Each task that
executes memory-mapped I/O must have a page (or pages) for the memory-mapped
address space. The V86 monitor may control memory-mapped I/O by any of
these means:

  ■  Assign the memory-mapped page to appropriate physical addresses.
     Different tasks may have different physical addresses, thereby
     preventing the tasks from interfering with each other.

  ■  Cause a trap to the monitor by forcing a page fault on the
     memory-mapped page. Read-only pages trap writes. Not-present pages trap
     both reads and writes.

Intervention for every I/O might be excessive for some kinds of I/O
devices. A page fault can still be used in this case to cause intervention
on the first I/O operation. The monitor can then at least make sure that the
task has exclusive access to the device. Then the monitor can change the
page status to present and read/write, allowing subsequent I/O to proceed at
full speed.


15.5.3  Special I/O Buffers

Buffers of intelligent controllers (for example, a bit-mapped graphics
buffer) can also be virtualized via page mapping. The linear space for the
buffer can be mapped to a different physical space for each virtual 8086
task. The V86 monitor can then assume responsibility for spooling the data
or assigning the virtual buffer to the real buffer at appropriate times.


15.6  Differences From 8086

In general, V86 mode will correctly execute software designed for the 8086,
8088, 80186, and 80188. Following is a list of the minor differences between
8086 execution on the 80386 and on an 8086.

  1.  Instruction clock counts.

      The 80386 takes fewer clocks for most instructions than the
      8086/8088. The areas most likely to be affected are:

      ■  Delays required by I/O devices between I/O operations.

      ■  Assumed delays with 8086/8088 operating in parallel with an 8087.

  2.  Divide exceptions point to the DIV instruction.

      Divide exceptions on the 80386 always leave the saved CS:IP value
      pointing to the instruction that failed. On the 8086/8088, the CS:IP
      value points to the next instruction.

  3.  Undefined 8086/8088 opcodes.

      Opcodes that were not defined for the 8086/8088 will cause exception
      6 or will execute one of the new instructions defined for the 80386.

  4.  Value written by PUSH SP.

      The 80386 pushes a different value on the stack for PUSH SP than the
      8086/8088. The 80386 pushes the value of SP before SP is incremented
      as part of the push operation; the 8086/8088 pushes the value of SP
      after it is incremented. If the value pushed is important, replace
      PUSH SP instructions with the following three instructions:

      PUSH  BP
      MOV   BP, SP
      XCHG  BP, [BP]

      This code functions as the 8086/8088 PUSH SP instruction on the
      80386.

  5.  Shift or rotate by more than 31 bits.

      The 80386 masks all shift and rotate counts to the low-order five
      bits. This MOD 32 operation limits the count to a maximum of 31 bits,
      thereby limiting the time that interrupt response is delayed while
      the instruction is executing.

  6.  Redundant prefixes.

      The 80386 sets a limit of 15 bytes on instruction length. The only
      way to violate this limit is by putting redundant prefixes before an
      instruction. Exception 13 occurs if the limit on instruction length
      is violated. The 8086/8088 has no instruction length limit.

  7.  Operand crossing offset 0 or 65,535.

      On the 8086, an attempt to access a memory operand that crosses
      offset 65,535 (e.g., MOV a word to offset 65,535) or offset 0 (e.g.,
      PUSH a word when SP = 1) causes the offset to wrap around modulo
      65,536. The 80386 raises an exception in these cases──exception 13 if
      the segment is a data segment (i.e., if CS, DS, ES, FS, or GS is
      being used to address the segment), exception 12 if the segment is a
      stack segment (i.e., if SS is being used).

  8.  Sequential execution across offset 65,535.

      On the 8086, if sequential execution of instructions proceeds past
      offset 65,535, the processor fetches the next instruction byte from
      offset 0 of the same segment. On the 80386, the processor raises
      exception 13 in such a case.

  9.  LOCK is restricted to certain instructions.

      The LOCK prefix and its corresponding output signal should only be
      used to prevent other bus masters from interrupting a data movement
      operation. The 80386 always asserts the LOCK signal during an XCHG
      instruction with memory (even if the LOCK prefix is not used). LOCK
      may only be used with the following 80386 instructions when they
      update memory: BTS, BTR, BTC, XCHG, ADD, ADC, SUB, SBB, INC, DEC,
      AND, OR, XOR, NOT, and NEG. An undefined-opcode exception (interrupt
      6) results from using LOCK before any other instruction.

 10.  Single-stepping external interrupt handlers.

      The priority of the 80386 single-step exception is different from
      that of the 8086/8088. The change prevents an external interrupt
      handler from being single-stepped if the interrupt occurs while a
      program is being single-stepped. The 80386 single-step exception has
      higher priority that any external interrupt. The 80386 will still
      single-step through an interrupt handler invoked by the INT
      instructions or by an exception.

  11.  IDIV exceptions for quotients of 80H or 8000H.

      The 80386 can generate the largest negative number as a quotient for
      the IDIV instruction. The 8086/8088 causes exception zero instead.

 12.  Flags in stack.

      The setting of the flags stored by PUSHF, by interrupts, and by
      exceptions is different from that stored by the 8086 in bit positions
      12 through 15. On the 8086 these bits are stored as ones, but in V86
      mode bit 15 is always zero, and bits 14 through 12 reflect the last
      value loaded into them.

 13.  NMI interrupting NMI handlers.

      After an NMI is recognized on the 80386, the NMI interrupt is masked
      until an IRET instruction is executed.

 14.  Coprocessor errors vector to interrupt 16.

      Any 80386 system with a coprocessor must use interrupt vector 16 for
      the coprocessor error exception. If an 8086/8088 system uses another
      vector for the 8087 interrupt, both vectors should point to the
      coprocessor-error exception handler.

 15.  Numeric exception handlers should allow prefixes.

      On the 80386, the value of CS:IP saved for coprocessor exceptions
      points at any prefixes before an ESC instruction. On 8086/8088
      systems, the saved CS:IP points to the ESC instruction itself.

 16.  Coprocessor does not use interrupt controller.

      The coprocessor error signal to the 80386 does not pass through an
      interrupt controller (an 8087 INT signal does). Some instructions in
      a coprocessor error handler may need to be deleted if they deal with
      the interrupt controller.


15.7  Differences From 80286 Real-Address Mode

The 80286 processor implements the bus lock function differently than the
80386. This fact may or may not be apparent to 8086 programs, depending on
how the V86 monitor handles the LOCK prefix. LOCKed instructions are
sensitive to IOPL; therefore, software designers can choose to emulate its
function. If, however, 8086 programs are allowed to execute LOCK directly,
programs that use forms of memory locking specific to the 8086 may not
execute properly when transported to a specific application of the 80386.

The LOCK prefix and its corresponding output signal should only be used to
prevent other bus masters from interrupting a data movement operation. LOCK
may only be used with the following 80386 instructions when they modify
memory. An undefined-opcode exception results from using LOCK before any
other instruction.

  ■  Bit test and change: BTS, BTR, BTC.
  ■  Exchange: XCHG.
  ■  One-operand arithmetic and logical: INC, DEC, NOT, and NEG.
  ■  Two-operand arithmetic and logical: ADD, ADC, SUB, SBB, AND, OR, XOR.

A locked instruction is guaranteed to lock only the area of memory defined
by the destination operand, but may lock a larger memory area. For example,
typical 8086 and 80286 configurations lock the entire physical memory space.
With the 80386, the defined area of memory is guaranteed to be locked
against access by a processor executing a locked instruction on exactly the
same memory area, i.e., an operand with identical starting address and
identical length.


Chapter 16  Mixing 16-Bit and 32 Bit Code

────────────────────────────────────────────────────────────────────────────

The 80386 running in protected mode is a 32-bit microprocessor, but it is
designed to support 16-bit processing at three levels:

  1.  Executing 8086/80286 16-bit programs efficiently with complete
      compatibility.

  2.  Mixing 16-bit modules with 32-bit modules.

  3.  Mixing 16-bit and 32-bit addresses and operands within one module.

The first level of support for 16-bit programs has already been discussed
in Chapter 13, Chapter 14, and Chapter 15. This chapter shows how 16-bit
and 32-bit modules can cooperate with one another, and how one module can
utilize both 16-bit and 32-bit operands and addressing.

The 80386 functions most efficiently when it is possible to distinguish
between pure 16-bit modules and pure 32-bit modules. A pure 16-bit module
has these characteristics:

  ■  All segments occupy 64 Kilobytes or less.
  ■  Data items are either 8 bits or 16 bits wide.
  ■  Pointers to code and data have 16-bit offsets.
  ■  Control is transferred only among 16-bit segments.

A pure 32-bit module has these characteristics:

  ■  Segments may occupy more than 64 Kilobytes (zero bytes to 4
     gigabytes).

  ■  Data items are either 8 bits or 32 bits wide.

  ■  Pointers to code and data have 32-bit offsets.

  ■  Control is transferred only among 32-bit segments.

Pure 16-bit modules do exist; they are the modules designed for 16-bit
microprocessors. Pure 32-bit modules may exist in new programs designed
explicitly for the 80386. However, as systems designers move applications
from 16-bit processors to the 32-bit 80386, it will not always be possible
to maintain these ideals of pure 16-bit or 32-bit modules. It may be
expedient to execute old 16-bit modules in a new 32-bit environment without
making source-code changes to the old modules if any of the following
conditions is true:

  ■  Modules will be converted one-by-one from 16-bit environments to
     32-bit environments.

  ■  Older, 16-bit compilers and software-development tools will be
     utilized in the new32-bit operating environment until new 32-bit
     versions can be created.

  ■  The source code of 16-bit modules is not available for modification.

  ■  The specific data structures used by a given module inherently utilize
     16-bit words.

  ■  The native word size of the source language is 16 bits.

On the 80386, 16-bit modules can be mixed with 32-bit modules. To design a
system that mixes 16- and 32-bit code requires an understanding of the
mechanisms that the 80386 uses to invoke and control its 32-bit and 16-bit
features.


16.1  How the 80386 Implements 16-Bit and 32-Bit Features

The features of the architecture that permit the 80386 to work equally well
with 32-bit and 16-bit address and operand sizes include:

  ■  The D-bit (default bit) of code-segment descriptors, which determines
     the default choice of operand-size and address-size for the
     instructions of a code segment. (In real-address mode and V86 mode,
     which do not use descriptors, the default is 16 bits.) A code segment
     whose D-bit is set is known as a USE32 segment; a code segment whose
     D-bit is zero is a USE16 segment. The D-bit eliminates the need to
     encode the operand size and address size in instructions when all
     instructions use operands and effective addresses of the same size.

  ■  Instruction prefixes that explicitly override the default choice of
     operand size and address size (available in protected mode as well as
     in real-address mode and V86 mode).

  ■  Separate 32-bit and 16-bit gates for intersegment control transfers
     (including call gates, interrupt gates, and trap gates). The operand
     size for the control transfer is determined by the type of gate, not by
     the D-bit or prefix of the transfer instruction.

  ■  Registers that can be used both for 32-bit and 16-bit operands and
     effective-address calculations.

  ■  The B-bit (big bit) of data-segment descriptors, which determines the
     size of stack pointer (32-bit ESP or 16-bit SP) used by the CPU for
     implicit stack references.


16.2  Mixing 32-Bit and 16-Bit Operations

The 80386 has two instruction prefixes that allow mixing of 32-bit and
16-bit operations within one segment:

  ■  The operand-size prefix (66H)
  ■  The address-size prefix (67H)

These prefixes reverse the default size selected by the D-bit. For example,
the processor can interpret the word-move instruction MOV mem, reg in any of
four ways:

  ■  In a USE32 segment:

     1.  Normally moves 32 bits from a 32-bit register to a 32-bit
         effective address in memory.

     2.  If preceded by an operand-size prefix, moves 16 bits from a 16-bit
         register to 32-bit effective address in memory.

     3.  If preceded by an address-size prefix, moves 32 bits from a 32-bit
         register to a16-bit effective address in memory.

     4.  If preceded by both an address-size prefix and an operand-size
         prefix, moves 16 bits from a 16-bit register to a 16-bit effective
         address in memory.

  ■  In a USE16 segment:

     1.  Normally moves 16 bits from a 16-bit register to a 16-bit
         effective address in memory.

     2.  If preceded by an operand-size prefix, moves 32 bits from a 32-bit
         register to 16-bit effective address in memory.

     3.  If preceded by an address-size prefix, moves 16 bits from a 16-bit
         register to a32-bit effective address in memory.

     4.  If preceded by both an address-size prefix and an operand-size
         prefix, moves 32 bits from a 32-bit register to a 32-bit effective
         address in memory.

These examples illustrate that any instruction can generate any combination
of operand size and address size regardless of whether the instruction is in
a USE16 or USE32 segment. The choice of the USE16 or USE32 attribute for a
code segment is based upon these criteria:

  1.  The need to address instructions or data in segments that are larger
      than 64 Kilobytes.

  2.  The predominant size of operands.

  3.  The addressing modes desired. (Refer to Chapter 17 for an explanation
      of the additional addressing modes that are available when 32-bit
      addressing is used.)

Choosing a setting of the D-bit that is contrary to the predominant size of
operands requires the generation of an excessive number of operand-size
prefixes.


16.3  Sharing Data Segments Among Mixed Code Segments

Because the choice of operand size and address size is defined in code
segments and their descriptors, data segments can be shared freely among
both USE16 and USE32 code segments. The only limitation is the one imposed
by pointers with 16-bit offsets, which can only point to the first 64
Kilobytes of a segment. When a data segment that contains more than 64
Kilobytes is to be shared among USE32 and USE16 segments, the data that is
to be accessed by the USE16 segments must be located within the first 64
Kilobytes.

A stack that spans addresses less than 64K can be shared by both USE16 and
USE32 code segments. This class of stacks includes:

  ■  Stacks in expand-up segments with G=0 and B=0.

  ■  Stacks in expand-down segments with G=0 and B=0.

  ■  Stacks in expand-up segments with G=1 and B=0, in which the stack is
     contained completely within the lower 64 Kilobytes. (Offsets greater
     than 64K can be used for data, other than the stack, that is not
     shared.)

The B-bit of a stack segment cannot, in general, be used to change the size
of stack used by a USE16 code segment. The size of stack pointer used by the
processor for implicit stack references is controlled by the B-bit of the
data-segment descriptor for the stack. Implicit references are those caused
by interrupts, exceptions, and instructions such as PUSH, POP, CALL, and
RET. One might be tempted, therefore, to try to increase beyond 64K the
size of the stack used by 16-bit code simply by supplying a larger stack
segment with the B-bit set. However, the B-bit does not control explicit
stack references, such as accesses to parameters or local variables. A USE16
code segment can utilize a "big" stack only if the code is modified so that
all explicit references to the stack are preceded by the address-size
prefix, causing those references to use 32-bit addressing.

In big, expand-down segments (B=1, G=1, and E=1), all offsets are greater
than 64K, therefore USE16 code cannot utilize such a stack segment unless
the code segment is modified to employ 32-bit addressing. (Refer to Chapter
6 for a review of the B, G, and E bits.)


16.4  Transferring Control Among Mixed Code Segments

When transferring control among procedures in USE16 and USE32 code
segments, programmers must be aware of three points:

  ■  Addressing limitations imposed by pointers with 16-bit offsets.

  ■  Matching of operand-size attribute in effect for the CALL/RET pair and
     theInterrupt/IRET pair so as to manage the stack correctly.

  ■  Translation of parameters, especially pointer parameters.

Clearly, 16-bit effective addresses cannot be used to address data or code
located beyond 64K in a 32-bit segment, nor can large 32-bit parameters be
squeezed into a 16-bit word; however, except for these obvious limits, most
interfacing problems between 16-bit and 32-bit modules can be solved. Some
solutions involve inserting interface procedures between the procedures in
question.


16.4.1  Size of Code-Segment Pointer

For control-transfer instructions that use a pointer to identify the next
instruction (i.e., those that do not use gates), the size of the offset
portion of the pointer is determined by the operand-size attribute. The
implications of the use of two different sizes of code-segment pointer are:

  ■  JMP, CALL, or RET from 32-bit segment to 16-bit segment is always
     possible using a 32-bit operand size.

  ■  JMP, CALL, or RET from 16-bit segment using a 16-bit operand size
     cannot address the target in a 32-bit segment if the address of the
     target is greater than 64K.

An interface procedure can enable transfers from USE16 segments to 32-bit
addresses beyond 64K without requiring modifications any more extensive than
relinking or rebinding the old programs. The requirements for such an
interface procedure are discussed later in this chapter.


16.4.2  Stack Management for Control Transfers

Because stack management is different for 16-bit CALL/RET than for 32-bit
CALL/RET, the operand size of RET must match that of CALL. (Refer to Figure
16-1.) A 16-bit CALL pushes the 16-bit IP and (for calls between privilege
levels) the 16-bit SP register. The corresponding RET must also use a 16-bit
operand size to POP these 16-bit values from the stack into the 16-bit
registers. A 32-bit CALL pushes the 32-bit EIP and (for interlevel calls)
the 32-bit ESP register. The corresponding RET must also use a 32-bit
operand size to POP these 32-bit values from the stack into the 32-bit
registers. If the two halves of a CALL/RET pair do not have matching operand
sizes, the stack will not be managed correctly and the values of the
instruction pointer and stack pointer will not be restored to correct
values.

When the CALL and its corresponding RET are in segments that have D-bits
with the same values (i.e., both have 32-bit defaults or both have 16-bit
defaults), there is no problem. When the CALL and its corresponding RET are
in segments that have different D-bit values, however, programmers (or
program development software) must ensure that the CALL and RET match.

There are three ways to cause a 16-bit procedure to execute a 32-bit call:

  1.  Use a 16-bit call to a 32-bit interface procedure that then uses a
      32-bit call to invoke the intended target.

  2.  Bind the 16-bit call to a 32-bit call gate.

  3.  Modify the 16-bit procedure, inserting an operand-size prefix before
      the call, thereby changing it to a 32-bit call.

Likewise, there are three ways to cause a 32-bit procedure to execute a
16-bit call:

  1.  Use a 32-bit call to a 32-bit interface procedure that then uses a
      16-bit call to invoke the intended target.

  2.  Bind the 32-bit call to a 16-bit call gate.

  3.  Modify the 32-bit procedure, inserting an operand-size prefix before
      the call, thereby changing it to a 16-bit call. (Be certain that the
      return offset does not exceed 64K.)

Programmers can utilize any of the preceding methods to make a CALL in a
USE16 segment match the corresponding RET in a USE32 segment, or to make a
CALL in a USE32 segment match the corresponding RET in a USE16 segment.


Figure 16-1.  Stack after Far 16-Bit and 32-Bit Calls

                           WITHOUT PRIVILEGE TRANSITION

               AFTER 16-BIT CALL                AFTER 32-BIT CALL

               31             0               31             0
       D  O    ║               ║                ║               ║
       I  F    ╠═══════╪═══════╣                ╠═══════╪═══════╣
       R       ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║                ║▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒║
       E  E    ╠═══════╪═══════╣                ╠═══════╪═══════╣
       C  X    ║ PARM2 │ PARM1 ║                ║     PARM2     ║
       T  P    ╠═══════╪═══════╣                ╠═══════╪═══════╣
       I  A    ║  CS   │  IP   ║──SP           ║     PARM1     ║
       O  N    ╠═══════╪═══════╣                ╠═══════╪═══════╣
       N  S    ║               ║                ║▒▒▒▒▒▒▒│  CS   ║
          I    ╠═══════╪═══════╣                ╠═══════╪═══════╣
        │ O    ║               ║                ║      EIP      ║──ESP
        │ N    ╠═══════╪═══════╣                ╠═══════╪═══════╣
        │      ║               ║                ║               ║
                                                            

                           WITH PRIVILEGE TRANSITION

               AFTER 16-BIT CALL                AFTER 32-BIT CALL

       D  O     31            0                  31            0
       I  F    ╔═══════╪═══════╗                ╔═══════╪═══════╗
       R       ║   SS  │  SP   ║                ║▒▒▒▒▒▒▒│  SS   ║
       E  E    ╠═══════╪═══════╣                ╠═══════╪═══════╣
       C  X    ║ PARM2 │ PARM1 ║                ║      ESP      ║
       T  P    ╠═══════╪═══════╣                ╠═══════╪═══════╣
       I  A    ║  CS   │  IP   ║──SP           ║     PARM2     ║
       O  N    ╠═══════╪═══════╣                ╠═══════╪═══════╣
       N  S    ║               ║                ║     PARM1     ║
          I    ╠═══════╪═══════╣                ╠═══════╪═══════╣
        │ O    ║               ║                ║▒▒▒▒▒▒▒│  CS   ║
        │ N    ╠═══════╪═══════╣                ╠═══════╪═══════╣
        │      ║               ║                ║      EIP      ║──ESP
              ╠═══════╪═══════╣                ╠═══════╪═══════╣
               ║               ║                ║               ║
                                                             


16.4.2.1  Controlling the Operand-Size for a Call

When the selector of the pointer referenced by a CALL instruction selects a
segment descriptor, the operand-size attribute in effect for the CALL
instruction is determined by the D-bit in the segment descriptor and by any
operand-size instruction prefix.

When the selector of the pointer referenced by a CALL instruction selects a
gate descriptor, the type of call is determined by the type of call gate. A
call via an 80286 call gate (descriptor type 4)  always has a 16-bit
operand-size attribute; a call via an 80386 call gate (descriptor type 12)
always has a 32-bit operand-size attribute. The offset of the target
procedure is taken from the gate descriptor; therefore, even a 16-bit
procedure can call a procedure that is located more than 64 kilobytes from
the base of a 32-bit segment, because a 32-bit call gate contains a 32-bit
target offset.

An unmodified 16-bit code segment that has run successfully on an 8086 or
real-mode 80286 will always have a D-bit of zero and will not use
operand-size override prefixes; therefore, it will always execute 16-bit
versions of CALL. The only modification needed to make a16-bit procedure
effect a 32-bit call is to relink the call to an 80386 call gate.


16.4.2.2  Changing Size of Call

When adding 32-bit gates to 16-bit procedures, it is important to consider
the number of parameters. The count field of the gate descriptor specifies
the size of the parameter string to copy from the current stack to the stack
of the more privileged procedure. The count field of a 16-bit gate specifies
the number of words to be copied, whereas the count field of a 32-bit gate
specifies the number of doublewords to be copied; therefore, the 16-bit
procedure must use an even number of words as parameters.


16.4.3  Interrupt Control Transfers

With a control transfer due to an interrupt or exception, a gate is always
involved. The operand-size attribute for the interrupt is determined by the
type of IDT gate.

A 386 interrupt or trap gate (descriptor type 14 or 15) to a 32-bit
interrupt procedure can be used to interrupt either 32-bit or 16-bit
procedures. However, it is not generally feasible to permit an interrupt or
exception to invoke a 16-bit handler procedure when 32-bit code is
executing, because a 16-bit interrupt procedure has a return offset of only
16-bits on its stack. If the 32-bit procedure is executing at an address
greater than 64K, the 16-bit interrupt procedure cannot return correctly.


16.4.4  Parameter Translation

When segment offsets or pointers (which contain segment offsets) are passed
as parameters between 16-bit and 32-bit procedures, some translation is
required. Clearly, if a 32-bit procedure passes a pointer to data located
beyond 64K to a 16-bit procedure, the 16-bit procedure cannot utilize it.
Beyond this natural limitation, an interface procedure can perform any
format conversion between 32-bit and 16-bit pointers that may be needed.

Parameters passed by value between 32-bit and 16-bit code may also require
translation between 32-bit and 16-bit formats. Such translation requirements
are application dependent. Systems designers should take care to limit the
range of values passed so that such translations are possible.


16.4.5  The Interface Procedure

Interposing an interface procedure between 32-bit and 16-bit procedures can
be the solution to any of several interface requirements:

  ■  Allowing procedures in 16-bit segments to transfer control to
     instructions located beyond 64K in 32-bit segments.

  ■  Matching of operand size for CALL/RET.

  ■  Parameter translation.

Interface procedures between USE32 and USE16 segments can be constructed
with these properties:

  ■  The procedures reside in a code segment whose D-bit is set, indicating
     a default operand size of 32-bits.

  ■  All entry points that may be called by 16-bit procedures have offsets
     that are actually less than 64K.

  ■  All points to which called 16-bit procedures may return also lie
     within 64K.

The interface procedures do little more than call corresponding procedures
in other segments. There may be two kinds of procedures:

  ■  Those that are called by 16-bit procedures and call 32-bit procedures.
     These interface procedures are called by 16-bit CALLs and use the
     operand-size prefix before RET instructions to cause a 16-bit RET.
     CALLs to 32-bit segments are 32-bit calls (by default, because the
     D-bit is set), and the 32-bit code returns with 32-bit RET
     instructions.

  ■  Those that are called by 32-bit procedures and call 16-bit procedures.
     These interface procedures are called by 32-bit CALL instructions, and
     return with 32-bit RET instructions (by default, because the D-bit is
     set).  CALLs to 16-bit procedures use the operand-size prefix;
     procedures in the 16-bit code return with 16-bit RET instructions.


                         PART IV  INSTRUCTION SET


Chapter 17  80386 Instruction Set

────────────────────────────────────────────────────────────────────────────

This chapter presents instructions for the 80386 in alphabetical order. For
each instruction, the forms are given for each operand combination,
including object code produced, operands required, execution time, and a
description. For each instruction, there is an operational description and a
summary of exceptions generated.


17.1  Operand-Size and Address-Size Attributes

When executing an instruction, the 80386 can address memory using either 16
or 32-bit addresses. Consequently, each instruction that uses memory
addresses has associated with it an address-size attribute of either 16 or
32 bits. 16-bit addresses imply both the use of a 16-bit displacement in
the instruction and the generation of a 16-bit address offset (segment
relative address) as the result of the effective address calculation.
32-bit addresses imply the use of a 32-bit displacement and the generation
of a 32-bit address offset. Similarly, an instruction that accesses words
(16 bits) or doublewords (32 bits) has an operand-size attribute of either
16 or 32 bits.

The attributes are determined by a combination of defaults, instruction
prefixes, and (for programs executing in protected mode) size-specification
bits in segment descriptors.


17.1.1  Default Segment Attribute

For programs executed in protected mode, the D-bit in executable-segment
descriptors determines the default attribute for both address size and
operand size. These default attributes apply to the execution of all
instructions in the segment. A value of zero in the D-bit sets the default
address size and operand size to 16 bits; a value of one, to 32 bits.

Programs that execute in real mode or virtual-8086 mode have 16-bit
addresses and operands by default.


17.1.2  Operand-Size and Address-Size Instruction Prefixes

The internal encoding of an instruction can include two byte-long prefixes:
the address-size prefix, 67H, and the operand-size prefix, 66H. (A later
section, "Instruction Format," shows the position of the prefixes in an
instruction's encoding.) These prefixes override the default segment
attributes for the instruction that follows. Table 17-1 shows the effect of
each possible combination of defaults and overrides.


17.1.3  Address-Size Attribute for Stack

Instructions that use the stack implicitly (for example: POP EAX also have
a stack address-size attribute of either 16 or 32 bits. Instructions with a
stack address-size attribute of 16 use the 16-bit SP stack pointer register;
instructions with a stack address-size attribute of 32 bits use the 32-bit
ESP register to form the address of the top of the stack.

The stack address-size attribute is controlled by the B-bit of the
data-segment descriptor in the SS register. A value of zero in the B-bit
selects a stack address-size attribute of 16; a value of one selects a stack
address-size attribute of 32.


Table 17-1. Effective Size Attributes

Segment Default D = ...      0    0    0    0    1    1    1    1
Operand-Size Prefix 66H      N    N    Y    Y    N    N    Y    Y
Address-Size Prefix 67H      N    Y    N    Y    N    Y    N    Y

Effective Operand Size      16   16   32   32   32   32   16   16
Effective Address Size      16   32   16   32   32   16   32   16

Y = Yes, this instruction prefix is present
N = No, this instruction prefix is not present


17.2  Instruction Format

All instruction encodings are subsets of the general instruction format
shown in Figure 17-1. Instructions consist of optional instruction
prefixes, one or two primary opcode bytes, possibly an address specifier
consisting of the ModR/M byte and the SIB (Scale Index Base) byte, a
displacement, if required, and an immediate data field, if required.

Smaller encoding fields can be defined within the primary opcode or
opcodes. These fields define the direction of the operation, the size of the
displacements, the register encoding, or sign extension; encoding fields
vary depending on the class of operation.

Most instructions that can refer to an operand in memory have an addressing
form byte following the primary opcode byte(s). This byte, called the ModR/M
byte, specifies the address form to be used. Certain encodings of the ModR/M
byte indicate a second addressing byte, the SIB (Scale Index Base) byte,
which follows the ModR/M byte and is required to fully specify the
addressing form.

Addressing forms can include a displacement immediately following either
the ModR/M or SIB byte. If a displacement is present, it can be 8-, 16- or
32-bits.

If the instruction specifies an immediate operand, the immediate operand
always follows any displacement bytes. The immediate operand, if specified,
is always the last field of the instruction.

The following are the allowable instruction prefix codes:

   F3H    REP prefix (used only with string instructions)
   F3H    REPE/REPZ prefix (used only with string instructions
   F2H    REPNE/REPNZ prefix (used only with string instructions)
   F0H    LOCK prefix

The following are the segment override prefixes:

   2EH    CS segment override prefix
   36H    SS segment override prefix
   3EH    DS segment override prefix
   26H    ES segment override prefix
   64H    FS segment override prefix
   65H    GS segment override prefix
   66H    Operand-size override
   67H    Address-size override


Figure 17-1.  80386 Instruction Format

      ╔═══════════════╦═══════════════╦═══════════════╦═══════════════╗
      ║  INSTRUCTION  ║   ADDRESS-    ║    OPERAND-   ║   SEGMENT     ║
      ║    PREFIX     ║  SIZE PREFIX  ║  SIZE PREFIX  ║   OVERRIDE    ║
      ╠═══════════════╩═══════════════╩═══════════════╩═══════════════╣
      ║     0 OR 1         0 OR 1           0 OR 1         0 OR 1     ║
      ╟─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─╢
      ║                        NUMBER OF BYTES                        ║
      ╚═══════════════════════════════════════════════════════════════╝

      ╔══════════╦═══════════╦═══════╦══════════════════╦═════════════╗
      ║  OPCODE  ║  MODR/M   ║  SIB  ║   DISPLACEMENT   ║  IMMEDIATE  ║
      ║          ║           ║       ║                  ║             ║
      ╠══════════╩═══════════╩═══════╩══════════════════╩═════════════╣
      ║  1 OR 2     0 OR 1    0 OR 1      0,1,2 OR 4       0,1,2 OR 4 ║
      ╟─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─╢
      ║                        NUMBER OF BYTES                        ║
      ╚═══════════════════════════════════════════════════════════════╝


17.2.1  ModR/M and SIB Bytes

The ModR/M and SIB bytes follow the opcode byte(s) in many of the 80386
instructions. They contain the following information:

  ■  The indexing type or register number to be used in the instruction
  ■  The register to be used, or more information to select the instruction
  ■  The base, index, and scale information

The ModR/M byte contains three fields of information:

  ■  The mod field, which occupies the two most significant bits of the
     byte, combines with the r/m field to form 32 possible values: eight
     registers and 24 indexing modes

  ■  The reg field, which occupies the next three bits following the mod
     field, specifies either a register number or three more bits of opcode
     information. The meaning of the reg field is determined by the first
     (opcode) byte of the instruction.

  ■  The r/m field, which occupies the three least significant bits of the
     byte, can specify a register as the location of an operand, or can form
     part of the addressing-mode encoding in combination with the field as
     described above

The based indexed and scaled indexed forms of 32-bit addressing require the
SIB byte. The presence of the SIB byte is indicated by certain encodings of
the ModR/M byte. The SIB byte then includes the following fields:

  ■  The ss field, which occupies the two most significant bits of the
     byte, specifies the scale factor

  ■  The index field, which occupies the next three bits following the ss
     field and specifies the register number of the index register

  ■  The base field, which occupies the three least significant bits of the
     byte, specifies the register number of the base register

Figure 17-2 shows the formats of the ModR/M and SIB bytes.

