HPAVC/HPAVC CD-ROM.iso/pc/DOSREF33.ZIP/CHAPTER.004

Text File | 1994-01-20 | 222.9 KB | 4,535 lines

   **  Programmer's Technical Reference for MSDOS and the IBM PC **
            USA copyright TXG 392-616  ALL RIGHTS RESERVED
 ──────────────────────────┤ DOSREF (tm) ├───────────────────────────
                 ISBN 1-878830-02-3 (disk-based text)
                Copyright (c) 1987, 1994 Dave Williams
                   ┌─────────────────────────────┐
                   │ Shareware Version, 01/20/94 │
                   │  Please Register Your Copy  │
                   └─────────────────────────────┘

                       C H A P T E R    F O U R


                   DOS INTERRUPTS AND FUNCTION CALLS


                            C O N T E N T S

General Programming Guidelines .................................. 4**1
DOS Registers ................................................... 4**2
DOS Stacks ...................................................... 4**3
DOS Interrupts .................................................. 4**4
Interrupt 20h (Terminate) ....................................... 4**5
DOS Services (quick list) ....................................... 4**6
Calling the DOS Services ........................................ 4**7
Version Specific Information .................................... 4**8
Compatibility Problems With DOS 4.0+ ............................ 4**9
PCjr Cartridge Support .......................................... 4**10
eDOS 4.0 ........................................................ 4**11
DOS Services in Detail .......................................... 4**12


General Programming Guidelines .................................. 4**1

  Microsoft recommends avoiding the "old style" DOS 1.0 (01h-0Ch and
26h) system calls wherever possible.  Programmers are urged to use the
"new style" (DOS 2.0+) handle calls instead.

  Do not use "undocumented" functions unless they are critical to your
application and no other reasonable workaround can be found.  Remember
that your programs may have to run under various versions of DOS, DOS
clones such as Digital's DR-DOS, the OS/2 Compatibility Box, or Unix
DOS Emulation Window.  Such environments or OS simulations do not
always implement the undocumented calls.

  DOS 2.x and 3.x lack many of the enhancements found in later
versions.  Your application should check the minimum required DOS
version when specific features are required.  Do not test for a higher
version than necessary or you will cause problems for machines running
older versions of DOS, such as business machines which are seldom
upgraded, laptops with DOS in ROM, etc.

  Direct disk access via hardware or the BIOS should be avoided
unless your program will only be run on specific hardware or under
certain circumstances.  Some versions of DOS 2.x treat their disks in
a manner much like 3.x.  Some vendors added multiple DOS partitions or
oversize drive support prior to the "official" 3.3 release.  Not all
these systems work in the same way!  New-type SCSI or ESDI hard disk
controllers do not always emulate all the standard BIOS calls.


DOS Registers ................................................... 4**2

 DOS uses the following registers, pointers, and flags when it executes
interrupts and function calls:

┌──────────────────┬────────┬─────────────────────────────────────────┐
│GENERAL REGISTERS │register│               definition                │
│                  ├────────┼─────────────────────────────────────────┤
│                  │   AX   │  accumulator                   (16 bit) │
│                  │   AH   │  accumulator high-order byte   ( 8 bit) │
│                  │   AL   │  accumulator low order byte    ( 8 bit) │
│                  │   BX   │  base                          (16 bit) │
│                  │   BH   │  base high-order byte          ( 8 bit) │
│                  │   BL   │  base low-order byte           ( 8 bit) │
│                  │   CX   │  count                         (16 bit) │
│                  │   CH   │  count high order byte         ( 8 bit) │
│                  │   CL   │  count low order byte          ( 8 bit) │
│                  │   DX   │  data                          (16 bit) │
│                  │   DH   │  date high order byte          ( 8 bit) │
│                  │   DL   │  data low order byte           ( 8 bit) │
├──────────────────┼────────┼─────────────────────────────────────────┤
│SEGMENT REGISTERS │register│               definition                │
│                  ├────────┼─────────────────────────────────────────┤
│                  │   CS   │  code  segment (16 bit)                 │
│                  │   DS   │  data  segment (16 bit)                 │
│                  │   SS   │  stack segment (16 bit)                 │
│                  │   ES   │  extra segment (16 bit)                 │
├──────────────────┼────────┼─────────────────────────────────────────┤
│INDEX REGISTERS   │register│               definition                │
│                  ├────────┼─────────────────────────────────────────┤
│                  │   DI   │  destination index (16 bit)             │
│                  │   SI   │  source      index (16 bit)             │
├──────────────────┼────────┼─────────────────────────────────────────┤
│POINTERS          │register│               definition                │
│                  ├────────┼─────────────────────────────────────────┤
│                  │   SP   │  stack       pointer (16 bit)           │
│                  │   BP   │  base        pointer (16 bit)           │
│                  │   IP   │  instruction pointer (16 bit)           │
├──────────────────┴────────┴─────────────────────────────────────────┤
│FLAGS               AF, CF, DF, IF, OF, PF, SF, TF, ZF               │
└─────────────────────────────────────────────────────────────────────┘

  These registers, pointers, and flags all work on the "lowest common
denominator" 8088-8086 CPU.  DOS makes no attempt to use any of the
special or enhanced instructions available on the later CPUs which will
execute 8088 code, such as the 80186, 80286, 80386, or NEC V20, V30,
V40, or V50.

  User registers except AX are preserved unless information is passed
back to the register as indicated in specific function calls.

  For PC-MOS/386 on a 386 CPU, the task switching logic saves the
upper half of the 32-bit registers and the complete FS and GS
registers.

  The Intel 8088 processors supplied for the early IBM PCs were
defective and did not handle the stack register properly on an INT
instruction.  DOS 2.1 added extra logic around interrupt calls for
stack handling; this is the major difference between 2.0 and 2.1.


DOS Stacks ...................................................... 4**3

  When DOS takes control after a function call, it switches to an
internal stack.  Registers which are not used to return information
(other than AX) are preserved.  The calling program's stack must be
large enough to accomodate the interrupt system - at least 128 bytes
in addition to other interrupts.

 DOS actually maintains three stacks -

stack 1: 384 bytes (in DOS 3.1)
         for functions 00h and for 0Dh and up, and for ints 25h and
         26h.

stack 2: 384 bytes (in DOS 3.1)
         for function calls 01h through 0Ch.

stack 3: 48 bytes (in DOS 3.1)
         for functions 0Dh and above.  This stack is the initial stack
         used by the int 21h handler before it decides which of the
         other two to use.  It is also used by function 59h (get
         extended error), and 01h to 0Ch if they are called during an
         int 24h (critical error) handler.  Functions 33h (get/set
         break flag), 50h (set process ID), 51h (get process ID) and
         62h (get PSP address) do not use any DOS stack under DOS 3.x
         (under 2.x, 50h and 51h use stack number 2).

  IBM and Microsoft made a change back in DOS 3.0 or 3.1 to reduce the
size of DOS.  They reduced the space allocated for scratch areas when
interrupts are being processed.  The default seems to vary with the
DOS version and the machine, but 8 stack frames seems to be common.
That means that if you get more than 8 interrupts at the same time,
clock, disk, printer spooler, keyboard, com port, etc., the system will
crash.  It usually seems to happen on a network.

  DOS 3.2 does some different stack switching than previous versions.
The interrupts which are switched are 02h, 08h, 09h, 0Ah, 0Bh, 0Ch,
0Dh, 0Eh, 70h, 72h, 73h, 74h, 75h, 76h, and 77h.  DOS 3.2 has a
special check in the initialization code for a PCjr and don't enable
stack switching on that machine.  DOS 3.3 was changed so that no stack
switching occurs on PC, PC-XT, or the PC Portable, and defaults to 9
stacks of 128 bytes in an AT.

  Additional stacks can be allocated when DOS loads by using the
STACKS= command in CONFIG.SYS.  These stacks are in addition to the
3 internal DOS stacks.  STACKS=16,256 means allow 16 interrupts to
interrupt each other and allow 256 bytes for each for scratch area.


DOS Interrupts .................................................. 4**4

  Microsoft recommends that a program wishing to examine or set the
contents of any interrupt vector use the DOS function calls 35h and
25h provided for those purposes and avoid referencing the interrupt
vector locations directly.

  DOS reserves interrupt numbers 20h to 3Fh for its own use.  This
means absolute memory locations 80h to 0FFh are reserved by DOS.



Interrupt 20h - Terminate Current Program ....................... 4**5

(0:0080h)
  Issue int 20h to exit from a program.  This vector transfers to the
logic in DOS to restore the terminate address, the Ctrl-Break address,
and the critical error exit address to the values they had on entry to
the program.  All the file buffers are flushed and all handles are
closed.  You should close all files changed in length (see function
calls 10h and 3Eh) before issuing this interrupt.  If the changed file
is not closed, its length, time, and date are not recorded correctly
in the directory.

  This call is intended for use with .COM programs only.

  For a program to pass a completion code or an error code when
terminating, it must use either function call 4Ch (Terminate a
Process) or 31h (Terminate Process and Stay Resident).  These two
methods are preferred over using int 20h and the codes returned by
them can be interrogated in batch processing.

Important: Before you issue an interrupt 20h, your program must
           ensure that the CS register contains the segment of its
           Program Segment Prefix.

Interrupt 20h   DOS - Terminate Program
entry   no parameters
return  The following vectors are restored from the Program Segment
        Prefix:
        PSP offset  interrupt  description
            0Ah        22h     Program Terminate
            0Eh        23h     Control-C
            12h        24h     Critical Error
note 1) IBM and Microsoft recommend using int 21h Fn 4Ch.  Using int
        20h is officially frowned upon since the introduction of DOS
        2.0.
     2) In DOS 3.2 at least, int 20h merely calls int 21h, fn 00h.
     3) Supported by PC-MOS/386.



DOS Services (quick list) ....................................... 4**6

┌────────────────────────────────────────────────────────────────────┐
│Interrupt 21h  Function Call Request                                │
└────────────────────────────────────────────────────────────────────┘
(0:0084h)
  DOS provides a wide variety of function calls for character device
I/O, file management, memory management, date and time functions,
execution of other programs, and more.  They are grouped as follows:

          call              description
        00h           program terminate
        01h-0Ch       character device I/O, CP/M compatibility format
        0Dh-24h       file management,      CP/M compatibility format
        25h-26h       nondevice functions,  CP/M compatibility format
        27h-29h       file management,      CP/M compatibility format
        2Ah-2Eh       nondevice functions,  CP/M compatibility format
        2Fh-38h       extended functions
        39h-3Bh       directory group
        3Ch-46h       extended file management
        47h           directory group
        48h-4Bh       extended memory management
        54h-57h       extended functions
        5Eh-5Fh       networking
        60h-62h       extended functions
        63h-66h       enhanced foreign language support


List of DOS services:   * = undocumented
        00h     terminate program
        01h     get keyboard input
        02h     display character to STDIO
        03h     get character from STDAUX
        04h     output character to STDAUX
        05h     output character to STDPRN
        06h     direct console I/O - keyboard to screen
        07h     get char from std I/O without echo
        08h     get char from std I/O without echo, checks for ^C
        09h     display a string to STDOUT
        0Ah     buffered keyboard input
        0Bh     check STDIN status
        0Ch     clear keyboard buffer and invoke keyboard function
    o   0Dh     flush all disk buffers
    o   0Eh     select disk
        0Fh     open file with File Control Block
        10h     close file opened with File Control Block
        11h     search for first matching file entry
        12h     search for next matching file entry
        13h     delete file specified by File Control Block
        14h     sequential read from file specified by File Control
                Block
        15h     sequential write to file specified by File Control
                Block
        16h     find or create firectory entry for file
        17h     rename file specified by file control block
        18h*    unknown
    o   19h     return current disk drive
    o   1Ah     set disk transfer area (DTA)
        1Bh     get current disk drive FAT
        1Ch     get disk FAT for any drive
        1Dh*    unknown
        1Eh*    unknown
        1Fh     read DOS disk block, default drive
        20h*    unknown
        21h     random read from file specified by FCB
        22h     random write to file specified by FCB
        23h     return number of records in file specified by FCB
        24h     set relative file record size field for file specified
                by FCB
    o   25h     set interrupt vector
        26h     create new Program Segment Prefix (PSP)
        27h     random file block read from file specified by FCB
        28h     random file block write to file specified by FCB
        29h     parse the command line for file name
    o   2Ah     get the system date
    o   2Bh     set the system date
    o   2Ch     get the system time
    o   2Dh     set the system time
    o   2Eh     set/clear disk write VERIFY
    o   2Fh     get the Disk Transfer Address (DTA)
    o   30h     get DOS version number
        31h     TSR, files opened remain open
        32h     read DOS Disk Block
    o   33h     get or set Ctrl-Break
        34h     INDOS  Critical Section Flag
    o   35h     get segment and offset address for an interrupt
    o   36h     get free disk space
        37h*    get/set option marking character (SWITCHAR)
    o   38h     return country-dependent information
    o   39h     create subdirectory
    o   3Ah     remove subdirectory
    o   3Bh     change current directory
    o   3Ch     create and return file handle
    o   3Dh     open file and return file handle
    o   3Eh     close file referenced by file handle
    o   3Fh     read from file referenced by file handle
    o   40h     write to file referenced by file handle
    o   41h     delete file
    o   42h     move file pointer (move read-write pointer for file)
    o   43h     set/return file attributes
    o   44h     device IOCTL (I/O control) info
    o   45h     duplicate file handle
    o   46h     force a duplicate file handle
    o   47h     get current directory
    o   48h     allocate memory
    o   49h     release allocated memory
    o   4Ah     modify allocated memory
    o   4Bh     load or execute a program
    o   4Ch     terminate prog and return to DOS
    o   4Dh     get return code of subprocess created by 4Bh
    o   4Eh     find first matching file
    o   4Fh     find next matching file
        50h*    set new current Program Segment Prefix (PSP)
        51h*    puts current PSP into BX
        52h*    pointer to the DOS list of lists
        53h*    translates BPB (Bios Parameter Block, see below)
    o   54h     get disk verification status (VERIFY)
        55h*    create PSP: similar to function 26h
    o   56h     rename a file
    o   57h     get/set file date and time
        58h     get/set allocation strategy             (DOS 3.x)
    o   59h     get extended error information
    o   5Ah     create a unique filename
    o   5Bh     create a DOS file
    o   5Ch     lock/unlock file contents
        5Dh     DOS internal functions
        5Eh*    network printer
        5Fh*    network redirection
        60h*    parse pathname (TRUENAME)
        61h*    unknown
        62h     get program segment prefix (PSP)
        63h*    get lead byte table                     (DOS 2.25)
        64h*    unknown
        65h     get extended country information        (DOS 3.3)
        66h     get/set global code page table          (DOS 3.3)
        67h     set handle count                        (DOS 3.3)
        68h     commit file                             (DOS 3.3)
        69h     disk serial number                      (DOS 4.0)
        6Ah     unknown
        6Bh     unknown
        6Ch     extended open/create                    (DOS 4.0)

  Items marked with 'o' are explicitly supported in the OS/2 1.x DOS
Compatibility Box, according to Microsoft.  Most everything is
supported in the OS/2 2.0 Virtual DOS Machine, according to IBM.