The values and the corresponding addressing forms of the ModR/M and SIB
bytes are shown in Tables 17-2, 17-3, and 17-4. The 16-bit addressing
forms specified by the ModR/M byte are in Table 17-2. The 32-bit addressing
forms specified by ModR/M are in Table 17-3. Table 17-4 shows the 32-bit
addressing forms specified by the SIB byte


Figure 17-2.  ModR/M and SIB Byte Formats

                                 MODR/M BYTE

                     7    6    5    4    3    2    1    0
                    ╔════════╦═════════════╦═════════════╗
                    ║  MOD   ║ REG/OPCODE  ║     R/M     ║
                    ╚════════╩═════════════╩═════════════╝

                          SIB (SCALE INDEX BASE) BYTE

                     7    6    5    4    3    2    1    0
                    ╔════════╦═════════════╦═════════════╗
                    ║   SS   ║    INDEX    ║    BASE     ║
                    ╚════════╩═════════════╩═════════════╝


Table 17-2. 16-Bit Addressing Forms with the ModR/M Byte


r8(/r)                     AL    CL    DL    BL    AH    CH    DH    BH
r16(/r)                    AX    CX    DX    BX    SP    BP    SI    DI
r32(/r)                    EAX   ECX   EDX   EBX   ESP   EBP   ESI   EDI
/digit (Opcode)            0     1     2     3     4     5     6     7
REG =                      000   001   010   011   100   101   110   111

   Effective
┌───Address
disp8 denotes an 8-bit displacement following the ModR/M byte, to be
sign-extended and added to the index. disp16 denotes a 16-bit displacement
following the ModR/M byte, to be added to the index. Default segment
register is SS for the effective addresses containing a BP index, DS for
other effective addresses.──┐ ┌Mod R/M┐ ┌────────ModR/M Values in Hexadecimal────────┐

[BX + SI]            000   00    08    10    18    20    28    30    38
[BX + DI]            001   01    09    11    19    21    29    31    39
[BP + SI]            010   02    0A    12    1A    22    2A    32    3A
[BP + DI]            011   03    0B    13    1B    23    2B    33    3B
[SI]             00  100   04    0C    14    1C    24    2C    34    3C
[DI]                 101   05    0D    15    1D    25    2D    35    3D
disp16               110   06    0E    16    1E    26    2E    36    3E
[BX]                 111   07    0F    17    1F    27    2F    37    3F

[BX+SI]+disp8        000   40    48    50    58    60    68    70    78
[BX+DI]+disp8        001   41    49    51    59    61    69    71    79
[BP+SI]+disp8        010   42    4A    52    5A    62    6A    72    7A
[BP+DI]+disp8        011   43    4B    53    5B    63    6B    73    7B
[SI]+disp8       01  100   44    4C    54    5C    64    6C    74    7C
[DI]+disp8           101   45    4D    55    5D    65    6D    75    7D
[BP]+disp8           110   46    4E    56    5E    66    6E    76    7E
[BX]+disp8           111   47    4F    57    5F    67    6F    77    7F

[BX+SI]+disp16       000   80    88    90    98    A0    A8    B0    B8
[BX+DI]+disp16       001   81    89    91    99    A1    A9    B1    B9
[BX+SI]+disp16       010   82    8A    92    9A    A2    AA    B2    BA
[BX+DI]+disp16       011   83    8B    93    9B    A3    AB    B3    BB
[SI]+disp16      10  100   84    8C    94    9C    A4    AC    B4    BC
[DI]+disp16          101   85    8D    95    9D    A5    AD    B5    BD
[BP]+disp16          110   86    8E    96    9E    A6    AE    B6    BE
[BX]+disp16          111   87    8F    97    9F    A7    AF    B7    BF

EAX/AX/AL            000   C0    C8    D0    D8    E0    E8    F0    F8
ECX/CX/CL            001   C1    C9    D1    D9    E1    E9    F1    F9
EDX/DX/DL            010   C2    CA    D2    DA    E2    EA    F2    FA
EBX/BX/BL            011   C3    CB    D3    DB    E3    EB    F3    FB
ESP/SP/AH        11  100   C4    CC    D4    DC    E4    EC    F4    FC
EBP/BP/CH            101   C5    CD    D5    DD    E5    ED    F5    FD
ESI/SI/DH            110   C6    CE    D6    DE    E6    EE    F6    FE
EDI/DI/BH            111   C7    CF    D7    DF    E7    EF    F7    FF


───────────────────────────────────────────────────────────────────────────
NOTES:
  disp8 denotes an 8-bit displacement following the ModR/M byte, to be
  sign-extended and added to the index. disp16 denotes a 16-bit displacement
  following the ModR/M byte, to be added to the index. Default segment
  register is SS for the effective addresses containing a BP index, DS for
  other effective addresses.
───────────────────────────────────────────────────────────────────────────


Table 17-3. 32-Bit Addressing Forms with the ModR/M Byte


r8(/r)                     AL    CL    DL    BL    AH    CH    DH    BH
r16(/r)                    AX    CX    DX    BX    SP    BP    SI    DI
r32(/r)                    EAX   ECX   EDX   EBX   ESP   EBP   ESI   EDI
/digit (Opcode)            0     1     2     3     4     5     6     7
REG =                      000   001   010   011   100   101   110   111

   Effective
┌───Address
[--] [--] means a SIB follows the ModR/M byte. disp8 denotes an 8-bit
displacement following the SIB byte, to be sign-extended and added to the
index. disp32 denotes a 32-bit displacement following the ModR/M byte, to
be added to the index.──┐ ┌Mod R/M┐ ┌─────────ModR/M Values in Hexadecimal───────┐

[EAX]                000   00    08    10    18    20    28    30    38
[ECX]                001   01    09    11    19    21    29    31    39
[EDX]                010   02    0A    12    1A    22    2A    32    3A
[EBX]                011   03    0B    13    1B    23    2B    33    3B
[--] [--]        00  100   04    0C    14    1C    24    2C    34    3C
disp32               101   05    0D    15    1D    25    2D    35    3D
[ESI]                110   06    0E    16    1E    26    2E    36    3E
[EDI]                111   07    0F    17    1F    27    2F    37    3F

disp8[EAX]           000   40    48    50    58    60    68    70    78
disp8[ECX]           001   41    49    51    59    61    69    71    79
disp8[EDX]           010   42    4A    52    5A    62    6A    72    7A
disp8[EPX];          011   43    4B    53    5B    63    6B    73    7B
disp8[--] [--]   01  100   44    4C    54    5C    64    6C    74    7C
disp8[ebp]           101   45    4D    55    5D    65    6D    75    7D
disp8[ESI]           110   46    4E    56    5E    66    6E    76    7E
disp8[EDI]           111   47    4F    57    5F    67    6F    77    7F

disp32[EAX]          000   80    88    90    98    A0    A8    B0    B8
disp32[ECX]          001   81    89    91    99    A1    A9    B1    B9
disp32[EDX]          010   82    8A    92    9A    A2    AA    B2    BA
disp32[EBX]          011   83    8B    93    9B    A3    AB    B3    BB
disp32[--] [--]  10  100   84    8C    94    9C    A4    AC    B4    BC
disp32[EBP]          101   85    8D    95    9D    A5    AD    B5    BD
disp32[ESI]          110   86    8E    96    9E    A6    AE    B6    BE
disp32[EDI]          111   87    8F    97    9F    A7    AF    B7    BF

EAX/AX/AL            000   C0    C8    D0    D8    E0    E8    F0    F8
ECX/CX/CL            001   C1    C9    D1    D9    E1    E9    F1    F9
EDX/DX/DL            010   C2    CA    D2    DA    E2    EA    F2    FA
EBX/BX/BL            011   C3    CB    D3    DB    E3    EB    F3    FB
ESP/SP/AH        11  100   C4    CC    D4    DC    E4    EC    F4    FC
EBP/BP/CH            101   C5    CD    D5    DD    E5    ED    F5    FD
ESI/SI/DH            110   C6    CE    D6    DE    E6    EE    F6    FE
EDI/DI/BH            111   C7    CF    D7    DF    E7    EF    F7    FF


───────────────────────────────────────────────────────────────────────────
NOTES:
  [--] [--] means a SIB follows the ModR/M byte. disp8 denotes an 8-bit
  displacement following the SIB byte, to be sign-extended and added to the
  index. disp32 denotes a 32-bit displacement following the ModR/M byte, to
  be added to the index.
───────────────────────────────────────────────────────────────────────────


Table 17-4. 32-Bit Addressing Forms with the SIB Byte


   r32                      EAX   ECX   EDX   EBX   ESP   [*]
[*] means a disp32 with no base if MOD is 00, [ESP] otherwise. This provides
the following addressing modes:
      disp32[index]        (MOD=00)
      disp8[EBP][index]    (MOD=01)
      disp32[EBP][index]   (MOD=10)  ESI   EDI
   Base =                   0     1     2     3     4     5     6     7
   Base =                   000   001   010   011   100   101   110   111

┌Scaled Index
[*] means a disp32 with no base if MOD is 00, [ESP] otherwise. This provides
the following addressing modes:
      disp32[index]        (MOD=00)
      disp8[EBP][index]    (MOD=01)
      disp32[EBP][index]   (MOD=10)┐┌SS Index┐  ┌────────ModR/M Values in Hexadecimal────────┐

[EAX]                000    00    01    02    03    04    05    06    07
[ECX]                001    08    09    0A    0B    0C    0D    0E    0F
[EDX]                010    10    11    12    13    14    15    16    17
[EBX]                011    18    19    1A    1B    1C    1D    1E    1F
none             00  100    20    21    22    23    24    25    26    27
[EBP]                101    28    29    2A    2B    2C    2D    2E    2F
[ESI]                110    30    31    32    33    34    35    36    37
[EDI]                111    38    39    3A    3B    3C    3D    3E    3F

[EAX*2]              000    40    41    42    43    44    45    46    47
[ECX*2]              001    48    49    4A    4B    4C    4D    4E    4F
[ECX*2]              010    50    51    52    53    54    55    56    57
[EBX*2]              011    58    59    5A    5B    5C    5D    5E    5F
none             01  100    60    61    62    63    64    65    66    67
[EBP*2]              101    68    69    6A    6B    6C    6D    6E    6F
[ESI*2]              110    70    71    72    73    74    75    76    77
[EDI*2]              111    78    79    7A    7B    7C    7D    7E    7F

[EAX*4]              000    80    81    82    83    84    85    86    87
[ECX*4]              001    88    89    8A    8B    8C    8D    8E    8F
[EDX*4]              010    90    91    92    93    94    95    96    97
[EBX*4]              011    98    89    9A    9B    9C    9D    9E    9F
none             10  100    A0    A1    A2    A3    A4    A5    A6    A7
[EBP*4]              101    A8    A9    AA    AB    AC    AD    AE    AF
[ESI*4]              110    B0    B1    B2    B3    B4    B5    B6    B7
[EDI*4]              111    B8    B9    BA    BB    BC    BD    BE    BF

[EAX*8]              000    C0    C1    C2    C3    C4    C5    C6    C7
[ECX*8]              001    C8    C9    CA    CB    CC    CD    CE    CF
[EDX*8]              010    D0    D1    D2    D3    D4    D5    D6    D7
[EBX*8]              011    D8    D9    DA    DB    DC    DD    DE    DF
none             11  100    E0    E1    E2    E3    E4    E5    E6    E7
[EBP*8]              101    E8    E9    EA    EB    EC    ED    EE    EF
[ESI*8]              110    F0    F1    F2    F3    F4    F5    F6    F7
[EDI*8]              111    F8    F9    FA    FB    FC    FD    FE    FF


───────────────────────────────────────────────────────────────────────────
NOTES:
  [*] means a disp32 with no base if MOD is 00, [ESP] otherwise. This
  provides the following addressing modes:
      disp32[index]        (MOD=00)
      disp8[EBP][index]    (MOD=01)
      disp32[EBP][index]   (MOD=10)
───────────────────────────────────────────────────────────────────────────


17.2.2  How to Read the Instruction Set Pages

The following is an example of the format used for each 80386 instruction
description in this chapter:

CMC ── Complement Carry Flag

Opcode   Instruction         Clocks      Description

F5        CMC                  2            Complement carry flag

The above table is followed by paragraphs labelled "Operation,"
"Description," "Flags Affected," "Protected Mode Exceptions," "Real
Address Mode Exceptions," and, optionally, "Notes." The following sections
explain the notational conventions and abbreviations used in these
paragraphs of the instruction descriptions.


17.2.2.1  Opcode

The "Opcode" column gives the complete object code produced for each form
of the instruction. When possible, the codes are given as hexadecimal bytes,
in the same order in which they appear in memory. Definitions of entries
other than hexadecimal bytes are as follows:

/digit: (digit is between 0 and 7) indicates that the ModR/M byte of the
instruction uses only the r/m (register or memory) operand. The reg field
contains the digit that provides an extension to the instruction's opcode.

/r: indicates that the ModR/M byte of the instruction contains both a
register operand and an r/m operand.

cb, cw, cd, cp: a 1-byte (cb), 2-byte (cw), 4-byte (cd) or 6-byte (cp)
value following the opcode that is used to specify a code offset and
possibly a new value for the code segment register.

ib, iw, id: a 1-byte (ib), 2-byte (iw), or 4-byte (id) immediate operand to
the instruction that follows the opcode, ModR/M bytes or scale-indexing
bytes. The opcode determines if the operand is a signed value. All words and
doublewords are given with the low-order byte first.

+rb, +rw, +rd: a register code, from 0 through 7, added to the hexadecimal
byte given at the left of the plus sign to form a single opcode byte. The
codes are──

      rb         rw         rd
    AL = 0     AX = 0     EAX = 0
    CL = 1     CX = 1     ECX = 1
    DL = 2     DX = 2     EDX = 2
    BL = 3     BX = 3     EBX = 3
    AH = 4     SP = 4     ESP = 4
    CH = 5     BP = 5     EBP = 5
    DH = 6     SI = 6     ESI = 6
    BH = 7     DI = 7     EDI = 7


17.2.2.2  Instruction

The "Instruction" column gives the syntax of the instruction statement as
it would appear in an ASM386 program. The following is a list of the symbols
used to represent operands in the instruction statements:

rel8: a relative address in the range from 128 bytes before the end of the
instruction to 127 bytes after the end of the instruction.

rel16, rel32: a relative address within the same code segment as the
instruction assembled. rel16 applies to instructions with an operand-size
attribute of 16 bits; rel32 applies to instructions with an operand-size
attribute of 32 bits.

ptr16:16, ptr16:32: a FAR pointer, typically in a code segment different
from that of the instruction. The notation 16:16 indicates that the value of
the pointer has two parts. The value to the right of the colon is a 16-bit
selector or value destined for the code segment register. The value to the
left corresponds to the offset within the destination segment. ptr16:16 is
used when the instruction's operand-size attribute is 16 bits; ptr16:32 is
used with the 32-bit attribute.

r8: one of the byte registers AL, CL, DL, BL, AH, CH, DH, or BH.

r16: one of the word registers AX, CX, DX, BX, SP, BP, SI, or DI.

r32: one of the doubleword registers EAX, ECX, EDX, EBX, ESP, EBP, ESI, or
EDI.

imm8: an immediate byte value. imm8 is a signed number between -128 and
+127 inclusive. For instructions in which imm8 is combined with a word or
doubleword operand, the immediate value is sign-extended to form a word or
doubleword. The upper byte of the word is filled with the topmost bit of the
immediate value.

imm16: an immediate word value used for instructions whose operand-size
attribute is 16 bits. This is a number between -32768 and +32767 inclusive.

imm32: an immediate doubleword value used for instructions whose
operand-size attribute is 32-bits. It allows the use of a number between
[phone] and -2147483648.

r/m8: a one-byte operand that is either the contents of a byte register
(AL, BL, CL, DL, AH, BH, CH, DH), or a byte from memory.

r/m16: a word register or memory operand used for instructions whose
operand-size attribute is 16 bits. The word registers are: AX, BX, CX, DX,
SP, BP, SI, DI. The contents of memory are found at the address provided by
the effective address computation.

r/m32: a doubleword register or memory operand used for instructions whose
operand-size attribute is 32-bits. The doubleword registers are: EAX, EBX,
ECX, EDX, ESP, EBP, ESI, EDI. The contents of memory are found at the
address provided by the effective address computation.

m8: a memory byte addressed by DS:SI or ES:DI (used only by string
instructions).

m16: a memory word addressed by DS:SI or ES:DI (used only by string
instructions).

m32: a memory doubleword addressed by DS:SI or ES:DI (used only by string
instructions).

m16:16, M16:32: a memory operand containing a far pointer composed of two
numbers. The number to the left of the colon corresponds to the pointer's
segment selector. The number to the right corresponds to its offset.

m16 & 32, m16 & 16, m32 & 32: a memory operand consisting of data item pairs
whose sizes are indicated on the left and the right side of the ampersand.
All memory addressing modes are allowed. m16 & 16 and m32 & 32 operands are
used by the BOUND instruction to provide an operand containing an upper and
lower bounds for array indices. m16 & 32 is used by LIDT and LGDT to
provide a word with which to load the limit field, and a doubleword with
which to load the base field of the corresponding Global and Interrupt
Descriptor Table Registers.

moffs8, moffs16, moffs32: (memory offset) a simple memory variable of type
BYTE, WORD, or DWORD used by some variants of the MOV instruction. The
actual address is given by a simple offset relative to the segment base. No
ModR/M byte is used in the instruction. The number shown with moffs
indicates its size, which is determined by the address-size attribute of the
instruction.

Sreg: a segment register. The segment register bit assignments are ES=0,
CS=1, SS=2, DS=3, FS=4, and GS=5.


17.2.2.3  Clocks

The "Clocks" column gives the number of clock cycles the instruction takes
to execute. The clock count calculations makes the following assumptions:

  ■  The instruction has been prefetched and decoded and is ready for
     execution.

  ■  Bus cycles do not require wait states.

  ■  There are no local bus HOLD requests delaying processor access to the
     bus.

  ■  No exceptions are detected during instruction execution.

  ■  Memory operands are aligned.

Clock counts for instructions that have an r/m (register or memory) operand
are separated by a slash. The count to the left is used for a register
operand; the count to the right is used for a memory operand.

The following symbols are used in the clock count specifications:

  ■  n, which represents a number of repetitions.

  ■  m, which represents the number of components in the next instruction
     executed, where the entire displacement (if any) counts as one
     component, the entire immediate data (if any) counts as one component,
     and every other byte of the instruction and prefix(es) each counts as
     one component.

  ■  pm=, a clock count that applies when the instruction executes in
     Protected Mode. pm= is not given when the clock counts are the same for
     Protected and Real Address Modes.

When an exception occurs during the execution of an instruction and the
exception handler is in another task, the instruction execution time is
increased by the number of clocks to effect a task switch. This parameter
depends on several factors:

  ■  The type of TSS used to represent the current task (386 TSS or 286
     TSS).

  ■  The type of TSS used to represent the new task.

  ■  Whether the current task is in V86 mode.

  ■  Whether the new task is in V86 mode.

Table 17-5 summarizes the task switch times for exceptions.


Table 17-5. Task Switch Times for Exceptions

                       New Task

Old              386 TSS     286 TSS
Task             VM = 0

386   VM = 0       309        282
TSS

386   VM = 1       314        231
TSS

286                307        282
TSS


17.2.2.4  Description

The "Description" column following the "Clocks" column briefly explains the
various forms of the instruction. The "Operation" and "Description" sections
contain more details of the instruction's operation.


17.2.2.5  Operation

The "Operation" section contains an algorithmic description of the
instruction which uses a notation similar to the Algol or Pascal language.
The algorithms are composed of the following elements:

Comments are enclosed within the symbol pairs "(*" and "*)".

Compound statements are enclosed between the keywords of the "if" statement
(IF, THEN, ELSE, FI) or of the "do" statement (DO, OD), or of the "case"
statement (CASE ... OF, ESAC).

A register name implies the contents of the register. A register name
enclosed in brackets implies the contents of the location whose address is
contained in that register. For example, ES:[DI] indicates the contents of
the location whose ES segment relative address is in register DI. [SI]
indicates the contents of the address contained in register SI relative to
SI's default segment (DS) or overridden segment.

Brackets also used for memory operands, where they mean that the contents
of the memory location is a segment-relative offset. For example, [SRC]
indicates that the contents of the source operand is a segment-relative
offset.

A  B; indicates that the value of B is assigned to A.

The symbols =, <>, ≥, and ≤ are relational operators used to compare two
values, meaning equal, not equal, greater or equal, less or equal,
respectively. A relational expression such as A = B is TRUE if the value of
A is equal to B; otherwise it is FALSE.

The following identifiers are used in the algorithmic descriptions:

  ■  OperandSize represents the operand-size attribute of the instruction,
     which is either 16 or 32 bits. AddressSize represents the address-size
     attribute, which is either 16 or 32 bits. For example,

   IF instruction = CMPSW
   THEN OperandSize  16;
   ELSE
      IF instruction = CMPSD
      THEN OperandSize  32;
      FI;
   FI;

indicates that the operand-size attribute depends on the form of the CMPS
instruction used. Refer to the explanation of address-size and operand-size
attributes at the beginning of this chapter for general guidelines on how
these attributes are determined.

  ■  StackAddrSize represents the stack address-size attribute associated
     with the instruction, which has a value of 16 or 32 bits, as explained
     earlier in the chapter.

  ■  SRC represents the source operand. When there are two operands, SRC is
     the one on the right.

  ■  DEST represents the destination operand. When there are two operands,
     DEST is the one on the left.

  ■  LeftSRC, RightSRC distinguishes between two operands when both are
     source operands.

  ■  eSP represents either the SP register or the ESP register depending on
     the setting of the B-bit for the current stack segment.

The following functions are used in the algorithmic descriptions:

  ■  Truncate to 16 bits(value) reduces the size of the value to fit in 16
     bits by discarding the uppermost bits as needed.

  ■  Addr(operand) returns the effective address of the operand (the result
     of the effective address calculation prior to adding the segment base).

  ■  ZeroExtend(value) returns a value zero-extended to the operand-size
     attribute of the instruction. For example, if OperandSize = 32,
     ZeroExtend of a byte value of -10 converts the byte from F6H to
     doubleword with hexadecimal value 000000F6H. If the value passed to
     ZeroExtend and the operand-size attribute are the same size,
     ZeroExtend returns the value unaltered.

  ■  SignExtend(value) returns a value sign-extended to the operand-size
     attribute of the instruction. For example, if OperandSize = 32,
     SignExtend of a byte containing the value -10 converts the byte from
     F6H to a doubleword with hexadecimal value FFFFFFF6H. If the value
     passed to SignExtend and the operand-size attribute are the same size,
     SignExtend returns the value unaltered.

  ■  Push(value) pushes a value onto the stack. The number of bytes pushed
     is determined by the operand-size attribute of the instruction. The
     action of Push is as follows:

   IF StackAddrSize = 16
   THEN
      IF OperandSize = 16
      THEN
         SP  SP - 2;
         SS:[SP]  value; (* 2 bytes assigned starting at
                             byte address in SP *)
      ELSE (* OperandSize = 32 *)
         SP  SP - 4;
         SS:[SP]  value; (* 4 bytes assigned starting at
                             byte address in SP *)
      FI;
   ELSE (* StackAddrSize = 32 *)
      IF OperandSize = 16
      THEN
         ESP  ESP - 2;
         SS:[ESP]  value; (* 2 bytes assigned starting at
                              byte address in ESP*)
      ELSE (* OperandSize = 32 *)
         ESP  ESP - 4;
         SS:[ESP]  value; (* 4 bytes assigned starting at
                              byte address in ESP*)
      FI;
   FI;

  ■  Pop(value) removes the value from the top of the stack and returns it.
     The statement EAX  Pop( ); assigns to EAX the 32-bit value that Pop
     took from the top of the stack. Pop will return either a word or a
     doubleword depending on the operand-size attribute. The action of Pop
     is as follows:

   IF StackAddrSize = 16
   THEN
      IF OperandSize = 16
      THEN
         ret val  SS:[SP]; (* 2-byte value *)
         SP  SP + 2;
      ELSE (* OperandSize = 32 *)
         ret val  SS:[SP]; (* 4-byte value *)
         SP  SP + 4;
      FI;
   ELSE (* StackAddrSize = 32 *)
      IF OperandSize = 16
      THEN
         ret val  SS:[ESP]; (* 2 bytes value *)
         ESP  ESP + 2;
      ELSE (* OperandSize = 32 *)
         ret val  SS:[ESP]; (* 4 bytes value *)
         ESP  ESP + 4;
      FI;
   FI;
   RETURN(ret val); (*returns a word or doubleword*)

  ■  Bit[BitBase, BitOffset] returns the address of a bit within a bit
     string, which is a sequence of bits in memory or a register. Bits are
     numbered from low-order to high-order within registers and within
     memory bytes. In memory, the two bytes of a word are stored with the
     low-order byte at the lower address.

   If the base operand is a register, the offset can be in the range 0..31.
   This offset addresses a bit within the indicated register. An example,
   "BIT[EAX, 21]," is illustrated in Figure 17-3.

   If BitBase is a memory address, BitOffset can range from -2 gigabits to 2
   gigabits. The addressed bit is numbered (Offset MOD 8) within the byte at
   address (BitBase + (BitOffset DIV 8)), where DIV is signed division with
   rounding towards negative infinity, and MOD returns a positive number.
   This is illustrated in Figure 17-4.

  ■  I-O-Permission(I-O-Address, width) returns TRUE or FALSE depending on
   the I/O permission bitmap and other factors. This function is defined as
   follows:

   IF TSS type is 286 THEN RETURN FALSE; FI;
   Ptr  [TSS + 66]; (* fetch bitmap pointer *)
   BitStringAddr  SHR (I-O-Address, 3) + Ptr;
   MaskShift  I-O-Address AND 7;
   CASE width OF:
         BYTE: nBitMask  1;
         WORD: nBitMask  3;
         DWORD: nBitMask  15;
   ESAC;
   mask  SHL (nBitMask, MaskShift);
   CheckString  [BitStringAddr] AND mask;
   IF CheckString = 0
   THEN RETURN (TRUE);
   ELSE RETURN (FALSE);
   FI;

  ■  Switch-Tasks is the task switching function described in Chapter 7.


17.2.2.6  Description

The "Description" section contains further explanation of the instruction's
operation.


Figure 17-3.  Bit Offset for BIT[EAX, 21]

   31                    21                                               0
  ╔═════════════════════╦═╦═══════════════════════════════════════════════╗
  ║                     ║ ║                                               ║
  ╚═════════════════════╩═╩═══════════════════════════════════════════════╝

                         └────────────────────BITOFFSET = 21──────────────┘


Figure 17-4.  Memory Bit Indexing

                         BIT INDEXING (POSITIVE OFFSET)

               7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
             ╔════╤═╤═════════╤═══════════════╤════════════════╗
             ║    │ │         │               │                ║
             ╚════╧═╧═════════╧═══════════════╪════════════════╝
             │  BITBASE + 1   │    BITBASE    │  BITBASE - 1   │
                                             │
                   └────────OFFSET = 13───────┘

                         BIT INDEXING (NEGATIVE OFFSET)

               7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
             ╔════════════════╤═══════════════╤═══╤═╤══════════╗
             ║                │               │   │ │          ║
             ╚════════════════╪═══════════════╧═══╧═╧══════════╝
             │    BITBASE     │  BITBASE - 1  │  BITBASE - 2   │
                              │
                              └─────OFFSET = -11───┘


17.2.2.7  Flags Affected

The "Flags Affected" section lists the flags that are affected by the
instruction, as follows:

  ■  If a flag is always cleared or always set by the instruction, the
     value is given (0 or 1) after the flag name. Arithmetic and logical
     instructions usually assign values to the status flags in the uniform
     manner described in Appendix C. Nonconventional assignments are
     described in the "Operation" section.

  ■  The values of flags listed as "undefined" may be changed by the
     instruction in an indeterminate manner.

All flags not listed are unchanged by the instruction.


17.2.2.8  Protected Mode Exceptions

This section lists the exceptions that can occur when the instruction is
executed in 80386 Protected Mode. The exception names are a pound sign (#)
followed by two letters and an optional error code in parentheses. For
example, #GP(0) denotes a general protection exception with an error code of
0. Table 17-6 associates each two-letter name with the corresponding
interrupt number.

Chapter 9 describes the exceptions and the 80386 state upon entry to the
exception.

Application programmers should consult the documentation provided with
their operating systems to determine the actions taken when exceptions
occur.


Table 17-6. 80386 Exceptions

Mnemonic     Interrupt    Description

#UD           6           Invalid opcode
#NM           7           Coprocessor not available
#DF           8           Double fault
#TS          10           Invalid TSS
#NP          11           Segment or gate not present
#SS          12           Stack fault
#GP          13           General protection fault
#PF          14           Page fault
#MF          16           Math (coprocessor) fault


17.2.2.9  Real Address Mode Exceptions

Because less error checking is performed by the 80386 in Real Address Mode,
this mode has fewer exception conditions. Refer to Chapter 14 for further
information on these exceptions.


17.2.2.10  Virtual-8086 Mode Exceptions

Virtual 8086 tasks provide the ability to simulate Virtual 8086 machines.
Virtual 8086 Mode exceptions are similar to those for the 8086 processor,
but there are some differences. Refer to Chapter 15 for details.


AAA ── ASCII Adjust after Addition

Opcode    Instruction    Clocks    Description

37        AAA            4         ASCII adjust AL after addition


Operation

IF ((AL AND 0FH) > 9) OR (AF = 1)
THEN
   AL  (AL + 6) AND 0FH;
   AH  AH + 1;
   AF  1;
   CF  1;
ELSE
   CF  0;
   AF  0;
FI;

Description

Execute AAA only following an ADD instruction that leaves a byte result
in the AL register. The lower nibbles of the operands of the ADD instruction
should be in the range 0 through 9 (BCD digits). In this case, AAA adjusts
AL to contain the correct decimal digit result. If the addition produced a
decimal carry, the AH register is incremented, and the carry and auxiliary
carry flags are set to 1. If there was no decimal carry, the carry and
auxiliary flags are set to 0 and AH is unchanged. In either case, AL is left
with its top nibble set to 0. To convert AL to an ASCII result, follow the
AAA instruction with OR AL, 30H.

Flags Affected

AF and CF as described above; OF, SF, ZF, and PF are undefined

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


AAD ── ASCII Adjust AX before Division

Opcode    Instruction    Clocks    Description

D5 0A     AAD            19        ASCII adjust AX before division


Operation

AL  AH * 10 + AL;
AH  0;

Description

AAD is used to prepare two unpacked BCD digits (the least-significant
digit in AL, the most-significant digit in AH) for a division operation that
will yield an unpacked result. This is accomplished by setting AL to
AL + (10 * AH), and then setting AH to 0. AX is then equal to the binary
equivalent of the original unpacked two-digit number.

Flags Affected

SF, ZF, and PF as described in Appendix C; OF, AF, and CF are undefined

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


AAM ── ASCII Adjust AX after Multiply

Opcode    Instruction    Clocks    Description

D4 0A     AAM            17        ASCII adjust AX after multiply


Operation

AH  AL / 10;
AL  AL MOD 10;

Description

Execute AAM only after executing a MUL instruction between two unpacked
BCD digits that leaves the result in the AX register. Because the result is
less than 100, it is contained entirely in the AL register. AAM unpacks the
AL result by dividing AL by 10, leaving the quotient (most-significant
digit) in AH and the remainder (least-significant digit) in AL.

Flags Affected

SF, ZF, and PF as described in Appendix C; OF, AF, and CF are undefined

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


AAS ── ASCII Adjust AL after Subtraction

Opcode    Instruction    Clocks    Description

3F        AAS            4         ASCII adjust AL after subtraction


Operation

IF (AL AND 0FH) > 9 OR AF = 1
THEN
   AL  AL - 6;
   AL  AL AND 0FH;
   AH  AH - 1;
   AF  1;
   CF  1;
ELSE
   CF  0;
   AF  0;
FI;

Description

Execute AAS only after a SUB instruction that leaves the byte result in the
AL register. The lower nibbles of the operands of the SUB instruction must
have been in the range 0 through 9 (BCD digits). In this case, AAS adjusts
AL so it contains the correct decimal digit result. If the subtraction
produced a decimal carry, the AH register is decremented, and the carry and
auxiliary carry flags are set to 1. If no decimal carry occurred, the carry
and auxiliary carry flags are set to 0, and AH is unchanged. In either case,
AL is left with its top nibble set to 0. To convert AL to an ASCII result,
follow the AAS with OR AL, 30H.

Flags Affected

AF and CF as described above; OF, SF, ZF, and PF are undefined

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


ADC ── Add with Carry


Opcode    Instruction      Clocks    Description

14 ib     ADC AL,imm8      2         Add with carry immediate byte to AL
15 iw     ADC AX,imm16     2         Add with carry immediate word to AX
15 id     ADC EAX,imm32    2         Add with carry immediate dword to EAX
80 /2 ib  ADC r/m8,imm8    2/7       Add with carry immediate byte to r/m
                                     byte
81 /2 iw  ADC r/m16,imm16  2/7       Add with carry immediate word to r/m
                                     word
81 /2 id  ADC r/m32,imm32  2/7       Add with CF immediate dword to r/m
                                     dword
83 /2 ib  ADC r/m16,imm8   2/7       Add with CF sign-extended immediate
                                     byte to r/m word
83 /2 ib  ADC r/m32,imm8   2/7       Add with CF sign-extended immediate
                                     byte into r/m dword
10 /r     ADC r/m8,r8      2/7       Add with carry byte register to r/m
                                     byte
11 /r     ADC r/m16,r16    2/7       Add with carry word register to r/m
                                     word
11 /r     ADC r/m32,r32    2/7       Add with CF dword register to r/m dword
12 /r     ADC r8,r/m8      2/6       Add with carry r/m byte to byte
                                     register
13 /r     ADC r16,r/m16    2/6       Add with carry r/m word to word
                                     register
13 /r     ADC r32,r/m32    2/6       Add with CF r/m dword to dword register


Operation

DEST  DEST + SRC + CF;

Description

ADC performs an integer addition of the two operands DEST and SRC and the
carry flag, CF. The result of the addition is assigned to the first operand
(DEST), and the flags are set accordingly. ADC is usually executed as part
of a multi-byte or multi-word addition operation. When an immediate byte
value is added to a word or doubleword operand, the immediate value is first
sign-extended to the size of the word or doubleword operand.

Flags Affected

OF, SF, ZF, AF, CF, and PF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) if page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


ADD ── Add

Opcode    Instruction         Clocks    Description

04 ib     ADD AL,imm8          2        Add immediate byte to AL
05 iw     ADD AX,imm16         2        Add immediate word to AX
05 id     ADD EAX,imm32        2        Add immediate dword to EAX
80 /0 ib  ADD r/m8,imm8        2/7      Add immediate byte to r/m byte
81 /0 iw  ADD r/m16,imm16      2/7      Add immediate word to r/m word
81 /0 id  ADD r/m32,imm32      2/7      Add immediate dword to r/m dword
83 /0 ib  ADD r/m16,imm8       2/7      Add sign-extended immediate byte
                                        to r/m word
83 /0 ib  ADD r/m32,imm8       2/7      Add sign-extended immediate byte
                                        to r/m dword
00 /r     ADD r/m8,r8          2/7      Add byte register to r/m byte
01 /r     ADD r/m16,r16        2/7      Add word register to r/m word
01 /r     ADD r/m32,r32        2/7      Add dword register to r/m dword
02 /r     ADD r8,r/m8          2/6      Add r/m byte to byte register
03 /r     ADD r16,r/m16        2/6      Add r/m word to word register
03 /r     ADD r32,r/m32        2/6      Add r/m dword to dword register


Operation

DEST  DEST + SRC;

Description

ADD performs an integer addition of the two operands (DEST and SRC). The
result of the addition is assigned to the first operand (DEST), and the
flags are set accordingly.

When an immediate byte is added to a word or doubleword operand, the
immediate value is sign-extended to the size of the word or doubleword
operand.

Flags Affected

OF, SF, ZF, AF, CF, and PF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


AND ── Logical AND

Opcode    Instruction          Clocks    Description

24 ib     AND AL,imm8          2         AND immediate byte to AL
25 iw     AND AX,imm16         2         AND immediate word to AX
25 id     AND EAX,imm32        2         AND immediate dword to EAX
80 /4 ib  AND r/m8,imm8        2/7       AND immediate byte to r/m byte
81 /4 iw  AND r/m16,imm16      2/7       AND immediate word to r/m word
81 /4 id  AND r/m32,imm32      2/7       AND immediate dword to r/m dword
83 /4 ib  AND r/m16,imm8       2/7       AND sign-extended immediate byte
                                         with r/m word
83 /4 ib  AND r/m32,imm8       2/7       AND sign-extended immediate byte
                                         with r/m dword
20 /r     AND r/m8,r8          2/7       AND byte register to r/m byte
21 /r     AND r/m16,r16        2/7       AND word register to r/m word
21 /r     AND r/m32,r32        2/7       AND dword register to r/m dword
22 /r     AND r8,r/m8          2/6       AND r/m byte to byte register
23 /r     AND r16,r/m16        2/6       AND r/m word to word register
23 /r     AND r32,r/m32        2/6       AND r/m dword to dword register


Operation

DEST  DEST AND SRC;
CF  0;
OF  0;

Description

Each bit of the result of the AND instruction is a 1 if both corresponding
bits of the operands are 1; otherwise, it becomes a 0.

Flags Affected

CF = 0, OF = 0; PF, SF, and ZF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


ARPL ── Adjust RPL Field of Selector

Opcode    Instruction          Clocks    Description

63 /r     ARPL r/m16,r16       pm=20/21  Adjust RPL of r/m16 to not
                                         less than RPL of r16


Operation

IF RPL bits(0,1) of DEST < RPL bits(0,1) of SRC
THEN
   ZF  1;
   RPL bits(0,1) of DEST  RPL bits(0,1) of SRC;
ELSE
   ZF  0;
FI;

Description

The ARPL instruction has two operands. The first operand is a 16-bit
memory variable or word register that contains the value of a selector. The
second operand is a word register. If the RPL field ("requested privilege
level"──bottom two bits) of the first operand is less than the RPL field of
the second operand, the zero flag is set to 1 and the RPL field of the
first operand is increased to match the second operand. Otherwise, the zero
flag is set to 0 and no change is made to the first operand.

ARPL appears in operating system software, not in application programs. It
is used to guarantee that a selector parameter to a subroutine does not
request more privilege than the caller is allowed. The second operand of
ARPL is normally a register that contains the CS selector value of the
caller.

Flags Affected

ZF as described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 6; ARPL is not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


BOUND ── Check Array Index Against Bounds

Opcode    Instruction          Clocks    Description

62 /r     BOUND r16,m16&16     10        Check if r16 is within bounds
                                         (passes test)
62 /r     BOUND r32,m32&32     10        Check if r32 is within bounds
                                         (passes test)


Operation

IF (LeftSRC < [RightSRC] OR LeftSRC > [RightSRC + OperandSize/8])
   (* Under lower bound or over upper bound *)
THEN Interrupt 5;
FI;

Description

BOUND ensures that a signed array index is within the limits specified by a
block of memory consisting of an upper and a lower bound. Each bound uses
one word for an operand-size attribute of 16 bits and a doubleword for an
operand-size attribute of 32 bits. The first operand (a register) must be
greater than or equal to the first bound in memory (lower bound), and less
than or equal to the second bound in memory (upper bound). If the register
is not within bounds, an Interrupt 5 occurs; the return EIP points to the
BOUND instruction.

The bounds limit data structure is usually placed just before the array
itself, making the limits addressable via a constant offset from the
beginning of the array.

Flags Affected

None

Protected Mode Exceptions

Interrupt 5 if the bounds test fails, as described above; #GP(0) for an
illegal memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

The second operand must be a memory operand, not a register. If BOUND is
executed with a ModRM byte representing a register as the second operand,
#UD occurs.

Real Address Mode Exceptions

Interrupt 5 if the bounds test fails; Interrupt 13 if any part of the
operand would lie outside of the effective address space from 0 to 0FFFFH;
Interrupt 6 if the second operand is a register

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


BSF ── Bit Scan Forward

Opcode    Instruction          Clocks    Description

0F  BC    BSF r16,r/m16        10+3n     Bit scan forward on r/m word
0F  BC    BSF r32,r/m32        10+3n     Bit scan forward on r/m dword


Notes

 is the number of leading zero bits.

Operation

IF r/m = 0
THEN
   ZF  1;
   register  UNDEFINED;
ELSE
   temp  0;
   ZF  0;
   WHILE BIT[r/m, temp = 0]
   DO
      temp  temp + 1;
      register  temp;
   OD;
FI;

Description

BSF scans the bits in the second word or doubleword operand starting with
bit 0. The ZF flag is cleared if the bits are all 0; otherwise, the ZF flag
is set and the destination register is loaded with the bit index of the
first set bit.

Flags Affected

ZF as described above

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


BSR ── Bit Scan Reverse

Opcode    Instruction          Clocks    Description

0F  BD    BSR r16,r/m16        10+3n     Bit scan reverse on r/m word
0F  BD    BSR r32,r/m32        10+3n     Bit scan reverse on r/m dword


Operation

IF r/m = 0
THEN
   ZF  1;
   register  UNDEFINED;
ELSE
   temp  OperandSize - 1;
   ZF  0;
   WHILE BIT[r/m, temp] = 0
   DO
      temp  temp - 1;
      register  temp;
   OD;
FI;

Description

BSR scans the bits in the second word or doubleword operand from the most
significant bit to the least significant bit. The ZF flag is cleared if the
bits are all 0; otherwise, ZF is set and the destination register is loaded
with the bit index of the first set bit found when scanning in the reverse
direction.

Flags Affected

ZF as described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


BT ── Bit Test

Opcode         Instruction     Clocks    Description

0F  A3         BT r/m16,r16    3/12      Save bit in carry flag
0F  A3         BT r/m32,r32    3/12      Save bit in carry flag
0F  BA /4 ib   BT r/m16,imm8   3/6       Save bit in carry flag
0F  BA /4 ib   BT r/m32,imm8   3/6       Save bit in carry flag


Operation

CF  BIT[LeftSRC, RightSRC];

Description

BT saves the value of the bit indicated by the base (first operand) and the
bit offset (second operand) into the carry flag.

Flags Affected

CF as described above

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

The index of the selected bit can be given by the immediate constant in the
instruction or by a value in a general register. Only an 8-bit immediate
value is used in the instruction. This operand is taken modulo 32, so the
range of immediate bit offsets is 0..31. This allows any bit within a
register to be selected. For memory bit strings, this immediate field gives
only the bit offset within a word or doubleword. Immediate bit offsets
larger than 31 are supported by using the immediate bit offset field in
combination with the displacement field of the memory operand. The low-order
3 to 5 bits of the immediate bit offset are stored in the immediate bit
offset field, and the high-order 27 to 29 bits are shifted and combined with
the byte displacement in the addressing mode.

When accessing a bit in memory, the 80386 may access four bytes starting
from the memory address given by:

   Effective Address + (4 * (BitOffset DIV 32))

for a 32-bit operand size, or two bytes starting from the memory address
given by:

   Effective Address + (2 * (BitOffset DIV 16))

for a 16-bit operand size. It may do so even when only a single byte needs
to be accessed in order to reach the given bit. You must therefore avoid
referencing areas of memory close to address space holes. In particular,
avoid references to memory-mapped I/O registers. Instead, use the MOV
instructions to load from or store to these addresses, and use the register
form of these instructions to manipulate the data.


BTC ── Bit Test and Complement

Opcode        Instruction     Clocks  Description

0F  BB        BTC r/m16,r16   6/13    Save bit in carry flag and complement
0F  BB        BTC r/m32,r32   6/13    Save bit in carry flag and complement
0F  BA /7 ib  BTC r/m16,imm8  6/8     Save bit in carry flag and complement
0F  BA /7 ib  BTC r/m32,imm8  6/8     Save bit in carry flag and complement


Operation

CF  BIT[LeftSRC, RightSRC];
BIT[LeftSRC, RightSRC]  NOT BIT[LeftSRC, RightSRC];

Description

BTC saves the value of the bit indicated by the base (first operand) and the
bit offset (second operand) into the carry flag and then complements the
bit.

Flags Affected

CF as described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

The index of the selected bit can be given by the immediate constant in the
instruction or by a value in a general register. Only an 8-bit immediate
value is used in the instruction. This operand is taken modulo 32, so the
range of immediate bit offsets is 0..31. This allows any bit within a
register to be selected. For memory bit strings, this immediate field gives
only the bit offset within a word or doubleword. Immediate bit offsets
larger than 31 are supported by using the immediate bit offset field in
combination with the displacement field of the memory operand. The low-order
3 to 5 bits of the immediate bit offset are stored in the immediate bit
offset field, and the high-order 27 to 29 bits are shifted and combined with
the byte displacement in the addressing mode.

When accessing a bit in memory, the 80386 may access four bytes starting
from the memory address given by:

   Effective Address + (4 * (BitOffset DIV 32))

for a 32-bit operand size, or two bytes starting from the memory address
given by:

   Effective Address + (2 * (BitOffset DIV 16))

for a 16-bit operand size. It may do so even when only a single byte needs
to be accessed in order to reach the given bit. You must therefore avoid
referencing areas of memory close to address space holes. In particular,
avoid references to memory-mapped I/O registers. Instead, use the MOV
instructions to load from or store to these addresses, and use the register
form of these instructions to manipulate the data.


BTR ── Bit Test and Reset

Opcode        Instruction     Clocks  Description

0F  B3        BTR r/m16,r16   6/13    Save bit in carry flag and reset
0F  B3        BTR r/m32,r32   6/13    Save bit in carry flag and reset
0F  BA /6 ib  BTR r/m16,imm8  6/8     Save bit in carry flag and reset
0F  BA /6 ib  BTR r/m32,imm8  6/8     Save bit in carry flag and reset


Operation

CF  BIT[LeftSRC, RightSRC];
BIT[LeftSRC, RightSRC]  0;

Description

BTR saves the value of the bit indicated by the base (first operand) and the
bit offset (second operand) into the carry flag and then stores 0 in the
bit.

Flags Affected

CF as described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

The index of the selected bit can be given by the immediate constant in the
instruction or by a value in a general register. Only an 8-bit immediate
value is used in the instruction. This operand is taken modulo 32, so the
range of immediate bit offsets is 0..31. This allows any bit within a
register to be selected. For memory bit strings, this immediate field gives
only the bit offset within a word or doubleword. Immediate bit offsets
larger than 31 (or 15) are supported by using the immediate bit offset field
in combination with the displacement field of the memory operand. The
low-order 3 to 5 bits of the immediate bit offset are stored in the
immediate bit offset field, and the high-order 27 to 29 bits are shifted and
combined with the byte displacement in the addressing mode.

When accessing a bit in memory, the 80386 may access four bytes starting
from the memory address given by:

   Effective Address + 4 * (BitOffset DIV 32)

for a 32-bit operand size, or two bytes starting from the memory address
given by:

   Effective Address + 2 * (BitOffset DIV 16)

for a 16-bit operand size. It may do so even when only a single byte needs
to be accessed in order to reach the given bit. You must therefore avoid
referencing areas of memory close to address space holes. In particular,
avoid references to memory-mapped I/O registers. Instead, use the MOV
instructions to load from or store to these addresses, and use the register
form of these instructions to manipulate the data.


BTS ── Bit Test and Set

Opcode        Instruction     Clocks  Description

0F  AB        BTS r/m16,r16   6/13    Save bit in carry flag and set
0F  AB        BTS r/m32,r32   6/13    Save bit in carry flag and set
0F  BA /5 ib  BTS r/m16,imm8  6/8     Save bit in carry flag and set
0F  BA /5 ib  BTS r/m32,imm8  6/8     Save bit in carry flag and set


Operation

CF  BIT[LeftSRC, RightSRC];
BIT[LeftSRC, RightSRC]  1;

Description

BTS saves the value of the bit indicated by the base (first operand) and the
bit offset (second operand) into the carry flag and then stores 1 in the
bit.

Flags Affected

CF as described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

The index of the selected bit can be given by the immediate constant in the
instruction or by a value in a general register. Only an 8-bit immediate
value is used in the instruction. This operand is taken modulo 32, so the
range of immediate bit offsets is 0..31. This allows any bit within a
register to be selected. For memory bit strings, this immediate field gives
only the bit offset within a word or doubleword. Immediate bit offsets
larger than 31 are supported by using the immediate bit offset field in
combination with the displacement field of the memory operand. The
low-order 3 to 5 bits of the immediate bit offset are stored in the
immediate bit offset field, and the high order 27 to 29 bits are shifted and
combined with the byte displacement in the addressing mode.

When accessing a bit in memory, the processor may access four bytes starting
from the memory address given by:

   Effective Address + (4 * (BitOffset DIV 32))

for a 32-bit operand size, or two bytes starting from the memory address
given by:

   Effective Address + (2 * (BitOffset DIV 16))

for a 16-bit operand size. It may do this even when only a single byte needs
to be accessed in order to get at the given bit. Thus the programmer must be
careful to avoid referencing areas of memory close to address space holes.
In particular, avoid references to memory-mapped I/O registers. Instead, use
the MOV instructions to load from or store to these addresses, and use the
register form of these instructions to manipulate the data.


CALL ── Call Procedure


Opcode    Instruction     Clocks
  Values of ts are given by the following table:

                             New Task
              386 TSS         386 TSS         286 TSS
  Old         VM = 0          VM = 1
  Task                     Via Task Gate?

              N     Y         N     Y         N     Y

386          300   309       217   226       273   282
TSS VM=0

286          298   307       217   226       273   282
TSS         Description

E8  cw    CALL rel16       7+m            Call near, displacement relative
                                          to next instruction
FF  /2    CALL r/m16       7+m/10+m       Call near, register
                                          indirect/memory indirect
9A  cd    CALL ptr16:16    17+m,pm=34+m   Call intersegment, to full
                                          pointer given
9A  cd    CALL ptr16:16    pm=52+m        Call gate, same privilege
9A  cd    CALL ptr16:16    pm=86+m        Call gate, more privilege, no
                                          parameters
9A  cd    CALL ptr16:16    pm=94+4x+m     Call gate, more privilege, x
                                          parameters
9A  cd    CALL ptr16:16    ts             Call to task
FF  /3    CALL m16:16      22+m,pm=38+m   Call intersegment, address at
                                          r/m dword
FF  /3    CALL m16:16      pm=56+m        Call gate, same privilege
FF  /3    CALL m16:16      pm=90+m        Call gate, more privilege, no
                                          parameters
FF  /3    CALL m16:16      pm=98+4x+m     Call gate, more privilege, x
                                          parameters
FF  /3    CALL m16:16      5 + ts         Call to task
E8  cd    CALL rel32       7+m            Call near, displacement relative
                                          to next instruction
FF  /2    CALL r/m32       7+m/10+m       Call near, indirect
9A  cp    CALL ptr16:32    17+m,pm=34+m   Call intersegment, to full
                                          pointer given
9A  cp    CALL ptr16:32    pm=52+m        Call gate, same privilege
9A  cp    CALL ptr16:32    pm=86+m        Call gate, more privilege, no
                                          parameters
9A  cp    CALL ptr32:32    pm=94+4x+m     Call gate, more privilege, x
                                          parameters
9A  cp    CALL ptr16:32    ts             Call to task
FF  /3    CALL m16:32      22+m,pm=38+m   Call intersegment, address at
                                          r/m dword
FF  /3    CALL m16:32      pm=56+m        Call gate, same privilege
FF  /3    CALL m16:32      pm=90+m        Call gate, more privilege, no
                                          parameters
FF  /3    CALL m16:32      pm=98+4x+m     Call gate, more privilege, x
                                          parameters
FF  /3    CALL m16:32      5 + ts         Call to task


───────────────────────────────────────────────────────────────────────────
NOTE:
  Values of ts are given by the following table:

                             New Task
              386 TSS         386 TSS         286 TSS
  Old         VM = 0          VM = 1
  Task                     Via Task Gate?

              N     Y         N     Y         N     Y

386          300   309       217   226       273   282
TSS VM=0

286          298   307       217   226       273   282
TSS
───────────────────────────────────────────────────────────────────────────

Operation

IF rel16 or rel32 type of call
THEN (* near relative call *)
   IF OperandSize = 16
   THEN
      Push(IP);
      EIP  (EIP + rel16) AND 0000FFFFH;
   ELSE (* OperandSize = 32 *)
      Push(EIP);
      EIP  EIP + rel32;
   FI;
FI;

IF r/m16 or r/m32 type of call
THEN (* near absolute call *)
   IF OperandSize = 16
   THEN
      Push(IP);
      EIP  [r/m16] AND 0000FFFFH;
   ELSE (* OperandSize = 32 *)
      Push(EIP);
      EIP  [r/m32];
   FI;
FI;

IF (PE = 0 OR (PE = 1 AND VM = 1))
(* real mode or virtual 8086 mode *)
   AND instruction = far CALL
   (* i.e., operand type is m16:16, m16:32, ptr16:16, ptr16:32 *)
THEN
   IF OperandSize = 16
   THEN
      Push(CS);
      Push(IP); (* address of next instruction; 16 bits *)
   ELSE
      Push(CS); (* padded with 16 high-order bits *)
      Push(EIP); (* address of next instruction; 32 bits *)
   FI;
   IF operand type is m16:16 or m16:32
   THEN (* indirect far call *)
      IF OperandSize = 16
      THEN
         CS:IP  [m16:16];
         EIP  EIP AND 0000FFFFH; (* clear upper 16 bits *)
      ELSE (* OperandSize = 32 *)
         CS:EIP  [m16:32];
      FI;
   FI;
   IF operand type is ptr16:16 or ptr16:32
   THEN (* direct far call *)
      IF OperandSize = 16
      THEN
         CS:IP  ptr16:16;
         EIP  EIP AND 0000FFFFH; (* clear upper 16 bits *)
      ELSE (* OperandSize = 32 *)
         CS:EIP  ptr16:32;
      FI;
   FI;
FI;

IF (PE = 1 AND VM = 0) (* Protected mode, not V86 mode *)
   AND instruction = far CALL
THEN
   If indirect, then check access of EA doubleword;
      #GP(0) if limit violation;
   New CS selector must not be null else #GP(0);
   Check that new CS selector index is within its
      descriptor table limits; else #GP(new CS selector);
   Examine AR byte of selected descriptor for various legal values;
      depending on value:
      go to CONFORMING-CODE-SEGMENT;
      go to NONCONFORMING-CODE-SEGMENT;
      go to CALL-GATE;
      go to TASK-GATE;
      go to TASK-STATE-SEGMENT;
   ELSE #GP(code segment selector);
FI;

CONFORMING-CODE-SEGMENT:
   DPL must be ≤ CPL ELSE #GP(code segment selector);
   Segment must be present ELSE #NP(code segment selector);
   Stack must be big enough for return address ELSE #SS(0);
   Instruction pointer must be in code segment limit ELSE #GP(0);
   Load code segment descriptor into CS register;
   Load CS with new code segment selector;
   Load EIP with zero-extend(new offset);
   IF OperandSize=16 THEN EIP  EIP AND 0000FFFFH; FI;

NONCONFORMING-CODE-SEGMENT:
   RPL must be ≤ CPL ELSE #GP(code segment selector)
   DPL must be = CPL ELSE #GP(code segment selector)
   Segment must be present ELSE #NP(code segment selector)
   Stack must be big enough for return address ELSE #SS(0)
   Instruction pointer must be in code segment limit ELSE #GP(0)
   Load code segment descriptor into CS register
   Load CS with new code segment selector
   Set RPL of CS to CPL
   Load EIP with zero-extend(new offset);
   IF OperandSize=16 THEN EIP  EIP AND 0000FFFFH; FI;

CALL-GATE:
   Call gate DPL must be ≥ CPL ELSE #GP(call gate selector)
   Call gate DPL must be ≥ RPL ELSE #GP(call gate selector)
   Call gate must be present ELSE #NP(call gate selector)
   Examine code segment selector in call gate descriptor:
      Selector must not be null ELSE #GP(0)
      Selector must be within its descriptor table
         limits ELSE #GP(code segment selector)
   AR byte of selected descriptor must indicate code
      segment ELSE #GP(code segment selector)
   DPL of selected descriptor must be ≤ CPL ELSE
      #GP(code segment selector)
   IF non-conforming code segment AND DPL < CPL
   THEN go to MORE-PRIVILEGE
   ELSE go to SAME-PRIVILEGE
   FI;

MORE-PRIVILEGE:
   Get new SS selector for new privilege level from TSS
      Check selector and descriptor for new SS:
         Selector must not be null ELSE #TS(0)
         Selector index must be within its descriptor
            table limits ELSE #TS(SS selector)
         Selector's RPL must equal DPL of code segment
            ELSE #TS(SS selector)
         Stack segment DPL must equal DPL of code
            segment ELSE #TS(SS selector)
         Descriptor must indicate writable data segment
            ELSE #TS(SS selector)
         Segment present ELSE #SS(SS selector)
      IF OperandSize=32
      THEN
         New stack must have room for parameters plus 16 bytes
            ELSE #SS(0)
         EIP must be in code segment limit ELSE #GP(0)
         Load new SS:eSP value from TSS
         Load new CS:EIP value from gate
      ELSE
         New stack must have room for parameters plus 8 bytes ELSE #SS(0)
         IP must be in code segment limit ELSE #GP(0)
         Load new SS:eSP value from TSS
         Load new CS:IP value from gate
      FI;
      Load CS descriptor
      Load SS descriptor
      Push long pointer of old stack onto new stack
      Get word count from call gate, mask to 5 bits
      Copy parameters from old stack onto new stack
      Push return address onto new stack
      Set CPL to stack segment DPL
      Set RPL of CS to CPL

SAME-PRIVILEGE:
   IF OperandSize=32
   THEN
      Stack must have room for 6-byte return address (padded to 8 bytes)
         ELSE #SS(0)
      EIP must be within code segment limit ELSE #GP(0)
      Load CS:EIP from gate
   ELSE
      Stack must have room for 4-byte return address ELSE #SS(0)
      IP must be within code segment limit ELSE #GP(0)
      Load CS:IP from gate
   FI;
   Push return address onto stack
   Load code segment descriptor into CS register
   Set RPL of CS to CPL

TASK-GATE:
   Task gate DPL must be ≥ CPL ELSE #TS(gate selector)
   Task gate DPL must be ≥ RPL ELSE #TS(gate selector)
   Task Gate must be present ELSE #NP(gate selector)
   Examine selector to TSS, given in Task Gate descriptor:
      Must specify global in the local/global bit ELSE #TS(TSS selector)
      Index must be within GDT limits ELSE #TS(TSS selector)
      TSS descriptor AR byte must specify nonbusy TSS
         ELSE #TS(TSS selector)
      Task State Segment must be present ELSE #NP(TSS selector)
   SWITCH-TASKS (with nesting) to TSS
   IP must be in code segment limit ELSE #TS(0)

TASK-STATE-SEGMENT:
   TSS DPL must be ≥ CPL else #TS(TSS selector)
   TSS DPL must be ≥ RPL ELSE #TS(TSS selector)
   TSS descriptor AR byte must specify available TSS
      ELSE #TS(TSS selector)
   Task State Segment must be present ELSE #NP(TSS selector)
   SWITCH-TASKS (with nesting) to TSS
   IP must be in code segment limit ELSE #TS(0)

Description

The CALL instruction causes the procedure named in the operand to be
executed. When the procedure is complete (a return instruction is executed
within the procedure), execution continues at the instruction that follows
the CALL instruction.

The action of the different forms of the instruction are described below.

Near calls are those with destinations of type r/m16, r/m32, rel16, rel32;
changing or saving the segment register value is not necessary. The CALL
rel16 and CALL rel32 forms add a signed offset to the address of the
instruction following CALL to determine the destination. The rel16 form is
used when the instruction's operand-size attribute is 16 bits; rel32 is used
when the operand-size attribute is 32 bits. The result is stored in the
32-bit EIP register. With rel16, the upper 16 bits of EIP are cleared,
resulting in an offset whose value does not exceed 16 bits. CALL r/m16 and
CALL r/m32 specify a register or memory location from which the absolute
segment offset is fetched. The offset fetched from r/m is 32 bits for an
operand-size attribute of 32 (r/m32), or 16 bits for an operand-size of 16
(r/m16). The offset of the instruction following CALL is pushed onto the
stack. It will be popped by a near RET instruction within the procedure. The
CS register is not changed by this form of CALL.

The far calls, CALL ptr16:16 and CALL ptr16:32, use a four-byte or six-byte
operand as a long pointer to the procedure called. The CALL m16:16 and
m16:32 forms fetch the long pointer from the memory location
specified (indirection). In Real Address Mode or Virtual 8086 Mode, the long
pointer provides 16 bits for the CS register and 16 or 32 bits for the EIP
register (depending on the operand-size attribute). These forms of the
instruction push both CS and IP or EIP as a return address.

In Protected Mode, both long pointer forms consult the AR byte in the
descriptor indexed by the selector part of the long pointer. Depending on
the value of the AR byte, the call will perform one of the following types
of control transfers:

  ■  A far call to the same protection level
  ■  An inter-protection level far call
  ■  A task switch

For more information on Protected Mode control transfers, refer to
Chapter 6 and Chapter 7.

Flags Affected

All flags are affected if a task switch occurs; no flags are affected if a
task switch does not occur

Protected Mode Exceptions

For far calls: #GP, #NP, #SS, and #TS, as indicated in the list above

For near direct calls: #GP(0) if procedure location is beyond the code
segment limits; #SS(0) if pushing the return address exceeds the bounds of
the stack segment; #PF (fault-code) for a page fault

For a near indirect call: #GP(0) for an illegal memory operand effective
address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address
in the SS segment; #GP(0) if the indirect offset obtained is beyond the code
segment limits; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

Any far call from a 32-bit code segment to 16-bit code segments should be
made from the first 64K bytes of the 32-bit code segment, since the
operand-size attribute of the instruction is set to 16, thus allowing only a
16-bit return address offset to be saved.


CBW/CWDE ── Convert Byte to Word/Convert Word to Doubleword

Opcode    Instruction     Clocks          Description

98        CBW             3               AX  sign-extend of AL
98        CWDE            3               EAX  sign-extend of AX


Operation

IF OperandSize = 16 (* instruction = CBW *)
THEN AX  SignExtend(AL);
ELSE (* OperandSize = 32, instruction = CWDE *)
   EAX  SignExtend(AX);
FI;

Description

CBW converts the signed byte in AL to a signed word in AX by extending the
most significant bit of AL (the sign bit) into all of the bits of AH. CWDE
converts the signed word in AX to a doubleword in EAX by extending the most
significant bit of AX into the two most significant bytes of EAX. Note that
CWDE is different from CWD. CWD uses DX:AX rather than EAX as a destination.

Flags Affected

None

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


CLC ── Clear Carry Flag

Opcode    Instruction     Clocks          Description

F8        CLC             2               Clear carry flag


Operation

CF  0;

Description

CLC sets the carry flag to zero. It does not affect other flags or
registers.

Flags Affected

CF = 0

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


CLD ── Clear Direction Flag

Opcode    Instruction     Clocks   Description

FC        CLD             2        Clear direction flag; SI and DI
                                   will increment during string
                                   instructions


Operation

DF  0;

Description

CLD clears the direction flag. No other flags or registers are affected.
After CLD is executed, string operations will increment the index registers
(SI and/or DI) that they use.

Flags Affected

DF = 0

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


CLI ── Clear Interrupt Flag

Opcode    Instruction    Clocks   Description

FA        CLI            3        Clear interrupt flag; interrupts disabled


Operation

IF  0;

Description

CLI clears the interrupt flag if the current privilege level is at least as
privileged as IOPL. No other flags are affected. External interrupts are not
recognized at the end of the CLI instruction or from that point on until the
interrupt flag is set.

Flags Affected

IF = 0

Protected Mode Exceptions

#GP(0) if the current privilege level is greater (has less privilege) than
the IOPL in the flags register. IOPL specifies the least privileged level at
which I/O can be performed.

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

#GP(0) as for Protected Mode


CLTS ── Clear Task-Switched Flag in CR0

Opcode    Instruction    Clocks   Description

OF  06    CLTS           5        Clear task-switched flag


Operation

TS Flag in CR0  0;

Description

CLTS clears the task-switched (TS) flag in register CR0. This flag is set by
the 80386 every time a task switch occurs. The TS flag is used to manage
processor extensions as follows:

  ■  Every execution of an ESC instruction is trapped if the TS flag is set.

  ■  Execution of a WAIT instruction is trapped if the MP flag and the TS
     flag are both set.

Thus, if a task switch was made after an ESC instruction was begun, the
processor extension's context may need to be saved before a new ESC
instruction can be issued. The fault handler saves the context and resets
the TS flag.

CLTS appears in operating system software, not in application programs. It
is a privileged instruction that can only be executed at privilege level 0.

Flags Affected

TS = 0 (TS is in CR0, not the flag register)

Protected Mode Exceptions

#GP(0) if CLTS is executed with a current privilege level other than 0

Real Address Mode Exceptions

None (valid in Real Address Mode to allow initialization for Protected
Mode)

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode


CMC ── Complement Carry Flag

Opcode    Instruction    Clocks   Description

F5        CMC            2        Complement carry flag


Operation

CF  NOT CF;

Description

CMC reverses the setting of the carry flag. No other flags are affected.

Flags Affected

CF as described above

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


CMP ── Compare Two Operands


Opcode          Instruction        Clocks   Description

3C  ib          CMP AL,imm8        2        Compare immediate byte to AL
3D  iw          CMP AX,imm16       2        Compare immediate word to AX
3D  id          CMP EAX,imm32      2        Compare immediate dword to EAX
80  /7 ib       CMP r/m8,imm8      2/5      Compare immediate byte to r/m
                                            byte
81  /7 iw       CMP r/m16,imm16    2/5      Compare immediate word to r/m
                                            word
81  /7 id       CMP r/m32,imm32    2/5      Compare immediate dword to r/m
                                            dword
83  /7 ib       CMP r/m16,imm8     2/5      Compare sign extended immediate
                                            byte to r/m word
83  /7 ib       CMP r/m32,imm8     2/5      Compare sign extended immediate
                                            byte to r/m dword
38  /r          CMP r/m8,r8        2/5      Compare byte register to r/m
                                            byte
39  /r          CMP r/m16,r16      2/5      Compare word register to r/m
                                            word
39  /r          CMP r/m32,r32      2/5      Compare dword register to r/m
                                            dword
3A  /r          CMP r8,r/m8        2/6      Compare r/m byte to byte
                                            register
3B  /r          CMP r16,r/m16      2/6      Compare r/m word to word
                                            register
3B  /r          CMP r32,r/m32      2/6      Compare r/m dword to dword
                                            register


Operation

LeftSRC - SignExtend(RightSRC);
(* CMP does not store a result; its purpose is to set the flags *)

Description

CMP subtracts the second operand from the first but, unlike the SUB
instruction, does not store the result; only the flags are changed. CMP is
typically used in conjunction with conditional jumps and the SETcc
instruction. (Refer to Appendix D for the list of signed and unsigned flag
tests provided.) If an operand greater than one byte is compared to an
immediate byte, the byte value is first sign-extended.

Flags Affected

OF, SF, ZF, AF, PF, and CF as described in Appendix C

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


CMPS/CMPSB/CMPSW/CMPSD ── Compare String Operands

Opcode    Instruction        Clocks   Description

A6        CMPS m8,m8         10       Compare bytes ES:[(E)DI] (second
                                      operand) with   [(E)SI] (first
                                      operand)
A7        CMPS m16,m16       10       Compare words ES:[(E)DI] (second
                                      operand) with   [(E)SI] (first
                                      operand)
A7        CMPS m32,m32       10       Compare dwords ES:[(E)DI]
                                      (second operand) with [(E)SI]
                                      (first operand)
A6        CMPSB              10       Compare bytes ES:[(E)DI] with
                                      DS:[SI]
A7        CMPSW              10       Compare words ES:[(E)DI] with
                                      DS:[SI]
A7        CMPSD              10       Compare dwords ES:[(E)DI] with
                                      DS:[SI]


Operation

IF (instruction = CMPSD) OR
   (instruction has operands of type DWORD)
THEN OperandSize  32;
ELSE OperandSize  16;
FI;
IF AddressSize = 16
THEN
   use SI for source-index and DI for destination-index
ELSE (* AddressSize = 32 *)
   use ESI for source-index and EDI for destination-index;
FI;
IF byte type of instruction
THEN
   [source-index] - [destination-index]; (* byte comparison *)
   IF DF = 0 THEN IncDec  1 ELSE IncDec  -1; FI;
ELSE
   IF OperandSize = 16
   THEN
      [source-index] - [destination-index]; (* word comparison *)
      IF DF = 0 THEN IncDec  2 ELSE IncDec  -2; FI;
   ELSE (* OperandSize = 32 *)
      [source-index] - [destination-index]; (* dword comparison *)
      IF DF = 0 THEN IncDec  4 ELSE IncDec  -4; FI;
   FI;
FI;
source-index = source-index + IncDec;
destination-index = destination-index + IncDec;

Description

CMPS compares the byte, word, or doubleword pointed to by the source-index
register with the byte, word, or doubleword pointed to by the
destination-index register.

If the address-size attribute of this instruction is 16 bits, SI and DI
will be used for source- and destination-index registers; otherwise ESI and
EDI will be used. Load the correct index values into SI and DI (or ESI and
EDI) before executing CMPS.

The comparison is done by subtracting the operand indexed by
the destination-index register from the operand indexed by the source-index
register.

Note that the direction of subtraction for CMPS is [SI] - [DI] or
[ESI] - [EDI]. The left operand (SI or ESI) is the source and the right
operand (DI or EDI) is the destination. This is the reverse of the usual
Intel convention in which the left operand is the destination and the right
operand is the source.

The result of the subtraction is not stored; only the flags reflect the
change. The types of the operands determine whether bytes, words, or
doublewords are compared. For the first operand (SI or ESI), the DS register
is used, unless a segment override byte is present. The second operand (DI
or EDI) must be addressable from the ES register; no segment override is
possible.

After the comparison is made, both the source-index register and
destination-index register are automatically advanced. If the direction flag
is 0 (CLD was executed), the registers increment; if the direction flag is 1
(STD was executed), the registers decrement. The registers increment or
decrement by 1 if a byte is compared, by 2 if a word is compared, or by 4 if
a doubleword is compared.

CMPSB, CMPSW and CMPSD are synonyms for the byte, word, and
doubleword CMPS instructions, respectively.

CMPS can be preceded by the REPE or REPNE prefix for block comparison of CX
or ECX bytes, words, or doublewords. Refer to the description of the REP
instruction for more information on this operation.

Flags Affected

OF, SF, ZF, AF, PF, and CF as described in Appendix C

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF (fault-code) for a page fault


CWD/CDQ ── Convert Word to Doubleword/Convert Doubleword to
           Quadword

Opcode    Instruction        Clocks   Description

99        CWD                2        DX:AX  sign-extend of AX
99        CDQ                2        EDX:EAX  sign-extend of EAX


Operation

IF OperandSize = 16 (* CWD instruction *)
THEN
   IF AX < 0 THEN DX  0FFFFH; ELSE DX  0; FI;
ELSE (* OperandSize = 32, CDQ instruction *)
   IF EAX < 0 THEN EDX  0FFFFFFFFH; ELSE EDX  0; FI;
FI;

Description

CWD converts the signed word in AX to a signed doubleword in DX:AX
by extending the most significant bit of AX into all the bits of DX. CDQ
converts the signed doubleword in EAX to a signed 64-bit integer in the
register pair EDX:EAX by extending the most significant bit of EAX
(the sign bit) into all the bits of EDX. Note that CWD is different from
CWDE. CWDE uses EAX as a destination, instead of DX:AX.

Flags Affected

None

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


DAA ── Decimal Adjust AL after Addition

Opcode    Instruction        Clocks   Description

27        DAA                4        Decimal adjust AL after addition


Operation

IF ((AL AND 0FH) > 9) OR (AF = 1)
THEN
   AL  AL + 6;
   AF  1;
ELSE
   AF  0;
FI;
IF (AL > 9FH) OR (CF = 1)
THEN
   AL  AL + 60H;
   CF  1;
ELSE CF  0;
FI;

Description

Execute DAA only after executing an ADD instruction that leaves a
two-BCD-digit byte result in the AL register. The ADD operands should
consist of two packed BCD digits. The DAA instruction adjusts AL to
contain the correct two-digit packed decimal result.

Flags Affected

AF and CF as described above; SF, ZF, PF, and CF as described in
Appendix C.

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


DAS ── Decimal Adjust AL after Subtraction

Opcode    Instruction        Clocks   Description

2F        DAS                4        Decimal adjust AL after subtraction


Operation

IF (AL AND 0FH) > 9 OR AF = 1
THEN
   AL  AL - 6;
   AF  1;
ELSE
   AF  0;
FI;
IF (AL > 9FH) OR (CF = 1)
THEN
   AL  AL - 60H;
   CF  1;
ELSE CF  0;
FI;

Description

Execute DAS only after a subtraction instruction that leaves a
two-BCD-digit byte result in the AL register. The operands should consist
of two packed BCD digits. DAS adjusts AL to contain the correct packed
two-digit decimal result.

Flags Affected

AF and CF as described above; SF, ZF, and PF as described in Appendix C.

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


DEC ── Decrement by 1

Opcode    Instruction        Clocks   Description

FE /1     DEC r/m8           2/6      Decrement r/m byte by 1
FF /1     DEC r/m16          2/6      Decrement r/m word by 1
          DEC r/m32          2/6      Decrement r/m dword by 1
48+rw     DEC r16            2        Decrement word register by 1
48+rw     DEC r32            2        Decrement dword register by 1


Operation

DEST  DEST - 1;

Description

DEC subtracts 1 from the operand. DEC does not change the carry flag.
To affect the carry flag, use the SUB instruction with an immediate
operand of 1.

Flags Affected

OF, SF, ZF, AF, and PF as described in Appendix C.

Protected Mode Exceptions

#GP(0) if the result is a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


DIV ── Unsigned Divide

Opcode    Instruction        Clocks   Description

F6 /6     DIV AL,r/m8        14/17    Unsigned divide AX by r/m byte
                                      (AL=Quo, AH=Rem)
F7 /6     DIV AX,r/m16       22/25    Unsigned divide DX:AX by r/m
                                      word (AX=Quo, DX=Rem)
F7 /6     DIV EAX,r/m32      38/41    Unsigned divide EDX:EAX by r/m
                                      dword (EAX=Quo, EDX=Rem)


Operation

temp  dividend / divisor;
IF temp does not fit in quotient
THEN Interrupt 0;
ELSE
   quotient  temp;
   remainder  dividend MOD (r/m);
FI;

───────────────────────────────────────────────────────────────────────────
Note:
  Divisions are unsigned. The divisor is given by the r/m operand.
  The dividend, quotient, and remainder use implicit registers. Refer to
  the table under "Description."
───────────────────────────────────────────────────────────────────────────

Description

DIV performs an unsigned division. The dividend is implicit; only the
divisor is given as an operand. The remainder is always less than the
divisor. The type of the divisor determines which registers to use as
follows:

    Size    Dividend     Divisor   Quotient   Remainder
    byte    AX           r/m8       AL          AH
    word    DX:AX        r/m16      AX          DX
    dword   EDX:EAX      r/m32      EAX         EDX

Flags Affected

OF, SF, ZF, AR, PF, CF are undefined.

Protected Mode Exceptions

Interrupt 0 if the quotient is too large to fit in the designated register
(AL, AX, or EAX), or if the divisor is 0; #GP(0) for an illegal memory
operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0)
for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 0 if the quotient is too big to fit in the designated register
(AL, AX, or EAX), or if the divisor is 0; Interrupt 13 if any part of the
operand would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


ENTER ── Make Stack Frame for Procedure Parameters

Opcode      Instruction        Clocks     Description

C8 iw 00    ENTER imm16,0      10         Make procedure stack frame
C8 iw 01    ENTER imm16,1      12         Make stack frame for procedure
                                          parameters
C8 iw ib    ENTER imm16,imm8   15+4(n-1)  Make stack frame for
                                          procedure parameters


Operation

level  level MOD 32
IF OperandSize = 16 THEN Push(BP) ELSE Push (EBP) FI;
   (* Save stack pointer *)
frame-ptr  eSP
IF level > 0
THEN (* level is rightmost parameter *)
   FOR i  1 TO level - 1
   DO
      IF OperandSize = 16
      THEN
         BP  BP - 2;
         Push[BP]
      ELSE (* OperandSize = 32 *)
         EBP  EBP - 4;
         Push[EBP];
      FI;
   OD;
   Push(frame-ptr)
FI;
IF OperandSize = 16 THEN BP  frame-ptr ELSE EBP  frame-ptr; FI;
IF StackAddrSize = 16
THEN SP  SP - First operand;
ELSE ESP  ESP - ZeroExtend(First operand);
FI;

Description

ENTER creates the stack frame required by most block-structured
high-level languages. The first operand specifies the number of bytes of
dynamic storage allocated on the stack for the routine being entered.
The second operand gives the lexical nesting level (0 to 31) of the routine
within the high-level language source code. It determines the number of
stack frame pointers copied into the new stack frame from the preceding
frame. BP (or EBP, if the operand-size attribute is 32 bits) is the current
stack frame pointer.