Calling the DOS Services ........................................ 4**7

  The DOS services are invoked by placing the number of the desired
function in register AH, subfunction in AL, setting the other
registers to any specific requirements of the function, and invoking
int 21h.

  When the interrupt is called, all register and flag values are
pushed into the stack.  Int 21h contains a pointer into an absolute
address in the IBMDOS.COM file.  This address is the main loop for the
DOS command handler.  The handler pops the register values, compares
them to its list of functions, and executes the function if valid.
When the function is complete, it may pass values back to the command
handler. The handler will push the values into the stack and then
return control to the calling program.

  Most functions will return an error code; some return more
information.  Details are contained in the listings for the individual
functions.  Extended error return codes for most functions may be
obtained by calling function 59h.

  Register settings listed are the ones used by DOS.  Some functions
will return with garbage values in unused registers.  Do not test for
values in unspecified registers; your program may exhibit odd behavior.

  DS:DX pointers are the data segment register (DS) indexed to the DH
and DL registers (DX).  DX always contains the offset address, DS
contains the segment address.

  The File Control Block services (FCB services) were part of DOS 1.0.
Since the release of DOS 2.0, Microsoft has recommended that these
services not be used.  A set of considerably more enhanced services
(handle services) were introduced with DOS 2.0.  The handle services
provide support for wildcards and subdirectories, and enhanced error
detection via function 59h.  The FCB calls  also do not support the
sharing modes or file locking functions of the handle calls, making
them unsafe for use with networks or multitaskers.

  The data for the following calls was compiled from various Intel,
Microsoft, IBM, and other publications.  There are many subtle
differences between MSDOS and PCDOS and between the individual
versions.  Differences between the versions are noted as they occur.

  There are various ways of calling the DOS functions.  For all
methods, the function number is loaded into register AH, subfunctions
and/or parameters are loaded into AL or other registers, and call int
21h by one of the following methods:

 A) call interrupt 21h directly  (the recommended procedure)
 B) perform a long call to offset 50h in the program's PSP.
     1) This method will not work under DOS 1.x.
     2) Though recommended by Microsoft for DOS 2.0, this method takes
        more time and is no longer recommended.
 C) place the function number in CL and perform an intrasegment call
    to location 05h in the current code segment.  This location
    contains a long call to the DOS function dispatcher.
     1) IBM recommends this method be used only when using existing
        programs written for different calling conventions.  (such as
        converting CP/M programs).  This method should be avoided
        unless you have some specific use for it.
     2) AX is always destroyed by this method.
     3) This method is valid only for functions 00h-24h.
 D) PC-MOS/386' virtualization scheme results in 200-400+ clocks per
    interrupt when run on an 80286 machine since it has to flip in and
    out of protected mode at CPU ring 0.  TSL recommends the following
    procedure for apps that must run quickly on 286 machines:
       1) PUSHF
       2) CLI
       3) CALL DWORD PTR[vector-contents]
    This avoids flipping in and out of protected mode and the attendant
    overhead.  However, TSL specifies this technique should not be used
    with native-mode MOS applications.


  There are also various ways of exiting from a program. (assuming it
is not intended to be a TSR).  All methods except call 4Ch must ensure
that the segment register contains the segment address of the PSP.

 A) Interrupt 21h, function 4Ch (Terminate with Result Code). This is
    the "official" recommended method of returning to DOS.
 B) Interrupt 21h, function 00h (Exit Program).  This is the early
    style int 21h function call.  It simply calls int 20h.
 C) Interrupt 20h (Exit).
 D) A JMP instruction to offset 00h (int 20h vector) in the Program
    Segment Prefix.  This is just a roundabout method to call int 20h.
    This method was set up in DOS 1.0 for ease of conversion for CP/M
    programs.  It is no longer recommended for use.
 E) A JMP instruction to offset 05h (int 21 vector) in the Program
    Segment Prefix, with AH set to 00h or 4Ch.  This is another CP/M
    type function.



Version Specific Information .................................... 4**8

* Function Calls:

        DOS 2.x  supports function calls 00h to 57h.

        DOS 2.25 is the only version to support function 63h
                 (foreign keyboard)

        DOS 3.x  has more sophisticated error handling and detection
                 function calls available than 2.x.

        DOS 3.0  supports function calls 00h to 5Ch and 62h, including
                 new and changed function calls for version 3.0:
                   3Dh  Open File
                   59h  Get Extended Error
                   5Ah  Create Temporary File
                   5Bh  Create New File
                   5Ch  Lock/Unlock File Access
                   62h  Get Program Segment Prefix Address

        DOS 3.1  supports function calls 00h to 62h, including the
                 new  and changed function calls for DOS 3.1:
                   5E00h  Get Machine Name
                   5E02h  Set Printer Setup
                   5E03h  Get Printer Setup
                   5F02h  Get Redirection List Entry
                   5F03h  Redirect Device
                   5F04h  Cancel Redirection

        DOS 3.2  supports the following new functions:
                   44h    extended IOCTL functions

        DOS 3.3  supports the following new functions:
                   44h    extended IOCTL functions
                   65h    get extended country information (DOS 3.3)
                   66h    get/set global code page table (DOS 3.3)
                   67h    set handle count (DOS 3.3)
                   68h    commit file (DOS 3.3)

        DOS 4.0  supports the following new functions:
                   44h    extended IOCTL functions
                   69h    disk serial number
                   6Ch    extended open/create

        DOS 5.0  supports the following new int 21h functions:
                   30h     sub 00h Get OEM ID number
                           sub 01h Get version flag
                   33h     sub 06h Return "real" DOS version number
                   44h     sub 0Dh/68h, Sense Media Type
                           sub 10h Query IOCTL handle
                           sub 11h Query IOCTL device
                   4Bh     sub 01h load but don't execute
                                   (formerly undocumented)
                           sub 05h  Enter EXEC State
                   51h     Get Program Segment Prefix
                                   (formerly undocumented)
                   58h     sub 02h Get UMB Link Status
                           sub 03h Set UMB Link Status
                   65h     Get Extended Country Information
                           sub 05h  Get filename character table
                           sub 20h  Convert character
                           sub 21h  Convert string
                           sub 22h  Convert ASCIIZ string

                ...and the following other functions:
                   2Fh     sub 1680h MS-DOS Idle Call
                   Task Switcher API

        DOS 6.0  supports the following new int 21h functions:
                   44h     sub 04h adds DoubleSpace flush


PCjr Cartridge Support .......................................... 4**9

==========================
ibm.dos/secrets.2 #1337, from jswitzer, 525 chars, Sat Jul 29 08:48:15 1989
This is a comment to message 1334.
--------------------------
The PCJr ROM cartridges have a command table that lists the name of
each supported command and its vector in the ROM.  It does the ROM
match first before the internal command match, actually, so it will
execute what it finds there.  Now that I think about it, this means that
you could burn your own ROMs in the proper format and get your own
commands. Neat, I guess.

As to it being in command.com, my DOS 3.3 version has some of the code
about 1A4C.  Do a search with debug for "55 AA" and you'll find the
main routine.

Read:comment
Comment to message number 1337. Enter text. End with '.<CR>'
> 55 AA, that's right. That's the ID for a valid ROM. I'll fire up
> symdeb and poke through COMMAND if I can find my little white cane...


==========================
ibm.dos/secrets.2 #1776, from jswitzer, 319 chars, Tue Sep 19 01:33:09 1989
This is a comment to message 1773.
--------------------------
Actually, you can NOP all of the PCJr code without problems -- AST and
Toshiba both have COMMAND.COM without it, and no major problems.  The
interesting thing is that PC-DOS 4.0x still has it, and they explicity
say that DOS 4 is NOT supported on the PCJr (*they* meaning IBM, of
course).

What can you figure, huh?

Read:comment
Comment to message number 1776. Enter text. End with '.<CR>'
> I sold my PCjr this morning (I swear to Baud!) and therefore can't make
>a test to see what would happen with 4.0 on the Jr. If IBM still sells
>the PC/JX in Japan that might be the reason for continuing cartridge
>support, since the machine is PCjr-based (well, about the way a PS/2 is
>PC-based).
> I used to run Toshiba DOS 2.11V on the Jr since it was provably faster
>on screen updates and disk access than IBM 2.1 (with a Jr, it doesn't take
>much to make a noticable difference). The only cartridges I had were
>ColorPaint and BASIC, and I can't remember if I used 2.11 with them or
>not.
>
==========================
ibm.dos/secrets.2 #1777, from jswitzer, 251 chars, Tue Sep 19 01:34:24 1989
--------------------------
The PCJr code in COMMAND.COM is ONLY executed on the PCJr and the PCJr
doesn't support normal BIOS expansion ROMs, only the expansion cartridges.

So, no conflict.  If you patch out the check for the PCJr machine ID,
though, good luck!
        John Switzer
Read:comment
Comment to message number 1777. Enter text. End with '.<CR>'
> Ummm..... lemme go back and look at the listing again. The way it looked
>the first time, it seemed like ALL commands went through the loop.


Compatibility Problems With DOS 4.0 ............................. 4**10

  Compatibility problems with DOS 4.00 lie mainly in int 2Ah and int
2Fh.  While 2Ah was always reserved, some network software uses this
interrupt.  IBM and Microsoft documentation prior to 4.0 strongly
implies that int 2Fh functions not already used by PRINT.COM were open
for general use.  DOS 4.00 grabs a number of these functions. The
difference in disk handling when >32mb partitions are used causes
problems with some older software.

  Most DOS 4.00 external programs (ASSIGN, SUBST, etc) check for files
being printed - including LABEL.  This is part of the enhanced network
support.  Almost all DOS 4.00 externals make checks for NETBIOS too.

  DOS 5.0 ditched most of this peculiar stuff, making for a much more
stable product.



eDOS 4.0 ........................................................ 4**11

  Microsoft had announced their intent to build a multitasking,
multiuser version of MSDOS as early as 1982.   As mentioned in Chapter
1, the DOS 4.0 issued to selected OEMS in England and Europe in
'86/'87 is not the same code that was released here as DOS 4.0 in
1988.  Microsoft shipped betas of "DOS 4.0" in the US during the same
time period, but the product was never release in the US.  The
European DOS 4.0 (eDOS?) is a multitasking DOS written by Microsoft,
while the US DOS 4.0 is a single tasking DOS written by IBM.  eDOS 4.0
was released in Europe after 3.1, but before 3.2.

(abstracted from a pre-release document):

  `Microsoft Multitasking MS-DOS Product Specification'  dated April
28, 1986

  MS-DOS 4.0 is a multitasking operating system, developed from and
compatible with MS-DOS 3.1.  It supports true multitasking which gives
the user the illusion of and benefits from many independent computers.
Further, MS-DOS 4.0 allows most existing MS-DOS 2.x and 3.x
applications to run without change in the MS-DOS 4.0 multitasking
environment.

(end of abstract).

  Gordon Letwin of Microsoft had this to say about eDOS:

(excerpted from "Turning Off The Car To Change Gears" Microsoft Systems
 Journal, volume 2, number 2. May 1987)

  "DOS 4 was the first product to result from Microsoft's multitasking
DOS effort.  We began it even before IBM introduced the PC AT.  It was
an ambitious project that was originally to include a protected mode
with mode switching capabilities so it could run on the 8086 or the
286.
  A general-purpose multitasking system needs to run in both modes:
the unprotected 8086 mode so that we can run existing DOS
applications, and the protected 286 mode so that we can multitask
arbitrary, unrelated applications. But the architecture of the 286
caused some delays.  Although we knew the project would be difficult,
it was only after we'd gotten deeply into it that we realized just how
difficult it would be.
  As a result, DOS 4 became too complicated for our schedules.
Because of the pressure of customer demand as well as that of previous
commitments, we broke the project into two parts.  DOS 4 runs only in
real mode and provides multitasking only for specialized applications.
DOS 5, which has now been released as OS/2, includes the protected
mode and other features.  DOS 4 was delivered in the last half of 1986
and is being sold in special application environments, primarily in
Europe.  It is a specialized product that can share the market with
OS/2, because it runs on 8086 hardware, while OS/2 requires a 286.
The move from DOS 4 to OS/2 was a gradual evolutionary process."

(end of excerpt)

  eDOS 4.0 consisted of one main program (DOS2/3 compatible) and
several multitasking (background) programs that had to be written
specially.  The whole lot was constrained to fit in the single 640k
memory map.

int 21h functions:   (some of these appeared in later versions of DOS)

AEXEC       Identical to EXEC sub 4.
CREATMEM    creates a named area of memory which may be accessed by
            other processes.  (shared memory \SHAREMEM\...)
CRITENTER, CRITLEAVE (semaphore routines)
CRITERR     (This is the one that should have been in DOS 3)
            Enables hard error processing or automatically fails hard
            errors.
CWAIT       Waits for return code from asynchronous process.  Returns
            when any child process terminates.  Children may also be
            started as orphans in which case you cannot WAIT for them.
EXEC 4Bh    (sub function 4) - Start async process.
FREEZE      Stops a specified process running.
GETEXTENDEDERROR - As for DOS3 except it checks version number to
            specify the level of error handling - you may ask for a
            pointer to the hard error information packet for the latest
            hard error.
GETMEM      obtains access to a shared memory area previously created.
GETPID      Returns process ID and parents process ID.
GET/SET MEMORY PARTITION SIZE
            foreground memory is used for ordinary apps.  Background
            memory is used for DOS 4 apps that don't use the screen (or
            only use it through popup functions).  Background apps can
            also use foreground memory but not vice versa.
KILL        Terminates a process.
PBLOCK      Block a process until matching PRUN is given.  A timeout
            is also specified.  A memory location is passed which must
            be matched in the PRUN.  (semaphore)
PIPE        Create a pipe.  Access it through READ/WRITE/CLOSE (but not
            LSEEK).
PRUN        Release a blocked process.  (semaphore)
RELEASEMEM  Release access to shared memory.  If the reference count
            hits zero then the memory is freed.
RESUME      Thaws a frozen process.
SEND SIGNAL
            Signals were:
                  SIGINTR, SIGTERM, SIGPIPE, SIGUSER1, SIGUSER2.
                  Control C, End of program, Broken pipe, user def,
                  user def.
            Actions were:
                  Terminate process on receipt, ignore, accept,
                  sender gets error or acknowledge received signal.
           The last one of these returns to the sender immediately -
           it is intended for use when processing the signal will take
           a long time.
SETFILETABLE (86h)   Install a new file handle table. (more than 20
           files, as in DOS 3.3+?)
SETPRI      Sets process priority (for this process or entire subtree).
SET SIGNAL HANDLER
SLEEP       Suspends the current process for given number of
            milliseconds.  MSC 4.0 had a demo which called 'DOS_sleep'
            int 21h/fn 89h - does not appear to be implemented in
            known DOS versions.
WAIT        As for DOS 3.

int 2Fh functions:

CHECKPU    check for popup package installation
POSTPU     open/close a popup screen
SAVEPU     save popup screen
RESTOREPU  restore popup screen

Other useful features:

   eDOS 4.0 programs used the Windows .EXE format
   programs may share code segments
   improved device drivers
       the serial port had a full set of IOCTL calls (used through
          a new generic IOCTL call).
       device drivers could be multi-tasking.  They had appropriate
          support routines to queue multiple requests (just like OS/2
          later provided)
   interrupt driven serial ports
   sorted disk buffers before writing


The following were listed as possible future enhancements:

   File system protection and        (not yet available from MS,
     permissions                      though DRI has it)

   High performance file system      (HPFS was delivered in OS/2 1.2)

   Installable file systems          (IFS hook is present in US DOS
                                      4.0, deleted in DOS 5 according
                                      to Gordon Letwin of Microsoft)

   Symbolic links                    (they don't have these on OS/2
                                      even now do they?)