If the operand-size attribute is 16 bits, the processor uses BP as the
frame pointer and SP as the stack pointer. If the operand-size attribute is
32 bits, the processor uses EBP for the frame pointer and ESP for the stack
pointer.

If the second operand is 0, ENTER pushes the frame pointer (BP or
EBP) onto the stack; ENTER then subtracts the first operand from the
stack pointer and sets the frame pointer to the current stack-pointer
value.

For example, a procedure with 12 bytes of local variables would have an
ENTER 12,0 instruction at its entry point and a LEAVE instruction
before every RET. The 12 local bytes would be addressed as negative
offsets from the frame pointer.

Flags Affected

None

Protected Mode Exceptions

#SS(0) if SP or ESP would exceed the stack limit at any point during
instruction execution; #PF(fault-code) for a page fault

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


HLT ── Halt

Opcode      Instruction        Clocks     Description

F4          HLT                5          Halt


Operation

Enter Halt state;

Description

HALT stops instruction execution and places the 80386 in a HALT state.
An enabled interrupt, NMI, or a reset will resume execution. If an
interrupt (including NMI) is used to resume execution after HLT, the saved
CS:IP (or CS:EIP) value points to the instruction following HLT.

Flags Affected

None

Protected Mode Exceptions

HLT is a privileged instruction; #GP(0) if the current privilege level is
not 0

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

#GP(0); HLT is a privileged instruction


IDIV ── Signed Divide

Opcode      Instruction        Clocks   Description

F6 /7       IDIV r/m8          19       Signed divide AX by r/m byte
                                        (AL=Quo, AH=Rem)
F7 /7       IDIV AX,r/m16      27       Signed divide DX:AX by EA word
                                        (AX=Quo, DX=Rem)
F7 /7       IDIV EAX,r/m32     43       Signed divide EDX:EAX by DWORD
                                        byte (EAX=Quo, EDX=Rem)


Operation

temp  dividend / divisor;
IF temp does not fit in quotient
THEN Interrupt 0;
ELSE
   quotient  temp;
   remainder  dividend MOD (r/m);
FI;

───────────────────────────────────────────────────────────────────────────
Notes:
  Divisions are signed. The divisor is given by the r/m operand. The
  dividend, quotient, and remainder use implicit registers. Refer to the
  table under "Description."
───────────────────────────────────────────────────────────────────────────

Description

IDIV performs a signed division. The dividend, quotient, and remainder
are implicitly allocated to fixed registers. Only the divisor is given as
an explicit r/m operand. The type of the divisor determines which registers
to use as follows:

Size     Divisor    Quotient    Remainder  Dividend
byte     r/m8        AL           AH       AX
word     r/m16       AX           DX       DX:AX
dword    r/m32       EAX          EDX      EDX:EAX

If the resulting quotient is too large to fit in the destination, or if the
division is 0, an Interrupt 0 is generated. Nonintegral quotients are
truncated toward 0. The remainder has the same sign as the dividend
and the absolute value of the remainder is always less than the absolute
value of the divisor.

Flags Affected

OF, SF, ZF, AR, PF, CF are undefined.

Protected Mode Exceptions

Interrupt 0 if the quotient is too large to fit in the designated register
(AL or AX), or if the divisor is 0; #GP (0) for an illegal memory operand
effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an
illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 0 if the quotient is too large to fit in the designated register
(AL or AX), or if the divisor is 0; Interrupt 13 if any part of the operand
would lie outside of the effective address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


IMUL ── Signed Multiply


Opcode      Instruction            Clocks      Description

F6  /5      IMUL r/m8              9-14/12-17  AX AL * r/m byte
F7  /5      IMUL r/m16             9-22/12-25  DX:AX  AX * r/m word
F7  /5      IMUL r/m32             9-38/12-41  EDX:EAX  EAX * r/m dword
0F  AF /r   IMUL r16,r/m16         9-22/12-25  word register  word
                                               register * r/m word
0F  AF /r   IMUL r32,r/m32         9-38/12-41  dword register  dword
                                               register * r/m dword
6B  /r ib   IMUL r16,r/m16,imm8    9-14/12-17  word register  r/m16 *
                                               sign-extended immediate byte
6B  /r ib   IMUL r32,r/m32,imm8    9-14/12-17  dword register  r/m32 *
                                               sign-extended immediate byte
6B  /r ib   IMUL r16,imm8          9-14/12-17  word register  word
                                               register * sign-extended
                                               immediate byte
6B  /r ib   IMUL r32,imm8          9-14/12-17  dword register  dword
                                               register * sign-extended
                                               immediate byte
69  /r iw   IMUL r16,r/m16,imm16   9-22/12-25  word register  r/m16 *
                                               immediate word
69  /r id   IMUL r32,r/m32,imm32   9-38/12-41  dword register  r/m32 *
                                               immediate dword
69  /r iw   IMUL r16,imm16         9-22/12-25  word register  r/m16 *
                                               immediate word
69  /r id   IMUL r32,imm32         9-38/12-41  dword register  r/m32 *
                                               immediate dword


───────────────────────────────────────────────────────────────────────────
NOTES:
  The 80386 uses an early-out multiply algorithm. The actual number of
  clocks depends on the position of the most significant bit in the
  optimizing multiplier, shown underlined above. The optimization occurs for
  positive and negative values. Because of the early-out algorithm, clock
  counts given are minimum to maximum. To calculate the actual clocks, use
  the following formula:
───────────────────────────────────────────────────────────────────────────

  Actual clock = if m <> 0 then max(ceiling(log{2} │m│), 3) + 6 clocks
  Actual clock = if m = 0 then 9 clocks
  (where m is the multiplier)

Add three clocks if the multiplier is a memory operand.

Operation

result  multiplicand * multiplier;

Description

IMUL performs signed multiplication. Some forms of the instruction
use implicit register operands. The operand combinations for all forms
of the instruction are shown in the "Description" column above.

IMUL clears the overflow and carry flags under the following conditions:

   Instruction Form    Condition for Clearing CF and OF
   r/m8                AL = sign-extend of AL to 16 bits
   r/m16               AX = sign-extend of AX to 32 bits
   r/m32               EDX:EAX = sign-extend of EAX to 32 bits
   r16,r/m16           Result exactly fits within r16
   r/32,r/m32          Result exactly fits within r32
   r16,r/m16,imm16     Result exactly fits within r16
   r32,r/m32,imm32     Result exactly fits within r32

Flags Affected

OF and CF as described above; SF, ZF, AF, and PF are undefined

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exeptions as in Real Address Mode; #PF(fault-code) for a page
fault

Notes

When using the accumulator forms (IMUL r/m8, IMUL r/m16, or IMUL
r/m32), the result of the multiplication is available even if the overflow
flag is set because the result is two times the size of the multiplicand
and multiplier. This is large enough to handle any possible result.


IN ── Input from Port

Opcode    Instruction   Clocks            Description

E4  ib    IN AL,imm8    12,pm=6*/26**     Input byte from immediate port
                                          into AL
E5  ib    IN AX,imm8    12,pm=6*/26**     Input word from immediate port
                                          into AX
E5  ib    IN EAX,imm8   12,pm=6*/26**     Input dword from immediate port
                                          into EAX
EC        IN AL,DX      13,pm=7*/27**     Input byte from port DX into AL
ED        IN AX,DX      13,pm=7*/27**     Input word from port DX into AX
ED        IN EAX,DX     13,pm=7*/27**     Input dword from port DX into
                                          EAX


───────────────────────────────────────────────────────────────────────────
NOTES:
   *If CPL ≤ IOPL
  **If CPL > IOPL or if in virtual 8086 mode
───────────────────────────────────────────────────────────────────────────

Operation

IF (PE = 1) AND ((VM = 1) OR (CPL > IOPL))
THEN (* Virtual 8086 mode, or protected mode with CPL > IOPL *)
   IF NOT I-O-Permission (SRC, width(SRC))
   THEN #GP(0);
   FI;
FI;
DEST  [SRC]; (* Reads from I/O address space *)

Description

IN transfers a data byte or data word from the port numbered by the
second operand into the register (AL, AX, or EAX) specified by the first
operand. Access any port from 0 to 65535 by placing the port number
in the DX register and using an IN instruction with DX as the second
parameter. These I/O instructions can be shortened by using an 8-bit
port I/O in the instruction. The upper eight bits of the port address will
be 0 when 8-bit port I/O is used.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the current privilege level is larger (has less privilege) than
IOPL and any of the corresponding I/O permission bits in TSS equals 1

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

#GP(0) fault if any of the corresponding I/O permission bits in TSS
equals 1


INC ── Increment by 1

Opcode      Instruction        Clocks      Description

FE  /0      INC r/m8                       Increment r/m byte by 1
FF  /0      INC r/m16                      Increment r/m word by 1
FF  /6      INC r/m32                      Increment r/m dword by 1
40 + rw     INC r16                        Increment word register by 1
40 + rd     INC r32                        Increment dword register by 1


Operation

DEST  DEST + 1;

Description

INC adds 1 to the operand. It does not change the carry flag. To affect
the carry flag, use the ADD instruction with a second operand of 1.

Flags Affected

OF, SF, ZF, AF, and PF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the operand is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


INS/INSB/INSW/INSD ── Input from Port to String

Opcode  Instruction    Clocks         Description

6C      INS r/m8,DX    15,pm=9*/29**  Input byte from port DX into ES:(E)DI
6D      INS r/m16,DX   15,pm=9*/29**  Input word from port DX into ES:(E)DI
6D      INS r/m32,DX   15,pm=9*/29**  Input dword from port DX into ES:(E)DI
6C      INSB           15,pm=9*/29**  Input byte from port DX into ES:(E)DI
6D      INSW           15,pm=9*/29**  Input word from port DX into ES:(E)DI
6D      INSD           15,pm=9*/29**  Input dword from port DX into ES:(E)DI


───────────────────────────────────────────────────────────────────────────
NOTES:
   *If CPL ≤ IOPL
  **If CPL > IOPL or if in virtual 8086 mode
───────────────────────────────────────────────────────────────────────────

Operation

IF AddressSize = 16
THEN use DI for dest-index;
ELSE (* AddressSize = 32 *)
   use EDI for dest-index;
FI;
IF (PE = 1) AND ((VM = 1) OR (CPL > IOPL))
THEN (* Virtual 8086 mode, or protected mode with CPL > IOPL *)
   IF NOT I-O-Permission (SRC, width(SRC))
   THEN #GP(0);
   FI;
FI;
IF byte type of instruction
THEN
   ES:[dest-index]  [DX]; (* Reads byte at DX from I/O address space *)
   IF DF = 0 THEN IncDec  1 ELSE IncDec  -1; FI;
FI;
IF OperandSize = 16
THEN
   ES:[dest-index]  [DX]; (* Reads word at DX from I/O address space *)
   IF DF = 0 THEN IncDec  2 ELSE IncDec  -2; FI;
FI;
IF OperandSize = 32
THEN
   ES:[dest-index]  [DX]; (* Reads dword at DX from I/O address space *)
   IF DF = 0 THEN IncDec  4 ELSE IncDec  -4; FI;
FI;
dest-index  dest-index + IncDec;

Description

INS transfers data from the input port numbered by the DX register to
the memory byte or word at ES:dest-index. The memory operand must
be addressable from ES; no segment override is possible. The destination
register is DI if the address-size attribute of the instruction is 16 bits,
or EDI if the address-size attribute is 32 bits.

INS does not allow the specification of the port number as an immediate
value. The port must be addressed through the DX register value. Load
the correct value into DX before executing the INS instruction.

The destination address is determined by the contents of the destination
index register. Load the correct index into the destination index register
before executing INS.

After the transfer is made, DI or EDI advances automatically. If the
direction flag is 0 (CLD was executed), DI or EDI increments; if the
direction flag is 1 (STD was executed), DI or EDI decrements. DI
increments or decrements by 1 if a byte is input, by 2 if a word is input,
or by 4 if a doubleword is input.

INSB, INSW and INSD are synonyms of the byte, word, and doubleword
INS instructions. INS can be preceded by the REP prefix for block input of
CX bytes or words. Refer to the REP instruction for details of this
operation.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if CPL is numerically greater than IOPL and any of the
corresponding I/O permission bits in TSS equals 1; #GP(0) if the
destination is in a nonwritable segment; #GP(0) for an illegal memory
operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for
an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

#GP(0) fault if any of the corresponding I/O permission bits in TSS
equals 1; #PF(fault-code) for a page fault


INT/INTO ── Call to Interrupt Procedure


Opcode    Instruction  Clocks          Description

CC        INT 3        33              Interrupt 3--trap to debugger
CC        INT 3        pm=59           Interrupt 3--Protected Mode, same
                                       privilege
CC        INT 3        pm=99           Interrupt 3--Protected Mode, more
                                       privilege
CC        INT 3        pm=119          Interrupt 3--from V86 mode to PL 0
CC        INT 3        ts              Interrupt 3--Protected Mode, via
                                       task gate
CD ib     INT imm8     37              Interrupt numbered by immediate
                                       byte
CD ib     INT imm8     pm=59           Interrupt--Protected Mode, same
                                       privilege
CD ib     INT imm8     pm=99           Interrupt--Protected Mode, more
                                       privilege
CD ib     INT imm8     pm=119          Interrupt--from V86 mode to PL 0
CD ib     INT imm8     ts              Interrupt--Protected Mode, via task
                                       gate
CE        INTO         Fail:3,pm=3;
                       Pass:35         Interrupt 4--if overflow flag is 1
CE        INTO         pm=59           Interrupt 4--Protected Mode, same
                                       privilege
CE        INTO         pm=99           Interrupt 4--Protected Mode, more
                                       privilege
CE        INTO         pm=119          Interrupt 4--from V86 mode to PL 0
CE        INTO         ts              Interrupt 4--Protected Mode, via
                                       task gate


───────────────────────────────────────────────────────────────────────────
NOTE:
  Approximate values of ts are given by the following table:

                            New Task

Old Task       386 TSS       386 TSS       286 TSS
               VM = 0        VM = 1

386
TSS VM=0         309           226           282

386
TSS VM=1         314           231           287

286
TSS              307           224           280
───────────────────────────────────────────────────────────────────────────

Operation

───────────────────────────────────────────────────────────────────────────
NOTE:
  The following operational description applies not only to the
  above instructions but also to external interrupts and exceptions.
───────────────────────────────────────────────────────────────────────────

IF PE = 0
THEN GOTO REAL-ADDRESS-MODE;
ELSE GOTO PROTECTED-MODE;
FI;

REAL-ADDRESS-MODE:
   Push (FLAGS);
   IF  0; (* Clear interrupt flag *)
   TF  0; (* Clear trap flag *)
   Push(CS);
   Push(IP);
   (* No error codes are pushed *)
   CS  IDT[Interrupt number * 4].selector;
   IP  IDT[Interrupt number * 4].offset;

PROTECTED-MODE:
   Interrupt vector must be within IDT table limits,
      else #GP(vector number * 8+2+EXT);
   Descriptor AR byte must indicate interrupt gate, trap gate, or task gate,
      else #GP(vector number * 8+2+EXT);
   IF software interrupt (* i.e. caused by INT n, INT 3, or INTO *)
   THEN
      IF gate descriptor DPL < CPL
      THEN #GP(vector number * 8+2+EXT);
      FI;
   FI;
   Gate must be present, else #NP(vector number * 8+2+EXT);
   IF trap gate OR interrupt gate
   THEN GOTO TRAP-GATE-OR-INTERRUPT-GATE;
   ELSE GOTO TASK-GATE;
   FI;

TRAP-GATE-OR-INTERRUPT-GATE:
   Examine CS selector and descriptor given in the gate descriptor;
   Selector must be non-null, else #GP (EXT);
   Selector must be within its descriptor table limits
      ELSE #GP(selector+EXT);
   Descriptor AR byte must indicate code segment
      ELSE #GP(selector + EXT);
   Segment must be present, else #NP(selector+EXT);
   IF code segment is non-conforming AND DPL < CPL
   THEN GOTO INTERRUPT-TO-INNER-PRIVILEGE;
   ELSE
      IF code segment is conforming OR code segment DPL = CPL
      THEN GOTO INTERRUPT-TO-SAME-PRIVILEGE-LEVEL;
      ELSE #GP(CS selector + EXT);
      FI;
   FI;

INTERRUPT-TO-INNER-PRIVILEGE:
   Check selector and descriptor for new stack in current TSS;
      Selector must be non-null, else #GP(EXT);
      Selector index must be within its descriptor table limits
         ELSE #TS(SS selector+EXT);
      Selector's RPL must equal DPL of code segment, else #TS(SS
         selector+EXT);
      Stack segment DPL must equal DPL of code segment, else #TS(SS
         selector+EXT);
      Descriptor must indicate writable data segment, else #TS(SS
         selector+EXT);
      Segment must be present, else #SS(SS selector+EXT);
   IF 32-bit gate
   THEN New stack must have room for 20 bytes else #SS(0)
   ELSE New stack must have room for 10 bytes else #SS(0)
   FI;
   Instruction pointer must be within CS segment boundaries else #GP(0);
   Load new SS and eSP value from TSS;
   IF 32-bit gate
   THEN CS:EIP  selector:offset from gate;
   ELSE CS:IP  selector:offset from gate;
   FI;
   Load CS descriptor into invisible portion of CS register;
   Load SS descriptor into invisible portion of SS register;
   IF 32-bit gate
   THEN
      Push (long pointer to old stack) (* 3 words padded to 4 *);
      Push (EFLAGS);
      Push (long pointer to return location) (* 3 words padded to 4*);
   ELSE
      Push (long pointer to old stack) (* 2 words *);
      Push (FLAGS);
      Push (long pointer to return location) (* 2 words *);
   FI;
   Set CPL to new code segment DPL;
   Set RPL of CS to CPL;
   IF interrupt gate THEN IF  0 (* interrupt flag to 0 (disabled) *); FI;
   TF  0;
   NT  0;

INTERRUPT-FROM-V86-MODE:
   TempEFlags  EFLAGS;
   VM  0;
   TF  0;
   IF service through Interrupt Gate THEN IF  0;
   TempSS  SS;
   TempESP  ESP;
   SS  TSS.SS0; (* Change to level 0 stack segment *)
   ESP  TSS.ESP0; (* Change to level 0 stack pointer *)
   Push(GS); (* padded to two words *)
   Push(FS); (* padded to two words *)
   Push(DS); (* padded to two words *)
   Push(ES); (* padded to two words *)
   GS  0;
   FS  0;
   DS  0;
   ES  0;
   Push(TempSS); (* padded to two words *)
   Push(TempESP);
   Push(TempEFlags);
   Push(CS); (* padded to two words *)
   Push(EIP);
   CS:EIP  selector:offset from interrupt gate;
   (* Starts execution of new routine in 80386 Protected Mode *)

INTERRUPT-TO-SAME-PRIVILEGE-LEVEL:
   IF 32-bit gate
   THEN Current stack limits must allow pushing 10 bytes, else #SS(0);
   ELSE Current stack limits must allow pushing 6 bytes, else #SS(0);
   FI;
   IF interrupt was caused by exception with error code
   THEN Stack limits must allow push of two more bytes;
   ELSE #SS(0);
   FI;
   Instruction pointer must be in CS limit, else #GP(0);
   IF 32-bit gate
   THEN
      Push (EFLAGS);
      Push (long pointer to return location); (* 3 words padded to 4 *)
      CS:EIP  selector:offset from gate;
   ELSE (* 16-bit gate *)
      Push (FLAGS);
      Push (long pointer to return location); (* 2 words *)
      CS:IP  selector:offset from gate;
   FI;
   Load CS descriptor into invisible portion of CS register;
   Set the RPL field of CS to CPL;
   Push (error code); (* if any *)
   IF interrupt gate THEN IF  0; FI;
   TF  0;
   NT  0;

TASK-GATE:
   Examine selector to TSS, given in task gate descriptor;
      Must specify global in the local/global bit, else #TS(TSS selector);
      Index must be within GDT limits, else #TS(TSS selector);
      AR byte must specify available TSS (bottom bits 00001),
         else #TS(TSS selector;
      TSS must be present, else #NP(TSS selector);
   SWITCH-TASKS with nesting to TSS;
   IF interrupt was caused by fault with error code
   THEN
      Stack limits must allow push of two more bytes, else #SS(0);
      Push error code onto stack;
   FI;
   Instruction pointer must be in CS limit, else #GP(0);

Description

The INT  instruction generates via software a call to an interrupt
handler. The immediate operand, from 0 to 255, gives the index number
into the Interrupt Descriptor Table (IDT) of the interrupt routine to be
called. In Protected Mode, the IDT consists of an array of eight-byte
descriptors; the descriptor for the interrupt invoked must indicate an
interrupt, trap, or task gate. In Real Address Mode, the IDT is an array
of four byte-long pointers. In Protected and Real Address Modes, the
base linear address of the IDT is defined by the contents of the IDTR.

The INTO conditional software instruction is identical to the INT
interrupt instruction except that the interrupt number is implicitly 4,
and the interrupt is made only if the 80386 overflow flag is set.

The first 32 interrupts are reserved by Intel for system use. Some of
these interrupts are use for internally generated exceptions.

INT n generally behaves like a far call except that the flags register is
pushed onto the stack before the return address. Interrupt procedures
return via the IRET instruction, which pops the flags and return address
from the stack.

In Real Address Mode, INT n pushes the flags, CS, and the return IP
onto the stack, in that order, then jumps to the long pointer indexed by
the interrupt number.

Flags Affected

None

Protected Mode Exceptions

#GP, #NP, #SS, and #TS as indicated under "Operation" above

Real Address Mode Exceptions

None; if the SP or ESP = 1, 3, or 5 before executing INT or INTO,
the 80386 will shut down due to insufficient stack space

Virtual 8086 Mode Exceptions

#GP(0) fault if IOPL is less than 3, for INT  only, to permit emulation;
Interrupt 3 (0CCH) generates Interrupt 3; INTO generates Interrupt 4
if the overflow flag equals 1


IRET/IRETD ── Interrupt Return

Opcode  Instruction  Clocks       Description

CF      IRET         22,pm=38     Interrupt return (far return and pop
                                  flags)
CF      IRET         pm=82        Interrupt return to lesser privilege
CF      IRET         ts           Interrupt return, different task (NT = 1)
CF      IRETD        22,pm=38     Interrupt return (far return and pop
                                  flags)
CF      IRETD        pm=82        Interrupt return to lesser privilege
CF      IRETD        pm=60        Interrupt return to V86 mode
CF      IRETD        ts           Interrupt return, different task (NT = 1)


───────────────────────────────────────────────────────────────────────────
NOTE:
  Values of ts are given by the following table:

                            New Task

Old Task       386 TSS       386 TSS       286 TSS
               VM = 0        VM = 1

386
TSS VM=0         275           224           271

286
TSS              265           214           232
───────────────────────────────────────────────────────────────────────────

Operation

IF PE = 0
THEN (* Real-address mode *)
   IF OperandSize = 32 (* Instruction = IRETD *)
   THEN EIP  Pop();
   ELSE (* Instruction = IRET *)
      IP  Pop();
   FI;
   CS  Pop();
   IF OperandSize = 32 (* Instruction = IRETD *)
   THEN EFLAGS  Pop();
   ELSE (* Instruction = IRET *)
      FLAGS  Pop();
   FI;
ELSE (* Protected mode *)
   IF VM = 1
   THEN #GP(0);
   ELSE
      IF NT = 1
      THEN GOTO TASK-RETURN;
      ELSE
         IF VM = 1 in flags image on stack
         THEN GO TO STACK-RETURN-TO-V86;
         ELSE GOTO STACK-RETURN;
         FI;
      FI;
   FI;
FI;STACK-RETURN-TO-V86: (* Interrupted procedure was in V86 mode *)
   IF return CS selector RPL < > 3
   THEN #GP(Return selector);
   FI;
   IF top 36 bytes of stack not within limits
   THEN #SS(0);
   FI;
   Examine return CS selector and associated descriptor:
      IF selector is null, THEN #GP(0); FI;
      IF selector index not within its descriptor table limits;
      THEN #GP(Return selector);
      FI;
      IF AR byte does not indicate code segment
      THEN #GP(Return selector);
      FI;
      IF code segment DPL not = 3;
      THEN #GP(Return selector);
      FI;
      IF code segment not present
      THEN #NP(Return selector);
      FI;

   Examine return SS selector and associated descriptor:
      IF selector is null THEN #GP(0); FI;
      IF selector index not within its descriptor table limits
      THEN #GP(SS selector);
      FI;
      IF selector RPL not = RPL of return CS selector
      THEN #GP(SS selector);
      FI;
      IF AR byte does not indicate a writable data segment
      THEN #GP(SS selector);
      FI;
      IF stack segment DPL not = RPL of return CS selector
      THEN #GP(SS selector);
      FI;
      IF SS not present
      THEN #NP(SS selector);
      FI;

   IF instruction pointer not within code segment limit  THEN #GP(0);
   FI;
   EFLAGS  SS:[eSP + 8]; (* Sets VM in interrupted routine *)
   EIP  Pop();
   CS  Pop(); (* CS behaves as in 8086, due to VM = 1 *)
   throwaway  Pop(); (* pop away EFLAGS already read *)
   ES  Pop(); (* pop 2 words; throw away high-order word *)
   DS  Pop(); (* pop 2 words; throw away high-order word *)
   FS  Pop(); (* pop 2 words; throw away high-order word *)
   GS  Pop(); (* pop 2 words; throw away high-order word *)
   IF CS.RPL > CPL
   THEN
      TempESP  Pop();
      TempSS  Pop();
      SS:ESP  TempSS:TempESP;
   FI;

   (* Resume execution in Virtual 8086 mode *)

TASK-RETURN:
   Examine Back Link Selector in TSS addressed by the current task
      register:
      Must specify global in the local/global bit, else #TS(new TSS
         selector);
      Index must be within GDT limits, else #TS(new TSS selector);
      AR byte must specify TSS, else #TS(new TSS selector);
      New TSS must be busy, else #TS(new TSS selector);
      TSS must be present, else #NP(new TSS selector);
   SWITCH-TASKS without nesting to TSS specified by back link selector;
   Mark the task just abandoned as NOT BUSY;
   Instruction pointer must be within code segment limit ELSE #GP(0);

STACK-RETURN:
   IF OperandSize=32
   THEN Third word on stack must be within stack limits, else #SS(0);
   ELSE Second word on stack must be within stack limits, else #SS(0);
   FI;
   Return CS selector RPL must be ≥ CPL, else #GP(Return selector);
   IF return selector RPL = CPL
   THEN GOTO RETURN-SAME-LEVEL;
   ELSE GOTO RETURN-OUTER-LEVEL;
   FI;

RETURN-SAME-LEVEL:
   IF OperandSize=32
   THEN
      Top 12 bytes on stack must be within limits, else #SS(0);
      Return CS selector (at eSP+4) must be non-null, else #GP(0);
   ELSE
      Top 6 bytes on stack must be within limits, else #SS(0);
      Return CS selector (at eSP+2) must be non-null, else #GP(0);
   FI;
   Selector index must be within its descriptor table limits, else #GP
      (Return selector);
   AR byte must indicate code segment, else #GP(Return selector);
   IF non-conforming
   THEN code segment DPL must = CPL;
   ELSE #GP(Return selector);
   FI;
   IF conforming
   THEN code segment DPL must be ≤ CPL, else #GP(Return selector);
   Segment must be present, else #NP(Return selector);
   Instruction pointer must be within code segment boundaries, else #GP(0);
   FI;
   IF OperandSize=32
   THEN
      Load CS:EIP from stack;
      Load CS-register with new code segment descriptor;
      Load EFLAGS with third doubleword from stack;
      Increment eSP by 12;
   ELSE
      Load CS-register with new code segment descriptor;
      Load FLAGS with third word on stack;
      Increment eSP by 6;
   FI;

RETURN-OUTER-LEVEL:
   IF OperandSize=32
   THEN Top 20 bytes on stack must be within limits, else #SS(0);
   ELSE Top 10 bytes on stack must be within limits, else #SS(0);
   FI;
   Examine return CS selector and associated descriptor:
      Selector must be non-null, else #GP(0);
      Selector index must be within its descriptor table limits;
         ELSE #GP(Return selector);
      AR byte must indicate code segment, else #GP(Return selector);
      IF non-conforming
      THEN code segment DPL must = CS selector RPL;
      ELSE #GP(Return selector);
      FI;
      IF conforming
      THEN code segment DPL must be > CPL;
      ELSE #GP(Return selector);
      FI;
      Segment must be present, else #NP(Return selector);
   Examine return SS selector and associated descriptor:
      Selector must be non-null, else #GP(0);
      Selector index must be within its descriptor table limits
         ELSE #GP(SS selector);
      Selector RPL must equal the RPL of the return CS selector
         ELSE #GP(SS selector);
      AR byte must indicate a writable data segment, else #GP(SS selector);
      Stack segment DPL must equal the RPL of the return CS selector
         ELSE #GP(SS selector);
      SS must be present, else #NP(SS selector);

   Instruction pointer must be within code segment limit ELSE #GP(0);
   IF OperandSize=32
   THEN
      Load CS:EIP from stack;
      Load EFLAGS with values at (eSP+8);
   ELSE
      Load CS:IP from stack;
      Load FLAGS with values at (eSP+4);
   FI;
   Load SS:eSP from stack;
   Set CPL to the RPL of the return CS selector;
   Load the CS register with the CS descriptor;
   Load the SS register with the SS descriptor;
   FOR each of ES, FS, GS, and DS
   DO;
      IF the current value of the register is not valid for the outer level;
      THEN zero the register and clear the valid flag;
      FI;
      To be valid, the register setting must satisfy the following
         properties:
         Selector index must be within descriptor table limits;
         AR byte must indicate data or readable code segment;
         IF segment is data or non-conforming code,
         THEN DPL must be ≥ CPL, or DPL must be ≥ RPL;
   OD;

Description

In Real Address Mode, IRET pops the instruction pointer, CS, and the
flags register from the stack and resumes the interrupted routine.

In Protected Mode, the action of IRET depends on the setting of the
nested task flag (NT) bit in the flag register. When popping the new
flag image from the stack, the IOPL bits in the flag register are changed
only when CPL equals 0.

If NT equals 0, IRET returns from an interrupt procedure without a
task switch. The code returned to must be equally or less privileged than
the interrupt routine (as indicated by the RPL bits of the CS selector
popped from the stack). If the destination code is less privileged, IRET
also pops the stack pointer and SS from the stack.

If NT equals 1, IRET reverses the operation of a CALL or INT that
caused a task switch. The updated state of the task executing IRET is
saved in its task state segment. If the task is reentered later, the code
that follows IRET is executed.

Flags Affected

All; the flags register is popped from stack

Protected Mode Exceptions

#GP, #NP, or #SS, as indicated under "Operation" above

Real Address Mode Exceptions

Interrupt 13 if any part of the operand being popped lies beyond address
0FFFFH

Virtual 8086 Mode Exceptions

#GP(0) fault if IOPL is less than 3, to permit emulation


Jcc ── Jump if Condition is Met


Opcode         Instruction       Clocks   Description

77  cb         JA rel8           7+m,3    Jump short if above (CF=0 and
                                          ZF=0)
73  cb         JAE rel8          7+m,3    Jump short if above or equal
                                          (CF=0)
72  cb         JB rel8           7+m,3    Jump short if below (CF=1)
76  cb         JBE rel8          7+m,3    Jump short if below or equal
                                          (CF=1 or ZF=1)
72  cb         JC rel8           7+m,3    Jump short if carry (CF=1)
E3  cb         JCXZ rel8         9+m,5    Jump short if CX register is 0
E3  cb         JECXZ rel8        9+m,5    Jump short if ECX register is 0
74  cb         JE rel8           7+m,3    Jump short if equal (ZF=1)
74  cb         JZ rel8           7+m,3    Jump short if 0 (ZF=1)
7F  cb         JG rel8           7+m,3    Jump short if greater (ZF=0 and
                                          SF=OF)
7D  cb         JGE rel8          7+m,3    Jump short if greater or equal
                                          (SF=OF)
7C  cb         JL rel8           7+m,3    Jump short if less (SF<>OF)
7E  cb         JLE rel8          7+m,3    Jump short if less or equal
                                          (ZF=1 and SF<>OF)
76  cb         JNA rel8          7+m,3    Jump short if not above (CF=1 or
                                          ZF=1)
72  cb         JNAE rel8         7+m,3    Jump short if not above or equal
                                          (CF=1)
73  cb         JNB rel8          7+m,3    Jump short if not below (CF=0)
77  cb         JNBE rel8         7+m,3    Jump short if not below or equal
                                          (CF=0 and ZF=0)
73  cb         JNC rel8          7+m,3    Jump short if not carry (CF=0)
75  cb         JNE rel8          7+m,3    Jump short if not equal (ZF=0)
7E  cb         JNG rel8          7+m,3    Jump short if not greater (ZF=1
                                          or SF<>OF)
7C  cb         JNGE rel8         7+m,3    Jump short if not greater or
                                          equal (SF<>OF)
7D  cb         JNL rel8          7+m,3    Jump short if not less (SF=OF)
7F  cb         JNLE rel8         7+m,3    Jump short if not less or equal
                                          (ZF=0 and SF=OF)
71  cb         JNO rel8          7+m,3    Jump short if not overflow
                                          (OF=0)
7B  cb         JNP rel8          7+m,3    Jump short if not parity (PF=0)
79  cb         JNS rel8          7+m,3    Jump short if not sign (SF=0)
75  cb         JNZ rel8          7+m,3    Jump short if not zero (ZF=0)
70  cb         JO rel8           7+m,3    Jump short if overflow (OF=1)
7A  cb         JP rel8           7+m,3    Jump short if parity (PF=1)
7A  cb         JPE rel8          7+m,3    Jump short if parity even (PF=1)
7B  cb         JPO rel8          7+m,3    Jump short if parity odd (PF=0)
78  cb         JS rel8           7+m,3    Jump short if sign (SF=1)
74  cb         JZ rel8           7+m,3    Jump short if zero (ZF = 1)
0F  87 cw/cd   JA rel16/32       7+m,3    Jump near if above (CF=0 and
                                          ZF=0)
0F  83 cw/cd   JAE rel16/32      7+m,3    Jump near if above or equal
                                          (CF=0)
0F  82 cw/cd   JB rel16/32       7+m,3    Jump near if below (CF=1)
0F  86 cw/cd   JBE rel16/32      7+m,3    Jump near if below or equal
                                          (CF=1 or ZF=1)
0F  82 cw/cd   JC rel16/32       7+m,3    Jump near if carry (CF=1)
0F  84 cw/cd   JE rel16/32       7+m,3    Jump near if equal (ZF=1)
0F  84 cw/cd   JZ rel16/32       7+m,3    Jump near if 0 (ZF=1)
0F  8F cw/cd   JG rel16/32       7+m,3    Jump near if greater (ZF=0 and
                                          SF=OF)
0F  8D cw/cd   JGE rel16/32      7+m,3    Jump near if greater or equal
                                          (SF=OF)
0F  8C cw/cd   JL rel16/32       7+m,3    Jump near if less (SF<>OF)
0F  8E cw/cd   JLE rel16/32      7+m,3    Jump near if less or equal (ZF=1
                                          and SF<>OF)
0F  86 cw/cd   JNA rel16/32      7+m,3    Jump near if not above (CF=1 or
                                          ZF=1)
0F  82 cw/cd   JNAE rel16/32     7+m,3    Jump near if not above or equal
                                          (CF=1)
0F  83 cw/cd   JNB rel16/32      7+m,3    Jump near if not below (CF=0)
0F  87 cw/cd   JNBE rel16/32     7+m,3    Jump near if not below or equal
                                          (CF=0 and ZF=0)
0F  83 cw/cd   JNC rel16/32      7+m,3    Jump near if not carry (CF=0)
0F  85 cw/cd   JNE rel16/32      7+m,3    Jump near if not equal (ZF=0)
0F  8E cw/cd   JNG rel16/32      7+m,3    Jump near if not greater (ZF=1
                                          or SF<>OF)
0F  8C cw/cd   JNGE rel16/32     7+m,3    Jump near if not greater or
                                          equal (SF<>OF)
0F  8D cw/cd   JNL rel16/32      7+m,3    Jump near if not less (SF=OF)
0F  8F cw/cd   JNLE rel16/32     7+m,3    Jump near if not less or equal
                                          (ZF=0 and SF=OF)
0F  81 cw/cd   JNO rel16/32      7+m,3    Jump near if not overflow (OF=0)
0F  8B cw/cd   JNP rel16/32      7+m,3    Jump near if not parity (PF=0)
0F  89 cw/cd   JNS rel16/32      7+m,3    Jump near if not sign (SF=0)
0F  85 cw/cd   JNZ rel16/32      7+m,3    Jump near if not zero (ZF=0)
0F  80 cw/cd   JO rel16/32       7+m,3    Jump near if overflow (OF=1)
0F  8A cw/cd   JP rel16/32       7+m,3    Jump near if parity (PF=1)
0F  8A cw/cd   JPE rel16/32      7+m,3    Jump near if parity even (PF=1)
0F  8B cw/cd   JPO rel16/32      7+m,3    Jump near if parity odd (PF=0)
0F  88 cw/cd   JS rel16/32       7+m,3    Jump near if sign (SF=1)
0F  84 cw/cd   JZ rel16/32       7+m,3    Jump near if 0 (ZF=1)


───────────────────────────────────────────────────────────────────────────
NOTES:
  The first clock count is for the true condition (branch taken); the
  second clock count is for the false condition (branch not taken). rel16/32
  indicates that these instructions map to two; one with a 16-bit relative
  displacement, the other with a 32-bit relative displacement, depending on
  the operand-size attribute of the instruction.
───────────────────────────────────────────────────────────────────────────

Operation

IF condition
THEN
   EIP  EIP + SignExtend(rel8/16/32);
   IF OperandSize = 16
   THEN EIP  EIP AND 0000FFFFH;
   FI;
FI;

Description

Conditional jumps (except JCXZ) test the flags which have been set by
a previous instruction. The conditions for each mnemonic are given in
parentheses after each description above. The terms "less" and "greater"
are used for comparisons of signed integers; "above" and "below" are
used for unsigned integers.