   Undelete                          (added with DOS 5.0)

   Long names, lowercase names,      (OS/2 HPFS, NT NTFS)
   access/creation date/time, name
   of app creating file, revision
   number etc.


  According to bits of information picked up from BIX, Wang tested an
alpha version of eDOS 4.0 for use on a laptop project sometime in 1985
but gave up because the OS was unable to cope with "ill behaved"
programs in a reasonable fashion.  I talked with one person who had
beta-tested it for MS, who commented "the only thing it would run was
COMMAND.COM."  Microsoft has evidently either squashed eDOS completely
or (likely) incorporated it into OS/2.  I've been unable to determine
if the French post office still uses eDOS, and to the best I can find
out Apricot Computer never did much with it.

  DOSREF user John Dallman contributed the following:  (July 1992)

  "You put a few notes in DOSREF about the multi-tasking MS-DOS 4.0
that was released in Europe during 1986/87.  Notably, you wondered if
it ever reached any customers.  I'm sorry to tell you that the answer
is yes - briefly.  It was withdrawn owing to serious reliability
problems: Apricot and ICL sold it in the UK, and I've had email from
someone who bought a retail copy in Hong Kong."

  Ray Duncan had this to say about eDOS' metamorphosis: (PC Magazine,
October 16, 1990 excerpted from Power Programming, page 464)

  "OS/2 as it exists today is a vastly different system than the
protected-mode successor to DOS first envisioned by Microsoft in 1984
and 1985.  The earliest version, internally known as DOS 5.0 or
286DOS, was small and relatively fast - it could even be booted from a
floppy disk on a 1mb 80286 machine.  The 286DOS application program
interface (API), which was a proper superset of the DOS Int 21h,
Microsoft Mouse Int 33h, and ROM BIOS video Int 10h and keyboard Int
16h interfaces, was small enough to be easily understood and allowed
the straightforward porting of any DOS application.  286DOS was not
burdened with a built-in graphical user interface; the original plan
was to make a protected-mode version of Microsoft Windows available as
a separate product that the user could run on top of 286DOS as an
option.
 But somewhere along the torturous path from the original,
experimental implementations of 286DOS to the retail product now known
as OS/2, things went badly awry..."

  Microsoft Press' "MSDOS Encyclopedia" shows a reproduction of a late
DOS 1.25 OEM brochure.  Microsoft was touting future enhancements to
1.25 including Xenix-compatible pipes, process forks, and
multitasking, as well as "graphics and cursor positioning, kanji
support, multi-user and hard disk support, and networking."  Microsoft
certainly thought big, but, alas, the forks, multitasking, and
multiuser support never came about, at least in US versions of DOS.
Oddly, the flyer claims that...

 "MS-DOS has no practical limit on disk size.  MS-DOS uses 4-byte
XENIX OS compatible pointers for file and disk capacity up to 4
gigabytes."

  Umm... yeah.  One sort of gets the idea nobody at Microsoft had a
hard disk larger than 32 megabytes...

 For the record they actually delivered:

Xenix-compatible pipes             DOS 2.0  ("|" operator)
process forks, and multitasking   eDOS 4.0  (not delivered in the US)
multi-user                         never delivered
graphics and cursor positioning    DOS 2.0  (ANSI.SYS, more than likely)
kanji support                      DOS 2.01, 2.25 (double-byte char set)
hard disk support                  DOS 2.0  (subdirectories)
networking                         DOS 3.1  (file locking, MS Networks)

  Early Microsoft ads pumped DOS' Xenix-like features and promised
Xenix functionality in future releases.

  We'll probably never know what the real story was behind eDOS - DOS
4 - DOS 5 - 286DOS - OS/2.  Despite Gordon Letwin's acid comments
about problems with the 80286 processor, I doubt the '286 was the
barrier between users and a multitasking MSDOS.  I also doubt there
was any shortage of programming talent at Microsoft - Digital
Research's Concurrent DOS and Software Link's PC-MOS were developed
without undue trouble.

  Though it's highly unlikely anyone would ever need programming
information for eDOS, I find the entire subject fascinating.  IBM had
not only promised the product, they ran ads for it in 1984-85. Who
axed the project?  Why?  We'll likely never find out.



DOS Services in Detail .......................................... 4**12


INT 21H   DOS services
          Function (hex)

* Indicates Functions not documented in the IBM DOS Technical
Reference.

  Note some functions have been documented in other Microsoft or
licensed OEM documentation.


Function  00h   Terminate Program
      Ends program, updates, FAT, flushes buffers, restores registers
entry   AH      00h
        CS      segment address of PSP
return  none
note 1) Program must place the segment address of the PSP control
        block in CS before calling this function.
     2) The terminate, ctrl-break,and critical error exit addresses
        (0Ah, 0Eh, 12h) are restored to the values they had on entry
        to the terminating program, from the values saved in the
        program segment prefix at locations PSP:000Ah, PSP:000Eh, and
        PSP:0012h.
     3) All file buffers are flushed and the handles opened by the
        process are closed.
     4) Any files that have changed in length and are not closed are
        not recorded properly in the directory.
     5) Control transfers to the terminate address.
     6) This call performs exactly the same function as int 20h.
     7) All memory used by the program is returned to DOS.  DOS just
        goes up the chain of memory blocks and marks any that are
        owned by the PSP which is terminating as free.
     8) TOS: $00 TERM.  Returns system control to the program from
        which it started.  If EXECed from a program, returns to that
        program.  If started from DeskTop, returns to DeskTop.  This
        is important for chaining program segments.
     9) Files opened with FCBs are not automatically closed.
    10) Supported in PC-MOS/386 compatibility mode, but not available
        for native MOS 386-mode applications.


Function  01h     Get Keyboard Input
        Waits for char at STDIN (if necessary), echoes to STDOUT
entry   AH      01h
return  AL      ASCII character from STDIN (8 bits)
note 1) Checks char for Ctrl-C, if char is Ctrl-C, executes int 23h.
     2) For function call 06h, extended ASCII codes require two
        function calls.  The first call returns 00h as an indicator
        that the next call will be an extended ASCII code.
     3) Input and output are redirectable.  If redirected, there is
        no way to detect EOF.
     4) TOS: $1 CONIN.  Char returns in D0. ASCII code of char in
        high byte, keyboard scan code in low byte.  Will return
        scancode on keys which have no ASCII value.


Function  02h   Display Output
      Outputs char in DL to STDOUT
entry   AH      02h
        DL      8 bit data (usually ASCII character)
return  none
note 1) If char is 08 (backspace) the cursor is moved 1 char to the
        left (nondestructive backspace).
     2) If Ctrl-C is detected after input, int 23h is executed.
     3) Input and output are redirectable.  If redirected, there is no
        way to detect disk full.
     4) TOS: $2 CONOUT.  Char must be placed on the stack as the first
        word.  The ASCII value of the char goes in the low byte and
        the high byte is zero.  Chars are output to DEVICE #2, normal
        console output.  Control characters and escape sequences are
        interpreted normally.
     5) Under DOS 1.x, 02h sends a character to the active display.
        Under DOS 2.x and later, the char goes to STDOUT.


Function  03h   Auxiliary Input
      Get (or wait until) character from STDAUX
entry   AH      03h
return  AL      ASCII char from auxiliary device
note 1) AUX, COM1, COM2 is unbuffered and not interrupt driven.
     2) This function call does not return status or error codes.
        For greater control it is recommended that you use the ROM BIOS
        routines (int 14h) or write an AUX device driver and use IOCTL.
     3) At startup, PC-DOS initializes the first auxiliary port (COM1)
        to 2400 baud, no parity, one stop bit, and an 8-bit word.
        Versions of MSDOS may differ.
     4) If Ctrl-C is has been entered from STDIN, int 23h is executed.
     5) TOS: $03 AUXILIARY INPUT.  Function returns when the character
        has been received.  Char is returned in the low byte of D0.
     6) Under DOS 1.x, the character is read from COM1.  Under DOS
        2.x and later, it is read from STDAUX.


Function  04h   Auxiliary Output
      Write character to STDAUX
entry   AH      04h
        DL      ASCII char to send to AUX
return  none
note 1) This function call does not return status or error codes.
        For greater control it is recommended that you use the ROM
        BIOS routine (int 14h) or write an AUX device driver and use
        IOCTL.
     2) If Ctrl-C is has been entered from STDIN, int 23h is executed.
     3) Default is COM1 unless redirected by DOS.
     4) If the device is busy, this function will wait until it is
        ready.
     5) TOS: $04 AUXILIARY OUTPUT.  High byte should be zero, low byte
        is ASCII code.
     6) Under DOS 1.x, the character is sent to COM1.  Under DOS 2.x
        and later, it goes to STDAUX.


Function  05h   Printer Output
      Write character to STDPRN
entry   AL      05h
        DL      ASCII code for character to send
return  none
note 1) If Ctrl-C is has been entered from STDIN, int 23h is executed.
     2) Default is PRN or LPT1 unless redirected with the MODE command.
     3) If the printer is busy, this function will wait until it is
        ready.
     5) TOS: $05 PRINTER OUTPUT.  High byte of D0 is zero, low byte is
        character.  Returns -1 if character is sent successfully.
        Times out after 30 seconds and resets D0 to zero.
     6) Under DOS 1.x, this function writes to LPT1.  Under DOS 2.x
        and higher it writes to STDPRN.


Function  06h   Direct Console I/O
      Get character from STDIN; echo character to STDOUT
entry   AH      06h
        DL      0FFh for console input, or 00h-0FEh for console output
return  ZF      set     no character available
                clear   character recieved
        AL      ASCII code for character
note 1) Extended ASCII codes require two function calls.  The first
        call returns 00h to indicate the next call will return an
        extended code.
     2) If DL is not 0FFh, DL is assumed to have a valid character
        that is output to STDOUT.
     3) This function does not check for Ctrl-C or Ctrl-PrtSc.
     4) Does not echo input to screen.
     5) If I/O is redirected, EOF or disk full cannot be detected.
     6) TOS: $06 RAWCONIO.  Get character from keyboard by calling
        with $FF in D0.  ASCII and scan codes are returned as per
        CONIN.  If a value other than $FF is used, it is printed to
        STDOUT at the current cursor position.  This call interprets
        all control chars and escape sequences.
     7) Under DOS 1.x, this call handles the keyboard and display
        directly.  Under DOS 2.x and later, calls are routed to the
        STDOUT driver.


Function  07h   Direct Console Input Without Echo
      Get or wait for char at STDIN, returns char in AL
      (does not check BREAK)
entry   AH      07h
return  AL      ASCII character from standard input device
note 1) Extended ASCII codes require two function calls.  The first
        call returns 00h to indicate the next call will return an
        extended code.
     2) No checking for Ctrl-C or Ctrl-PrtSc is done.
     3) Input is redirectable.
     4) TOS: $07 DIRECT CONIN WITHOUT ECHO.  Same as function 01h
        only does not echo character to screen.
     5) Under DOS 1.x the keyboard is read directly.  Under DOS 2.x
        and later the STDIN is polled.


Function  08h   Console Input Without Echo
       Get or Wait for char at STDIN, return char in AL
       (checks BREAK)
entry   AH      08h
return  AL      char from standard input device
note 1) Char is checked for ctrl-C. If ctrl-C is detected, executes
        int 23h.
     2) For function call 08h, extended ASCII characters require two
        function calls.  The first call returns 00h to signify an
        extended ASCII code.  The next call returns the actual code.
     3) Input is redirectable.  If redirected, there is no way to
        check EOF.
     4) TOS: $08 CONIN WITHOUT ECHO.  Same as previous call. (no
        checking for control characters).
     5) Under DOS 1.x the keyboard is read directly.  Under DOS 2.x
        and later the STDIN is polled.


Function  09h   Print String
      Outputs Characters in the Print String to the STDOUT
entry   AH      09h
        DS:DX   pointer to the Character String to be displayed
return  none
note 1) The character string in memory must be terminated by a $
        character. (ASCII 24h)
     2) The $ is not displayed but remains in AL forever unless popped.
     3) Output to STDOUT is the same as function call 02h.
     4) TOS: $09 PRINT LINE.  The address of the ASCIIZ string is
        placed on the stack as a parameter and is printed at the
        current cursor position.  There is no limit to the size of the
        string.  D0 contains the number of characters to be printed.
        Control and escape codes are evaluated.
     5) Under DOS 1.x, this function writes to the screen.  Under DOS
        2.x and higher it writes to STDOUT.


Function  0Ah   Buffered Keyboard Input
      Reads characters from STDIN and places them in the buffer
      beginning at the third byte.
entry   AH      0Ah
        DS:DX   pointer to an input buffer
return  none
note 1) Min buffer size = 1, max = 255.
     2) Char is checked for ctrl-C. If ctrl-C is detected, executes
        int 23h.
     3) Format of buffer DX:
        byte       contents
         1      Maximum number of chars the buffer will take,
                including CR.  Reading STDIN and filling the buffer
                continues until a carriage return (<Enter> or 0Dh) is
                read.  If the buffer fills to one less than the maximum
                number the buffer can hold, each additional number read
                is ignored and ASCII 7 (BEL) is output to the display
                until a carriage return is read. (you must set this
                value)
         2      Actual number of characters received, excluding the
                carriage return, which is always the last character.
                (the function sets this value)
         3-n    Characters received are placed into the buffer
                starting here.  Buffer must be at least as long as the
                number in byte 1.
         n+1    Carriage return character (0Dh).
     4) Input is redirectable.  If redirected, there is no way to
        check EOF.
     5) The string may be edited with the standard DOS editing
        commands as it is being entered.
     6) Extended ASCII characters are stored as 2 bytes, the first
        byte being zero.
     7) TOS: $0A READLINE.  Fetches CR-terminated line from CONIN.
        The address of the buffer is passed as a parameter.  Byte 1
        is the maximum length of the line, byte 2 is the number of
        characters entered, byte 3 is the first character.  Escape
        codes are not interpreted, but the normal editing control
        characters are.
     8) Under DOS 1.x the keyboard is read directly.  Under DOS 2.x
        and later the STDIN is polled.


Function  0Bh   Check Standard Input (STDIN) status
      Checks for character available at STDIN
entry   AH      0Bh
return  AL      00h     if no character is available from STDIN
                0FFh    if a character is available from STDIN
note 1) Checks for Ctrl-C. If Ctrl-C is detected, int 23h is executed.
     2) Input can be redirected.
     3) Checks for character only, it is not read into the application.
     4) IBM reports that this call does not work properly under the
        DOSSHELL program in DOS 4.00 and 4.01.  DOSSHELL will return
        all zeroes.  This function works correctly from the command
        line or application.
     5) TOS: $0B CONSTAT.  Checks 64-byte OS input buffer.
     6) Under DOS 1.x, the type-ahead buffer is checked.  Under DOS
        2.x and later, the type-ahead buffer is checked unless STDIN
        has been redirected.