If the given condition is true, a jump is made to the location provided as
the operand. Instruction coding is most efficient when the target for the
conditional jump is in the current code segment and within -128 to
+127 bytes of the next instruction's first byte. The jump can also target
-32768 thru +32767 (segment size attribute 16) or -2^(31) thru +2^(31) -1
(segment size attribute 32) relative to the next instruction's first byte.
When the target for the conditional jump is in a different segment, use
the opposite case of the jump instruction (i.e., JE and JNE), and then
access the target with an unconditional far jump to the other segment.
For example, you cannot code──

JZ FARLABEL;

You must instead code──

   JNZ BEYOND;
   JMP FARLABEL;
BEYOND:

Because there can be several ways to interpret a particular state of the
flags, ASM386 provides more than one mnemonic for most of the
conditional jump opcodes. For example, if you compared two characters in
AX and want to jump if they are equal, use JE; or, if you ANDed AX
with a bit field mask and only want to jump if the result is 0, use JZ, a
synonym for JE.

JCXZ differs from other conditional jumps because it tests the contents of
the CX or ECX register for 0, not the flags. JCXZ is useful at the beginning
of a conditional loop that terminates with a conditional loop instruction
(such as LOOPNE TARGET LABEL. The JCXZ prevents entering the loop with CX or
ECX equal to zero, which would cause the loop to execute 64K or 32G times
instead of zero times.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the offset jumped to is beyond the limits of the code segment

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


JMP ── Jump


Opcode    Instruction     Clocks          Description

EB  cb    JMP rel8        7+m             Jump short
E9  cw    JMP rel16       7+m             Jump near, displacement relative
                                          to next instruction
FF  /4    JMP r/m16       7+m/10+m        Jump near indirect
EA  cd    JMP ptr16:16    12+m,pm=27+m    Jump intersegment, 4-byte
                                          immediate address
EA  cd    JMP ptr16:16    pm=45+m         Jump to call gate, same
                                          privilege
EA  cd    JMP ptr16:16    ts              Jump via task state segment
EA  cd    JMP ptr16:16    ts              Jump via task gate
FF  /5    JMP m16:16      43+m,pm=31+m    Jump r/m16:16 indirect and
                                          intersegment
FF  /5    JMP m16:16      pm=49+m         Jump to call gate, same
                                          privilege
FF  /5    JMP m16:16      5 + ts          Jump via task state segment
FF  /5    JMP m16:16      5 + ts          Jump via task gate
E9  cd    JMP rel32       7+m             Jump near, displacement relative
                                          to next instruction
FF  /4    JMP r/m32       7+m,10+m        Jump near, indirect
EA  cp    JMP ptr16:32    12+m,pm=27+m    Jump intersegment, 6-byte
                                          immediate address
EA  cp    JMP ptr16:32    pm=45+m         Jump to call gate, same
                                          privilege
EA  cp    JMP ptr16:32    ts              Jump via task state segment
EA  cp    JMP ptr16:32    ts              Jump via task gate
FF  /5    JMP m16:32      43+m,pm=31+m    Jump intersegment, address at
                                          r/m dword
FF  /5    JMP m16:32      pm=49+m         Jump to call gate, same
                                          privilege
FF  /5    JMP m16:32      5 + ts          Jump via task state segment
FF  /5    JMP m16:32      5 + ts          Jump via task gate


───────────────────────────────────────────────────────────────────────────
NOTE:
Values of ts are given by the following table:

                               New Task

                386 TSS       386 TASK       286 TSS
                VM = 0        VM = 1

Old Task                   Via Task Gate?

                N     Y       N      Y       N     Y
386
TSS VM=0       303   312     220    229     276   285

286
TSS            301   310     218    227     274   283
───────────────────────────────────────────────────────────────────────────

Operation

IF instruction = relative JMP
   (* i.e. operand is rel8, rel16, or rel32 *)
THEN
   EIP  EIP + rel8/16/32;
   IF OperandSize = 16
   THEN EIP  EIP AND 0000FFFFH;
   FI;
FI;
IF instruction = near indirect JMP
   (* i.e. operand is r/m16 or r/m32 *)
THEN
   IF OperandSize = 16
   THEN
      EIP  [r/m16] AND 0000FFFFH;
   ELSE (* OperandSize = 32 *)
      EIP  [r/m32];
   FI;
FI;

IF (PE = 0 OR (PE = 1 AND VM = 1)) (* real mode or V86 mode *)
   AND instruction = far JMP
   (* i.e., operand type is m16:16, m16:32, ptr16:16, ptr16:32 *)
THEN GOTO REAL-OR-V86-MODE;
   IF operand type = m16:16 or m16:32
   THEN (* indirect *)
      IF OperandSize = 16
      THEN
         CS:IP  [m16:16];
         EIP  EIP AND 0000FFFFH; (* clear upper 16 bits *)
      ELSE (* OperandSize = 32 *)
         CS:EIP  [m16:32];
      FI;
   FI;
   IF operand type = ptr16:16 or ptr16:32
   THEN
      IF OperandSize = 16
      THEN
         CS:IP  ptr16:16;
         EIP  EIP AND 0000FFFFH; (* clear upper 16 bits *)
      ELSE (* OperandSize = 32 *)
         CS:EIP  ptr16:32;
      FI;
   FI;
FI;

IF (PE = 1 AND VM = 0) (* Protected mode, not V86 mode *)
   AND instruction = far JMP
THEN
   IF operand type = m16:16 or m16:32
   THEN (* indirect *)
      check access of EA dword;
      #GP(0) or #SS(0) IF limit violation;
   FI;
   Destination selector is not null ELSE #GP(0)
   Destination selector index is within its descriptor table limits ELSE
#GP(selector)
   Depending on AR byte of destination descriptor:
      GOTO CONFORMING-CODE-SEGMENT;
      GOTO NONCONFORMING-CODE-SEGMENT;
      GOTO CALL-GATE;
      GOTO TASK-GATE;
      GOTO TASK-STATE-SEGMENT;
   ELSE #GP(selector); (* illegal AR byte in descriptor *)
FI;

CONFORMING-CODE-SEGMENT:
   Descriptor DPL must be ≤ CPL ELSE #GP(selector);
   Segment must be present ELSE #NP(selector);
   Instruction pointer must be within code-segment limit ELSE #GP(0);
   IF OperandSize = 32
   THEN Load CS:EIP from destination pointer;
   ELSE Load CS:IP from destination pointer;
   FI;
   Load CS register with new segment descriptor;

NONCONFORMING-CODE-SEGMENT:
   RPL of destination selector must be ≤ CPL ELSE #GP(selector);
   Descriptor DPL must be = CPL ELSE #GP(selector);
   Segment must be present ELSE # NP(selector);
   Instruction pointer must be within code-segment limit ELSE #GP(0);
   IF OperandSize = 32
   THEN Load CS:EIP from destination pointer;
   ELSE Load CS:IP from destination pointer;
   FI;
   Load CS register with new segment descriptor;
   Set RPL field of CS register to CPL;

CALL-GATE:
   Descriptor DPL must be ≥ CPL ELSE #GP(gate selector);
   Descriptor DPL must be ≥ gate selector RPL ELSE #GP(gate selector);
   Gate must be present ELSE #NP(gate selector);
   Examine selector to code segment given in call gate descriptor:
      Selector must not be null ELSE #GP(0);
      Selector must be within its descriptor table limits ELSE
         #GP(CS selector);
      Descriptor AR byte must indicate code segment
         ELSE #GP(CS selector);
      IF non-conforming
      THEN code-segment descriptor, DPL must = CPL
      ELSE #GP(CS selector);
      FI;
      IF conforming
      THEN code-segment descriptor DPL must be ≤ CPL;
      ELSE #GP(CS selector);
      Code segment must be present ELSE #NP(CS selector);
      Instruction pointer must be within code-segment limit ELSE #GP(0);
      IF OperandSize = 32
      THEN Load CS:EIP from call gate;
      ELSE Load CS:IP from call gate;
      FI;
   Load CS register with new code-segment descriptor;
   Set RPL of CS to CPL

TASK-GATE:
   Gate descriptor DPL must be ≥ CPL ELSE #GP(gate selector);
   Gate descriptor DPL must be ≥ gate selector RPL ELSE #GP(gate
     selector);
   Task Gate must be present ELSE #NP(gate selector);
   Examine selector to TSS, given in Task Gate descriptor:
   Must specify global in the local/global bit ELSE #GP(TSS selector);
   Index must be within GDT limits ELSE #GP(TSS selector);
   Descriptor AR byte must specify available TSS (bottom bits 00001);
      ELSE #GP(TSS selector);
   Task State Segment must be present ELSE #NP(TSS selector);
SWITCH-TASKS (without nesting) to TSS;
Instruction pointer must be within code-segment limit ELSE #GP(0);

TASK-STATE-SEGMENT:
   TSS DPL must be ≥ CPL ELSE #GP(TSS selector);
   TSS DPL must be ≥ TSS selector RPL ELSE #GP(TSS selector);
   Descriptor AR byte must specify available TSS (bottom bits 00001)
      ELSE #GP(TSS selector);
   Task State Segment must be present ELSE #NP(TSS selector);
   SWITCH-TASKS (without nesting) to TSS;
   Instruction pointer must be within code-segment limit ELSE #GP(0);

Description

The JMP instruction transfers control to a different point in the
instruction stream without recording return information.

The action of the various forms of the instruction are shown below.

Jumps with destinations of type r/m16, r/m32, rel16, and rel32 are near
jumps and do not involve changing the segment register value.

The JMP rel16 and JMP rel32 forms of the instruction add an offset to
the address of the instruction following the JMP to determine the
destination. The rel16 form is used when the instruction's operand-size
attribute is 16 bits (segment size attribute 16 only); rel32 is used when
the operand-size attribute is 32 bits (segment size attribute 32 only). The
result is stored in the 32-bit EIP register. With rel16, the upper 16 bits
of EIP are cleared, which results in an offset whose value does not exceed
16 bits.

JMP r/m16 and JMP r/m32 specifies a register or memory location from which
the absolute offset from the procedure is fetched. The offset fetched from
r/m is 32 bits for an operand-size attribute of 32 bits (r/m32), or 16 bits
for an operand-size attribute of 16 bits (r/m16).

The JMP ptr16:16 and ptr16:32 forms of the instruction use a four-byte
or six-byte operand as a long pointer to the destination. The JMP
and  forms fetch the long pointer from the memory location
specified (indirection). In Real Address Mode or Virtual 8086 Mode,
the long pointer provides 16 bits for the CS register and 16 or 32 bits
for the EIP register (depending on the operand-size attribute). In
Protected Mode, both long pointer forms consult the Access Rights (AR)
byte in the descriptor indexed by the selector part of the long pointer.

Depending on the value of the AR byte, the jump will perform one of
the following types of control transfers:

  ■  A jump to a code segment at the same privilege level
  ■  A task switch

For more information on protected mode control transfers, refer to
Chapter 6 and Chapter 7.

Flags Affected

All if a task switch takes place; none if no task switch occurs

Protected Mode Exceptions

Far jumps: #GP, #NP, #SS, and #TS, as indicated in the list above.

Near direct jumps: #GP(0) if procedure location is beyond the code
segment limits.

Near indirect jumps: #GP(0) for an illegal memory operand effective
address in the CS, DS, ES, FS, or GS segments: #SS(0) for an illegal
address in the SS segment; #GP if the indirect offset obtained is beyond
the code segment limits; #PF(fault-code) for a page fault.

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would be outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as under Real Address Mode; #PF(fault-code) for a
page fault


LAHF ── Load Flags into AH Register

Opcode  Instruction   Clocks   Description

9F      LAHF          2        Load: AH = flags SF ZF xx AF xx PF xx CF


Operation

AH  SF:ZF:xx:AF:xx:PF:xx:CF;

Description

LAHF transfers the low byte of the flags word to AH. The bits, from
MSB to LSB, are sign, zero, indeterminate, auxiliary, carry,
indeterminate, parity, indeterminate, and carry.

Flags Affected

None

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


LAR ── Load Access Rights Byte

Opcode        Instruction      Clocks      Description

0F  02 /r     LAR r16,r/m16    pm=15/16    r16  r/m16 masked by FF00
0F  02 /r     LAR r32,r/m32    pm=15/16    r32  r/m32 masked by 00FxFF00


Description

The LAR instruction stores a marked form of the second doubleword of
the descriptor for the source selector if the selector is visible at the
CPL (modified by the selector's RPL) and is a valid descriptor type. The
destination register is loaded with the high-order doubleword of the
descriptor masked by 00FxFF00, and ZF is set to 1. The x indicates that the
four bits corresponding to the upper four bits of the limit are undefined in
the value loaded by LAR. If the selector is invisible or of the wrong type,
ZF is cleared.

If the 32-bit operand size is specified, the entire 32-bit value is loaded
into the 32-bit destination register. If the 16-bit operand size is
specified, the lower 16-bits of this value are stored in the 16-bit
destination register.

All code and data segment descriptors are valid for LAR.

The valid special segment and gate descriptor types for LAR are given
in the following table:

Type   Name                     Valid/Invalid

  0    Invalid                  Invalid
  1    Available 80286 TSS      Valid
  2    LDT                      Valid
  3    Busy 80286 TSS           Valid
  4    80286 call gate          Valid
  5    80286/80386 task gate    Valid
  6    80286 trap gate          Valid
  7    80286 interrupt gate     Valid
  8    Invalid                  Invalid
  9    Available 80386 TSS      Valid
  A    Invalid                  Invalid
  B    Busy 80386 TSS           Valid
  C    80386 call gate          Valid
  D    Invalid                  Invalid
  E    80386 trap gate          Valid
  F    80386 interrupt gate     Valid

Flags Affected

ZF as described above

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 6; LAR is unrecognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode


LEA ── Load Effective Address

Opcode  Instruction  Clocks  Description

8D  /r  LEA r16,m    2       Store effective address for m in register r16
8D  /r  LEA r32,m    2       Store effective address for m in register r32
8D  /r  LEA r16,m    2       Store effective address for m in register r16
8D  /r  LEA r32,m    2       Store effective address for m in register r32


Operation

IF OperandSize = 16 AND AddressSize = 16
THEN r16  Addr(m);
ELSE
   IF OperandSize = 16 AND AddressSize = 32
   THEN
      r16  Truncate_to_16bits(Addr(m));   (* 32-bit address *)
   ELSE
      IF OperandSize = 32 AND AddressSize = 16
      THEN
         r32  Truncate_to_16bits(Addr(m));
      ELSE
         IF OperandSize = 32 AND AddressSize = 32
         THEN  r32  Addr(m);
         FI;
      FI;
   FI;
FI;

Description

LEA calculates the effective address (offset part) and stores it in the
specified register. The operand-size attribute of the instruction
(represented by OperandSize in the algorithm under "Operation" above) is
determined by the chosen register. The address-size attribute (represented
by AddressSize) is determined by the USE attribute of the segment containing
the second operand. The address-size and operand-size attributes affect the
action performed by LEA, as follows:

Operand Size  Address Size  Action Performed

    16            16        16-bit effective address is calculated and
                            stored in requested 16-bit register
                            destination.

    16            32        32-bit effective address is calculated. The
                            lower 16 bits of the address are stored in
                            the requested 16-bit register destination.

    32            16        16-bit effective address is calculated. The
                            16-bit address is zero-extended and stored
                            in the requested 32-bit register destination.

    32            32        32-bit effective address is calculated and
                            stored in the requested 32-bit register
                            destination.

Flags Affected

None

Protected Mode Exceptions

#UD if the second operand is a register

Real Address Mode Exceptions

Interrupt 6 if the second operand is a register

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode


LEAVE ── High Level Procedure Exit

Opcode  Instruction  Clocks  Description

C9      LEAVE        4       Set SP to BP, then pop BP
C9      LEAVE        4       Set ESP to EBP, then pop EBP


Operation

IF StackAddrSize = 16
THEN
   SP  BP;
ELSE (* StackAddrSize = 32 *)
   ESP  EBP;
FI;
IF OperandSize = 16
THEN
   BP  Pop();
ELSE (* OperandSize = 32 *)
   EBP  Pop();
FI;

Description

LEAVE reverses the actions of the ENTER instruction. By copying the
frame pointer to the stack pointer, LEAVE releases the stack space used
by a procedure for its local variables. The old frame pointer is popped
into BP or EBP, restoring the caller's frame. A subsequent RET
instruction removes any arguments pushed onto the stack of the exiting
procedure.

Flags Affected

None

Protected Mode Exceptions

#SS(0) if BP does not point to a location within the limits of the current
stack segment

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode


LGDT/LIDT ── Load Global/Interrupt Descriptor Table Register

Opcode       Instruction      Clocks        Description

0F  01 /2    LGDT m16&32      11            Load m into GDTR
0F  01 /3    LIDT m16&32      11            Load m into IDTR


Operation

IF instruction = LIDT
THEN
   IF OperandSize = 16
   THEN IDTR.Limit:Base  m16:24 (* 24 bits of base loaded *)
   ELSE IDTR.Limit:Base  m16:32
   FI;
ELSE (* instruction = LGDT *)
   IF OperandSize = 16
   THEN GDTR.Limit:Base  m16:24 (* 24 bits of base loaded *)
   ELSE GDTR.Limit:Base  m16:32;
   FI;
FI;

Description

The LGDT and LIDT instructions load a linear base address and limit
value from a six-byte data operand in memory into the GDTR or IDTR,
respectively. If a 16-bit operand is used with LGDT or LIDT, the
register is loaded with a 16-bit limit and a 24-bit base, and the
high-order eight bits of the six-byte data operand are not used. If a 32-bit
operand is used, a 16-bit limit and a 32-bit base is loaded; the high-order
eight bits of the six-byte operand are used as high-order base address bits.

The SGDT and SIDT instructions always store into all 48 bits of the
six-byte data operand. With the 80286, the upper eight bits are undefined
after SGDT or SIDT is executed. With the 80386, the upper eight bits
are written with the high-order eight address bits, for both a 16-bit
operand and a 32-bit operand. If LGDT or LIDT is used with a 16-bit
operand to load the register stored by SGDT or SIDT, the upper eight
bits are stored as zeros.

LGDT and LIDT appear in operating system software; they are not used
in application programs. They are the only instructions that directly load
a linear address (i.e., not a segment relative address) in 80386 Protected
Mode.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the current privilege level is not 0; #UD if the source operand
is a register; #GP(0) for an illegal memory operand effective address in
the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in
the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH; Interrupt 6 if the source operand is a
register

───────────────────────────────────────────────────────────────────────────
Note:
  These instructions are valid in Real Address Mode to allow
  power-up initialization for Protected Mode
───────────────────────────────────────────────────────────────────────────

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


LGS/LSS/LDS/LES/LFS ── Load Full Pointer

Opcode      Instruction      Clocks   Description

C5  /r      LDS r16,m16:16   7,p=22   Load DS:r16 with pointer from memory
C5  /r      LDS r32,m16:32   7,p=22   Load DS:r32 with pointer from memory
0F  B2 /r   LSS r16,m16:16   7,p=22   Load SS:r16 with pointer from memory
0F  B2 /r   LSS r32,m16:32   7,p=22   Load SS:r32 with pointer from memory
C4  /r      LES r16,m16:16   7,p=22   Load ES:r16 with pointer from memory
C4  /r      LES r32,m16:32   7,p=22   Load ES:r32 with pointer from memory
0F  B4 /r   LFS r16,m16:16   7,p=25   Load FS:r16 with pointer from memory
0F  B4 /r   LFS r32,m16:32   7,p=25   Load FS:r32 with pointer from memory
0F  B5 /r   LGS r16,m16:16   7,p=25   Load GS:r16 with pointer from memory
0F  B5 /r   LGS r32,m16:32   7,p=25   Load GS:r32 with pointer from memory


Operation

CASE instruction OF
   LSS: Sreg is SS; (* Load SS register *)
   LDS: Sreg is DS; (* Load DS register *)
   LES: Sreg is ES; (* Load ES register *)
   LFS: Sreg is FS; (* Load FS register *)
   LGS: Sreg is DS; (* Load GS register *)
ESAC;
IF (OperandSize = 16)
THEN
   r16  [Effective Address]; (* 16-bit transfer *)
   Sreg  [Effective Address + 2]; (* 16-bit transfer *)
   (* In Protected Mode, load the descriptor into the segment register *)
ELSE (* OperandSize = 32 *)
   r32  [Effective Address]; (* 32-bit transfer *)
   Sreg  [Effective Address + 4]; (* 16-bit transfer *)
   (* In Protected Mode, load the descriptor into the segment register *)
FI;

Description

These instructions read a full pointer from memory and store it in the
selected segment register:register pair. The full pointer loads 16 bits
into the segment register SS, DS, ES, FS, or GS. The other register loads 32
bits if the operand-size attribute is 32 bits, or loads 16 bits if the
operand-size attribute is 16 bits. The other 16- or 32-bit register to be
loaded is determined by the r16 or r32 register operand specified.

When an assignment is made to one of the segment registers, the
descriptor is also loaded into the segment register. The data for the
register is obtained from the descriptor table entry for the selector
given.

A null selector (values 0000-0003) can be loaded into DS, ES, FS, or
GS registers without causing a protection exception. (Any subsequent
reference to a segment whose corresponding segment register is loaded
with a null selector to address memory causes a #GP(0) exception. No
memory reference to the segment occurs.)

The following is a listing of the Protected Mode checks and actions taken in
the loading of a segment register:

IF SS is loaded:
   IF selector is null THEN #GP(0); FI;
   Selector index must be within its descriptor table limits ELSE
      #GP(selector);
   Selector's RPL must equal CPL ELSE #GP(selector);
   AR byte must indicate a writable data segment ELSE #GP(selector);
   DPL in the AR byte must equal CPL ELSE #GP(selector);
   Segment must be marked present ELSE #SS(selector);
   Load SS with selector;
   Load SS with descriptor;
IF DS, ES, FS, or GS is loaded with non-null selector:
   Selector index must be within its descriptor table limits ELSE
      #GP(selector);
   AR byte must indicate data or readable code segment ELSE
      #GP(selector);
   IF data or nonconforming code
   THEN both the RPL and the CPL must be less than or equal to DPL in
      AR byte;
   ELSE #GP(selector);
   Segment must be marked present ELSE #NP(selector);
Load segment register with selector and RPL bits;
Load segment register with descriptor;
IF DS, ES, FS or GS is loaded with a null selector:
   Clear descriptor valid bit;

Flags Affected

None

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
the second operand must be a memory operand, not a register; #GP(0)
if a null selector is loaded into SS; #PF(fault-code) for a page fault

Real Address Mode Exceptions

The second operand must be a memory operand, not a register; Interrupt
13 if any part of the operand would lie outside of the effective address
space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


LLDT ── Load Local Descriptor Table Register

Opcode      Instruction      Clocks   Description

0F  00 /2   LLDT r/m16       20       Load selector r/m16 into LDTR


Operation

LDTR  SRC;

Description

LLDT loads the Local Descriptor Table register (LDTR). The word
operand (memory or register) to LLDT should contain a selector to the
Global Descriptor Table (GDT). The GDT entry should be a Local Descriptor
Table. If so, then the LDTR is loaded from the entry. The descriptor
registers DS, ES, SS, FS, GS, and CS are not affected. The LDT field in the
task state segment does not change.

The selector operand can be 0; if so, the LDTR is marked invalid. All
descriptor references (except by the LAR, VERR, VERW or LSL
instructions) cause a #GP fault.

LLDT is used in operating system software; it is not used in application
programs.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the current privilege level is not 0; #GP(selector) if the
selector operand does not point into the Global Descriptor Table, or if the
entry in the GDT is not a Local Descriptor Table; #NP(selector) if the
LDT descriptor is not present; #GP(0) for an illegal memory operand
effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an
illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 6; LLDT is not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode (because the instruction is
not recognized, it will not execute or perform a memory reference)

Note

The operand-size attribute has no effect on this instruction.


LMSW ── Load Machine Status Word

Opcode      Instruction      Clocks   Description

0F  01 /6   LMSW r/m16       10/13    Load r/m16 in machine status word


Operation

MSW  r/m16; (* 16 bits is stored in the machine status word *)

Description

LMSW loads the machine status word (part of CR0) from the source
operand. This instruction can be used to switch to Protected Mode; if so,
it must be followed by an intrasegment jump to flush the instruction
queue. LMSW will not switch back to Real Address Mode.

LMSW is used only in operating system software. It is not used in
application programs.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the current privilege level is not 0; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault

Notes

The operand-size attribute has no effect on this instruction. This
instruction is provided for compatibility with the 80286; 80386 programs
should use MOV CR0, ... instead.


LOCK ── Assert LOCK# Signal Prefix

Opcode  Instruction  Clocks  Description

F0      LOCK         0       Assert LOCK# signal for the next instruction


Description

The LOCK prefix causes the LOCK# signal of the 80386 to be asserted
during execution of the instruction that follows it. In a multiprocessor
environment, this signal can be used to ensure that the 80386 has
exclusive use of any shared memory while LOCK# is asserted. The
read-modify-write sequence typically used to implement test-and-set on the
80386 is the BTS instruction.

The LOCK prefix functions only with the following instructions:

BT, BTS, BTR, BTC                   mem, reg/imm
XCHG                                reg, mem
XCHG                                mem, reg
ADD, OR, ADC, SBB, AND, SUB, XOR    mem, reg/imm
NOT, NEG, INC, DEC                  mem

An undefined opcode trap will be generated if a LOCK prefix is used
with any instruction not listed above.

XCHG always asserts LOCK# regardless of the presence or absence of
the LOCK prefix.

The integrity of the LOCK is not affected by the alignment of the
memory field. Memory locking is observed for arbitrarily misaligned
fields.

Locked access is not assured if another 80386 processor is executing an
instruction concurrently that has one of the following characteristics:

  ■  Is not preceded by a LOCK prefix

  ■  Is not one of the instructions in the preceding list

  ■  Specifies a memory operand that does not exactly overlap the
     destination operand. Locking is not guaranteed for partial overlap,
     even if one memory operand is wholly contained within another.

Flags Affected

None

Protected Mode Exceptions

#UD if LOCK is used with an instruction not listed in the "Description"
section above; other exceptions can be generated by the subsequent
(locked) instruction

Real Address Mode Exceptions

Interrupt 6 if LOCK is used with an instruction not listed in the
"Description" section above; exceptions can still be generated by the
subsequent (locked) instruction

Virtual 8086 Mode Exceptions

#UD if LOCK is used with an instruction not listed in the "Description"
section above; exceptions can still be generated by the subsequent (locked)
instruction


LODS/LODSB/LODSW/LODSD ── Load String Operand

Opcode  Instruction   Clocks   Description

AC      LODS m8       5        Load byte [(E)SI] into AL
AD      LODS m16      5        Load word [(E)SI] into AX
AD      LODS m32      5        Load dword [(E)SI] into EAX
AC      LODSB         5        Load byte DS:[(E)SI] into AL
AD      LODSW         5        Load word DS:[(E)SI] into AX
AD      LODSD         5        Load dword DS:[(E)SI] into EAX


Operation

IF AddressSize = 16
THEN use SI for source-index
ELSE (* AddressSize = 32 *)
   use ESI for source-index;
FI;
IF byte type of instruction
THEN
   AL  [source-index]; (* byte load *)
   IF DF = 0 THEN IncDec  1 ELSE IncDec  -1; FI;
ELSE
   IF OperandSize = 16
   THEN
      AX  [source-index]; (* word load *)
      IF DF = 0 THEN IncDec  2 ELSE IncDec  -2; FI;
   ELSE (* OperandSize = 32 *)
      EAX  [source-index]; (* dword load *)
      IF DF = 0 THEN IncDec  4 ELSE IncDec  -4; FI;
   FI;
FI;
source-index  source-index + IncDec

Description

LODS loads the AL, AX, or EAX register with the memory byte, word,
or doubleword at the location pointed to by the source-index register.
After the transfer is made, the source-index register is automatically
advanced. If the direction flag is 0 (CLD was executed), the source index
increments; if the direction flag is 1 (STD was executed), it decrements.
The increment or decrement is 1 if a byte is loaded, 2 if a word is loaded,
or 4 if a doubleword is loaded.

If the address-size attribute for this instruction is 16 bits, SI is used
for the source-index register; otherwise the address-size attribute is 32
bits, and the ESI register is used. The address of the source data is
determined solely by the contents of ESI/SI. Load the correct index value
into SI before executing the LODS instruction. LODSB, LODSW, LODSD are
synonyms for the byte, word, and doubleword LODS instructions.

LODS can be preceded by the REP prefix; however, LODS is used more typically
within a LOOP construct, because further processing of the data moved into
EAX, AX, or AL is usually necessary.

Flags Affected

None

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


LOOP/LOOPcond ── Loop Control with CX Counter

Opcode   Instruction  Clocks  Description

E2  cb   LOOP rel8    11+m    DEC count; jump short if count <> 0
E1  cb   LOOPE rel8   11+m    DEC count; jump short if count <> 0 and ZF=1
E1  cb   LOOPZ rel8   11+m    DEC count; jump short if count <> 0 and ZF=1
E0  cb   LOOPNE rel8  11+m    DEC count; jump short if count <> 0 and ZF=0
E0  cb   LOOPNZ rel8  11+m    DEC count; jump short if count <> 0 and ZF=0


Operation

IF AddressSize = 16 THEN CountReg is CX ELSE CountReg is ECX; FI;
CountReg  CountReg - 1;
IF instruction <> LOOP
THEN
   IF (instruction = LOOPE) OR (instruction = LOOPZ)
   THEN BranchCond  (ZF = 1) AND (CountReg <> 0);
   FI;
   IF (instruction = LOOPNE) OR (instruction = LOOPNZ)
   THEN BranchCond  (ZF = 0) AND (CountReg <> 0);
   FI;
FI;

IF BranchCond
THEN
   IF OperandSize = 16
   THEN
      IP  IP + SignExtend(rel8);
   ELSE (* OperandSize = 32 *)
      EIP  EIP + SignExtend(rel8);
   FI;
FI;

Description

LOOP decrements the count register without changing any of the flags.
Conditions are then checked for the form of LOOP being used. If the
conditions are met, a short jump is made to the label given by the operand
to LOOP. If the address-size attribute is 16 bits, the CX register is used
as the count register; otherwise the ECX register is used. The operand
of LOOP must be in the range from 128 (decimal) bytes before the
instruction to 127 bytes ahead of the instruction.

The LOOP instructions provide iteration control and combine loop index
management with conditional branching. Use the LOOP instruction by
loading an unsigned iteration count into the count register, then code the
LOOP at the end of a series of instructions to be iterated. The
destination of LOOP is a label that points to the beginning of the
iteration.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the offset jumped to is beyond the limits of the current code
segment

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


LSL ── Load Segment Limit

Opcode       Instruction      Clocks      Description

0F  03 /r    LSL r16,r/m16    pm=20/21    Load: r16  segment limit,
                                          selector r/m16 (byte granular)
0F  03 /r    LSL r32,r/m32    pm=20/21    Load: r32  segment limit,
                                          selector r/m32 (byte granular)
0F  03 /r    LSL r16,r/m16    pm=25/26    Load: r16  segment limit,
                                          selector r/m16 (page granular)
0F  03 /r    LSL r32,r/m32    pm=25/26    Load: r32  segment limit,
                                          selector r/m32 (page granular)


Description

The LSL instruction loads a register with an unscrambled segment limit,
and sets ZF to 1, provided that the source selector is visible at the CPL
weakened by RPL, and that the descriptor is a type accepted by LSL.
Otherwise, ZF is cleared to 0, and the destination register is unchanged.
The segment limit is loaded as a byte granular value. If the descriptor
has a page granular segment limit, LSL will translate it to a byte limit
before loading it in the destination register (shift left 12 the 20-bit
"raw" limit from descriptor, then OR with 00000FFFH).

The 32-bit forms of this instruction store the 32-bit byte granular limit
in the 16-bit destination register.

Code and data segment descriptors are valid for LSL.

The valid special segment and gate descriptor types for LSL are given
in the following table:

Type   Name                      Valid/Invalid

  0    Invalid                   Invalid
  1    Available 80286 TSS       Valid
  2    LDT                       Valid
  3    Busy 80286 TSS            Valid
  4    80286 call gate           Invalid
  5    80286/80386 task gate     Invalid
  6    80286 trap gate           Invalid
  7    80286 interrupt gate      Invalid
  8    Invalid                   Valid
  9    Available 80386 TSS       Valid
  A    Invalid                   Invalid
  B    Busy 80386 TSS            Valid
  C    80386 call gate           Invalid
  D    Invalid                   Invalid
  E    80386 trap gate           Invalid
  F    80386 interrupt gate      Invalid

Flags Affected

ZF as described above

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 6; LSL is not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode


LTR ── Load Task Register

Opcode       Instruction    Clocks    Description

0F  00 /3    LTR r/m16      pm=23/27  Load EA word into task register


Description

LTR loads the task register from the source register or memory location
specified by the operand. The loaded task state segment is marked busy.
A task switch does not occur.

LTR is used only in operating system software; it is not used in
application programs.

Flags Affected

None

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#GP(0) if the current privilege level is not 0; #GP(selector) if the object
named by the source selector is not a TSS or is already busy;
#NP(selector) if the TSS is marked "not present"; #PF(fault-code) for
a page fault

Real Address Mode Exceptions

Interrupt 6; LTR is not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode

Notes

The operand-size attribute has no effect on this instruction.