Function  0Ch   Clear Keyboard Buffer & Invoke a Keyboard Function
      Dumps buffer, executes function in AL                  (FCB)
entry   AH      0Ch
        AL      function number (must be 01h, 06h, 07h, 08h, or 0Ah)
                note: the DOS 5.0 TR specifies 0Ah as reserved and
                       "must not be used"
return  AL      00h     buffer was flushed, no other processing
                        performed
                other   any other value has no meaning
note 1) Forces system to wait until a character is typed.
     2) Flushes all typeahead input, then executes function specified
        by AL (by moving it to AH and repeating the int 21 call).
     3) If AL contains a value not in the list above, (such as 00h)
        the input buffer is flushed and no other action is taken.  Note
        that this is the STDIN buffer, not the actual keyboard buffer.
        The keyboard buffer will not be flushed if input is redirected.
     4) Under DOS 1.x, the type-ahead buffer is emptied before the
        function in AL is performed.


Function  0Dh   Disk Reset
      Flushes all currently open file buffers to disk
entry   AH      0Dh
return          none
note 1) Does not close files.  Does not update directory entries;
        files changed in size but not closed are not properly
        recorded in the directory.
     2) Sets DTA address to DS:0080h
     3) Should be used before a disk change, Ctrl-C handlers, and to
        flush the buffers to disk.
     4) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     5) IBM TopView saves DTA information during task switches.


Function  0Eh   Select Disk
      Sets the drive specified in DL (if valid) as the default drive
entry   AL      0Eh
        DL      new default drive number (0=A:,1=B:,2=C:,etc.)
return  AL      number of physical drives found unless LASTDRIVE= is
                included in CONFIG.SYS and specifies a higher number
                than that of the last physical drive.
note 1) For DOS 1.x and 2.x, the minimum value for AL is 2.
     2) For DOS 3.x and 4.x, the minimum value for AL is 5.
     3) The drive number returned is not necessarily a valid drive.
     4) For DOS 1.x: 16 logical drives are available, A-P. (0-0Fh)
        For DOS 2.x: 63 logical drives are available. (Letters are only
                     used for the first 26 drives.  If more than 26
                     logical drives are used, further drive letters
                     will be other ASCII characters, such as {, ],
                     etc. (0-3Fh)
        For DOS 3.x, +: 26 logical drives are available, A-Z. (0-19h)
     5) TOS: $0E SETDRV.  A 16-bit parameter with drivespec is passed
        to TOS.  On exit, D0 contains the number of the drive active
        before the call. (not the total number of drives)
     6) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     7) For DOS 3.3, LASTDRIVE= is not needed when accessing
        additional hard disk partitions past E:.  I haven't done any
        further checking, but it seems that LASTDRIVE= may be fairly
        useless to most programs.
     6) IBM TopView saves DTA information during task switches.


Function  0Fh   Open Disk File                                  (FCB)
      Searches current directory for specified filename and places
      relevantinformation into the File Control Block.
entry   AH      0Fh
        DS:DX   pointer to an unopened FCB
return  AL      00h     if file found
                0FFh    if file not not found
note 1) If the drive code was 0 (default drive) it is changed to the
        actual drive used (1=A:,2=B:,3=C:, etc).  This allows changing
        the default drive without interfering with subsequent
        operations on this file.
     2) The current block field (FCB bytes C-D, offset 0Ch) is set to
        zero.
     3) The size of the record to be worked with (FCB bytes E-F, offset
        0Eh) is set to the system default of 80h.  The size of the file
        (offset 10h) and the date (offset 14h) are set from information
        obtained in the root directory.  You can change the default
        value for the record size (FCB bytes E-F) or set the random
        record size and/or current record field.  Perform these actions
        after the open but before any disk operations.
     4) With DOS 3.x the file is opened in compatibility mode.
        (network)
     5) Microsoft recommends handle function call 3Dh be used instead.
     6) This call is also used by the APPEND command in DOS 3.2 and
        higher.
     7) Before performing a sequential disk operation on the file, you
        must set the Current Record field (offset 20h).  Before
        performing a random disk operation on the file, you must set
        the Relative Record field (offset 21h).  If the default record
        size of 128 bytes is incorrect, set it to the correct value.


Function  10h  Close File                                     (FCB)
     Closes a File After a File Write
entry   AH      10h
        DS:DX   pointer to an opened FCB
return  AL      00h     if the file is found and closed
                0FFh    if the file is not found in the current
                        directory
note 1) This function call must be done on open files that are no
        longer needed, and after file writes to insure all directory
        information is updated.
     2) If the file is not found in its correct position in the current
        directory, it is assumed that the diskette was changed and AL
        returns 0FFh.  This error return is reportedly not completely
        reliable with DOS version 2.x.
     3) If found, the directory is updated to reflect the status in the
        FCB, the buffers to that file are flushed, and AL returns 00h.
     4) There is a subtle but dangerous quirk to this function.  If a
        Close request is issued using a File Control Block that has
        not been previously activated by a successful Open command, the
        file's length will be truncated to zero and the clusters
        previously assigned to the file are left floating. (lost
        clusters)
     5) This function works for files in the current directory only.
     6) For DOS 3.1 and later, files opened with this call are set to
        compatibility mode and the access code is set to read/write.
     7) For DOS 3.0 and later, int 21h fn 59h (Get Extended Error
        Information) may be used to determine errors.


Function  11h   Search For First Matching Entry                  (FCB)
      Searches current disk & directory for first matching filename
entry   AH      11h
        DS:DX   pointer to address of FCB
return  AL      00h     successful match
                0FFh    no matching filename found
note 1) The FCB may contain the wildcard character ? under DOS 2.x,
        and ? or * under 3.x and later.
     2) The original FCB at DS:DX contains information to continue the
        search with function 12h, and should not be modified.
     3) If a matching filename is found, AL returns 00h and the
        locations at the Disk Transfer Address are set as follows:
        a) If the FCB provided for searching was an extended FCB, then
           the first byte at the disk transfer address is set to 0FFh
           followed by 5 bytes of zeroes, then the attribute byte from
           the search FCB, then the drive number used (1=A, 2=B, etc)
           then the 32 bytes of the directory entry.  Thus, the disk
           transfer address contains a valid unopened FCB with the same
           search attributes as the search FCB.
        b) If the FCB provided for searching was a standard FCB, then
           the first byte is set to the drive number used (1=A, 2=b,
           etc), and the next 32 bytes contain the matching directory
           entry.  Thus, the disk transfer address contains a valid
           unopened normal FCB.
     4) If an extended FCB is used, the following search pattern is
        used:
        a) If the FCB attribute byte is zero, only normal file entries
           are found.  Entries for volume label, subdirectories, hidden
           or system files, are not returned.
        b) If the attribute byte is set for hidden or system files, or
           subdirectory entries, it is to be considered as an inclusive
           search.  All normal file entries plus all entries matching
           the specified attributes are returned.  To look at all
           directory entries except the volume label, the attribute byte
           may be set to hidden + system + directory (all 3 bits on).
        c) If the attribute field is set for the volume label, it is
           considered an exclusive search, and ONLY the volume label
           entry is returned.
     5) This call is also used by the APPEND command in DOS 3.2+


Function  12h   Search For Next Entry Using FCB                 (FCB)
      Search for next matching filename
entry   AH      12h
        DS:DX   pointer to the unopened FCB specified from the previous
                Search
                First (11h) or Search Next (12h)
return  AL      00h     if matching filename found
                0FFh    if matching filename was not found
note 1) After a matching filename has been found using function call
        11h, function 12h may be called to find the next match to an
        ambiguous request.  For DOS 2.x, ?'s are allowed in the
        filename.  For DOS 3.x and 4.x, global (*) filename characters
        are allowed.
     2) The DTA contains info from the previous Search First or Search
        Next.
     3) Do not perform any disk operations with this FCB between a
        previous function 11h or 12h call and this one.  "Undocumented"
        fields in the FCB are used to keep information necessary for
        continuing the search, and some disk operations may overwrite
        these areas.
     4) If the file is found, an FCB is created at the DTA address and
        set up to open or delete it.


Function  13h   Delete File Via FCB                              (FCB)
      Deletes file specified in FCB from current directory
entry   AH      13h
        DS:DX   pointer to address of FCB
return  AL      00h     file deleted
                0FFh    if file not found or was read-only
note 1) All matching current directory entries are deleted. The global
        filename character "?" is allowed in the filename.
     2) Will not delete files with read-only attribute set.
     3) Close open files before deleting them.
     4) For DOS 3.1+, requires network Create rights to the sub-
        directory.
     5) This call supports wildcards and is very fast.  The "new"
        handle call 41h is supposed to replace this one since it knows
        about subdirectories.  Unfortunately, fn 41h doesn't know about
        wildcards.


Function  14h   Sequential Disk File Read                         (FCB)
      Reads record sequentially from disk via FCB
entry   AH  14h
        DS:DX   pointer to an opened FCB
return  AL      00h     successful read
                01h     end of file (no data read)
                02h     Data Transfer Area too small for record size
                        specified or segment overflow
                03h     partial record read, EOF found
note 1) The record size is set to the value at offset 0Eh in the FCB.
     2) The record pointed to by the Current Block (offset 0Ch) and the
        Current Record (offset 20h) fields is loaded at the DTA, then
        the Current Block and Current Record fields are incremented.
     3) The record is read into memory at the current DTA address as
        specified by the most recent call to function 1Ah.  If the size
        of the record and location of the DTA are such that a segment
        overflow or wraparound would occur, the error return is set to
        AL=02h.
     4) If a partial record is read at the end of the file, it is passed
        to the requested size with zeroes and the error return is set to
        AL=03h.
     5) For DOS 3.1+ networks, requires Read rights to the subdirectory.


Function  15h   Sequential Disk Write                              (FCB)
      Writes record specified by FCB sequentially to disk
entry   AH      15h
        DS:DX   pointer to address of FCB
return  AL      00h     successful write
                01h     diskette full, write canceled
                02h     disk transfer area (DTA) too small or segment
                        wrap
note 1) The data to write is obtained from the disk transfer area.
     2) The record size is set to the value at offset 0Eh in the FCB.
     3) This service cannot write to files set as read-only.
     4) The record pointed to by the Current Block (offset 0Ch) and the
        Current Record (offset 20h) fields is loaded at the DTA, then
        the Current Block and Current Record fields are incremented.
     5) If the record size is less than a sector, the data in the DTA
        is written to a buffer; the buffer is written to disk when it
        contains a full sector of data, the file is closed, or a Reset
        Disk (function 0Dh) is issued.
     6) The record is written to disk at the current DTA address as
        specified by the most recent call to function 1Ah.  If the size
        of the record and location of the DTA are such that a segment
        overflow or wraparound would occur, the error return is set to
        AL=02h.
     5) For DOS 3.1+ networks, requires Write rights to the subdirectory.


Function  16h   Create A Disk File                                (FCB)
      Search and open or create directory entry for file
entry   AH      16h
        DS:DX   pointer to an FCB
return  AL      00h     successful creation
                0FFh    no room in directory
note 1) If a matching directory entry is found, the file is truncated
        to zero bytes.
     2) If there is no matching filename, a filename is created.
     3) This function calls function 0Fh (Open File) after creating or
        truncating a file.
     4) A hidden file can be created by using an extended FCB with the
        attribute byte (offset FCB-1) set to 2.
     5) The corresponding handle call is 3Ch.
     6) For DOS 3.1+ networks, requires  Create rights to the sub-
        directory.
     7) For DOS 3.0 and later, int 21h function 59h (Get Extended Error
        Information) may be used to determine errors.
     8) Pathnames and wildcards are not supported.


Function  17h   Rename File Specified by File Control Block       (FCB)
      Renames file in current directory
entry   AH      17h
        DS:DX   pointer to an FCB (see note 4)
return  AL      00h     successfully renamed
                0FFh    file not found or filename already exists
note 1) This service cannot rename read-only files.
     2) The "?" wildcard may be used.
     3) If the "?" wildcard is used in the second filename, the
        corresponding letters in the filename of the directory entry are
        not changed.
     4) A special modified FCB is used. It must have a drive number,
        filename, and extension in the usual position, and a second
        filename starting 6 bytes after the first, at offset 11h.  This
        is normally a "reserved" area.
        Modified FCB format:
        bytes           contents
        00h       drive number
        01h-08h   old filename (blank padded if required)
        09h-0Bh   old file extension (blank padded if reqired)
        0Ch-10h   zeroed out
        11h-18h   new filename (blank padded if required)
        19h-1Bh   new file extension (blank padded if required)
        1Ch-24h   zeroed out
     5) The two filenames cannot have the same name.
     6) FCB contains new name starting at byte 17h.
     7) Under DOS 2.0+, subdirectories may be renamed with this call.


Function  18h  Unknown - reportedly not used
entry   AH      18h
return  AL      00h


Function  19h   Get Current Disk Drive
      Return designation of current default disk drive
entry   AH      19h
return  AL      current default drive (0=A, 1=B,etc.)
note 1) Some other DOS functions use 0 for default, 1=A, 2=B, etc.
     2) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     3) TOS: $19 CURRENT DISK. D0 returns number of drive (0=A:)


Function  1Ah   Set Disk Transfer Area Address (DTA)
      Sets DTA address to the address specified in DS:DX.
      Required for functions 4Eh and 4Fh.
entry   AH      1Ah
        DS:DX   pointer to DTA buffer
return  none
note 1) The default DTA is 128 bytes at offset 80h in the PSP.  You may
        set up your own DTA with any size and location.  The DTA should
        be large enough to handle the largest record you intend to write.
     2) Registers are unchanged.
     3) No error codes are returned.
     4) Disk transfers cannot wrap around from the end of the segment
        to the beginning or overflow into another segment.
     5) DOS uses the DTA for file I/O. (see Chapter 6)
     6) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     7) TOS: $1A SET DISK TRANSFER ADDRESS. Sets up a 44-byte buffer for
        disk operations.
     8) PC-MOS/386: not supported for native 386 mode MOS applications.


Function  1Bh   Get Current Drive File Allocation Table Information
      Returns information from the FAT on the current drive
entry   AH      1Bh
return  AL      number of sectors per allocation unit (cluster)
        CX      number of bytes per sector
        DS:BX   address of the current drive's media descriptor byte
        DX      number of allocation units (clusters) for default drive
note 1) Save DS before calling this function.
     2) This call returned a pointer to the FAT in DOS 1.x.  Beginning
        with DOS 2.00, it returns a pointer only to the table's ID byte.
     3) IBM recommends programmers avoid this call and use int 25h
        instead.
     4) Offset DS:[BX-1] is the dirty byte for the table: 00=clean,
        01=dirty.
     5) DOS 2.x+ and above do not keep the FAT in RAM.  Do not use this
        call to access the FAT.  Function 36 is preferred.
     6) If unsuccessful, AL returns with 0FFh.