MOV ── Move Data


Opcode   Instruction       Clocks        Description

88  /r   MOV r/m8,r8       2/2           Move byte register to r/m byte
89  /r   MOV r/m16,r16     2/2           Move word register to r/m word
89  /r   MOV r/m32,r32     2/2           Move dword register to r/m dword
8A  /r   MOV r8,r/m8       2/4           Move r/m byte to byte register
8B  /r   MOV r16,r/m16     2/4           Move r/m word to word register
8B  /r   MOV r32,r/m32     2/4           Move r/m dword to dword register
8C  /r   MOV r/m16,Sreg    2/2           Move segment register to r/m word
8D  /r   MOV Sreg,r/m16    2/5,pm=18/19  Move r/m word to segment register
A0       MOV AL,moffs8     4             Move byte at (seg:offset) to AL
A1       MOV AX,moffs16    4             Move word at (seg:offset) to AX
A1       MOV EAX,moffs32   4             Move dword at (seg:offset) to EAX
A2       MOV moffs8,AL     2             Move AL to (seg:offset)
A3       MOV moffs16,AX    2             Move AX to (seg:offset)
A3       MOV moffs32,EAX   2             Move EAX to (seg:offset)
B0 + rb  MOV reg8,imm8     2             Move immediate byte to register
B8 + rw  MOV reg16,imm16   2             Move immediate word to register
B8 + rd  MOV reg32,imm32   2             Move immediate dword to register
C6       MOV r/m8,imm8     2/2           Move immediate byte to r/m byte
C7       MOV r/m16,imm16   2/2           Move immediate word to r/m word
C7       MOV r/m32,imm32   2/2           Move immediate dword to r/m dword


───────────────────────────────────────────────────────────────────────────
NOTES:
  moffs8, moffs16, and moffs32 all consist of a simple offset relative
  to the segment base. The 8, 16, and 32 refer to the size of the data. The
  address-size attribute of the instruction determines the size of the
  offset, either 16 or 32 bits.
───────────────────────────────────────────────────────────────────────────

Operation

DEST  SRC;

Description

MOV copies the second operand to the first operand.

If the destination operand is a segment register (DS, ES, SS, etc.), then
data from a descriptor is also loaded into the register. The data for the
register is obtained from the descriptor table entry for the selector
given. A null selector (values 0000-0003) can be loaded into DS and ES
registers without causing an exception; however, use of DS or ES causes a
#GP(0), and no memory reference occurs.

A MOV into SS inhibits all interrupts until after the execution of the
next instruction (which is presumably a MOV into eSP).

Loading a segment register under 80386 Protected Mode results in special
checks and actions, as described in the following listing:

IF SS is loaded;
THEN
   IF selector is null THEN #GP(0);
FI;
   Selector index must be within its descriptor table limits else
      #GP(selector);
   Selector's RPL must equal CPL else #GP(selector);
AR byte must indicate a writable data segment else #GP(selector);
   DPL in the AR byte must equal CPL else #GP(selector);
   Segment must be marked present else #SS(selector);
   Load SS with selector;
   Load SS with descriptor.
FI;
IF DS, ES, FS or GS is loaded with non-null selector;
THEN
   Selector index must be within its descriptor table limits
      else #GP(selector);
   AR byte must indicate data or readable code segment else
      #GP(selector);
   IF data or nonconforming code segment
   THEN both the RPL and the CPL must be less than or equal to DPL in
      AR byte;
   ELSE #GP(selector);
   FI;
   Segment must be marked present else #NP(selector);
   Load segment register with selector;
   Load segment register with descriptor;
FI;
IF DS, ES, FS or GS is loaded with a null selector;
THEN
   Load segment register with selector;
   Clear descriptor valid bit;
FI;

Flags Affected

None

Protected Mode Exceptions

#GP, #SS, and #NP if a segment register is being loaded; otherwise,
#GP(0) if the destination is in a nonwritable segment; #GP(0) for an
illegal memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


MOV ── Move to/from Special Registers

Opcode      Instruction           Clocks   Description

0F  20 /r   MOV r32,CR0/CR2/CR3   6        Move (control register) to
                                           (register)
0F  22 /r   MOV CR0/CR2/CR3,r32   10/4/5   Move (register) to (control
                                           register)
0F  21 /r   MOV r32,DR0 -- 3      22       Move (debug register) to
                                           (register)
0F  21 /r   MOV r32,DR6/DR7       14       Move (debug register) to
                                           (register)
0F  23 /r   MOV DR0 -- 3,r32      22       Move (register) to (debug
                                           register)
0F  23 /r   MOV DR6/DR7,r32       16       Move (register) to (debug
                                           register)
0F  24 /r   MOV r32,TR6/TR7       12       Move (test register) to
                                           (register)
0F  26 /r   MOV TR6/TR7,r32       12       Move (register) to (test
                                           register)


Operation

DEST  SRC;

Description

The above forms of MOV store or load the following special registers in
or from a general purpose register:

  ■  Control registers CR0, CR2, and CR3
  ■  Debug Registers DR0, DR1, DR2, DR3, DR6, and DR7
  ■  Test Registers TR6 and TR7

32-bit operands are always used with these instructions, regardless of the
operand-size attribute.

Flags Affected

OF, SF, ZF, AF, PF, and CF are undefined

Protected Mode Exceptions

#GP(0) if the current privilege level is not 0

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

#GP(0) if instruction execution is attempted

Notes

The instructions must be executed at privilege level 0 or in real-address
mode; otherwise, a protection exception will be raised.

The reg field within the ModRM byte specifies which of the special
registers in each category is involved. The two bits in the  field are
always 11. The r/m field specifies the general register involved.


MOVS/MOVSB/MOVSW/MOVSD ── Move Data from String to String

Opcode  Instruction      Clocks   Description

A4      MOVS m8,m8       7        Move byte [(E)SI] to ES:[(E)DI]
A5      MOVS m16,m16     7        Move word [(E)SI] to ES:[(E)DI]
A5      MOVS m32,m32     7        Move dword [(E)SI] to ES:[(E)DI]
A4      MOVSB            7        Move byte DS:[(E)SI] to ES:[(E)DI]
A5      MOVSW            7        Move word DS:[(E)SI] to ES:[(E)DI]
A5      MOVSD            7        Move dword DS:[(E)SI] to ES:[(E)DI]


Operation

IF (instruction = MOVSD) OR (instruction has doubleword operands)
THEN OperandSize  32;
ELSE OperandSize  16;
IF AddressSize = 16
THEN use SI for source-index and DI for destination-index;
ELSE (* AddressSize = 32 *)
   use ESI for source-index and EDI for destination-index;
FI;
IF byte type of instruction
THEN
   [destination-index]  [source-index]; (* byte assignment *)
   IF DF = 0 THEN IncDec  1 ELSE IncDec  -1; FI;
ELSE
   IF OperandSize = 16
   THEN
      [destination-index]  [source-index]; (* word assignment *)
      IF DF = 0 THEN IncDec  2 ELSE IncDec  -2; FI;
   ELSE (* OperandSize = 32 *)
      [destination-index]  [source-index]; (* doubleword assignment *)
      IF DF = 0 THEN IncDec  4 ELSE IncDec  -4; FI;
   FI;
FI;
source-index  source-index + IncDec;
destination-index  destination-index + IncDec;

Description

MOVS copies the byte or word at [(E)SI] to the byte or word at
ES:[(E)DI]. The destination operand must be addressable from the ES
register; no segment override is possible for the destination. A segment
override can be used for the source operand; the default is DS.

The addresses of the source and destination are determined solely by the
contents of (E)SI and (E)DI. Load the correct index values into (E)SI
and (E)DI before executing the MOVS instruction. MOVSB, MOVSW,
and MOVSD are synonyms for the byte, word, and doubleword MOVS
instructions.

After the data is moved, both (E)SI and (E)DI are advanced
automatically. If the direction flag is 0 (CLD was executed), the registers
are incremented; if the direction flag is 1 (STD was executed), the
registers are decremented. The registers are incremented or decremented by 1
if a byte was moved, 2 if a word was moved, or 4 if a doubleword was moved.

MOVS can be preceded by the REP prefix for block movement of CX
bytes or words. Refer to the REP instruction for details of this operation.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


MOVSX ── Move with Sign-Extend

Opcode     Instruction        Clocks   Description

0F  BE /r  MOVSX r16,r/m8     3/6      Move byte to word with sign-extend
0F  BE /r  MOVSX r32,r/m8     3/6      Move byte to dword, sign-extend
0F  BF /r  MOVSX r32,r/m16    3/6      Move word to dword, sign-extend


Operation

DEST  SignExtend(SRC);

Description

MOVSX reads the contents of the effective address or register as a byte
or a word, sign-extends the value to the operand-size attribute of the
instruction (16 or 32 bits), and stores the result in the destination
register.

Flags Affected

None

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


MOVZX ── Move with Zero-Extend

Opcode      Instruction        Clocks   Description

0F  B6 /r   MOVZX r16,r/m8     3/6      Move byte to word with zero-extend
0F  B6 /r   MOVZX r32,r/m8     3/6      Move byte to dword, zero-extend
0F  B7 /r   MOVZX r32,r/m16    3/6      Move word to dword, zero-extend


Operation

DEST  ZeroExtend(SRC);

Description

MOVZX reads the contents of the effective address or register as a byte
or a word, zero extends the value to the operand-size attribute of the
instruction (16 or 32 bits), and stores the result in the destination
register.

Flags Affected

None

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


MUL ── Unsigned Multiplication of AL or AX

Opcode  Instruction     Clocks       Description

F6  /4  MUL AL,r/m8     9-14/12-17   Unsigned multiply (AX  AL * r/m byte)
F7  /4  MUL AX,r/m16    9-22/12-25   Unsigned multiply (DX:AX  AX * r/m
                                     word)
F7  /4  MUL EAX,r/m32   9-38/12-41   Unsigned multiply (EDX:EAX  EAX * r/m
                                     dword)


───────────────────────────────────────────────────────────────────────────
NOTES:
  The 80386 uses an early-out multiply algorithm. The actual number of
  clocks depends on the position of the most significant bit in the
  optimizing multiplier, shown underlined above. The optimization occurs
  for positive and negative multiplier values. Because of the early-out
  algorithm, clock counts given are minimum to maximum. To calculate the
  actual clocks, use the following formula:

    Actual clock = if  <> 0 then max(ceiling(log{2} │m│), 3) + 6 clocks;

    Actual clock = if  = 0 then 9 clocks

  where m is the multiplier.
───────────────────────────────────────────────────────────────────────────

Operation

IF byte-size operation
THEN AX  AL * r/m8
ELSE (* word or doubleword operation *)
   IF OperandSize = 16
   THEN DX:AX  AX * r/m16
   ELSE (* OperandSize = 32 *)
      EDX:EAX  EAX * r/m32
   FI;
FI;

Description

MUL performs unsigned multiplication. Its actions depend on the size
of its operand, as follows:

  ■  A byte operand is multiplied by AL; the result is left in AX. The
     carry and overflow flags are set to 0 if AH is 0; otherwise, they are
     set to 1.

  ■  A word operand is multiplied by AX; the result is left in DX:AX.
     DX contains the high-order 16 bits of the product. The carry and
     overflow flags are set to 0 if DX is 0; otherwise, they are set to 1.

  ■  A doubleword operand is multiplied by EAX and the result is left in
     EDX:EAX. EDX contains the high-order 32 bits of the product. The
     carry and overflow flags are set to 0 if EDX is 0; otherwise, they are
     set to 1.

Flags Affected

OF and CF as described above; SF, ZF, AF, PF, and CF are undefined

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


NEG ── Two's Complement Negation

Opcode  Instruction   Clocks    Description

F6  /3  NEG r/m8      2/6       Two's complement negate r/m byte
F7  /3  NEG r/m16     2/6       Two's complement negate r/m word
F7  /3  NEG r/m32     2/6       Two's complement negate r/m dword


Operation

IF r/m = 0 THEN CF  0 ELSE CF  1; FI;
r/m  - r/m;

Description

NEG replaces the value of a register or memory operand with its two's
complement. The operand is subtracted from zero, and the result is placed
in the operand.

The carry flag is set to 1, unless the operand is zero, in which case the
carry flag is cleared to 0.

Flags Affected

CF as described above; OF, SF, ZF, and PF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page
fault


NOP ── No Operation

Opcode  Instruction   Clocks    Description

90      NOP           3         No operation


Description

NOP performs no operation. NOP is a one-byte instruction that takes
up space but affects none of the machine context except (E)IP.

NOP is an alias mnemonic for the XCHG (E)AX, (E)AX instruction.

Flags Affected

None

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


NOT ── One's Complement Negation

Opcode    Instruction   Clocks    Description

F6   /2   NOT r/m8       2/6      Reverse each bit of r/m byte
F7   /2   NOT r/m16      2/6      Reverse each bit of r/m word
F7   /2   NOT r/m32      2/6      Reverse each bit of r/m dword


Operation

r/m  NOT r/m;

Description

NOT inverts the operand; every 1 becomes a 0, and vice versa.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page
fault


OR ── Logical Inclusive OR

Opcode       Instruction       Clocks    Description

0C  ib       OR AL,imm8        2         OR immediate byte to AL
0D  iw       OR AX,imm16       2         OR immediate word to AX
0D  id       OR EAX,imm32      2         OR immediate dword to EAX
80  /1 ib    OR r/m8,imm8      2/7       OR immediate byte to r/m byte
81  /1 iw    OR r/m16,imm16    2/7       OR immediate word to r/m word
81  /1 id    OR r/m32,imm32    2/7       OR immediate dword to r/m dword
83  /1 ib    OR r/m16,imm8     2/7       OR sign-extended immediate byte
                                         with r/m word
83  /1 ib    OR r/m32,imm8     2/7       OR sign-extended immediate byte
                                         with r/m dword
08  /r       OR r/m8,r8        2/6       OR byte register to r/m byte
09  /r       OR r/m16,r16      2/6       OR word register to r/m word
09  /r       OR r/m32,r32      2/6       OR dword register to r/m dword
0A  /r       OR r8,r/m8        2/7       OR byte register to r/m byte
0B  /r       OR r16,r/m16      2/7       OR word register to r/m word
0B  /r       OR r32,r/m32      2/7       OR dword register to r/m dword


Operation

DEST  DEST OR SRC;
CF  0;
OF  0

Description

OR computes the inclusive OR of its two operands and places the result
in the first operand. Each bit of the result is 0 if both corresponding
bits of the operands are 0; otherwise, each bit is 1.

Flags Affected

OF  0, CF  0; SF, ZF, and PF as described in Appendix C; AF is
undefined

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page
fault


OUT ── Output to Port

Opcode    Instruction     Clocks          Description

E6  ib    OUT imm8,AL     10,pm=4*/24**   Output byte AL to immediate port
                                          number
E7  ib    OUT imm8,AX     10,pm=4*/24**   Output word AL to immediate port
                                          number
E7  ib    OUT imm8,EAX    10,pm=4*/24**   Output dword AL to immediate
                                          port number
EE        OUT DX,AL       11,pm=5*/25**   Output byte AL to port number in
DX
EF        OUT DX,AX       11,pm=5*/25**   Output word AL to port number in
DX
EF        OUT DX,EAX      11,pm=5*/25**   Output dword AL to port number
                                          in DX


───────────────────────────────────────────────────────────────────────────
NOTES:
   *If CPL ≤ IOPL
  **If CPL > IOPL or if in virtual 8086 mode
───────────────────────────────────────────────────────────────────────────

Operation

IF (PE = 1) AND ((VM = 1) OR (CPL > IOPL))
THEN (* Virtual 8086 mode, or protected mode with CPL > IOPL *)
   IF NOT I-O-Permission (DEST, width(DEST))
   THEN #GP(0);
   FI;
FI;
[DEST]  SRC; (* I/O address space used *)

Description

OUT transfers a data byte or data word from the register (AL, AX, or
EAX) given as the second operand to the output port numbered by the
first operand. Output to any port from 0 to 65535 is performed by placing
the port number in the DX register and then using an OUT instruction
with DX as the first operand. If the instruction contains an eight-bit port
ID, that value is zero-extended to 16 bits.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the current privilege level is higher (has less privilege) than
IOPL and any of the corresponding I/O permission bits in TSS equals 1

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

#GP(0) fault if any of the corresponding I/O permission bits in TSS
equals 1


OUTS/OUTSB/OUTSW/OUTSD ── Output String to Port

Opcode   Instruction     Clocks          Description

6E       OUTS DX,r/m8    14,pm=8*/28**   Output byte [(E)SI] to port in DX
6F       OUTS DX,r/m16   14,pm=8*/28**   Output word [(E)SI] to port in DX
6F       OUTS DX,r/m32   14,pm=8*/28**   Output dword [(E)SI] to port in DX
6E       OUTSB           14,pm=8*/28**   Output byte DS:[(E)SI] to port in
                                         DX
6F       OUTSW           14,pm=8*/28**   Output word DS:[(E)SI] to port in
                                         DX
6F       OUTSD           14,pm=8*/28**   Output dword DS:[(E)SI] to port in
                                         DX


───────────────────────────────────────────────────────────────────────────
NOTES:
   *If CPL ≤ IOPL
  **If CPL > IOPL or if in virtual 8086 mode
───────────────────────────────────────────────────────────────────────────

Operation

IF AddressSize = 16
THEN use SI for source-index;
ELSE (* AddressSize = 32 *)
   use ESI for source-index;
FI;

IF (PE = 1) AND ((VM = 1) OR (CPL > IOPL))
THEN (* Virtual 8086 mode, or protected mode with CPL > IOPL *)
   IF NOT I-O-Permission (DEST, width(DEST))
   THEN #GP(0);
   FI;
FI;
IF byte type of instruction
THEN
   [DX]  [source-index]; (* Write byte at DX I/O address *)
   IF DF = 0 THEN IncDec  1 ELSE IncDec  -1; FI;
FI;
IF OperandSize = 16
THEN
   [DX]  [source-index]; (* Write word at DX I/O address *)
   IF DF = 0 THEN IncDec  2 ELSE IncDec  -2; FI;
FI;
IF OperandSize = 32
THEN
   [DX]  [source-index]; (* Write dword at DX I/O address *)
   IF DF = 0 THEN IncDec  4 ELSE IncDec  -4; FI;
   FI;
FI;
source-index  source-index + IncDec;

Description

OUTS transfers data from the memory byte, word, or doubleword at the
source-index register to the output port addressed by the DX register. If
the address-size attribute for this instruction is 16 bits, SI is used for
the source-index register; otherwise, the address-size attribute is 32 bits,
and ESI is used for the source-index register.

OUTS does not allow specification of the port number as an immediate value.
The port must be addressed through the DX register value. Load the correct
value into DX before executing the OUTS instruction.

The address of the source data is determined by the contents of
source-index register. Load the correct index value into SI or ESI before
executing the OUTS instruction.

After the transfer, source-index register is advanced automatically. If
the direction flag is 0 (CLD was executed), the source-index register is
incremented; if the direction flag is 1 (STD was executed), it is
decremented. The amount of the increment or decrement is 1 if a byte is
output, 2 if a word is output, or 4 if a doubleword is output.

OUTSB, OUTSW, and OUTSD are synonyms for the byte, word, and
doubleword OUTS instructions. OUTS can be preceded by the REP
prefix for block output of CX bytes or words. Refer to the REP
instruction for details on this operation.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if CPL is greater than IOPL and any of the corresponding I/O
permission bits in TSS equals 1; #GP(0) for an illegal memory operand
effective address in the CS, DS, or ES segments; #SS(0) for an illegal
address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

#GP(0) fault if any of the corresponding I/O permission bits in TSS
equals 1; #PF(fault-code) for a page fault


POP ── Pop a Word from the Stack

Opcode      Instruction   Clocks     Description

8F   /0     POP m16       5          Pop top of stack into memory word
8F   /0     POP m32       5          Pop top of stack into memory dword
58 + rw     POP r16       4          Pop top of stack into word register
58 + rd     POP r32       4          Pop top of stack into dword register
1F          POP DS        7,pm=21    Pop top of stack into DS
07          POP ES        7,pm=21    Pop top of stack into ES
17          POP SS        7,pm=21    Pop top of stack into SS
0F   A1     POP FS        7,pm=21    Pop top of stack into FS
0F   A9     POP GS        7,pm=21    Pop top of stack into GS


Operation

IF StackAddrSize = 16
THEN
   IF OperandSize = 16
   THEN
      DEST  (SS:SP); (* copy a word *)
      SP  SP + 2;
   ELSE (* OperandSize = 32 *)
      DEST  (SS:SP); (* copy a dword *)
      SP  SP + 4;
   FI;
ELSE (* StackAddrSize = 32 * )
   IF OperandSize = 16
   THEN
      DEST  (SS:ESP); (* copy a word *)
      ESP  ESP + 2;
   ELSE (* OperandSize = 32 *)
      DEST  (SS:ESP); (* copy a dword *)
      ESP  ESP + 4;
   FI;
FI;

Description

POP replaces the previous contents of the memory, the register, or the
segment register operand with the word on the top of the 80386 stack,
addressed by SS:SP (address-size attribute of 16 bits) or SS:ESP
(addresssize attribute of 32 bits). The stack pointer SP is incremented
by 2 for an operand-size of 16 bits or by 4 for an operand-size of 32 bits.
It then points to the new top of stack.

POP CS is not an 80386 instruction. Popping from the stack into the CS
register is accomplished with a RET instruction.

If the destination operand is a segment register (DS, ES, FS, GS, or
SS), the value popped must be a selector. In protected mode, loading the
selector initiates automatic loading of the descriptor information
associated with that selector into the hidden part of the segment register;
loading also initiates validation of both the selector and the descriptor
information.

A null value (0000-0003) may be popped into the DS, ES, FS, or GS
register without causing a protection exception. An attempt to reference
a segment whose corresponding segment register is loaded with a null
value causes a #GP(0) exception. No memory reference occurs. The saved
value of the segment register is null.

A POP SS instruction inhibits all interrupts, including NMI, until after
execution of the next instruction. This allows sequential execution of POP
SS and POP eSP instructions without danger of having an invalid stack
during an interrupt. However, use of the LSS instruction is the preferred
method of loading the SS and eSP registers.

Loading a segment register while in protected mode results in special
checks and actions, as described in the following listing:

IF SS is loaded:
   IF selector is null THEN #GP(0);
   Selector index must be within its descriptor table limits ELSE
      #GP(selector);
   Selector's RPL must equal CPL ELSE #GP(selector);
   AR byte must indicate a writable data segment ELSE #GP(selector);
   DPL in the AR byte must equal CPL ELSE #GP(selector);
   Segment must be marked present ELSE #SS(selector);
   Load SS register with selector;
   Load SS register with descriptor;

IF DS, ES, FS or GS is loaded with non-null selector:
   AR byte must indicate data or readable code segment ELSE
      #GP(selector);
   IF data or nonconforming code
   THEN both the RPL and the CPL must be less than or equal to DPL in
      AR byte
   ELSE #GP(selector);
   FI;
   Segment must be marked present ELSE #NP(selector);
   Load segment register with selector;
   Load segment register with descriptor;

IF DS, ES, FS, or GS is loaded with a null selector:
   Load segment register with selector
   Clear valid bit in invisible portion of register

Flags Affected

None

Protected Mode Exceptions

#GP, #SS, and #NP if a segment register is being loaded; #SS(0) if the
current top of stack is not within the stack segment; #GP(0) if the result
is in a nonwritable segment; #GP(0) for an illegal memory operand
effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an
illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page
fault


POPA/POPAD ── Pop all General Registers

Opcode   Instruction   Clocks   Description

61       POPA          24       Pop DI, SI, BP, SP, BX, DX, CX, and AX
61       POPAD         24       Pop EDI, ESI, EBP, ESP, EDX, ECX, and EAX


Operation

IF OperandSize = 16 (* instruction = POPA *)
THEN
   DI  Pop();
   SI  Pop();
   BP  Pop();
   throwaway  Pop (); (* Skip SP *)
   BX  Pop();
   DX  Pop();
   CX  Pop();
   AX  Pop();
ELSE (* OperandSize = 32, instruction = POPAD *)
   EDI  Pop();
   ESI  Pop();
   EBP  Pop();
   throwaway  Pop (); (* Skip ESP *)
   EBX  Pop();
   EDX  Pop();
   ECX  Pop();
   EAX  Pop();
FI;

Description

POPA pops the eight 16-bit general registers. However, the SP value is
discarded instead of loaded into SP. POPA reverses a previous PUSHA,
restoring the general registers to their values before PUSHA was
executed. The first register popped is DI.

POPAD pops the eight 32-bit general registers. The ESP value is
discarded instead of loaded into ESP. POPAD reverses the previous
PUSHAD, restoring the general registers to their values before PUSHAD
was executed. The first register popped is EDI.

Flags Affected

None

Protected Mode Exceptions

#SS(0) if the starting or ending stack address is not within the stack
segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page
fault


POPF/POPFD ── Pop Stack into FLAGS or EFLAGS Register

Opcode   Instruction   Clocks   Description

9D       POPF          5        Pop top of stack FLAGS
9D       POPFD         5        Pop top of stack into EFLAGS


Operation

Flags  Pop();

Description

POPF/POPFD pops the word or doubleword on the top of the stack and
stores the value in the flags register. If the operand-size attribute of
the instruction is 16 bits, then a word is popped and the value is stored in
FLAGS. If the operand-size attribute is 32 bits, then a doubleword is popped
and the value is stored in EFLAGS.

Refer to Chapter 2 and Chapter 4 for information about the FLAGS
and EFLAGS registers. Note that bits 16 and 17 of EFLAGS, called
VM and RF, respectively, are not affected by POPF or POPFD.

The I/O privilege level is altered only when executing at privilege level
0. The interrupt flag is altered only when executing at a level at least as
privileged as the I/O privilege level. (Real-address mode is equivalent to
privilege level 0.) If a POPF instruction is executed with insufficient
privilege, an exception does not occur, but the privileged bits do not
change.

Flags Affected

All flags except VM and RF

Protected Mode Exceptions

#SS(0) if the top of stack is not within the stack segment

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

#GP(0) fault if IOPL is less than 3, to permit emulation


PUSH ── Push Operand onto the Stack

Opcode     Instruction   Clocks   Description

FF   /6    PUSH m16      5        Push memory word
FF   /6    PUSH m32      5        Push memory dword
50 + /r    PUSH r16      2        Push register word
50 + /r    PUSH r32      2        Push register dword
6A         PUSH imm8     2        Push immediate byte
68         PUSH imm16    2        Push immediate word
68         PUSH imm32    2        Push immediate dword
0E         PUSH CS       2        Push CS
16         PUSH SS       2        Push SS
1E         PUSH DS       2        Push DS
06         PUSH ES       2        Push ES
0F   A0    PUSH FS       2        Push FS
OF   A8    PUSH GS       2        Push GS


Operation

IF StackAddrSize = 16
THEN
   IF OperandSize = 16 THEN
      SP  SP - 2;
      (SS:SP)  (SOURCE); (* word assignment *)
   ELSE
      SP  SP - 4;
      (SS:SP)  (SOURCE); (* dword assignment *)
   FI;
ELSE (* StackAddrSize = 32 *)
   IF OperandSize = 16
   THEN
      ESP  ESP - 2;
      (SS:ESP)  (SOURCE); (* word assignment *)
   ELSE
      ESP  ESP - 4;
      (SS:ESP)  (SOURCE); (* dword assignment *)
   FI;
FI;

Description

PUSH decrements the stack pointer by 2 if the operand-size attribute of
the instruction is 16 bits; otherwise, it decrements the stack pointer by
4. PUSH then places the operand on the new top of stack, which is
pointed to by the stack pointer.

The 80386 PUSH eSP instruction pushes the value of eSP as it existed
before the instruction. This differs from the 8086, where PUSH SP
pushes the new value (decremented by 2).

Flags Affected

None

Protected Mode Exceptions

#SS(0) if the new value of SP or ESP is outside the stack segment limit;
#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

None; if SP or ESP is 1, the 80386 shuts down due to a lack of stack
space

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page
fault


PUSHA/PUSHAD ── Push all General Registers

Opcode  Instruction  Clocks   Description

60      PUSHA        18       Push AX, CX, DX, BX, original SP, BP, SI, and
                              DI
60      PUSHAD       18       Push EAX, ECX, EDX, EBX, original ESP, EBP,
                              ESI, and EDI


Operation

IF OperandSize = 16 (* PUSHA instruction *)
THEN
   Temp  (SP);
   Push(AX);
   Push(CX);
   Push(DX);
   Push(BX);
   Push(Temp);
   Push(BP);
   Push(SI);
   Push(DI);
ELSE (* OperandSize = 32, PUSHAD instruction *)
   Temp  (ESP);
   Push(EAX);
   Push(ECX);
   Push(EDX);
   Push(EBX);
   Push(Temp);
   Push(EBP);
   Push(ESI);
   Push(EDI);
FI;

Description

PUSHA and PUSHAD save the 16-bit or 32-bit general registers,
respectively, on the 80386 stack. PUSHA decrements the stack pointer
(SP) by 16 to hold the eight word values. PUSHAD decrements the
stack pointer (ESP) by 32 to hold the eight doubleword values. Because
the registers are pushed onto the stack in the order in which they were
given, they appear in the 16 or 32 new stack bytes in reverse order. The
last register pushed is DI or EDI.

Flags Affected

None

Protected Mode Exceptions

#SS(0) if the starting or ending stack address is outside the stack segment
limit; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Before executing PUSHA or PUSHAD, the 80386 shuts down if SP or
ESP equals 1, 3, or 5; if SP or ESP equals 7, 9, 11, 13, or 15, exception
13 occurs

Virtual 8086 Mode Exceptions

Same exceptions as in real-address mode; #PF(fault-code) for a page
fault


PUSHF/PUSHFD ── Push Flags Register onto the Stack

Opcode  Instruction  Clocks   Description

9C      PUSHF        4        Push FLAGS
9C      PUSHFD       4        Push EFLAGS


Operation

IF OperandSize = 32
THEN push(EFLAGS);
ELSE push(FLAGS);
FI;

Description

PUSHF decrements the stack pointer by 2 and copies the FLAGS
register to the new top of stack; PUSHFD decrements the stack pointer by
4, and the 80386 EFLAGS register is copied to the new top of stack
which is pointed to by SS:eSP. Refer to Chapter 2 and Chapter 4 for
information on the EFLAGS register.

Flags Affected

None

Protected Mode Exceptions

#SS(0) if the new value of eSP is outside the stack segment boundaries

Real Address Mode Exceptions

None; the 80386 shuts down due to a lack of stack space

Virtual 8086 Mode Exceptions

#GP(0) fault if IOPL is less than 3, to permit emulation


RCL/RCR/ROL/ROR ── Rotate


Opcode       Instruction       Clocks  Description

D0  /2       RCL r/m8,1        9/10    Rotate 9 bits (CF,r/m byte) left
                                       once
D2  /2       RCL r/m8,CL       9/10    Rotate 9 bits (CF,r/m byte) left CL
                                       times
C0  /2 ib    RCL r/m8,imm8     9/10    Rotate 9 bits (CF,r/m byte) left
                                       imm8 times
D1  /2       RCL r/m16,1       9/10    Rotate 17 bits (CF,r/m word) left
                                       once
D3  /2       RCL r/m16,CL      9/10    Rotate 17 bits (CF,r/m word) left
                                       CL times
C1  /2 ib    RCL r/m16,imm8    9/10    Rotate 17 bits (CF,r/m word) left
                                       imm8 times
D1  /2       RCL r/m32,1       9/10    Rotate 33 bits (CF,r/m dword) left
                                       once
D3  /2       RCL r/m32,CL      9/10    Rotate 33 bits (CF,r/m dword) left
                                       CL times
C1  /2 ib    RCL r/m32,imm8    9/10    Rotate 33 bits (CF,r/m dword) left
                                       imm8 times
D0  /3       RCR r/m8,1        9/10    Rotate 9 bits (CF,r/m byte) right
                                       once
D2  /3       RCR r/m8,CL       9/10    Rotate 9 bits (CF,r/m byte) right
                                       CL times
C0  /3 ib    RCR r/m8,imm8     9/10    Rotate 9 bits (CF,r/m byte) right
                                       imm8 times
D1  /3       RCR r/m16,1       9/10    Rotate 17 bits (CF,r/m word) right
                                       once
D3  /3       RCR r/m16,CL      9/10    Rotate 17 bits (CF,r/m word) right
                                       CL times
C1  /3 ib    RCR r/m16,imm8    9/10    Rotate 17 bits (CF,r/m word) right
                                       imm8 times
D1  /3       RCR r/m32,1       9/10    Rotate 33 bits (CF,r/m dword) right
                                       once
D3  /3       RCR r/m32,CL      9/10    Rotate 33 bits (CF,r/m dword) right
                                       CL times
C1  /3 ib    RCR r/m32,imm8    9/10    Rotate 33 bits (CF,r/m dword) right
                                       imm8 times
D0  /0       ROL r/m8,1        3/7     Rotate 8 bits r/m byte left once
D2  /0       ROL r/m8,CL       3/7     Rotate 8 bits r/m byte left CL
                                       times
C0  /0 ib    ROL r/m8,imm8     3/7     Rotate 8 bits r/m byte left imm8
                                       times
D1  /0       ROL r/m16,1       3/7     Rotate 16 bits r/m word left once
D3  /0       ROL r/m16,CL      3/7     Rotate 16 bits r/m word left CL
                                       times
C1  /0 ib    ROL r/m16,imm8    3/7     Rotate 16 bits r/m word left imm8
                                       times
D1  /0       ROL r/m32,1       3/7     Rotate 32 bits r/m dword left once
D3  /0       ROL r/m32,CL      3/7     Rotate 32 bits r/m dword left CL
                                       times
C1  /0 ib    ROL r/m32,imm8    3/7     Rotate 32 bits r/m dword left imm8
                                       times
D0  /1       ROR r/m8,1        3/7     Rotate 8 bits r/m byte right once
D2  /1       ROR r/m8,CL       3/7     Rotate 8 bits r/m byte right CL
                                       times
C0  /1 ib    ROR r/m8,imm8     3/7     Rotate 8 bits r/m word right imm8
                                       times
D1  /1       ROR r/m16,1       3/7     Rotate 16 bits r/m word right once
D3  /1       ROR r/m16,CL      3/7     Rotate 16 bits r/m word right CL
                                       times
C1  /1 ib    ROR r/m16,imm8    3/7     Rotate 16 bits r/m word right imm8
                                       times
D1  /1       ROR r/m32,1       3/7     Rotate 32 bits r/m dword right once
D3  /1       ROR r/m32,CL      3/7     Rotate 32 bits r/m dword right CL
                                       times
C1  /1 ib    ROR r/m32,imm8    3/7     Rotate 32 bits r/m dword right imm8
                                       times


Operation

(* ROL - Rotate Left *)
temp  COUNT;
WHILE (temp <> 0)
DO
   tmpcf  high-order bit of (r/m);
   r/m  r/m * 2 + (tmpcf);
   temp  temp - 1;
OD;
IF COUNT = 1
THEN
   IF high-order bit of r/m <> CF
   THEN OF  1;
   ELSE OF  0;
   FI;
ELSE OF  undefined;
FI;
(* ROR - Rotate Right *)
temp  COUNT;
WHILE (temp <> 0 )
DO
   tmpcf  low-order bit of (r/m);
   r/m  r/m / 2 + (tmpcf * 2^(width(r/m)));
   temp  temp - 1;
DO;
IF COUNT = 1
THEN
   IF (high-order bit of r/m) <> (bit next to high-order bit of r/m)
   THEN OF  1;
   ELSE OF  0;
   FI;
ELSE OF  undefined;
FI;

Description

Each rotate instruction shifts the bits of the register or memory operand
given. The left rotate instructions shift all the bits upward, except for
the top bit, which is returned to the bottom. The right rotate instructions
do the reverse: the bits shift downward until the bottom bit arrives at
the top.

For the RCL and RCR instructions, the carry flag is part of the rotated
quantity. RCL shifts the carry flag into the bottom bit and shifts the top
bit into the carry flag; RCR shifts the carry flag into the top bit and
shifts the bottom bit into the carry flag. For the ROL and ROR
instructions, the original value of the carry flag is not a part of the
result, but the carry flag receives a copy of the bit that was shifted from
one end to the other.

The rotate is repeated the number of times indicated by the second
operand, which is either an immediate number or the contents of the CL
register. To reduce the maximum instruction execution time, the 80386
does not allow rotation counts greater than 31. If a rotation count greater
than 31 is attempted, only the bottom five bits of the rotation are used.
The 8086 does not mask rotation counts. The 80386 in Virtual 8086 Mode does
mask rotation counts.

The overflow flag is defined only for the single-rotate forms of the
instructions (second operand = 1). It is undefined in all other cases. For
left shifts/rotates, the CF bit after the shift is XORed with the
high-order result bit. For right shifts/rotates, the high-order two bits of
the result are XORed to get OF.