Function  1Ch   Get File Allocation Table Information for Drive
      Returns information on specified drive
entry   AH      1Ch
        DL      drive number (1=A, 2=B, 3=C, etc)
return  AL      number of sectors per allocation unit (cluster)
                0FFh    invalid drive specification
        DS:BX   address of media descriptor byte for drive in DL
        CX      sector size in bytes
        DX      number of allocation units (clusters)
note 1) Set DL = 0 for default.
     2) Save DS before calling this function.
     3) Format of media-descriptor byte:
        bits:   0       0   (clear)   not double sided
                        1   (set)     double sided
                1       0   (clear)   not 8 sector
                        1   (set)     8 sector
                2       0   (clear)   nonremovable device
                        1   (set)     removable device
                3-7     always set (1)
     4) This call returned a pointer to the FAT in DOS 1.x.  Beginning
        with DOS 2.00, it returns a pointer only to the table's ID byte.
     5) IBM recommends programmers avoid this call and use int 25h
        instead.
     6) This function reads the BPB only and is very fast.
     7) If unsuccessful, AL returns with 0FFh.
     8) PC-MOS/386: native mode should use DS:EBX instead of DS:BX.


Function  1Dh   Not Documented by Microsoft
 *    Unknown - reportedly not used
entry   AH      1Dh
return  AL      00h


Function  1Eh   Not Documented by Microsoft
 *    Unknown - reportedly not used
entry   AH      1Eh
return  AL      00h
note    Apparently does nothing.


Function  1Fh   Get Default Drive Parameter Block
    Same as function call 32h (below), except that the table is
    accessed from the default drive
entry   AH      1Fh
        other registers unknown
return  AL      00h     no error
                0FFh    error
        DS:BX   pointer to DOS Disk Parameter Block for default drive.
note    For DOS 2, 3, 4.x, this just invokes function 32h (Read DOS
        Disk Block) with DL=0.


Function  20h  Unknown
 *   Internal - does nothing?
entry   AH      20h
return  AL      00h


Function  21h  Random Read from File Specified by File Control Block (FCB)
     Reads one record as specified in the FCB into the current DTA.
entry   AH      21h
        DS:DX   address of the opened FCB
return  AL      00h     successful read operation
                01h     end of file (EOF), no data read
                02h     DTA too small for the record size specified
                03h     end of file (EOF), partial data read
note 1) The current block and current record fields are set to agree
        with the random record field.  Then the record addressed by
        these fields is read into memory at the current Disk Transfer
        Address.
     2) The current file pointers are NOT incremented this function.
     3) If the DTA is larger than the file, the file is padded to the
        requested length with zeroes.


Function  22h  Random Write to File Specified by FCB              (FCB)
     Writes one record as specified in the FCB to the current DTA
entry   AH      22h
        DS:DX   address of the opened FCB
return  AL      00h     successful write operation
                01h     disk full; no data written (write was canceled)
                02h     DTA too small for the record size specified
                        (write was canceled)
note 1) This service cannot write to read-only files.
     2) The record pointed to by the Current Block (offset 0Ch) and the
        Current Record (offset 20h) fields is loaded at the DTA, then
        the Current Block and Current Record fields are incremented.
     3) If the record size is less than a sector, the data in the DTA is
        written to a buffer; the buffer is written to disk when it
        contains a full sector of data, the file is closed, or a Reset
        Disk (function 0Dh) is issued.
     4) The current file pointers are NOT incremented this function.
     5) The record is written to disk at the current DTA address as
        specified by the most recent call to function 1Ah.  If the size
        of the record and location of the DTA are such that a segment
        overflow or wraparound would occur, the error return is set to
        AL=02h.
     6) Under networks running DOS 3.1 or later, the user must have
        Write access rights to the subdirectory.


Function  23h  Get File Size                                       (FCB)
     Searches current subdir for matching file, returns size in FCB
entry   AH      23h
        DS:DX   address of an unopened FCB
return  AL      00h file found
                0FFh file not found
note 1) Record size field (offset 0Eh) must be set before invoking this
        function.
     2) The disk directory is searched for the matching entry.  If a
        matching entry is found, the random record field is set to the
        number of records in the file.  If the value of the Record Size
        field is not an even divisor of the file size, the value set in
        the relative record field is rounded up.  This gives a returned
        value larger than the actual file size.
     3) This call is used by the APPEND command in DOS 3.2+.
     4) Record numbers start with zero.


Function  24h  Set Relative Record Field                           (FCB)
     Set random record field specified by an FCB
entry   AH      24h
        DS:DX   address of an opened FCB
return  Random Record Field of FCB is set to be same as Current Block
        and Current Record.
note 1) You must invoke this function before performing random file
        access.
     2) The relative record field of FCB (offset 21h) is set to be same
        as the Current Block (offset 0Ch) and Current Record (offset
        20h).
     3) No error codes are returned.
     4) The FCB must already be opened.


Function  25h  Set Interrupt Vector
entry   AH      25h
        AL      interrupt number to reassign the handler to
        DS:DX   address of new interrupt vector
return  none
note 1) Registers are unchanged.
     2) No error codes are returned.
     3) The interrupt vector table for the interrupt number specified
        in AL is set to the address contained in DS:DX.  Use function
        35h (Get Vector) to get the contents of the interrupt vector
        and save it for later use.
     4) When you use function 25 to set an interrupt vector, DOS 3.2
        doesn't point the actual interrupt vector to what you requested.
        Instead, it sets the interrupt vector to point to a routine
        inside DOS, which does this:
                1. Save old stack pointer
                2. Switch to new stack pointer allocated from DOS's
                   stack pool
                3. Call your routine
                4. Restore old stack pointer
        The purpose for this was to avoid possible stack overflows when
        there are a large number of active interrupts.  IBM was
        concerned (this was an IBM change, not Microsoft) that on a
        Token Ring network there would be a lot of interrupts going on,
        and applications that hadn't allocated very much stack space
        would get clobbered.
     5) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     6) This call is used to access the Phar Lap DOS extender's
        protected mode functions.  The Phar Lap function number is
        placed in AL.


Function  26h  Create New Program Segment Prefix (PSP)
     This service copies the current program-segment prefix to a new
     memory location for the creation of a new program or overlay.
     Once the new PSP is in place, a DOS program can read a DOS .COM
     or overlay file into the memory location immediately following
     the new PSP and pass control to it.
entry   AH      26h
        DX      segment number for the new PSP
return  Current PSP is copied to specified segment
note 1) Microsoft recommends you use the newer DOS service 4Bh (EXEC)
        instead.
     2) The entire 100h area at location 0 in the current PSP is copied
        into location 0 of the new PSP.  The memory size information at
        location 6 in the new segment is updated and the current
        termination, ctrl-break, and critical error addresses from the
        interrupt vector table entries for ints 22h, 23h, and 24 are
        saved in the new program segment starting at 0Ah.  They are
        restored from this area when the program terminates.
     3) The PSP structure is found in Chapter 6.


Function  27h  Random Block Read From File Specified by FCB
     Similar to 21h (Random Read) except allows multiple files to be
     read.
entry   AH      27h
        CX      number of records to be read
        DS:DX   address of an opened FCB
return  AL      00h     successful read
                01h     end of file, no data read
                02h     DTA too small for record size specified
                        (read canceled)
                03h     end of file
        CX      actual number of records read (includes partial if
                AL=03h)
note 1) The record size is specified in the FCB.  The service updates
        the Current Block (offset 0Ch) and Current Record (offset 20h)
        fields to the next record not read.
     2) If CX contained 0 on entry, this is a NOP.
     3) If the DTA is larger than the file, the file is padded to the
        requested length with zeroes.
     4) This function assumes that the FCB record size field (0Eh) is
        correctly set.  If not set by the user, the default is 128
        bytes.
     5) The record is written to disk at the current DTA address as
        specified by the most recent call to function 1Ah.  If the size
        of the record and location of the DTA are such that a segment
        overflow or wraparound would occur, the error return is set to
        AL=02h.
     6) Under networks running DOS 3.1 or later, the user must have
        Read access rights to the subdirectory.


Function  28h  Random Block Write to File Specified in FCB
     Similar to 27h (Random Write)
entry   AH      28h
        CX      number of records to write
        DS:DX   address of an opened FCB
return  AL      00h     successful write
                01h     disk full, no data written
                02h     DTA too small for record size specified
                        (write canceled)
        CX      number of records written
note 1) The record size is specified in the FCB.
     2) This service allocates disk clusters as required.
     3) This function assumes that the FCB Record Size field (offset
        0Eh) is correctly set.  If not set by the user, the default is
        128 bytes.
     4) The record size is specified in the FCB.  The service updates
        the Current Block (offset 0Ch) and Current Record (offset 20h)
        fields to the next record not read.
     5) The record is written to disk at the current DTA address as
        specified by the most recent call to function 1Ah.  If the size
        of the record and location of the DTA are such that a segment
        overflow or wraparound would occur, the error return is set to
        AL=02h.
     6) If called with CX=0, no records are written, but the FCB's File
        Size entry (offset 1Ch) is set to the size specified by the
        FCB's Relative Record field (offset 21h).
     7) Under networks running DOS 3.1 or later, the user must have
        Write access rights to the subdirectory.


Function  29h  Parse the Command Line for Filename
     Parses a text string into the fields of a File Control Block
entry   AH      29h
        AL      bit mask to control parsing
            bit 0    0     parsing stops if file seperator found
                     1     causes service to scan past leading chars
                           such as blanks.  Otherwise assumes the
                           filename begins in the first byte
                1    0     drive number in FCB set to default (0) if
                           the string contains no drive number
                     1     drive number in FCB not changed
                2    0     filename in FCB set to 8 blanks if no
                           filename in string
                     1     filename in FCB not changed if string does
                           not contain a filename
                3    0     extension in FCB set to 3 blanks if no
                           extension in string
                     1     extension left unchanged
                4-7  must be zero
        DS:SI   pointer to string to parse
        ES:DI   pointer to memory buffer to fill with unopened FCB
return  AL      00h     no wildcards in name or extension
                01h     wildcards appeared in name or extension
                0FFh    invalid drive specifier
        DS:SI   pointer to the first byte after the parsed string
        ES:DI   pointer to a buffer filled with the unopened FCB
note 1) If the * wildcard characters are found in the command line, this
        service will replace all subsequent chars in the FCB with
        question marks.
     2) This service uses the characters as filename separators
        DOS 1       : ; . , + / [ ] = " TAB SPACE
        DOS 2,3,4   : ; . , + = TAB SPACE
     3) This service uses the characters
        : ; . , + < > | / \ [ ] = " TAB SPACE
        or any control characters as valid filename separators.
     4) A filename cannot contain a filename terminator.  If one is
        encountered, all processing stops.  The handle functions will
        allow use of some of these characters.
     5) If no valid filename was found on the command line, ES:DI +1
        points to a blank (ASCII 32).
     6) This call will not handle pathnames.
     7) Parsing is in the form D:FILENAME.EXT.  If one is found, a
        corresponding unopened FCB is built at ES:DI.


Function  2Ah  Get Date
     Returns day of the week, year, month, and date
entry   AH      2Ah
return  CX      year    (1980-2099)
        DH      month   (1-12)
        DL      day     (1-31)
        AL      weekday 00h     Sunday
                        01h     Monday
                        02h     Tuesday
                        03h     Wednesday
                        04h     Thursday
                        05h     Friday
                        06h     Saturday
note 1) Date is adjusted automatically if clock rolls over to the next
        day, and takes leap years and number of days in each month into
        account.
     2) Although DOS cannot set an invalid date, it can read one, such
        as 91/32/80, etc.
     3) DESQview's DOS subfunctions also accept CX = 4445h and DX =
        5351h, i.e. 'DESQ' as valid.
     4) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     5) DOS will accept CH=0 (midnight) as a valid time.  Through DOS
        3.31, if a file's time is set to within 2 seconds of midnight
        the time will not be displayed by the DIR command.  For DOS
        4.0+ and DR-DOS the creation time will always be displayed.
     6) TOS: $2A GET DATE.


Function  2Bh  Set Date
     set current system date
entry   AH      2Bh
        CX      year    (1980-2099)
        DH      month   (1-12)
        DL      day     (1-31)
return  AL      00h     no error (valid date)
                0FFh    invalid date specified
note 1) On entry, CX:DX must have a valid date in the same format as
        returned by function call 2Ah.
     2) DOS 3.3+ also sets CMOS clock.
     3) Under the DESQview system shell, this is the DV_GET_VERSION
        check:
        entry   AH      2Bh
                AL      01h     DESQ call
                CX      4445h   'DE'               (invalid date used
                DX      5351h   'SQ'                for DesQview ID)
        return  AH      major version
                AL      minor version
                AX      0FFh    DESQ not installed (DOS error code)
     4) For DESQview 2.00+, installation check
        entry   AH      2Bh
                AL      subfunction (DV v2.00+)
                        01h     Get Version
        return  BX      version (BH = major, BL = minor)
        note    Early copies of v2.00 return 0002h.
                        02h     Get Shadow Buffer Info, and Start
                                Shadowing
        return  BH      rows in shadow buffer
                BL      columns in shadow buffer
                DX      segment of shadow buffer
                        04h     Get Shadow Buffer Info
        return  BH      rows in shadow buffer
                BL      columns in shadow buffer
                DX      segment of shadow buffer
                        05h     Stop Shadowing
                CX      4445h ('DE')
                DX      5351h ('SQ')
        return  AL      0FFh    if DESQview not installed
        note    In DESQview v1.x, there were no subfunctions; this
                call only identified whether or not DESQview was loaded.
     5) PC-Tools PC-Cache 5.1 (Multisoft cache) installation check.
        entry   CX      4358h ('CX')
        return  AL      00h     installed
                                CX      6378h
                        0FFh    not installed
     6) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     7) TOS: GET DATE.


Function  2Ch  Get Time
     Get current system time from CLOCK$ driver
entry   AH      2Ch
return  CH      hours   (0-23)
        CL      minutes (0-59)
        DH      seconds (0-59)
        DL      hundredths of a second (0-99)
note 1) Time is updated every 5/100 second.
     2) The date and time are in binary format.
     3) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     4) TOS: GET TIME.


Function  2Dh  Set Time
     Sets current system time
entry   AH      2Dh
        CH      hours   (0-23)
        CL      minutes (0-59)
        DH      seconds (0-59)
        DL      hundredths of seconds (0-99)
return  AL      00h     if no error
                0FFh    if bad value sent to routine
note 1) DOS 3.3+ also sets CMOS clock.
     2) CX and DX must contain a valid time in binary.
     3) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     4) TOS: SET TIME.


Function  2Eh  Set/Reset Verify Switch
     Set verify flag
entry   AH      2Eh
        AL      00      to turn verify off (default)
                01      to turn verify on
        DL      00h     (DOS 1.x and 2.x only, according to Microsoft's
                         MS-DOS Encyclopedia.  My IBM 2.0 and Intel 2.0
                         manuals don't show DL being used)
return  none
note 1) This is the call invoked by the DOS VERIFY command.
     2) The setting of the Verify switch can be obtained by calling
        function 54h.
     3) This call is not supported on network drives.
     4) DOS checks this flag each time it accesses a disk or block
        device.
     5) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.


Function  2Fh  Get Disk Transfer Address (DTA)
     Returns current DTA used by all DOS read/write operations
entry   AH      2Fh
return  ES:BX   address of DTA
note 1) The DTA is set by function call 1Ah
     2) Default DTA address is a 128 byte buffer at offset 80h in that
        program's Program Segment Prefix.
     3) See Chapter 6 for a description of the DTA.
     4) This call is explicitly supported in the OS/2 1.x
        Compatibility Box.
     5) TOS: GET DTA. Returns segment address of current DTA in D0.


Function  30h  Get DOS Version Number
     Return DOS version and/or user number
entry   AH      30h
 (5.0+) AL      00h     Get OEM ID number
                01h     Get version flag
return  AH      minor version number  (i.e., DOS 2.10 returns AX = 0A02h)
                                      (AH undefined for DOS 1.x)
        AL      major version number  (0 for DOS 1.x, 02h for 2.x, 05h
                                       for 5.x)

(DOS 2.0 through 4.01)
        BH      OEM ID number
                00h     IBM DOS
                16h     DEC DOS
                0FFh    MS-DOS (generic) (Also Toshiba DOS 5.0)

(DOS 5.0+ called with AL=00h)
        BH      OEM ID number
                (see above)

(DOS 5.0+ called with AL=01h)
        BH      version flag
                08h     DOS 5.0 or higher running out of ROM
                        (all other bits are reserved and set to zero)

        BL:CX   24-bit user serial number (optional, OEM dependent.
                If not used this field returns zeroes)
note 1) If AL returns a major version number of zero, the DOS version
        is below 1.28 for MSDOS and below 2.00 for PCDOS.
     2) IBM PC-DOS always returns 0000h in BX and CX, as does DR-DOS.
     3) Due to OS/2 returning version numbers over 10 and the fact
        that some European versions of DOS carried higher version
        numbers than IBM's DOS, utilities which check for a DOS version
        should not abort if a higher version than required is found
        unless some specific problems are known.
     4) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     5) TOS: $30 GET VERSION NUMBER.  Returns GEMDOS version number
        in D0.
     6) Under PC-MOS/386, if AX=BX=CX=DX, the PC-MOS/386 version number
        is returned.  If all registers to not hold the same value, an
        MSDOS equivalent version number is returned.  The PC-MOS/386
        version number is always different from the MSDOS version number.
     7) DOS version equivalents:
        3.20    PC-MOS/386 v3.0
        3.31    Digital Research DR-DOS 3.40, 3.41, 5.0, 6.0
        10      OS/2 v1.0 Compatibility Box  (major version number)
     8) DOS 5.0 adds the AL parameter to the command, but the ID number
        returned in BL has been available since MSDOS 2.0, as
        documented by Intel's DOS 2.0 Technical Reference.
     9) DOS 5.0's SETVER command can modify the value returned in AX.
        You must use int 21h/3306h to obtain the true version number.
    10) OS/2 1.0's Compatibility Box returns version 10.  OS/2 1.1
        returns version 10.1, etc.  OS/2 2.0 returns version 20.
    11) If running in a Microsoft Windows 3.0 or later DOS box, the
        string "windir" will be in the DOS environment.  Note
        lowercase!
    12) It may sometimes be necessary to identify DOS-compatible
        operating systems or replacement command interpreters.  This
        doesn't appear to be a simple task.

        COMMAND.COM replacements:
        1) 4DOS can be identified with int 2Fh, fn 44DDh.
        2) Command Plus   (unknown)
        3) FlexShell      (unknown)

        DOS replacements:
        1) DR-DOS 3.4x has no easy way to be identified, which is a
           pity.  Though it has some MS4.0-like characteristics, it
           reports 3.31 with the DOS call, though VER will return
           whatever the DRDOS revision is.
           To identify DRDOS I've noted the following:
              The string "OS=" appears in the default
                      environment (unless the user nulls it out)
              The OS= string is generated by COMMAND.COM and does not
                      appear when the user is running 4DOS as a command
                      interpreter.
              The internal DOS version call will report 3.31, but the
                      string returned by VER is at minimum 3.4.
             For DRDOS 3.x:
               SHARE and FASTOPEN are always loaded.
             For DRDOS 5 and 6:
               SHARE and FASTOPEN are NOT always loaded.
           The DR-DOS Programmer's Reference doesn't show a call to
                      identify DR-DOS specifically.
        2) PC-MOS/386: unknown. Fail an API call?
        3) Concurrent DOS: unknown. Fail an API call?
        4) DR Multiuser DOS inserts the string: OS=DRMDOS into the
                      environment.  Since DRMDOS doesn't allow
                      replacement of the command interpreter via the
                      SHELL= statement, OS= will be there unless nulled
                      by the user.

Function  31h  Terminate Process and Stay Resident      (KEEP)
entry   AH      31h
        AL      exit code
        DX      program memory requirement in 16 byte paragraphs
return  AX      return code (retrieveable by function 4Dh)
note 1) Files opened by the application are not closed when this call
        is made.
     2) Memory can be used more efficiently if the block containing the
        copy of the DOS environment is deallocated before terminating.
        This can be done by loading ES with the segment contained in 2Ch
        of the PSP and issuing function call 49h (Free Allocated Memory).
     3) Unlike int 27h, more than 64k may be made resident with this
        call.
     4) TOS: $31 KEEP PROCESS.
     5) Programs larger than 64k may be made resident with this call.
     6) For PC-MOS/386, tasks made resident with this call are subject
        to being swapped out of memory by the kernel for room to process
        other applications.  If your code needs to poll the keyboard or
        a port, you need to use a device driver instead.  PC-MOS does
        not swap device drivers.


Function  32h  Read DOS Disk Parameter Block
     Retrieve the pointer to the drive parameter block for a drive
entry   AH      32h
        DL      drive (0=default, 1=A:, etc.).
return  AL      0FFh    if drive is not valid or other error
                00h     if drive is valid and:
        DS:BX   pointer to DOS Drive Parameter Table. Format of block:
              ┌────────┬──────┬────────────────────────────────────────
              │ Bytes  │ Type │                     Value
              ├────────┼──────┼────────────────────────────────────────
              │ 00h    │ byte │ Drive: 0=A:, 1=B:, etc.
              │ 01h    │ byte │ Unit within device driver (0, 1, 2, etc.)
              │ 02h-03h│ word │ Bytes per sector
              │ 04h    │ byte │ Sectors per cluster - 1
              │ 05h    │ byte │ Sectors per cluster as powers of 2
              │ 06h-07h│ word │ First sector containing FAT
              │ 08h    │ byte │ Number of copies of the FAT
              │ 09h-0Ah│ word │ Number of root directory entries
              │ 0Bh-0Ch│ word │ First sector of first cluster
              │ 0Dh-0Eh│ word │ Number of clusters on drive + 1
              ├────────┴──────┼────────────────────────────────────────
              │ DOS 2.x only  │
              ├────────┬──────┼────────────────────────────────────────
              │ 0Fh    │ byte │ Number of sectors for one copy of the FAT
              │ 10h-11h│ word │ Number of first sector of root directory
              │ 12h-15h│ dword│ Address of device driver header for this
              │        │      │ drive (beginning of device driver)
              │ 16h    │ byte │ Media Descriptor Byte for this drive
              │ 17h    │ byte │ 0FFh indicates block must be rebuilt
              │ 18h-1Bh│ dword│ address of next DOS Disk Block (0FFFFh
              │        │      │ means last in chain)
              │ 1Ch    │ word │ starting cluster of current dir (0 = root)
              │ 1Eh    │64byts│ ASCIIZ current directory path string
              ├────────┴──────┼────────────────────────────────────────
              │    DOS 3.x    │
              ├────────┬──────┼────────────────────────────────────────
              │ 0Fh    │ byte │ number of sectors in one FAT copy
              │ 10h    │ word │ first sector of root directory
              │ 12h    │dword │ address of device driver for this drive
              │ 16h    │ byte │ media descriptor byte for medium
              │ 17h    │ byte │ 0FFh = block must be rebuilt, 00h
              │        │      │ indicates block accessed
              │ 18h    │dword │ address of next device block, offset
              │        │      │ = 0FFFFh indicates last
              │ 1Ch    │ word │ cluster at which to start search for free
              │        │      │ space when writing
              │ 1Eh    │ word │ number of free clusters on drive, 0FFFFh
              │        │      │ unknown?
              ├────────┴──────┼────────────────────────────────────────
              │  DOS 4.0, 5.0 │  (from MS 5.0 TR)
              ├────────┬──────┼────────────────────────────────────────
              │ 0Fh    │ word │ number of sectors in one FAT copy
              │ 11h    │ word │ first sector of containing directory
              │ 13h    │dword │ address of device driver for this drive
              │ 17h    │ byte │ media descriptor byte for drive
              │ 18h    │ byte │ 0FFh = block must be rebuilt, 00h
              │        │      │ indicates block accessed
              │ 19h    │dword │ address of next device block, offset
              │        │      │  = 0FFFFh indicates last
              │ 1Dh    │ word │ last allocated cluster
              │ 1Fh    │ word │ number of free clusters on drive
              └────────┴──────┴────────────────────────────────────────
note 1) Use [BX+0Dh] to find no. of clusters (>1000h, 16-bit FAT; if
        not, 12-bit (exact dividing line is probably a little below
        1000h to allow for bad sectors, EOF markers, etc.)
     2) Short article by C.Petzold, PC Magazine  Vol.5, no.8.
     3) Some information from the article "Finding Disk Parameters" in
        the May 1986 issue of PC Tech Journal.
     4) This call is mostly supported in OS/2 1.0's DOS Compatibility
        Box.  The dword at 12h will not return the address of the next
        device driver when in the Compatibility Box.
     5) Used by CHKDSK.


Function  33h  Control-Break Check
     Get or set control-break checking at CON
entry   AH      33h
        AL      00h     see if ^C checking is active
                01h     to set break checking
                        DL      00h     to disable break checking
                                01h     to enable break checking
                02h     internal, called by PRINT.COM (DOS 3.1)
                03h     unknown
                04h     unknown - DOS 4.0's CPSW command in CONFIG.SYS
                        calls this function
                05h     (DOS 4.0+) get boot drive
                06h     (DOS 5.0+) return "real" DOS version number
                        instead of number returned by SETVER/int 21h
                        fn 30h.
return  (if AL=00h)  break setting
        DL      00h     if break=off
                01h     if break=on

        (if AL=05h)  boot drive
        DL      01h     A:
                02h     B:  (etc.)

        (if AL=06h)  real DOS version
        BH      major version (05h=DOS5)
        BL      minor version (00h=.00)
        DH      0Bh     DOS is in ROM
                10h     DOS is in HMA
        DL      DOS subversion number (0-7, VER/R reports as A-G)

        (all)
        AL      0FFh    error
note 1) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     2) When ^C checking is on, DOS checks for ^C after each DOS call.
        When off, DOS only checks after each DOS character I/O function
        from 01h to 0Ch.


Function  34h  Return INDOS Flag
     Returns ES:BX pointing to Critical Section Flag, byte indicating
     whether it is safe to interrupt DOS.
entry   AH      34h
return  ES:BX   points to 1-byte DOS "critical section flag"
note 1) If this byte is 0, it is safe to interrupt DOS.
     2) Supported OS/2 1.x Compatibility Box.
     3) For PC-MOS/386, this flag indicates whether MOS is using its
        stack or a task's stack.  If the flag is 0 it is safe to
        interrupt MOS.


Function  35h  Get Interrupt Vector
     Get interrupt vector
entry   AH      35h
        AL      interrupt number (hexadecimal)
return  ES:BX   address of interrupt vector
note 1) Use function call 25h to set the interrupt vectors.
     2) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.


Function  36h  Get Disk Free Space
     get information on specified drive
entry   AH      36h
        DL      drive number (0=default, 1=A:, 2=B:, etc)
return  AX      number of sectors per cluster
                0FFFFh means drive specified in DL is invalid
        BX      number of available clusters
        CX      bytes per sector
        DX      clusters per drive
note 1) Multiply AX * CX * BX for free space on disk.
     2) Multiply AX * CX * DX for total disk space.
     3) Function 36h returns an incorrect value after an ASSIGN command.
        Prior to ASSIGN, the DX register contains 0943h on return, which
        is the free space in clusters on the HC diskette. After ASSIGN,
        even with no parameters, 0901h is returned in the DX register;
        this is an incorrect value.  Similar results occur with DD
        diskettes on a PC-XT or a PC-AT.  This occurs only when the disk
        is not the default drive.  Results are as expected when the
        drive is the default drive.  Therefore, the circumvention is to
        make the desired drive the default drive prior to issuing this
        function call.
     4) This function supercedes functions 1Bh and 1Ch.
     5) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     6) TOS: $36 GET DISK FREE SPACE.


Function  37h  SWITCHAR / AVAILDEV
 *   Get/set option marking character (is usually "/"), and device type
entry   AH      37h
        AL      00h     read switch character (returns current character
                        in DL)
                01h     set character in DL as new switch character
    (DOS 2.x)   02h     read device availability (as set by function
                        AL=3) into DL.  A 0 means devices that devices
                        must be accessed in file I/O calls by /dev/device.
                        A non-zero value means that devices are
                        accessible at every level of the directory tree
                        (e.g., PRN is the printer and not a file PRN).
                        AL=2 to return flag in DL, AL=3 to set from DL
                        (0 = set, 1 = not set).
    (DOS 2.x)   03h     get device availability, where:
                        DL      00h     /dev/ must precede device names
                                01h     /dev/ need not precede device
                                        names
return  DL      switch character (if AL=0 or 1)
                device availability flag (if AL=2 or 3)
        AL      0FFh    the value in AL was not in the range 0-3
note 1) Functions 2 & 3 appear not to be implemented for DOS 3.x.
     2) It is documented on page 4.324 of the MS-DOS (version 2)
        Programmer's Utility Pack (Microsoft - published by Zenith).
     3) Works on all versions of IBM PC-DOS from 2.0 through 3.3.1.
     4) The SWITCHAR is the character used for "switches" in DOS command
        arguments (defaults to '/', as in "DIR/P"). '-' is popular to
        make a system look more like UNIX; if the SWITCHAR is anything
        other than '/', then '/' may be used instead of '\' for
        pathnames.
     5) Ignored by XCOPY, PKARC, LIST, used by PKZIP, LHARC.
     6) SWITCHAR may not be set to any character used in a filename.
        Legal filename characters vary in DOS 1.x through 6.x.
     7) In DOS 3.x you can still read the "AVAILDEV" byte with
        subfunction 02h but it always returns 0FFh even if you try to
        change it to 0 with subfunction 03h.
     8) AVAILDEV=0 means that devices must be referenced in an
        imaginary subdirectory "\dev" (similar to UNIX's /dev/*); a
        filename "PRN.DAT" can be created on disk and manipulated like
        any other.  If AVAILDEV != 0 then device names are recognized
        anywhere (this is the default): "PRN.DAT" is synonymous with
        "PRN:".
     9) These functions reportedly are not supported in the same
        fashion in various implementations of DOS.
    10) Used in DOS 3.3 by CHKDSK, BASIC, and DEBUG.
    11) SWITCHAR is not supported in the OS/2 1.x Compatibility Box.
    12) Not supported by the aftermarket 4DOS command interpreter in
        versions prior to 3.0.
    13) The MSDOS Encyclopedia reports: "XENIX used a forward slash as
        a separator, but versions 1.x of MS-DOS, borrowing from the
        tradition of DEC operating systems, already used the forward
        slash for switches on the command line, so Microsoft, at IBM's
        request, decided to use the backslash as the separator instead."
    14) SWITCHAR is not supported in MS-DOS 5.0.