Flags Affected

OF only for single rotates; OF is undefined for multi-bit rotates; CF as
described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


REP/REPE/REPZ/REPNE/REPNZ ── Repeat Following String Operation


Opcode    Instruction         Clocks           Description

F3  6C    REP INS r/m8, DX    13+6*(E)CX,
                              pm=7+6*(E)CX
If CPL ≤ IOPL/
                              27+6*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Input (E)CX bytes from port
                                               DX into ES:[(E)DI]
F3  6D    REP INS r/m16,DX    13+6*(E)CX,
                              pm=7+6*(E)CX
If CPL ≤ IOPL/
                              27+6*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Input (E)CX words from port
                                               DX into ES:[(E)DI]
F3  6D    REP INS r/m32,DX    13+6*(E)CX,
                              pm=7+6*(E)CX
If CPL ≤ IOPL/
                              27+6*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Input (E)CX dwords from port
                                               DX into ES:[(E)DI]
F3  A4    REP MOVS m8,m8      5+4*(E)CX        Move (E)CX bytes from
                                               [(E)SI] to ES:[(E)DI]
F3  A5    REP MOVS m16,m16    5+4*(E)CX        Move (E)CX words from
                                               [(E)SI] to ES:[(E)DI]
F3  A5    REP MOVS m32,m32    5+4*(E)CX        Move (E)CX dwords from
                                               [(E)SI] to ES:[(E)DI]
F3  6E    REP OUTS DX,r/m8    5+12*(E)CX,
                              pm=6+5*(E)CX
If CPL ≤ IOPL/
                              26+5*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Output (E)CX bytes from
                                               [(E)SI] to port DX
F3  6F    REP OUTS DX,r/m16   5+12*(E)CX,
                              pm=6+5*(E)CX
If CPL ≤ IOPL/
                              26+5*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Output (E)CX words from
                                               [(E)SI] to port DX
F3  6F    REP OUTS DX,r/m32   5+12*(E)CX,
                              pm=6+5*(E)CX
If CPL ≤ IOPL/
                              26+5*(E)CX
If CPL > IOPL or if in virtual 8086 mode      Output (E)CX dwords from
                                               [(E)SI] to port DX
F3  AA    REP STOS m8         5+5*(E)CX        Fill (E)CX bytes at
                                               ES:[(E)DI] with AL
F3  AB    REP STOS m16        5+5*(E)CX        Fill (E)CX words at
                                               ES:[(E)DI] with AX
F3  AB    REP STOS m32        5+5*(E)CX        Fill (E)CX dwords at
                                               ES:[(E)DI] with EAX
F3  A6    REPE CMPS m8,m8     5+9*N            Find nonmatching bytes in
                                               ES:[(E)DI] and [(E)SI]
F3  A7    REPE CMPS m16,m16   5+9*N            Find nonmatching words in
                                               ES:[(E)DI] and [(E)SI]
F3  A7    REPE CMPS m32,m32   5+9*N            Find nonmatching dwords in
                                               ES:[(E)DI] and [(E)SI]
F3  AE    REPE SCAS m8        5+8*N            Find non-AL byte starting
                                               at ES:[(E)DI]
F3  AF    REPE SCAS m16       5+8*N            Find non-AX word starting
                                               at ES:[(E)DI]
F3  AF    REPE SCAS m32       5+8*N            Find non-EAX dword starting
                                               at ES:[(E)DI]
F2  A6    REPNE CMPS m8,m8    5+9*N            Find matching bytes in
                                               ES:[(E)DI] and [(E)SI]
F2  A7    REPNE CMPS m16,m16  5+9*N            Find matching words in
                                               ES:[(E)DI] and [(E)SI]
F2  A7    REPNE CMPS m32,m32  5+9*N            Find matching dwords in
                                               ES:[(E)DI] and [(E)SI]
F2  AE    REPNE SCAS m8       5+8*N            Find AL, starting at
                                               ES:[(E)DI]
F2  AF    REPNE SCAS m16      5+8*N            Find AX, starting at
                                               ES:[(E)DI]
F2  AF    REPNE SCAS m32      5+8*N            Find EAX, starting at
                                               ES:[(E)DI]


Operation

IF AddressSize = 16
THEN use CX for CountReg;
ELSE (* AddressSize = 32 *) use ECX for CountReg;
FI;
WHILE CountReg <> 0
DO
   service pending interrupts (if any);
   perform primitive string instruction;
   CountReg  CountReg - 1;
   IF primitive operation is CMPB, CMPW, SCAB, or SCAW
   THEN
      IF (instruction is REP/REPE/REPZ) AND (ZF=1)
      THEN exit WHILE loop
      ELSE
         IF (instruction is REPNZ or REPNE) AND (ZF=0)
         THEN exit WHILE loop;
         FI;
      FI;
   FI;
OD;

Description

REP, REPE (repeat while equal), and REPNE (repeat while not equal)
are prefix that are applied to string operation. Each prefix cause the
string instruction that follows to be repeated the number of times
indicated in the count register or (for REPE and REPNE) until the
indicated condition in the zero flag is no longer met.

Synonymous forms of REPE and REPNE are REPZ and REPNZ,
respectively.

The REP prefixes apply only to one string instruction at a time. To repeat
a block of instructions, use the LOOP instruction or another looping
construct.

The precise action for each iteration is as follows:

  1.  If the address-size attribute is 16 bits, use CX for the count
      register; if the address-size attribute is 32 bits, use ECX for the
      count register.

  2.  Check CX. If it is zero, exit the iteration, and move to the next
      instruction.

  3.  Acknowledge any pending interrupts.

  4.  Perform the string operation once.

  5.  Decrement CX or ECX by one; no flags are modified.

  6.  Check the zero flag if the string operation is SCAS or CMPS. If
      the repeat condition does not hold, exit the iteration and move to
      the next instruction. Exit the iteration if the prefix is REPE and ZF
      is 0 (the last comparison was not equal), or if the prefix is REPNE
      and ZF is one (the last comparison was equal).

  7.  Return to step 1 for the next iteration.

Repeated CMPS and SCAS instructions can be exited if the count is
exhausted or if the zero flag fails the repeat condition. These two cases
can be distinguished by using either the JCXZ instruction, or by using
the conditional jumps that test the zero flag (JZ, JNZ, and JNE).

Flags Affected

ZF by REP CMPS and REP SCAS as described above

Protected Mode Exceptions

#UD if a repeat prefix is used before an instruction that is not in the
list above; further exceptions can be generated when the string operation is
executed; refer to the descriptions of the string instructions themselves

Real Address Mode Exceptions

Interrupt 6 if a repeat prefix is used before an instruction that is not in
the list above; further exceptions can be generated when the string
operation is executed; refer to the descriptions of the string instructions
themselves

Virtual 8086 Mode Exceptions

#UD if a repeat prefix is used before an instruction that is not in the
list above; further exceptions can be generated when the string operation is
executed; refer to the descriptions of the string instructions themselves

Notes

Not all input/output ports can handle the rate at which the REP INS
and REP OUTS instructions execute.


RET ── Return from Procedure

Opcode     Instruction  Clocks         Description

C3         RET          10+m           Return (near) to caller
CB         RET          18+m,pm=32+m   Return (far) to caller, same
                                       privilege
CB         RET          pm=68          Return (far), lesser privilege,
                                       switch stacks
C2  iw     RET imm16    10+m           Return (near), pop imm16 bytes of
                                       parameters
CA  iw     RET imm16    18+m,pm=32+m   Return (far), same privilege, pop
                                       imm16 bytes
CA  iw     RET imm16    pm=68          Return (far), lesser privilege, pop
                                       imm16 bytes


Operation

IF instruction = near RET
THEN;
   IF OperandSize = 16
   THEN
      IP  Pop();
      EIP  EIP AND 0000FFFFH;
   ELSE (* OperandSize = 32 *)
      EIP  Pop();
   FI;
   IF instruction has immediate operand THEN eSP  eSP + imm16; FI;
FI;

IF (PE = 0 OR (PE = 1 AND VM = 1))
   (* real mode or virtual 8086 mode *)
   AND instruction = far RET
THEN;
   IF OperandSize = 16
   THEN
      IP  Pop();
      EIP  EIP AND 0000FFFFH;
      CS  Pop(); (* 16-bit pop *)
   ELSE (* OperandSize = 32 *)
      EIP  Pop();
      CS  Pop(); (* 32-bit pop, high-order 16-bits discarded *)
   FI;
   IF instruction has immediate operand THEN eSP  eSP + imm16; FI;
FI;

IF (PE = 1 AND VM = 0) (* Protected mode, not V86 mode *)
   AND instruction = far RET
THEN
   IF OperandSize=32
   THEN Third word on stack must be within stack limits else #SS(0);
   ELSE Second word on stack must be within stack limits else #SS(0);
   FI;
   Return selector RPL must be ≥ CPL ELSE #GP(return selector)
   IF return selector RPL = CPL
   THEN GOTO SAME-LEVEL;
   ELSE GOTO OUTER-PRIVILEGE-LEVEL;
   FI;
FI;

SAME-LEVEL:
   Return selector must be non-null ELSE #GP(0)
   Selector index must be within its descriptor table limits ELSE
      #GP(selector)
   Descriptor AR byte must indicate code segment ELSE #GP(selector)
   IF non-conforming
   THEN code segment DPL must equal CPL;
   ELSE #GP(selector);
   FI;
   IF conforming
   THEN code segment DPL must be ≤ CPL;
   ELSE #GP(selector);
   FI;
   Code segment must be present ELSE #NP(selector);
   Top word on stack must be within stack limits ELSE #SS(0);
   IP must be in code segment limit ELSE #GP(0);
   IF OperandSize=32
   THEN
      Load CS:EIP from stack
      Load CS register with descriptor
      Increment eSP by 8 plus the immediate offset if it exists
   ELSE (* OperandSize=16 *)
      Load CS:IP from stack
      Load CS register with descriptor
      Increment eSP by 4 plus the immediate offset if it exists
   FI;

OUTER-PRIVILEGE-LEVEL:
   IF OperandSize=32
   THEN Top (16+immediate) bytes on stack must be within stack limits
      ELSE #SS(0);
   ELSE Top (8+immediate) bytes on stack must be within stack limits ELSE
      #SS(0);
   FI;
   Examine return CS selector and associated descriptor:
      Selector must be non-null ELSE #GP(0);
      Selector index must be within its descriptor table limits ELSE
         #GP(selector)
      Descriptor AR byte must indicate code segment ELSE #GP(selector);
      IF non-conforming
      THEN code segment DPL must equal return selector RPL
      ELSE #GP(selector);
      FI;
      IF conforming
      THEN code segment DPL must be ≤ return selector RPL;
      ELSE #GP(selector);
      FI;
      Segment must be present ELSE #NP(selector)
   Examine return SS selector and associated descriptor:
      Selector must be non-null ELSE #GP(0);
      Selector index must be within its descriptor table limits
         ELSE #GP(selector);
      Selector RPL must equal the RPL of the return CS selector ELSE
         #GP(selector);
      Descriptor AR byte must indicate a writable data segment ELSE
         #GP(selector);
      Descriptor DPL must equal the RPL of the return CS selector ELSE
         #GP(selector);
      Segment must be present ELSE #NP(selector);
   IP must be in code segment limit ELSE #GP(0);
   Set CPL to the RPL of the return CS selector;
   IF OperandMode=32
   THEN
      Load CS:EIP from stack;
      Set CS RPL to CPL;
      Increment eSP by 8 plus the immediate offset if it exists;
      Load SS:eSP from stack;
   ELSE (* OperandMode=16 *)
      Load CS:IP from stack;
      Set CS RPL to CPL;
      Increment eSP by 4 plus the immediate offset if it exists;
      Load SS:eSP from stack;
   FI;
   Load the CS register with the return CS descriptor;
   Load the SS register with the return SS descriptor;
   For each of ES, FS, GS, and DS
   DO
      IF the current register setting is not valid for the outer level,
         set the register to null (selector  AR  0);
      To be valid, the register setting must satisfy the following
         properties:
         Selector index must be within descriptor table limits;
         Descriptor AR byte must indicate data or readable code segment;
         IF segment is data or non-conforming code, THEN
            DPL must be ≥ CPL, or DPL must be ≥ RPL;
      FI;
   OD;

Description

RET transfers control to a return address located on the stack. The
address is usually placed on the stack by a CALL instruction, and the
return is made to the instruction that follows the CALL.

The optional numeric parameter to RET gives the number of stack bytes
(OperandMode=16) or words (OperandMode=32) to be released after the return
address is popped. These items are typically used as input parameters to the
procedure called.

For the intrasegment (near) return, the address on the stack is a segment
offset, which is popped into the instruction pointer. The CS register is
unchanged. For the intersegment (far) return, the address on the stack
is a long pointer. The offset is popped first, followed by the selector.

In real mode, CS and IP are loaded directly. In Protected Mode, an
intersegment return causes the processor to check the descriptor
addressed by the return selector. The AR byte of the descriptor must
indicate a code segment of equal or lesser privilege (or greater or equal
numeric value) than the current privilege level. Returns to a lesser
privilege level cause the stack to be reloaded from the value saved beyond
the parameter block.

The DS, ES, FS, and GS segment registers can be set to 0 by the RET
instruction during an interlevel transfer. If these registers refer to
segments that cannot be used by the new privilege level, they are set to
0 to prevent unauthorized access from the new privilege level.

Flags Affected

None

Protected Mode Exceptions

#GP, #NP, or #SS, as described under "Operation" above; #PF(fault-code) for
a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would be outside the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


SAHF ── Store AH into Flags

Opcode  Instruction  Clocks   Description

9E      SAHF         3        Store AH into flags SF ZF xx AF xx PF xx CF


Operation

SF:ZF:xx:AF:xx:PF:xx:CF  AH;

Description

SAHF loads the flags listed above with values from the AH register,
from bits 7, 6, 4, 2, and 0, respectively.

Flags Affected

SF, ZF, AF, PF, and CF as described above

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


SAL/SAR/SHL/SHR ── Shift Instructions


Opcode          Instruction       Clocks  Description

D0   /4         SAL r/m8,1        3/7     Multiply r/m byte by 2, once
D2   /4         SAL r/m8,CL       3/7     Multiply r/m byte by 2, CL times
C0   /4 ib      SAL r/m8,imm8     3/7     Multiply r/m byte by 2, imm8
                                          times
D1   /4         SAL r/m16,1       3/7     Multiply r/m word by 2, once
D3   /4         SAL r/m16,CL      3/7     Multiply r/m word by 2, CL times
C1   /4 ib      SAL r/m16,imm8    3/7     Multiply r/m word by 2, imm8
                                          times
D1   /4         SAL r/m32,1       3/7     Multiply r/m dword by 2, once
D3   /4         SAL r/m32,CL      3/7     Multiply r/m dword by 2, CL
                                          times
C1   /4 ib      SAL r/m32,imm8    3/7     Multiply r/m dword by 2, imm8
                                          times
D0   /7         SAR r/m8,1        3/7     Signed divide^(1) r/m byte by 2,
                                          once
D2   /7         SAR r/m8,CL       3/7     Signed divide^(1) r/m byte by 2,
                                          CL times
C0   /7 ib      SAR r/m8,imm8     3/7     Signed divide^(1) r/m byte by 2,
                                          imm8 times
D1   /7         SAR r/m16,1       3/7     Signed divide^(1) r/m word by 2,
                                          once
D3   /7         SAR r/m16,CL      3/7     Signed divide^(1) r/m word by 2,
                                          CL times
C1   /7 ib      SAR r/m16,imm8    3/7     Signed divide^(1) r/m word by 2,
                                          imm8 times
D1   /7         SAR r/m32,1       3/7     Signed divide^(1) r/m dword by 2,
                                          once
D3   /7         SAR r/m32,CL      3/7     Signed divide^(1) r/m dword by 2,
                                          CL times
C1   /7 ib      SAR r/m32,imm8    3/7     Signed divide^(1) r/m dword by 2,
                                          imm8 times
D0   /4         SHL r/m8,1        3/7     Multiply r/m byte by 2, once
D2   /4         SHL r/m8,CL       3/7     Multiply r/m byte by 2, CL times
C0   /4 ib      SHL r/m8,imm8     3/7     Multiply r/m byte by 2, imm8
                                          times
D1   /4         SHL r/m16,1       3/7     Multiply r/m word by 2, once
D3   /4         SHL r/m16,CL      3/7     Multiply r/m word by 2, CL times
C1   /4 ib      SHL r/m16,imm8    3/7     Multiply r/m word by 2, imm8
                                          times
D1   /4         SHL r/m32,1       3/7     Multiply r/m dword by 2, once
D3   /4         SHL r/m32,CL      3/7     Multiply r/m dword by 2, CL
                                          times
C1   /4 ib      SHL r/m32,imm8    3/7     Multiply r/m dword by 2, imm8
                                          times
D0   /5         SHR r/m8,1        3/7     Unsigned divide r/m byte by 2,
                                          once
D2   /5         SHR r/m8,CL       3/7     Unsigned divide r/m byte by 2,
                                          CL times
C0   /5 ib      SHR r/m8,imm8     3/7     Unsigned divide r/m byte by 2,
                                          imm8 times
D1   /5         SHR r/m16,1       3/7     Unsigned divide r/m word by 2,
                                          once
D3   /5         SHR r/m16,CL      3/7     Unsigned divide r/m word by 2,
                                          CL times
C1   /5 ib      SHR r/m16,imm8    3/7     Unsigned divide r/m word by 2,
                                          imm8 times
D1   /5         SHR r/m32,1       3/7     Unsigned divide r/m dword by 2,
                                          once
D3   /5         SHR r/m32,CL      3/7     Unsigned divide r/m dword by 2,
                                          CL times
C1   /5 ib      SHR r/m32,imm8    3/7     Unsigned divide r/m dword by 2,
                                          imm8 times


Not the same division as IDIV; rounding is toward negative infinity.

Operation

(* COUNT is the second parameter *)
(temp)  COUNT;
WHILE (temp <> 0)
DO
   IF instruction is SAL or SHL
   THEN CF  high-order bit of r/m;
   FI;
   IF instruction is SAR or SHR
   THEN CF  low-order bit of r/m;
   FI;
   IF instruction = SAL or SHL
   THEN r/m  r/m * 2;
   FI;
   IF instruction = SAR
   THEN r/m  r/m /2 (*Signed divide, rounding toward negative infinity*);
   FI;
   IF instruction = SHR
   THEN r/m  r/m / 2; (* Unsigned divide *);
   FI;
   temp  temp - 1;
OD;
(* Determine overflow for the various instructions *)
IF COUNT = 1
THEN
   IF instruction is SAL or SHL
   THEN OF  high-order bit of r/m <> (CF);
   FI;
   IF instruction is SAR
   THEN OF  0;
   FI;
   IF instruction is SHR
   THEN OF  high-order bit of operand;
   FI;
ELSE OF  undefined;
FI;

Description

SAL (or its synonym, SHL) shifts the bits of the operand upward. The
high-order bit is shifted into the carry flag, and the low-order bit is set
to 0.

SAR and SHR shift the bits of the operand downward. The low-order
bit is shifted into the carry flag. The effect is to divide the operand by
2. SAR performs a signed divide with rounding toward negative infinity (not
the same as IDIV); the high-order bit remains the same. SHR performs an
unsigned divide; the high-order bit is set to 0.

The shift is repeated the number of times indicated by the second
operand, which is either an immediate number or the contents of the CL
register. To reduce the maximum execution time, the 80386 does not
allow shift counts greater than 31. If a shift count greater than 31 is
attempted, only the bottom five bits of the shift count are used. (The
8086 uses all eight bits of the shift count.)

The overflow flag is set only if the single-shift forms of the instructions
are used. For left shifts, OF is set to 0 if the high bit of the answer is
the same as the result of the carry flag (i.e., the top two bits of the
original operand were the same); OF is set to 1 if they are different. For
SAR, OF is set to 0 for all single shifts. For SHR, OF is set to the
high-order bit of the original operand.

Flags Affected

OF for single shifts; OF is undefined for multiple shifts; CF, ZF, PF,
and SF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


SBB ── Integer Subtraction with Borrow


Opcode       Instruction       Clocks  Description

1C  ib       SBB AL,imm8       2       Subtract with borrow immediate byte
                                       from AL
1D  iw       SBB AX,imm16      2       Subtract with borrow immediate word
                                       from AX
1D  id       SBB EAX,imm32     2       Subtract with borrow immediate
                                       dword from EAX
80  /3 ib    SBB r/m8,imm8     2/7     Subtract with borrow immediate byte
                                       from r/m byte
81  /3 iw    SBB r/m16,imm16   2/7     Subtract with borrow immediate word
                                       from r/m word
81  /3 id    SBB r/m32,imm32   2/7     Subtract with borrow immediate
                                       dword from r/m dword
83  /3 ib    SBB r/m16,imm8    2/7     Subtract with borrow sign-extended
                                       immediate byte from r/m word
83  /3 ib    SBB r/m32,imm8    2/7     Subtract with borrow sign-extended
                                       immediate byte from r/m dword
18  /r       SBB r/m8,r8       2/6     Subtract with borrow byte register
                                       from r/m byte
19  /r       SBB r/m16,r16     2/6     Subtract with borrow word register
                                       from r/m word
19  /r       SBB r/m32,r32     2/6     Subtract with borrow dword register
                                       from r/m dword
1A  /r       SBB r8,r/m8       2/7     Subtract with borrow byte register
                                       from r/m byte
1B  /r       SBB r16,r/m16     2/7     Subtract with borrow word register
                                       from r/m word
1B  /r       SBB r32,r/m32     2/7     Subtract with borrow dword register
                                       from r/m dword


Operation

IF SRC is a byte and DEST is a word or dword
THEN DEST = DEST - (SignExtend(SRC) + CF)
ELSE DEST  DEST - (SRC + CF);

Description

SBB adds the second operand (DEST) to the carry flag (CF) and
subtracts the result from the first operand (SRC). The result of the
subtraction is assigned to the first operand (DEST), and the flags are
set accordingly.

When an immediate byte value is subtracted from a word operand, the
immediate value is first sign-extended.

Flags Affected

OF, SF, ZF, AF, PF, and CF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


SCAS/SCASB/SCASW/SCASD ── Compare String Data

Opcode  Instruction  Clocks  Description

AE      SCAS m8      7       Compare bytes AL-ES:[DI], update (E)DI
AF      SCAS m16     7       Compare words AX-ES:[DI], update (E)DI
AF      SCAS m32     7       Compare dwords EAX-ES:[DI], update (E)DI
AE      SCASB        7       Compare bytes AL-ES:[DI], update (E)DI
AF      SCASW        7       Compare words AX-ES:[DI], update (E)DI
AF      SCASD        7       Compare dwords EAX-ES:[DI], update (E)DI


Operation

IF AddressSize = 16
THEN use DI for dest-index;
ELSE (* AddressSize = 32 *) use EDI for dest-index;
FI;
IF byte type of instruction
THEN
   AL - [dest-index]; (* Compare byte in AL and dest *)
   IF DF = 0 THEN IndDec  1 ELSE IncDec  -1; FI;
ELSE
   IF OperandSize = 16
   THEN
      AX - [dest-index]; (* compare word in AL and dest *)
      IF DF = 0 THEN IncDec  2 ELSE IncDec  -2; FI;
   ELSE (* OperandSize = 32 *)
      EAX - [dest-index];(* compare dword in EAX & dest *)
      IF DF = 0 THEN IncDec  4 ELSE IncDec  -4; FI;
   FI;
FI;
dest-index = dest-index + IncDec

Description

SCAS subtracts the memory byte or word at the destination register from
the AL, AX or EAX register. The result is discarded; only the flags are set.
The operand must be addressable from the ES segment; no segment override is
possible.

If the address-size attribute for this instruction is 16 bits, DI is used
as the destination register; otherwise, the address-size attribute is 32
bits and EDI is used.

The address of the memory data being compared is determined solely by the
contents of the destination register, not by the operand to SCAS. The
operand validates ES segment addressability and determines the data type.
Load the correct index value into DI or EDI before executing SCAS.

After the comparison is made, the destination register is automatically
updated. If the direction flag is 0 (CLD was executed), the destination
register is incremented; if the direction flag is 1 (STD was executed), it
is decremented. The increments or decrements are by 1 if bytes are compared,
by 2 if words are compared, or by 4 if doublewords are compared.

SCASB, SCASW, and SCASD are synonyms for the byte, word and
doubleword SCAS instructions that don't require operands. They are
simpler to code, but provide no type or segment checking.

SCAS can be preceded by the REPE or REPNE prefix for a block search
of CX or ECX bytes or words. Refer to the REP instruction for further
details.

Flags Affected

OF, SF, ZF, AF, PF, and CF as described in Appendix C

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


SETcc ── Byte Set on Condition


Opcode   Instruction  Clocks  Description

0F  97   SETA r/m8    4/5     Set byte if above (CF=0 and ZF=0)
0F  93   SETAE r/m8   4/5     Set byte if above or equal (CF=0)
0F  92   SETB r/m8    4/5     Set byte if below (CF=1)
0F  96   SETBE r/m8   4/5     Set byte if below or equal (CF=1 or (ZF=1)
0F  92   SETC r/m8    4/5     Set if carry (CF=1)
0F  94   SETE r/m8    4/5     Set byte if equal (ZF=1)
0F  9F   SETG r/m8    4/5     Set byte if greater (ZF=0 or SF=OF)
0F  9D   SETGE r/m8   4/5     Set byte if greater or equal (SF=OF)
0F  9C   SETL r/m8    4/5     Set byte if less (SF<>OF)
0F  9E   SETLE r/m8   4/5     Set byte if less or equal (ZF=1 and
                              SF<>OF)
0F  96   SETNA r/m8   4/5     Set byte if not above (CF=1)
0F  92   SETNAE r/m8  4/5     Set byte if not above or equal (CF=1)
0F  93   SETNB r/m8   4/5     Set byte if not below (CF=0)
0F  97   SETNBE r/m8  4/5     Set byte if not below or equal (CF=0 and
                              ZF=0)
0F  93   SETNC r/m8   4/5     Set byte if not carry (CF=0)
0F  95   SETNE r/m8   4/5     Set byte if not equal (ZF=0)
0F  9E   SETNG r/m8   4/5     Set byte if not greater (ZF=1 or SF<>OF)
0F  9C   SETNGE r/m8  4/5     Set if not greater or equal (SF<>OF)
0F  9D   SETNL r/m8   4/5     Set byte if not less (SF=OF)
0F  9F   SETNLE r/m8  4/5     Set byte if not less or equal (ZF=1 and
                              SF<>OF)
0F  91   SETNO r/m8   4/5     Set byte if not overflow (OF=0)
0F  9B   SETNP r/m8   4/5     Set byte if not parity (PF=0)
0F  99   SETNS r/m8   4/5     Set byte if not sign (SF=0)
0F  95   SETNZ r/m8   4/5     Set byte if not zero (ZF=0)
0F  90   SETO r/m8    4/5     Set byte if overflow (OF=1)
0F  9A   SETP r/m8    4/5     Set byte if parity (PF=1)
0F  9A   SETPE r/m8   4/5     Set byte if parity even (PF=1)
0F  9B   SETPO r/m8   4/5     Set byte if parity odd (PF=0)
0F  98   SETS r/m8    4/5     Set byte if sign (SF=1)
0F  94   SETZ r/m8    4/5     Set byte if zero (ZF=1)


Operation

IF condition THEN r/m8  1 ELSE r/m8  0; FI;

Description

SETcc stores a byte at the destination specified by the effective address
or register if the condition is met, or a 0 byte if the condition is not
met.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the result is in a non-writable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


SGDT/SIDT ── Store Global/Interrupt Descriptor Table Register

Opcode       Instruction   Clocks   Description

0F  01 /0    SGDT m        9        Store GDTR to m
0F  01 /1    SIDT m        9        Store IDTR to m


Operation

DEST  48-bit BASE/LIMIT register contents;

Description

SGDT/SIDT copies the contents of the descriptor table register the six
bytes of memory indicated by the operand. The LIMIT field of the
register is assigned to the first word at the effective address. If the
operand-size attribute is 32 bits, the next three bytes are assigned the
BASE field of the register, and the fourth byte is written with zero. The
last byte is undefined. Otherwise, if the operand-size attribute is 16
bits, the next four bytes are assigned the 32-bit BASE field of the
register.

SGDT and SIDT are used only in operating system software; they are
not used in application programs.

Flags Affected

None

Protected Mode Exceptions

Interrupt 6 if the destination operand is a register; #GP(0) if the
destination is in a nonwritable segment; #GP(0) for an illegal memory
operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for
an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 6 if the destination operand is a register; Interrupt 13 if any
part of the operand would lie outside of the effective address space from
0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault

Compatability Note

The 16-bit forms of the SGDT/SIDT instructions are compatible with
the 80286, if the value in the upper eight bits is not referenced. The
80286 stores 1's in these upper bits, whereas the 80386 stores 0's if the
operand-size attribute is 16 bits. These bits were specified as undefined
by the SGDT/SIDT instructions in the iAPX 286 Programmer's
Reference Manual.


SHLD ── Double Precision Shift Left

Opcode   Instruction          Clocks   Description

0F  A4   SHLD r/m16,r16,imm8  3/7      r/m16 gets SHL of r/m16 concatenated
                                       with r16
0F  A4   SHLD r/m32,r32,imm8  3/7      r/m32 gets SHL of r/m32 concatenated
                                       with r32
0F  A5   SHLD r/m16,r16,CL    3/7      r/m16 gets SHL of r/m16 concatenated
                                       with r16
0F  A5   SHLD r/m32,r32,CL    3/7      r/m32 gets SHL of r/m32 concatenated
                                       with r32


Operation

(* count is an unsigned integer corresponding to the last operand of the
instruction, either an immediate byte or the byte in register CL *)
ShiftAmt  count MOD 32;
inBits  register; (* Allow overlapped operands *)
IF ShiftAmt = 0
THEN no operation
ELSE
   IF ShiftAmt ≥ OperandSize
   THEN (* Bad parameters *)
      r/m  UNDEFINED;
      CF, OF, SF, ZF, AF, PF  UNDEFINED;
   ELSE (* Perform the shift *)
      CF  BIT[Base, OperandSize - ShiftAmt];
         (* Last bit shifted out on exit *)
   FOR i  OperandSize - 1 DOWNTO ShiftAmt
   DO
      BIT[Base, i]  BIT[Base, i - ShiftAmt];
   OF;
   FOR i  ShiftAmt - 1 DOWNTO 0
   DO
      BIT[Base, i]  BIT[inBits, i - ShiftAmt + OperandSize];
   OD;
   Set SF, ZF, PF (r/m);
      (* SF, ZF, PF are set according to the value of the result *)
   AF  UNDEFINED;
   FI;
FI;

Description

SHLD shifts the first operand provided by the r/m field to the left as
many bits as specified by the count operand. The second operand (r16 or r32)
provides the bits to shift in from the right (starting with bit 0). The
result is stored back into the r/m operand. The register remains unaltered.

The count operand is provided by either an immediate byte or the contents
of the CL register. These operands are taken MODULO 32 to provide a number
between 0 and 31 by which to shift. Because the bits to shift are provided
by the specified registers, the operation is useful for multiprecision
shifts (64 bits or more). The SF, ZF and PF flags are set according to the
value of the result. CS is set to the value of the last bit shifted out. OF
and AF are left undefined.

Flags Affected

OF, SF, ZF, PF, and CF as described above; AF and OF are undefined

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


SHRD ── Double Precision Shift Right

Opcode   Instruction           Clocks  Description

0F  AC   SHRD r/m16,r16,imm8   3/7     r/m16 gets SHR of r/m16 concatenated
                                       with r16
0F  AC   SHRD r/m32,r32,imm8   3/7     r/m32 gets SHR of r/m32 concatenated
                                       with r32
0F  AD   SHRD r/m16,r16,CL     3/7     r/m16 gets SHR of r/m16 concatenated
                                       with r16
0F  AD   SHRD r/m32,r32,CL     3/7     r/m32 gets SHR of r/m32 concatenated
                                       with r32


Operation

(* count is an unsigned integer corresponding to the last operand of the
instruction, either an immediate byte or the byte in register CL *)
ShiftAmt  count MOD 32;
inBits  register; (* Allow overlapped operands *)
IF ShiftAmt = 0
THEN no operation
ELSE
   IF ShiftAmt ≥ OperandSize
   THEN (* Bad parameters *)
      r/m  UNDEFINED;
      CF, OF, SF, ZF, AF, PF  UNDEFINED;
   ELSE (* Perform the shift *)
      CF  BIT[r/m, ShiftAmt - 1]; (* last bit shifted out on exit *)
      FOR i  0 TO OperandSize - 1 - ShiftAmt
      DO
         BIT[r/m, i]  BIT[r/m, i - ShiftAmt];
      OD;
      FOR i  OperandSize - ShiftAmt TO OperandSize - 1
      DO
         BIT[r/m,i]  BIT[inBits,i+ShiftAmt - OperandSize];
      OD;
      Set SF, ZF, PF (r/m);
         (* SF, ZF, PF are set according to the value of the result *)
      Set SF, ZF, PF (r/m);
      AF  UNDEFINED;
   FI;
FI;

Description

SHRD shifts the first operand provided by the r/m field to the right as many
bits as specified by the count operand. The second operand (r16 or r32)
provides the bits to shift in from the left (starting with bit 31). The
result is stored back into the r/m operand. The register remains unaltered.

The count operand is provided by either an immediate byte or the contents
of the CL register. These operands are taken MODULO 32 to provide a number
between 0 and 31 by which to shift. Because the bits to shift are provided
by the specified register, the operation is useful for multi-precision
shifts (64 bits or more). The SF, ZF and PF flags are set according to the
value of the result. CS is set to the value of the last bit shifted out. OF
and AF are left undefined.

Flags Affected

OF, SF, ZF, PF, and CF as described above; AF and OF are undefined

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


SLDT ── Store Local Descriptor Table Register

Opcode      Instruction   Clocks      Description

0F  00 /0   SLDT r/m16    pm=2/2      Store LDTR to EA word


Operation

r/m16  LDTR;

Description

SLDT stores the Local Descriptor Table Register (LDTR) in the two-byte
register or memory location indicated by the effective address operand.
This register is a selector that points into the Global Descriptor Table.

SLDT is used only in operating system software. It is not used in
application programs.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 6; SLDT is not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

The operand-size attribute has no effect on the operation of the
instruction.


SMSW ── Store Machine Status Word

Opcode      Instruction     Clocks          Description

0F  01 /4   SMSW r/m16      2/3,pm=2/2      Store machine status word to EA
                                            word


Operation

r/m16  MSW;

Description

SMSW stores the machine status word (part of CR0) in the two-byte register
or memory location indicated by the effective address operand.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

Notes

This instruction is provided for compatibility with the 80286; 80386
programs should use MOV ..., CR0.


STC ── Set Carry Flag

Opcode      Instruction     Clocks      Description

F9          STC             2           Set carry flag


Operation

CF  1;

Description

STC sets the carry flag to 1.

Flags Affected

CF = 1

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


STD ── Set Direction Flag

Opcode  Instruction   Clocks    Description

FD      STD           2         Set direction flag so (E)SI and/or (E)DI
                                decrement


Operation

DF  1;

Description

STD sets the direction flag to 1, causing all subsequent string operations
to decrement the index registers, (E)SI and/or (E)DI, on which they
operate.

Flags Affected

DF = 1

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


STI ── Set Interrupt Flag

Opcode  Instruction   Clocks   Description

F13     STI           3        Set interrupt flag; interrupts enabled at the
                               end of the next instruction


Operation

IF  1

Description

STI sets the interrupt flag to 1. The 80386 then responds to external
interrupts after executing the next instruction if the next instruction
allows the interrupt flag to remain enabled. If external interrupts are
disabled and you code STI, RET (such as at the end of a subroutine),
the RET is allowed to execute before external interrupts are recognized.
Also, if external interrupts are disabled and you code STI, CLI, then
external interrupts are not recognized because the CLI instruction clears
the interrupt flag during its execution.