Function  38h   Return Country-Dependent Information
                (PCDOS 2.0, 2.1, MSDOS 2.00 only)
entry   AH      38h
        AL      function code  (must be 0 in DOS 2.x)
        DS:DX   pointer to 32 byte memory buffer for returned information
return  CF      set on error
                AX      error code (02h)
        BX      country code
        DS:DX   pointer to buffer filled with country information:
          bytes 00h,01h date/time format
                        0000h   USA standard       H:M:S   M-D-Y
                        0001h   European standard  H:M:S   D/M/Y
                        0002h   Japanese standard  H:M:S   D:M:Y
                02h     ASCIIZ string currency symbol
                03h     byte of zeroes
                04h     ASCIIZ string thousands separator
                05h     byte of zeroes
                06h     ASCIIZ string decimal separator
                07h     byte of zeroes
       24 bytes 08h-1Fh reserved


Function  38h   Get Country-Dependent Information
                (PCDOS 3.x+, MSDOS 2.01+)
entry   AH      38h
        AL      function code
                00h     to get current country information
                01h-0FEh country code to get information for, for
                        countries with codes less than 255
                0FFh    to get country information for countries with
                        a code greater than 255
                        BX      16 bit country code if AL=0FFh
        DS:DX   pointer to the memory buffer where the data will be
                returned
        DX      0FFFFh if setting country code rather than getting info
return  CF      0 (clear) function completed
                1 (set) error
                        AX   error code
                             02h     invalid country code (no table
                                     for it)
        (if DX <> 0FFFFh)
        BX      country code (usually international telephone code)
        DS:DX   pointer to country data buffer
          bytes 0,1     date/time format
                        0       USA standard       H:M:S   M/D/Y
                        1       European standard  H:M:S   D/M/Y
                        2       Japanese standard  H:M:S   D:M:Y
          bytes 02h-06h ASCIIZ currency symbol
          byte  07h     ASCIIZ thousands separator
          byte  08h     byte of zeroes
          byte  09h     ASCIIZ decimal separator
          byte  0Ah     byte of zeroes
          byte  0Bh     ASCIIZ date separator
          byte  0Ch     byte of zeroes
          byte  0Dh     ASCIIZ time separator
          byte  0Eh     byte of zeroes
          byte  0Fh     currency format byte
                     bit 0      0  if currency symbol precedes the value
                                1  if currency symbol is after the value
                         1      0  no spaces between value and currency
                                   symbol
                                1  one space between value and currency
                                   symbol
                         2      1  if currency symbol replaces decimal
                                   point
                         3-7    not defined by Microsoft
          byte  10h     number of significant decimal digits in currency
                        (number of places to right of decimal point)
          byte  11h     time format byte
                    bit 0       0       12 hour clock
                                1       24 hour clock
                        1-7     unknown, probably not used
          bytes 12h-15h address of case map routine (FAR CALL, AL=char
                                                     to map)
                        entry   AL  ASCII code of character to be
                                    converted to uppercase
                        return  AL  ASCII code of the uppercase input
                                    character
          byte  16h     data-list separator character
          byte  17h     zeroes
          bytes 18h-21h 5 words reserved
note 1) When an alternate keyboard handler is invoked, the keyboard
        routine is loaded into user memory starting at the lowest
        portion of available user memory.  The BIOS interrupt vector
        that services the keyboard is redirected to the memory area
        where the new routine resides.  Each new routine takes up about
        1.6K of memory and has lookup tables that return values unique
        to each language.  (KEYBxx in the DOS book)
          Once the keyboard interrupt vector is changed by the DOS
        keyboard routine, the new routine services all calls unless the
        system is returned to the US format by the ctrl-alt-F1 keystroke
        combination.  This does not change the interrupt vector back to
        the BIOS location; it merely passes the table lookup to the ROM
        locations.
     2) Ctrl-Alt-F1 will only change systems with US ROMS to the US
        layout.  Some systems are delivered with non-US keyboard
        handler routines in ROM.  (Amstrad machines)
     3) Case mapping call: the segment/offset of a FAR procedure that
        performs country-specific lower-to-upper case mapping on ASCII
        characters 80h to 0FFh.  It is called with the character to be
        mapped in AL.  If there is an uppercase code for the letter, it
        is returned in AL, if there is no code or the function was
        called with a value of less than 80h AL is returned unchanged.
     4) This call is fully implemented in MS-DOS version 2.01 and
        higher.  It is in version 2.00 but not fully implemented
        (according to Microsoft).
     5) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     6) For PC-MOS/386 native mode apps, use DS:EDX instead of DS:DX.


Function  38h   Set Country Dependent Information
entry   AH      38h
        AL      code    country code to set information for, for
                        countries with codes less than 255
                0FFh    to set country information for countries with
                        a code greater than 255
        BX      16 bit country code if AL=0FFh
        DX      0FFFFh
return  CF      clear   successful
                set     if error
                        AX      error code (02h)
note 1) Some country codes are:
        061     Australia       ("International English" in DOS 5.0)
        032     Belgium
        055     Brazil          (DOS 5.0)
        002     French-Canadian
        042     Czechoslovakia  (DOS 5.0)
        045     Denmark
        358     Finland
        033     France
        049     Germany
        036     Hungary         (DOS 5.0)
        972     Israel
        039     Italy
        081     Japan
        758     Middle East
        031     Netherlands
        047     Norway
        048     Poland          (DOS 5.0)
        351     Portugal
        003     Latin America
        034     Spain
        046     Sweden
        041     Switzerland
        088     Taiwan          (MS 4.0+, but not DRDOS 5 or 6)
        044     U.K.
        001     USA
        038     Yugoslavia      (DOS 5.0)
     2) The country code is the same as the 3-digit international
        telephone code for that country.
     3) The documentation for COUNTRY= will tell you which codes are
        valid for your particular DOS version.  Different OEM
        implementations of the same DOS version may not support the
        same countries.
     4) TSL offers support for developing special country code drivers
        for PC-MOS/386.  Version 4.10 comes with:
        061     Australia
        032     Belgium
        002     French-Canadian
        045     Denmark
        358     Finland
        033     France
        049     Germany
        972     Israel
        039     Italy
        758     Middle East
        031     Netherlands
        047     Norway
        351     Portugal
        034     Spain
        046     Sweden
        041     Switzerland
        044     U.K.
        001     USA
     5) For PC-MOS/386 native mode apps, use DS:EDX instead of DS:DX.


Function  39h   Create Subdirectory                     (MKDIR)
                Makes a subdirectory along the indicated path
entry   AH      39h
        DS:DX   address of ASCIIZ directory pathname string
return  flag CF 0       successful
                1       error
                        AX      error code if any  (03h, 05h)
                                        (DOS 5.0)  (02h, 03h, 05h)
note 1) The ASCIIZ string contains the drive and subdirectory.
     2) Drive may be any valid drive (not necessarily current drive).
     3) The pathname cannot exceed 64 characters.  (same with
        PC-MOS/386)
     4) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     5) The characters [, ] , =, and " may not be used in subdirectory
        names.
     6) TOS: $39 MKDIR.
     7) For DOS 3.1+ networks, the user must have Create access to the
        subdirectory.
     8) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  3Ah   Remove Subdirectory                     (RMDIR)
entry   AH      3Ah
        DS:DX   address of ASCIIZ pathname string
return  CF      clear     successful
                set       AX      error code if any  (03h, 05h, 10h)
note 1) The ASCIIZ string contains the drive and subdirectory.
     2) Drive may be any valid drive (not necessarily current drive).
     3) The pathname cannot exceed 64 characters.
     4) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     5) TOS: $3A RMDIR.
     7) For DOS 3.1+ networks, the user must have Delete access to the
        subdirectory.
     8) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  3Bh   Change Current Directory                (CHDIR)
entry   AH      3Bh
        DS:DX   address of ASCIIZ string
return  flag CF 0       successful
                1       error
        AX      error code if any (03h)
note 1) The pathname cannot exceed 64 characters including separators.
     2) The ASCIIZ string may contain drive and subdirectory.
     3) Drive may be any valid drive (not necessarily current drive).
        This call will not change the current logged drive.
     4) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     5) TOS: $3B CHDIR.
     6) The pathspec may not contain wildcards.
     7) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  3Ch   Create A File                           (CREAT)
                Create a file with handle
entry   AH      3Ch
        CX      byte, attributes for file
                00h     normal read/write
                01h     read only
                02h     hidden
                04h     system
                08h     volume label
                20h     archive bit
        DS:DX   address of ASCIIZ filename string
return  CF      0       successful creation
                1       error
        AX      16 bit file handle
                or error code  (03h, 04h, 05h)
note 1) The ASCIIZ string may contain drive and subdirectory.
     2) Drive may be any valid drive (not necessarily current drive).
     3) If the volume label or subdirectory bits are set in CX, they are
        ignored.
     4) The file is opened in read/write mode
     5) If the file does not exist, it is created. If one of the same
        name exists, it is truncated to a length of 0.
     6) Good practice is to attempt to open a file with fn 3Dh and jump
        to an error routine if successful, create file if 3Dh fails.
        That way an existing file will not be truncated and overwritten.
     7) If the application will run only on DOS 3.x or higher, fn 5Bh
        should be used as it automatically creates the file if it does
        not exist, preventing problems with networks or multitaskers.
     8) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     9) TOS: $3C CREAT.
    10) For DOS 3.1+ networks, the user must have Create access to the
        subdirectory.
    11) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  3Dh   Open A File
                Open disk file with handle
entry   AH      3Dh
        AL      access code byte
(DOS 2.x)  bits 0-2     file attribute
                        000     read only
                        001     write only
                        010     read/write (default)
                3-7     reserved, should be set to zero
(DOS 3.x)  bits 0-2     file attribute
                        000     read only
                        001     write only
                        010     read/write (default)
                3       reserved, should be set to zero
                4-6     sharing mode (network)
                        000     compatibility mode       (default)
                        001     read/write access denied (exclusive)
                        010     write access denied
                        011     read access denied
                        100     full access permitted
                7       inheritance flag
                        0       file inherited by child process
                        1       file private to child process
        DS:DX   address of ASCIIZ drive/path/filename string
return  CF set on error
                AX      error code - MS-DOS (01h, 02h, 03h, 04h, 05h,
                                            0Ch)
                                   - DR-DOS (02h, 04h, 05h, 0Ch) and
                                            MS-DOS 5.0
        AX      16 bit DOS file handle
note 1) Opens any normal, system, or hidden file.
     2) Files that end in a colon are not opened.
     3) The read/write pointer is set at the first byte of the file and
        the record size of the file is 1 byte (the read/write pointer
        can be changed through function call 42h).  The returned file
        handle must be used for all subsequent input and output to the
        file.
     4) If the file handle was inherited from a parent process or was
        duplicated by DUP or FORCEDUP, all sharing and access
        restrictions are also inherited.
     5) A file sharing error (error 01h) causes an int 24h to execute
        with an error code of 02h.
     6) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     7) TOS: $3D OPEN.
     8) If any process opens a file that denies a level of access, all
        subsequent requests to open the file at that level of access
        will fail.
     9) Any attempt to open a file with a sharing mode that is already
        breached by an existing process will always fail.
    10) Used by APPEND in DOS 3.2,+.
    11) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  3Eh   Close A File Handle
                Close a file and release handle for reuse
entry   AH      3Eh
        BX      file handle (generated by 3Dh)
return  flag CF clear   successful close
                set     error
        AX      error code if error (06h)
note 1) When executed, the file is closed, the directory is updated,
        and all buffers for that file are flushed.  If the file was
        changed, the time and date stamps are changed to reflect the
        current time.
     2) If called with the handle 00000h, it will close STDIN (normally
        the keyboard).
     3) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     4) TOS: $3E CLOSE.
     5) In most versions of DOS only 15 file handles are available for
        user processes.  It is good practice to close a file handle
        when you are not using it.
     6) For DOS 3.1+ networks, all file locks must be removed before
        the file is closed.  Closing a file with active locks is
        unpredictable.
     7) Although closing a file invalidates the corresponding handle,
        DOS may reuse the handle to identify a file that is subsequently
        opened or created.  You can use int 21h/440Ah (Is File or Device
        Remote) to tell if a given handle is valid.


Function  3Fh   Read From A File Or Device
                Read from file with handle
entry   AH      3Fh
        BX      file handle
        CX      number of bytes to read
        DS:DX   address of buffer
return  flag CF clear   successful read
                set     error
        AX      00h     pointer was already at end of file
                     or number of bytes read
                     or error code (05h, 06h)
note 1) This function attempts to transfer the number of bytes specified
        in CX to a buffer location.  It is not guaranteed that all bytes
        will be read.
     2) If performed from STDIN (file handle 0000), the input can be
        redirected.
     3) If used to read the keyboard, it will only read to the first CR.
     4) The file pointer is incremented to the last byte read.
     5) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     6) TOS: $3F READ.
     7) If AX < CX a partial record was read.
     8) For DOS 3.1+ networks, the user must have Read access to the
        subdirectory.
    11) For PC-MOS/386 native mode, use DS:EDX and ECX instead of DS:DX
        and CX.


Function  40h   Write To A File Or Device
                Write to file with handle
entry   AH      40h
        BX      file handle
        CX      number of bytes to write
        DS:DX   address of buffer
return  CF      clear   successful write
                set     error
        AX      number of bytes written
                or error code  (05h, 06h)
note 1) This call attempts to transfer the number of bytes indicated
        in CX from a buffer to a file.  If CX and AX do not match after
        the write, an error has taken place; however no error code will
        be returned for this problem.  This is usually caused by a full
        disk.
     2) If the write is performed to STDOUT (handle 0001), it may be
        redirected.
     3) To truncate the file at the current position of the file
        pointer, set the number of bytes in CX to zero before calling
        int 21h.  The pointer can be moved to any desired position with
        function 42h.
     4) This function will not write to a file or device marked
        read-only.
     5) May also be used to display strings to CON instead of fn 09h.
        This function will write CX bytes and stop; fn 09h will
        continue to write until a '$' character is found.
     6) This is the call that DOS actually uses to write to the screen
        in DOS 2.x and above.
     7) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     8) TOS: $40 WRITE.
     9) For PC-MOS/386 native mode, use DS:EDX and ECX instead of DS:DX
        and CX.


Function  41h   Delete A File From A Specified Subdirectory  (UNLINK)
entry   AH      41h
        DS:DX   pointer to ASCIIZ filespec to delete
return  CF      clear   successful
                set     error
                AX      error code if any  (02h, 05h)
note 1) This function will not work on a file marked read-only.
     2) Wildcards are not accepted.
     3) For deleting multiple files, function 13h is faster.
     4) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     5) TOS: $41 UNLINK.
     6) For DOS 3.1+ networks, the user must have Delete access to the
        subdirectory.
     7) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  42h   Move a File Read/Write Pointer               (LSEEK)
entry   AH      42h
        AL      method code byte
                00h     offset from beginning of file
                01h     offset from present location
                02h     offset from end of file
        BX      file handle
        CX:DX   offset into file (high/low word) in bytes
return  AX:DX   new file pointer (segment/offset)
        CF      set     error
                        AX      error code (01h, 06h)
                clear   successful move
note 1) If pointer is at end of file, reflects file size in bytes.
     2) The value in DX:AX is the absolute 32 bit byte offset from the
        beginning of the file.
     3) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     4) TOS: $42 LSEEK.
     5) File most be open.
     6) For PC-MOS/386 native mode, use ECX instead of CX.