Flags Affected

IF = 1

Protected Mode Exceptions

#GP(0) if the current privilege level is greater (has less privilege) than
the I/O privilege level

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None


STOS/STOSB/STOSW/STOSD ── Store String Data

Opcode  Instruction  Clocks   Description

AA      STOS m8      4        Store AL in byte ES:[(E)DI], update (E)DI
AB      STOS m16     4        Store AX in word ES:[(E)DI], update (E)DI
AB      STOS m32     4        Store EAX in dword ES:[(E)DI], update (E)DI
AA      STOSB        4        Store AL in byte ES:[(E)DI], update (E)DI
AB      STOSW        4        Store AX in word ES:[(E)DI], update (E)DI
AB      STOSD        4        Store EAX in dword ES:[(E)DI], update (E)DI


Operation

IF AddressSize = 16
THEN use ES:DI for DestReg
ELSE (* AddressSize = 32 *) use ES:EDI for DestReg;
FI;
IF byte type of instruction
THEN
   (ES:DestReg)  AL;
   IF DF = 0
   THEN DestReg  DestReg + 1;
   ELSE DestReg  DestReg - 1;
   FI;
ELSE IF OperandSize = 16
   THEN
      (ES:DestReg)  AX;
      IF DF = 0
      THEN DestReg  DestReg + 2;
      ELSE DestReg  DestReg - 2;
      FI;
   ELSE (* OperandSize = 32 *)
      (ES:DestReg)  EAX;
      IF DF = 0
      THEN DestReg  DestReg + 4;
      ELSE DestReg  DestReg - 4;
      FI;
   FI;
FI;

Description

STOS transfers the contents of all AL, AX, or EAX register to the memory
byte or word given by the destination register relative to the ES segment.
The destination register is DI for an address-size attribute of 16 bits or
EDI for an address-size attribute of 32 bits.

The destination operand must be addressable from the ES register. A segment
override is not possible.

The address of the destination is determined by the contents of the
destination register, not by the explicit operand of STOS. This operand is
used only to validate ES segment addressability and to determine the data
type. Load the correct index value into the destination register before
executing STOS.

After the transfer is made, DI is automatically updated. If the direction
flag is 0 (CLD was executed), DI is incremented; if the direction flag is
1 (STD was executed), DI is decremented. DI is incremented or decremented by
1 if a byte is stored, by 2 if a word is stored, or by 4 if a doubleword is
stored.

STOSB, STOSW, and STOSD are synonyms for the byte, word, and doubleword STOS
instructions, that do not require an operand. They are simpler to use, but
provide no type or segment checking.

STOS can be preceded by the REP prefix for a block fill of CX or ECX bytes,
words, or doublewords. Refer to the REP instruction for further details.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


STR ── Store Task Register

Opcode        Instruction   Clocks       Description

0F  00 /1     STR r/m16     pm=23/27     Load EA word into task register


Operation

r/m  task register;

Description

The contents of the task register are copied to the two-byte register or
memory location indicated by the effective address operand.

STR is used only in operating system software. It is not used in application
programs.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 6; STR is not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode

Notes

The operand-size attribute has no effect on this instruction.


SUB ── Integer Subtraction

Opcode      Instruction      Clocks   Description

2C  ib      SUB AL,imm8      2        Subtract immediate byte from AL
2D  iw      SUB AX,imm16     2        Subtract immediate word from AX
2D  id      SUB EAX,imm32    2        Subtract immediate dword from EAX
80  /5 ib   SUB r/m8,imm8    2/7      Subtract immediate byte from r/m byte
81  /5 iw   SUB r/m16,imm16  2/7      Subtract immediate word from r/m word
81  /5 id   SUB r/m32,imm32  2/7      Subtract immediate dword from r/m
                                      dword
83  /5 ib   SUB r/m16,imm8   2/7      Subtract sign-extended immediate byte
                                      from r/m word
83  /5 ib   SUB r/m32,imm8   2/7      Subtract sign-extended immediate byte
                                      from r/m dword
28  /r      SUB r/m8,r8      2/6      Subtract byte register from r/m byte
29  /r      SUB r/m16,r16    2/6      Subtract word register from r/m word
29  /r      SUB r/m32,r32    2/6      Subtract dword register from r/m
                                      dword
2A  /r      SUB r8,r/m8      2/7      Subtract byte register from r/m byte
2B  /r      SUB r16,r/m16    2/7      Subtract word register from r/m word
2B  /r      SUB r32,r/m32    2/7      Subtract dword register from r/m
                                      dword


Operation

IF SRC is a byte and DEST is a word or dword
THEN DEST = DEST - SignExtend(SRC);
ELSE DEST  DEST - SRC;
FI;

Description

SUB subtracts the second operand (SRC) from the first operand (DEST). The
first operand is assigned the result of the subtraction, and the flags are
set accordingly.

When an immediate byte value is subtracted from a word operand, the
immediate value is first sign-extended to the size of the destination
operand.

Flags Affected

OF, SF, ZF, AF, PF, and CF as described in Appendix C

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


TEST ── Logical Compare

Opcode       Instruction       Clocks   Description

A8   ib      TEST AL,imm8      2        AND immediate byte with AL
A9   iw      TEST AX,imm16     2        AND immediate word with AX
A9   id      TEST EAX,imm32    2        AND immediate dword with EAX
F6   /0 ib   TEST r/m8,imm8    2/5      AND immediate byte with r/m byte
F7   /0 iw   TEST r/m16,imm16  2/5      AND immediate word with r/m word
F7   /0 id   TEST r/m32,imm32  2/5      AND immediate dword with r/m dword
84   /r      TEST r/m8,r8      2/5      AND byte register with r/m byte
85   /r      TEST r/m16,r16    2/5      AND word register with r/m word
85   /r      TEST r/m32,r32    2/5      AND dword register with r/m dword


Operation

DEST : = LeftSRC AND RightSRC;
CF  0;
OF  0;

Description

TEST computes the bit-wise logical AND of its two operands. Each bit
of the result is 1 if both of the corresponding bits of the operands are 1;
otherwise, each bit is 0. The result of the operation is discarded and only
the flags are modified.

Flags Affected

OF = 0, CF = 0; SF, ZF, and PF as described in Appendix C

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


VERR, VERW ── Verify a Segment for Reading or Writing

Opcode       Instruction   Clocks      Description

0F  00 /4    VERR r/m16    pm=10/11    Set ZF=1 if segment can be read,
                                       selector in r/m16
0F  00 /5    VERW r/m16    pm=15/16    Set ZF=1 if segment can be written,
                                       selector in r/m16


Operation

IF segment with selector at (r/m) is accessible
   with current protection level
   AND ((segment is readable for VERR) OR
      (segment is writable for VERW))
THEN ZF  0;
ELSE ZF  1;
FI;

Description

The two-byte register or memory operand of VERR and VERW contains
the value of a selector. VERR and VERW determine whether the
segment denoted by the selector is reachable from the current privilege
level and whether the segment is readable (VERR) or writable (VERW).
If the segment is accessible, the zero flag is set to 1; if the segment is
not accessible, the zero flag is set to 0. To set ZF, the following
conditions must be met:

  ■  The selector must denote a descriptor within the bounds of the table
     (GDT or LDT); the selector must be "defined."

  ■  The selector must denote the descriptor of a code or data segment
     (not that of a task state segment, LDT, or a gate).

  ■  For VERR, the segment must be readable. For VERW, the segment
     must be a writable data segment.

  ■  If the code segment is readable and conforming, the descriptor
     privilege level (DPL) can be any value for VERR. Otherwise, the
     DPL must be greater than or equal to (have less or the same
     privilege as) both the current privilege level and the selector's RPL.

The validation performed is the same as if the segment were loaded into
DS, ES, FS, or GS, and the indicated access (read or write) were
performed. The zero flag receives the result of the validation. The
selector's value cannot result in a protection exception, enabling the
software to anticipate possible segment access problems.

Flags Affected

ZF as described above

Protected Mode Exceptions

Faults generated by illegal addressing of the memory operand that
contains the selector, the selector is not loaded into any segment
register, and no faults attributable to the selector operand are generated

#GP(0) for an illegal memory operand effective address in the CS, DS,
ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 6; VERR and VERW are not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


WAIT ── Wait until BUSY# Pin is Inactive (HIGH)

Opcode   Instruction   Clocks     Description

9B       WAIT          6 min.     Wait until BUSY pin is inactive (HIGH)


Description

WAIT suspends execution of 80386 instructions until the BUSY# pin is
inactive (high). The BUSY# pin is driven by the 80287 numeric processor
extension.

Flags Affected

None

Protected Mode Exceptions

#NM if the task-switched flag in the machine status word (the lower 16 bits
of register CR0) is set; #MF if the ERROR# input pin is asserted (i.e., the
80287 has detected an unmasked numeric error)

Real Address Mode Exceptions

Same exceptions as in Protected Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Protected Mode


XCHG ── Exchange Register/Memory with Register

Opcode    Instruction      Clocks     Description

90 + r    XCHG AX,r16      3          Exchange word register with AX
90 + r    XCHG r16,AX      3          Exchange word register with AX
90 + r    XCHG EAX,r32     3          Exchange dword register with EAX
90 + r    XCHG r32,EAX     3          Exchange dword register with EAX
86  /r    XCHG r/m8,r8     3          Exchange byte register with EA byte
86  /r    XCHG r8,r/m8     3/5        Exchange byte register with EA byte
87  /r    XCHG r/m16,r16   3          Exchange word register with EA word
87  /r    XCHG r16,r/m16   3/5        Exchange word register with EA word
87  /r    XCHG r/m32,r32   3          Exchange dword register with EA dword
87  /r    XCHG r32,r/m32   3/5        Exchange dword register with EA dword


Operation

temp  DEST
DEST  SRC
SRC  temp

Description

XCHG exchanges two operands. The operands can be in either order. If a
memory operand is involved, BUS LOCK is asserted for the duration of the
exchange, regardless of the presence or absence of the LOCK prefix or of the
value of the IOPL.

Flags Affected

None

Protected Mode Exceptions

#GP(0) if either operand is in a nonwritable segment; #GP(0) for an
illegal memory operand effective address in the CS, DS, ES, FS, or GS
segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code)
for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


XLAT/XLATB ── Table Look-up Translation

D7    XLAT m8    5     Set AL to memory byte DS:[(E)BX + unsigned AL]
D7    XLATB      5     Set AL to memory byte DS:[(E)BX + unsigned AL]


Operation

IF AddressSize = 16
THEN
   AL  (BX + ZeroExtend(AL))
ELSE (* AddressSize = 32 *)
   AL  (EBX + ZeroExtend(AL));
FI;

Description

XLAT changes the AL register from the table index to the table entry. AL
should be the unsigned index into a table addressed by DS:BX (for an
address-size attribute of 16 bits) or DS:EBX (for an address-size attribute
of 32 bits).

The operand to XLAT allows for the possibility of a segment override. XLAT
uses the contents of BX even if they differ from the offset of the operand.
The offset of the operand should have been moved intoBX/EBX with a previous
instruction.

The no-operand form, XLATB, can be used if the BX/EBX table will always
reside in the DS segment.

Flags Affected

None

Protected Mode Exceptions

#GP(0) for an illegal memory operand effective address in the CS, DS, ES,
FS, or GS segments; #SS(0) for an illegal address in the SS segment;
#PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault


XOR ── Logical Exclusive OR


Opcode      Instruction      Clocks   Description

34  ib      XOR AL,imm8      2        Exclusive-OR immediate byte to AL
35  iw      XOR AX,imm16     2        Exclusive-OR immediate word to AX
35  id      XOR EAX,imm32    2        Exclusive-OR immediate dword to EAX
80  /6 ib   XOR r/m8,imm8    2/7      Exclusive-OR immediate byte to r/m
                                      byte
81  /6 iw   XOR r/m16,imm16  2/7      Exclusive-OR immediate word to r/m
                                      word
81  /6 id   XOR r/m32,imm32  2/7      Exclusive-OR immediate dword to r/m
                                      dword
83  /6 ib   XOR r/m16,imm8   2/7      XOR sign-extended immediate byte
                                      with r/m word
83  /6 ib   XOR r/m32,imm8   2/7      XOR sign-extended immediate byte
                                      with r/m dword
30  /r      XOR r/m8,r8      2/6      Exclusive-OR byte register to r/m
                                      byte
31  /r      XOR r/m16,r16    2/6      Exclusive-OR word register to r/m
                                      word
31  /r      XOR r/m32,r32    2/6      Exclusive-OR dword register to r/m
                                      dword
32  /r      XOR r8,r/m8      2/7      Exclusive-OR byte register to r/m
                                      byte
33  /r      XOR r16,r/m16    2/7      Exclusive-OR word register to r/m
                                      word
33  /r      XOR r32,r/m32    2/7      Exclusive-OR dword register to r/m
                                      dword


Operation

DEST  LeftSRC XOR RightSRC
CF  0
OF  0

Description

XOR computes the exclusive OR of the two operands. Each bit of the result
is 1 if the corresponding bits of the operands are different; each bit is 0
if the corresponding bits are the same. The answer replaces the first
operand.

Flags Affected

CF = 0, OF = 0; SF, ZF, and PF as described in Appendix C; AF is undefined

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal
memory operand effective address in the CS, DS, ES, FS, or GS segments;
#SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page
fault

Real Address Mode Exceptions

Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault


Appendix A  Opcode Map

────────────────────────────────────────────────────────────────────────────

The opcode tables that follow aid in interpreting 80386 object code. Use
the high-order four bits of the opcode as an index to a row of the opcode
table; use the low-order four bits as an index to a column of the table. If
the opcode is 0FH, refer to the two-byte opcode table and use the second
byte of the opcode to index the rows and columns of that table.


Key to Abbreviations

Operands are identified by a two-character code of the form Zz. The first
character, an uppercase letter, specifies the addressing method; the second
character, a lowercase letter, specifies the type of operand.


Codes for Addressing Method

A  Direct address; the instruction has no modR/M byte; the address of the
   operand is encoded in the instruction; no base register, index register,
   or scaling factor can be applied; e.g., far JMP (EA).

C  The reg field of the modR/M byte selects a control register; e.g., MOV
   (0F20, 0F22).

D  The reg field of the modR/M byte selects a debug register; e.g., MOV
   (0F21,0F23).

E  A modR/M byte follows the opcode and specifies the operand. The operand
   is either a general register or a memory address. If it is a memory
   address, the address is computed from a segment register and any of the
   following values: a base register, an index register, a scaling factor,
   a displacement.

F  Flags Register.

G  The reg field of the modR/M byte selects a general register; e.g., ADD
   (00).

I  Immediate data. The value of the operand is encoded in subsequent bytes
   of the instruction.

J  The instruction contains a relative offset to be added to the
   instruction pointer register; e.g., JMP short, LOOP.

M  The modR/M byte may refer only to memory; e.g., BOUND, LES, LDS, LSS,
   LFS, LGS.

O  The instruction has no modR/M byte; the offset of the operand is coded as
   a word or double word (depending on address size attribute) in the
   instruction. No base register, index register, or scaling factor can be
   applied; e.g., MOV (A0-A3).

R  The mod field of the modR/M byte may refer only to a general register;
   e.g., MOV (0F20-0F24, 0F26).

S  The reg field of the modR/M byte selects a segment register; e.g., MOV
   (8C,8E).

T  The reg field of the modR/M byte selects a test register; e.g., MOV
   (0F24,0F26).

X  Memory addressed by DS:SI; e.g., MOVS, COMPS, OUTS, LODS, SCAS.

Y  Memory addressed by ES:DI; e.g., MOVS, CMPS, INS, STOS.


Codes for Operant Type

a  Two one-word operands in memory or two double-word operands in memory,
   depending on operand size attribute (used only by BOUND).

b  Byte (regardless of operand size attribute)

c  Byte or word, depending on operand size attribute.

d  Double word (regardless of operand size attribute)

p  32-bit or 48-bit pointer, depending on operand size attribute.

s  Six-byte pseudo-descriptor

v  Word or double word, depending on operand size attribute.

w  Word (regardless of operand size attribute)


Register Codes

When an operand is a specific register encoded in the opcode, the register
is identified by its name; e.g., AX, CL, or ESI. The name of the register
indicates whether the register is 32-, 16-, or 8-bits wide. A register
identifier of the form eXX is used when the width of the register depends on
the operand size attribute; for example, eAX indicates that the AX register
is used when the operand size attribute is 16 and the EAX register is used
when the operand size attribute is 32.


One-Byte Opcode Map


       0         1         2         3         4         5         6        7        8         9         A         B         C         D         E        F
 ╔═══════════════════════════════════════════════════════════╤════════╤════════╤═══════════════════════════════════════════════════════════╤════════╤════════╗
 ║                              ADD                          │  PUSH  │   POP  │                             OR                            │  PUSH  │ 2-byte ║
0╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │        ├─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │        ║
 ║  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │   ES   │   ES   │  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │   CS   │ escape ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╪════════╪════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╪════════╪════════╣
 ║                              ADC                          │  PUSH  │   POP  │                            SBB                            │  PUSH  │  POP   ║
1╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │        ├─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │        ║
 ║  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │   SS   │   SS   │  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │   DS   │   DS   ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╪════════╪════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╪════════╪════════╣
 ║                              AND                          │  SEG   │        │                            SUB                            │  SEG   │        ║
2╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │   DAA  ├─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │  DAS   ║
 ║  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │  =ES   │        │  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │  =CS   │        ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╪════════╪════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╪════════╪════════╣
 ║                              XOR                          │  SEG   │        │                            CMP                            │  SEG   │        ║
3╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │   AAA  ├─────────┬─────────┬─────────┬─────────┬─────────┬─────────┤        │  AAS   ║
 ║  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │  =SS   │        │  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  AL,Ib  │ eAX,Iv  │  =CS   │        ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╣
 ║                               INC general register                          │                              DEC general register                           ║
4╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────┼─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────╢
 ║   eAX   │   eCX   │   eDX   │   eBX   │   eSP   │   eBP   │  eSI   │  eDI   │   eAX   │   eCX   │   eDX   │   eBX   │   eSP   │   eBP   │   eSI  │  eDI   ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╣
 ║                               PUSH general register                         │                          POP into general register                          ║
5╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────┼─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────╢
 ║   eAX   │   eCX   │   eDX   │   eBX   │   eSP   │   eBP   │  eSI   │  eDI   │   eAX   │   eCX   │   eDX   │   eBX   │   eSP   │   eBP   │  eSI   │  eDI   ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │  BOUND  │  ARPL   │   SEG   │   SEG   │ Operand│ Address│  PUSH   │  IMUL   │  PUSH   │  IMUL   │  INSB   │ INSW/D  │ OUTSB  │OUTSW/D ║
6║  PUSHA  │  POPA   │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │  Gv,Ma  │  Ew,Rw  │   =FS   │   =GS   │  Size  │  Size  │   Ib    │ GvEvIv  │   Ib    │ GvEvIv  │  Yb,DX  │  Yb,DX  │ Dx,Xb  │ DX,Xv  ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╣
 ║                    Short displacement jump of condition (Jb)                │                   Short-displacement jump on condition(Jb)                  ║
7╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────┼─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────╢
 ║   JO    │   JNO   │   JB    │   JNB   │   JZ    │  JNZ    │   JBE  │  JNBE  │   JS    │   JNS   │   JP    │   JNP   │   JL    │  JNL    │  JLE   │  JNLE  ║
 ╠═════════╧═════════╪═════════╪═════════╪═════════╧═════════╪════════╧════════╪═════════╧═════════╧═════════╧═════════╪═════════╪═════════╪════════╪════════╣
 ║  Immediate Grpl   │         │  Grpl   │       TEST        │      XCNG       │                 MOV                   │   MOV   │   LEA   │  MOV   │  POP   ║
8╟─────────┬─────────┤         │         ├─────────┬─────────┼────────┬────────┼─────────┬─────────┬─────────┬─────────┤         │         │        │        ║
 ║  Eb,Ib  │  Ev,Iv  │         │  Ev,Iv  │  Eb,Gb  │  Ev,Gv  │  Eb,Gb │  Ev,Gv │  Eb,Gb  │  Ev,Gv  │  Gb,Eb  │  Gv,Ev  │  Ew,Sw  │  Gv,M   │  Sw,Ew │   Ev   ║
 ╠═════════╪═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │              XCHG word or double-word register with eAX           │         │         │  CALL   │         │  PUSHF  │  POPF   │        │        ║
9║   NOP   ├─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────┤   CBW   │   CWD   │         │  WAIT   │         │         │  SAHF  │  LAHF  ║
 ║         │   eCX   │   eDX   │   eBX   │   eSP   │   eBP   │  eSI   │  eDI   │         │         │   Ap    │         │   Fv    │   Fv    │        │        ║
 ╠═════════╧═════════╧═════════╧═════════╪═════════╪═════════╪════════╪════════╪═════════╧═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║                  MOV                  │  MOVSB  │ MOVSW/D │ CMPSB  │CMPSW/D │       TEST        │  STOSB  │ STOSW/D │  LODSB  │ LODSW/D │ SCASB  │SCASW/D ║
A╟─────────┬─────────┬─────────┬─────────┤         │         │        │        ├─────────┬─────────┤         │         │         │         │        │        ║
 ║  AL,Ob  │  eAX,Ov │  Ob,AL  │  Ov,eAX │  Xb,Yb  │  Xv,Yv  │  Xb,Yb │  Xv,Yv │  AL,Ib  │ eAX,Iv  │  Yb,AL  │  Yv,eAX │  AL,Xb  │ eAX,Xv  │  AL,Xb │eAX,Xv  ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╣
 ║                       MOV immediate byte into byte register                 │        MOV immediate word or double into word or double register            ║
B╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────┼─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────╢
 ║    AL   │   CL    │   DL    │   BL    │   AH    │   CH    │   DH   │   BH   │   eAX   │   eCX   │   eDX   │   eBX   │   eSP   │   eBP   │   eSI  │  eDI   ║
 ╠═════════╧═════════╪═════════╧═════════╪═════════╪═════════╪════════╧════════╪═════════╪═════════╪═════════╧═════════╪═════════╪═════════╪════════╪════════╣
 ║    Shift Grp2     │      RET near     │   LES   │   LDS   │       MOV       │  ENTER  │         │      RET far      │  INT    │  INT    │        │        ║
C╟─────────┬─────────┼─────────┬─────────┤         │         ├────────┬────────┤         │  LEAVE  ├─────────┬─────────┤         │         │  INTO  │  IRET  ║
 ║  Eb,Ib  │  Ev,Iv  │    Iw   │         │  Gv,Mp  │  Gv,Mp  │  Eb,Ib │  Ev,Iv │  Iw,Ib  │         │   Iw    │         │   3     │  Ib     │        │        ║
 ╠═════════╧═════════╧═════════╧═════════╪═════════╪═════════╪════════╪════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╣
 ║               Shift Grp2              │         │         │        │        │                                                                             ║
D╟─────────┬─────────┬─────────┬─────────┤   AAM   │   AAD   │        │  XLAT  │                  ESC(Escape to coprocessor instruction set)                 ║
 ║   Eb,1  │  Ev,1   │  Eb,CL  │  Ev,CL  │         │         │        │        │                                                                             ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╧═════════╪════════╧════════╪═════════╤═════════════════════════════╤═══════════════════╤═════════════════╣
 ║ LOOPNE  │  LOOPE  │   LOOP  │  JCXZ   │        IN         │       OUT       │   CALL  │             JNP             │        IN         │       OUT       ║
E║         │         │         │         ├─────────┬─────────┼────────┬────────┤         ├─────────┬─────────┬─────────┼─────────┬─────────┼────────┬────────╢
 ║   Jb    │   Jb    │    Jb   │   Jb    │  AL,Ib  │ eAX,Ib  │  Ib,AL │ Ib,eAX │    Av   │   Jv    │   Ap    │   Jb    │  AL,DX  │ eAX,DX  │ DX,AL  │ DX,eAX ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╧════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │   REP   │         │         │     Unary Grp3  │         │         │         │         │         │         │INC/DEC │Indirct ║
F║  LOCK   │         │  REPNE  │         │   HLT   │   CMC   ├────────┬────────┤   CLC   │   STC   │   CLI   │   STI   │   CLD   │   STD   │        │        ║
 ║         │         │         │  REPE   │         │         │   Eb   │   Ev   │         │         │         │         │         │         │  Grp4  │  Grp5  ║
 ╚═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╧═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╝


Two-Byte Opcode Map (first byte is 0FH)


      0         1         2         3         4         5         6        7        8         9         A         B         C         D         E        F
 ╔═════════╤═════════╤═════════╤═════════╤═════════╤═════════╤════════╤════════╤═════════╤═════════╤═════════╤═════════╤═════════╤═════════╤════════╤════════╗
 ║         │         │   LAR   │   LSL   │         │         │        │        │         │         │         │         │         │         │        │        ║
0║  Grp6   │  Grp7   │         │         │         │         │  CLTS  │        │         │         │         │         │         │         │        │        ║
 ║         │         │  Gw,Ew  │  Gv,Ew  │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
1║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║   MOV   │   MOV   │   MOV   │   MOV   │   MOV   │         │   MOV  │        │         │         │         │         │         │         │        │        ║
2║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║  Cd,Rd  │  Dd,Rd  │  Rd,Cd  │  Rd,Dd  │  Td,Rd  │         │  Rd,Td │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
3║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
4║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
5║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
6║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
7║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╪═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╣
 ║                      Long-displacement jump on condition (Jv)               │                 Long-displacement jump on condition (Jv)                    ║
8╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────┼─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────╢
 ║   JO    │   JNO   │   JB    │   JNB   │   JZ    │   JNZ   │   JBE  │  JNBE  │   JS    │   JNS   │   JP    │   JNP   │   JL    │   JNL   │   JLE  │  JNLE  ║
 ╠═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║                              Byte Set on condition (Eb)                     │         │         │         │         │         │         │        │        ║
9╟─────────┬─────────┬─────────┬─────────┬─────────┬─────────┬────────┬────────┤  SETS   │  SETNS  │  SETP   │  SETNP  │  SETL   │  SETNL  │  SETLE │ SETNLE ║
 ║  SETO   │  SETNO  │  SETB   │  SETNB  │  SETZ   │  SETNZ  │  SETBE │ SETNBE │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║  PUSH   │   POP   │         │   BT    │  SHLD   │  SHLD   │        │        │  PUSH   │   POP   │         │   BTS   │  SHRD   │  SHRD   │        │  IMUL  ║
A║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║   FS    │   FS    │         │  Ev,Gv  │ EvGvIb  │ EvGvCL  │        │        │   GS    │   GS    │         │  Ev,Gv  │ EvGvIb  │ EvGvCL  │        │ Gv,Ev  ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╧════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╧════════╣
 ║         │         │   LSS   │   BTR   │   LFS   │   LGS   │      MOVZX      │         │         │  Grp-8  │   BTC   │   BSF   │   BSR   │      MOVSX      ║
B║         │         │         │         │         │         ├────────┬────────┤         │         │         │         │         │         ├─────────────────╢
 ║         │         │   Mp    │  Ev,Gv  │   Mp    │   Mp    │ Gv,Eb  │ Gv,Ew  │         │         │  Ev,Ib  │  Ev,Gv  │  Gv,Ev  │  Gv,Ev  │  Gv,Eb   Gv,Ew  ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╤════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
C║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
D║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
E║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╠═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╪═════════╪═════════╪═════════╪═════════╪═════════╪═════════╪════════╪════════╣
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
F║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ║         │         │         │         │         │         │        │        │         │         │         │         │         │         │        │        ║
 ╚═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╧═════════╧═════════╧═════════╧═════════╧═════════╧═════════╧════════╧════════╝


Opcodes determined by bits 5,4,3 of modR/M byte:

     G                       ┌───────┬───────┬───────┐
     r                       │  mod  │  nnn  │  R/M  │
     o                       └───────┴───────┴───────┘
     u
     p   000     001     010     011     100     101     110     111
      ┌───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┐
     1│  ADD  │  OR   │  ADC  │  SBB  │  AND  │  SUB  │  XOR  │  CMP  │
      │       │       │       │       │       │       │       │       │
      ├───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┤
     2│  ROL  │  ROR  │  RCL  │  RCR  │  SHL  │  SHR  │       │  SAR  │
      │       │       │       │       │       │       │       │       │
      ├───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┤
     3│ TEST  │       │  NOT  │  NEG  │  MUL  │ IMUL  │  DIV  │ IDIV  │
      │ Ib/Iv │       │       │       │AL/eAX │AL/eAX │AL/eAX │AL/eAX │
      ├───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┤
     4│  INC  │  DEC  │       │       │       │       │       │       │
      │  Eb   │  Eb   │       │       │       │       │       │       │
      ├───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┤
     5│  INC  │  DEC  │ CALL  │ CALL  │  JMP  │  JMP  │ PUSH  │       │
      │  Ev   │  Ev   │  Ev   │  eP   │  Ev   │  Ep   │  Ev   │       │
      └───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┘


Opcodes determined by bits 5,4,3 of modR/M byte:

     G                       ┌───────┬───────┬───────┐
     r                       │  mod  │  nnn  │  R/M  │
     o                       └───────┴───────┴───────┘
     u
     p   000     001     010     011     100     101     110     111
      ┌───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────┐
     6│ SLDT  │  STR  │ LLDT  │  LTR  │ VERR  │ VERW  │       │       │
      │  Ew   │  Ew   │  Ew   │  Ew   │  Ew   │  Ew   │       │       │
      ├───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┤
     7│ SGDT  │ SIDT  │ LGDT  │ LIDT  │ SMSW  │       │ LMSW  │       │
      │  Ms   │  Ms   │  Ms   │   Ms  │  Ew   │       │  Ew   │       │
      ├───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┤
     8│       │       │       │       │  BT   │  BTS  │  BTR  │  BTC  │
      │       │       │       │       │       │       │       │       │
      └───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┘


Appendix B  Complete Flag Cross-Reference

────────────────────────────────────────────────────────────────────────────

Key to Codes

T     = instruction tests flag

M     = instruction modifies flag
        (either sets or resets depending on operands)

0     = instruction resets flag

1     = instruction sets flag

──    = instruction's effect on flag is undefined

R     = instruction restores prior value of flag

blank = instruction does not affect flag


Instruction            OF   SF   ZF   AF   PF   CF   TF   IF   DF   NT   RF

AAA                    ──   ──   ──   TM   ──   M
AAD                    ──   M    M    ──   M    ──
AAM                    ──   M    M    ──   M    ──
AAS                    ──   ──   ──   TM   ──   M
ADC                    M    M    M    M    M    TM
ADD                    M    M    M    M    M    M
AND                    0    M    M    ──   M    0
ARPL                             M
BOUND
BSF/BSR                ──   ──   M    ──   ──   ──
BT/BTS/BTR/BTC         ──   ──   ──   ──   ──   M
CALL
CBW
CLC                                             0
CLD                                                            0
CLI                                                       0
CLTS
CMC                                             M
CMP                    M    M    M    M    M    M
CMPS                   M    M    M    M    M    M              T
CWD
DAA                    ──   M    M    TM   M    TM
DAS                    ──   M    M    TM   M    TM
DEC                    M    M    M    M    M
DIV                    ──   ──   ──   ──   ──   ──
ENTER
ESC
HLT
IDIV                   ──   ──   ──   ──   ──   ──
IMUL                   M    ──   ──   ──   ──   M
IN
INC                    M    M    M    M    M
INS                                                            T
INT                                                  0              0
INTO                   T                             0              0
IRET                   R    R    R    R    R    R    R    R    R    T
Jcond                  T    T    T    T    T
JCXZ
JMP
LAHF
LAR                              M
LDS/LES/LSS/LFS/LGS
LEA
LEAVE
LGDT/LIDT/LLDT/LMSW
LOCK
LODS                                                           T
LOOP
LOOPE/LOOPNE                     T
LSL                              M
LTR
MOV
MOV control, debug     ──   ──   ──   ──   ──   ──
MOVS                                                           T
MOVSX/MOVZX
MUL                    M    ──   ──   ──   ──   M
NEG                    M    M    M    M    M    M
NOP
NOT
OR                     0    M    M    ──   M    0
OUT
OUTS                                                           T
POP/POPA
POPF                   R    R    R    R    R    R    R    R    R    R
PUSH/PUSHA/PUSHF
RCL/RCR 1              M                        TM
RCL/RCR count          ──                       TM
REP/REPE/REPNE
RET
ROL/ROR 1              M                        M
ROL/ROR count          ──                       M
SAHF                        R    R    R    R    R
SAL/SAR/SHL/SHR 1      M    M    M    ──   M    M
SAL/SAR/SHL/SHR count  ──   M    M    ──   M    M
SBB                    M    M    M    M    M    TM
SCAS                   M    M    M    M    M    M              T
SET cond               T    T    T         T    T
SGDT/SIDT/SLDT/SMSW
SHLD/SHRD              ──   M    M    ──   M    M
STC                                             1
STD                                                            1
STI                                                       1
STOS                                                           T
STR
SUB                    M    M    M    M    M    M
TEST                   0    M    M    ──   M    0
VERR/VERRW                       M
WAIT
XCHG
XLAT
XOR                    0    M    M    ──   M    0


Appendix C  Status Flag Summary

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Status Flags' Functions

Bit  Name   Function

 0   CF     Carry Flag ── Set on high-order bit carry or borrow; cleared
            otherwise.
 2   PF     Parity Flag ── Set if low-order eight bits of result contain
            an even number of 1 bits; cleared otherwise.
 4   AF     Adjust flag ── Set on carry from or borrow to the low order
            four bits of   AL; cleared otherwise. Used for decimal
            arithmetic.
 6   ZF     Zero Flag ── Set if result is zero; cleared otherwise.
 7   SF     Sign Flag ── Set equal to high-order bit of result (0 is
            positive, 1 if negative).
11   OF     Overflow Flag ── Set if result is too large a positive number
            or too small a negative number (excluding sign-bit) to fit in
            destination operand; cleared otherwise.

Key to Codes

T     = instruction tests flag
M     = instruction modifies flag
        (either sets or resets depending on operands)
0     = instruction resets flag
──    = instruction's effect on flag is undefined
blank = instruction does not affect flag



Instruction            OF   SF   ZF   AF   PF   CF
AAA                    ──   ──   ──   TM   ──   M
AAS                    ──   ──   ──   TM   ──   M
AAD                    ──   M    M    ──   M    ──
AAM                    ──   M    M    ──   M    ──
DAA                    ──   M    M    TM   M    TM
DAS                    ──   M    M    TM   M    TM
ADC                    M    M    M    M    M    TM
ADD                    M    M    M    M    M    M
SBB                    M    M    M    M    M    TM
SUB                    M    M    M    M    M    M
CMP                    M    M    M    M    M    M
CMPS                   M    M    M    M    M    M
SCAS                   M    M    M    M    M    M
NEG                    M    M    M    M    M    M
DEC                    M    M    M    M    M
INC                    M    M    M    M    M
IMUL                   M    ──   ──   ──   ──   M
MUL                    M    ──   ──   ──   ──   M
RCL/RCR 1              M                        TM
RCL/RCR count          ──                       TM
ROL/ROR 1              M                        M
ROL/ROR count          ──                       M
SAL/SAR/SHL/SHR 1      M    M    M    ──   M    M
SAL/SAR/SHL/SHR count  ──   M    M    ──   M    M
SHLD/SHRD              ──   M    M    ──   M    M
BSF/BSR                ──   ──   M    ──   ──   ──
BT/BTS/BTR/BTC         ──   ──   ──   ──   ──   M
AND                    0    M    M    ──   M    0
OR                     0    M    M    ──   M    0
TEST                   0    M    M    ──   M    0
XOR                    0    M    M    ──   M    0


Appendix D  Condition Codes

───────────────────────────────────────────────────────────────────────────

───────────────────────────────────────────────────────────────────────────
Note:
  The terms "above" and "below" refer to the relation between two
  unsigned values (neither SF nor OF is tested). The terms "greater" and
  "less" refer to the relation between two signed values (SF and OF are
  tested).
───────────────────────────────────────────────────────────────────────────

Definition of Conditions

(For conditional instructions Jcond, and SETcond)


                                       Instruction  Condition
Mnemonic  Meaning                      Subcode      Tested

O         Overflow                     0000         OF = 1

NO        No overflow                  0001         OF = 0

B         Below
NAE       Neither above nor equal      0010         CF = 1

NB        Not below
AE       Above or equal                0011         CF = 0

E         Equal
Z         Zero                         0100         ZF = 1

NE        Not equal
NZ        Not zero                     0101         ZF = 0

BE        Below or equal
NA        Not above                    0110         (CF or ZF) = 1

NBE       Neither below nor equal
NA        Above                        0111         (CF or ZF) = 0

S         Sign                         1000         SF = 1

NS        No sign                      1001         SF = 0

P         Parity
PE        Parity even                  1010         PF = 1

NP        No parity
PO        Parity odd                   1011         PF = 0

L         Less
NGE       Neither greater nor equal    1100         (SF xor OF) = 1

NL        Not less
GE        Greater or equal             1101         (SF xor OF) = 0

LE        Less or equal
NG        Not greater                  1110         ((SF xor OF) or ZF) = 1

NLE       Neither less nor equal
G         Greater                      1111         ((SF xor OF) or ZF) = 0