Function   43h  Get/Set file attributes                 (CHMOD)
entry   AH      43h
        AL      00h     get file attributes
                01h     set file attributes
                CX      file attributes to set
                    bit 0       read only
                        1       hidden file
                        2       system file
                        3       volume label
                        4       subdirectory
                        5       written since backup (archive bit)
                        6,7     not used
                        8       shareable (Novell NetWare)
                        9-F     not used
        DS:DX   segment/offset pointer to full ASCIIZ file name
return  CF      set if error
                AX      error code - MSDOS (01h, 02h, 03h, 05h)
                                     DRDOS (02h, 03h, 05h)
        CX      file attributes on get
                attributes:
                00h     normal
                01h     read only
                02h     hidden
                04h     system
                08h     volume label
                10h     file is subdirectory
                20h     archive
note 1) This call will not change the volume label or directory bits.
     2) Any combination of file attributes may be used.
     3) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     4) TOS: $43 CHANGE MODE (CHMOD).
     5) For DOS 3.1+ networks, the user must have Create access to the
        subdirectory.
     6) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  44h   I/O Control for Devices                 (IOCTL)
        provided with full version of DOSREF


Function  45h   Duplicate a File Handle                 (DUP)
entry   AH      45h
        BX      file handle to duplicate
return  CF      clear   AX      duplicate handle
                set     AX      error code  (04h, 06h)
note 1) If you move the pointer of one handle with 3Fh (Read), 40h
        (Write), or 42h (Move Pointer) the pointer of the other will
        also be moved.
     2) The handle in BX must be open.
     3) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     4) TOS: $45 DUP.
     5) This call can be used to update the directory entry of a file
        without the overhead of opening and closing it.  For example,
        this call followed by fn 3Eh (Close File) with the duplicate
        file handle will cause DOS to flush its buffers and update the
        directory entry while the original file remains open in read/
        write mode.


Function  46h   Force Duplicate of a Handle          (FORCEDUP or CDUP)
                Forces handle in CX to refer to the same file at the
                same position as BX
entry   AH      46h
        BX      existing open file handle to duplicate
        CX      new file handle
return  CF      clear   both handles now refer to existing file
                set     error
                AX      error code (04h, 06h)
note 1) If CX was an open file, it is closed first.
     2) If you move the pointer of one handle with 3Fh (Read), 40h
        (Write), or 42h (Move Pointer) the pointer of the other will
        also be moved.
     3) The handle in BX must be open.
     4) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     5) TOS $46 FORCE.
     6) This call may be used for redirecting input and output.


Function  47h   Get Current Directory
                Places full pathname of current directory/drive into
                a buffer
entry   AH      47h
        DL      drive (0=default, 1=A:, etc.)
        DS:SI   segment/offset pointer to 64-byte buffer area
return  CF      clear   DS:DI   pointer to ASCIIZ pathname of the current
                                directory
                set     AX      error code (0Fh)
note 1) String does not begin with a drive identifier or a backslash.
     2) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     3) TOS: $47 GETDIR.
     4) For PC-MOS/386 native mode, use DS:ESI instead of DS:SI.


Function  48h   Allocate Memory                         (MALLOC)
                Allocates the requested number of 16-byte paragraphs
                of memory
entry   AH      48h
        BX      number of 16-byte paragraphs desired
return  CF      clear   AX      segment address of allocated space
                        BX      maximum number paragraphs available
                set     AX      error code (07h, 08h)
note 1) BX indicates maximum memory available only if allocation fails.
     2) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     3) TOS: $48 MALLOC.
     4) Native-mode PC-MOS/386 applications allocate memory via MOS'
        interrupt 0D4h API function 11h.
     5) When in TopView this call is checked to ensure none of the
        addresses are outside the application's memory partition.


Function  49h   Free Allocated Memory
                Frees specified memory blocks
entry   AH      49h
        ES      segment address of area to be freed
return  CF      clear   successful
                set     AX      error code (07h, 09h)
note 1) This call is only valid when freeing memory obtained by
        function 48h.
     2) A program should not try to release memory not belonging to it.
     3) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     4) TOS: $49 MFREE.
     5) Native-mode PC-MOS/386 applications allocate memory via MOS'
        interrupt 0D4h API function 12h.
     6) When in TopView this call is checked to ensure none of the
        addresses are outside the application's memory partition.


Function  4Ah   Modify Allocated Memory Blocks          (SETBLOCK)
                Expand or shrink memory for a program
entry   AH      4Ah
        BX      new size in 16 byte paragraphs
        ES      segment address of block to change
return  CF      clear   nothing
                set     AX      error code (07h, 08h, 09h)
                    or  BX      maximum number of paragraphs available
note 1) Max number paragraphs available is returned only if the call
        fails.
     2) Memory can be expanded only if there is memory available.
     3) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
     4) TOS: $4A SETBLOCK.
     5) Native-mode PC-MOS/386 applications allocate memory via MOS'
        interrupt 0D4h API function 12h.
     6) When in TopView this call is checked to ensure none of the
        addresses are outside the application's memory partition.


Function  4Bh   Load or Execute a Program                  (EXEC)
entry   AH      4Bh
        AL      00h     load and execute program.  A PSP is built for
                        the program the ctrl-break and terminate
                        addresses are set to the new PSP.
  (DOS 5.0)    *01h     load but don't execute (internal, DOS 3.x &
                        DESQview)    (see note 1)
               *02h     load but do not execute (internal,
                        DOS 2.x only)
                03h     load overlay (do not create PSP, do not begin
                        execution)
               *04h     start async process (Euro-DOS 4.0 only)
                        (see note 12)
  (DOS 5.0)     05h     EnterExecState API (see note 17)
        DS:DX   segment/offset pointer to the ASCIIZ string with the
                drive, path, and filename to be loaded
        ES:BX   segment/offset pointer to a parameter block for the load
               (AL=00h) word    segment address of environment string
                                to be passed (0=use current)
                       dword    pointer to the command line to be
                                placed at PSP+80h
                       dword    pointer to default FCB to be passed at
                                PSP+5Ch
                       dword    pointer to default FCB to be passed at
                                PSP+6Ch
              (*AL=01h) word    segment of environment (0 = use current)
                       dword    pointer to command line
                       dword    pointer to FCB 1
                       dword    pointer to FCB 2
         (DOS 3.x+)    dword    will hold SS:SP on return
         (DOS 3.x+)    dword    will hold program entry point (CS:IP) on
                                return
              (*AL=02h) word    segment of environment (0 = use current)
                       dword    pointer to command line
                       dword    pointer to FCB 1
                       dword    pointer to FCB 2
               (AL=03h) word    segment address where file will be
                                loaded
                        word    relocation factor to be applied to the
                                image
return  CF      set     error
                        AX      error code (01h - 05h, 08h, 0Ah, 0Bh)
        CF      clear   if successful
                for fn 00h, process ID set to new program's PSP; get
                        with function 62h
                for fn 01h and DOS 3.x+ or DESQview, process ID set
                        to new program's PSP; get with function 62h
                for fn 01h and DOS 2.x, new program's initial stack
                        and entry point returned in registers
                for fn 02h, new program's initial stack and entry point
                        are returned in the registers
note 1) If you make this call with AL=1 the program will be loaded as
        if you made the call with AL=0 except that the program will not
        be executed.  Additionally, with AL=1 the stack segment and
        pointer along with the program's CS:IP entry point are returned
        to the program which made the 4B01h call.  These values are put
        in the four words at ES:BX+0Eh.  On entry to the call ES:BX
        points to the environment address, the command line and the
        two default FCBs.  This form of EXEC is used by DEBUG.COM.
     2) Application programs may invoke a secondary copy of the command
        processor (normally COMMAND.COM) by using the EXEC function.
        Your program may pass a DOS command as a parameter that the
        secondary command processor will execute as though it had been
        entered from the standard input device.
        The procedure is:
         A. Assure that adequate free memory (17k for 2.x and 3.0, 23k
            for 3.1) exists to contain the second copy of the command
            processor and the command it is to execute.  This is
            accomplished by executing function call 4Ah to shrink memory
            allocated to that of your current requirements.  Next,
            execute function call 48h with BX=0FFFFh.  This returns the
            amount of memory available.
        B. Build a parameter string for the secondary command processor
           in the form:
                         1 byte   length of parameter string
                        xx bytes  parameter string
                         1 byte   0Dh (carriage return)
           For example, the assembly language statement below would
           build the string to cause execution of the command FOO.EXE:
                              DB 19,"/C C:FOO",13
        C. Use the EXEC function call (4Bh), function value 0 to cause
           execution of the secondary copy of the command processor.
           (The drive, directory, and name of the command processor can
           be obtained from the COMSPEC variable in the DOS environment
           passed to you at PSP+2Ch.)
        D. Remember to set offset 2 of the EXEC control block to point
           to the string built above.
     3) All open files of a process are duplicated in the newly created
        process after an EXEC, except for files originally opened with
        the inheritance bit set to 1.
     4) The environment is a copy of the original command processor's
        environment.  Changes to the EXECed environment are not passed
        back to the original.  The environment is followed by a copy of
        the DS:DX filename passed to the child process.  A zero value
        will cause the child process to inherit the environment of the
        calling process.  The segment address of the environment is
        placed at offset 2Ch of the PSP of the program being invoked.
     5) This function uses the same resident part of COMMAND.COM, but
        makes a duplicate of the transient part.
     6) How EXEC knows where to return to: Basically the vector for int
        22h holds the Terminate address for the current process.  When
        a process gets started, the previous contents of int 22h get
        tucked away in the PSP for that process, then int 22h gets
        modified.  So if Process A EXECs process B, while Process B is
        running, the vector for int 22h holds the address to return to
        in Process A, while the save location in Process B's PSP holds
        the address that process A will return to when *it* terminates.
        When Process B terminates by one of the usual legal means, the
        contents of int 22h are (surmising) shoved onto the stack, the
        old terminate vector contents are copied back to int 22h vector
        from Process B's PSP, then a RETF or equivalent is executed to
        return control to process A.
     7) To load an overlay file with 4B: first, don't de-allocate the
        memory that the overlay will load into.  With the other 4Bh
        functions, the opposite is true - you have to free the memory
        first, with function 4Ah.  Second, the "segment address where
        the file will be loaded" (first item in the parameter block for
        sub-function 03) should be a paragraph boundary within your
        currently-allocated memory.  Third, if the procedures within
        the overlay are FAR procs (while they execute, CS will be
        equal to the segment address of the overlay area), the
        relocation factor should be set to zero.  On the other hand,
        if the CS register will be different from the overlay area's
        segment address, the relocation factor should be set to
        represent the difference.  You determine where in memory the
        overlay file will load by using the segment address mentioned
        above.  Overlay files are .EXEs (containing header, relocation
        table, and memory image).
     8) When function 00h returns, all registers are changed,
        including the stack.  You must resore SS, SP, and any other
        required registers.
     9) PCDOS EXEC function 3 (overlay) lives in the transient piece
        of COMMAND.COM and gets loaded when needed, thus the
        requirement for enough free space to load the EXEC loader
        (about 1.5k).  Under MS-DOS the EXEC system call lives in
        the system space.
    10) If you try to overlay an .EXE file with the high/low switch set
        to load the in the upper end of memory nothing will happen.
        The high/low switch is only for process creation, not for
        overlays.
    11) DOS 2.x destroys all registers, including SS:SP.
    12) (AL=04h) This is DOS 4.0 as released in 1987 to various
        European OEMs.  It is not related to US DOS 4.0.
    13) This call is explicitly supported by the OS/2 1.x DOS
        Compatibility Box.
    14) TOS: $4B EXEC.
    15) New with DOS 5.0.  Sets up for EXEC, including setting the DOS
        version number returned by SETVER/int 21h,fn 30h.  If DOS is
        installed in the HMA this function turns off the A20 line,
        making the HMA inaccessible.  If your program needs the HMA
        you must turn it back on before EXECing.  DOS normally turns
        the A20 line back on when called for normal system functions.
        Your program must call EXEC immediately after this subfunction.
        You may not call any DOS, BIOS, or system interrupts between
        this subfunction and loading your program.
    15) When in TopView this call is checked to ensure none of the
        addresses are outside the application's memory partition.
    16) This call is documented in the MS 5.0 TR, and in the PC-MOS/386
        TR.
    17) Some applications use their own EXEC scheme instead of calling
        this function.  Since DOS 5 enhanced the EXEC functions these
        programs might not work, so Microsoft added a new set of
        functions called EnterExecState.  If a self-EXECing program
        calls EnterExecState before EXECing, DOS 5 will do various
        internal fixups to keep things happy.


Function  4Ch   Terminate a Process                          (EXIT)
                Quit with ERRORLEVEL exit code
entry   AH      4Ch
        AL      exit code in AL when called, if any, is passed to next
                process
return  none
note 1) Control passes to DOS or calling program.
     2) Return code from AL can be retrieved by ERRORLEVEL or function
        4Dh.
     3) All files opened by this process are closed, buffers are
        flushed, memory is released, any network file region locks are
        released, and the disk directory is updated.
     4) Restores: Terminate vector from PSP:000Ah
                  Ctrl-C vector from PSP:000Eh
                  Critical Error vector from PSP:0012h
     5) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     6) TOS: $4C TERM. Returns 2-byte errorlevel to calling program
     7) Under DOS 3.1+ networks, all file locks should be removed
        before terminating the program.


Function  4Dh   Get Return Code of a Subprocess              (WAIT)
                Get return from functions 31h and 4Dh  (ERRORLEVEL)
entry   AH      4Dh
return  AH      circumstance which caused termination
                00h     normal termination
                01h     control-break or control-C
                02h     critical device error
                03h     terminate and stay resident (function 31h)
        AL      exit code of subprogram (functions 31h or 4Ch)
note 1) The exit code is only returned once (the first time).
     2) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     3) Many programmers have wondered where DOS stores this
        information, so they might access it multiple times or set it
        directly.  This is a version-dependent area that changes with
        every release of DOS.


Function  4Eh   Find First Matching File                  (FIND FIRST)
entry   AH      4Eh
        CX      search attributes  (see function 43h)
        DS:DX   segment/offset pointer to ASCIIZ filename
                (with attributes)
return  CF      set     AX      error code (02h, 03h, 12h)
                clear   data block written at current DTA
                        format of block is:  (info from BIX)
  documented by Micro-  |00h     1 byte   attribute byte of search
  soft as "reserved for |01h     1 byte   drive letter for search
  DOS' use on subsquent |02h    11 bytes  the search name used
  Find Next calls"      |0Ch     2 bytes  word value of last entry
  function 4Fh          |0Fh     4 bytes  dword pointer to this DTA
                        |13h     2 bytes  word directory start

                        | PC-DOS 3.10 (from INTERRUP.ARC)
                        |00h     1 byte   drive letter
                        |01h-0Bh 11 bytes search template
                        |0Ch     1 byte   search attributes

                        | DOS 2.x (and DOS 3.x except 3.1?)
                                  (from INTERRUP.ARC)
                        |00h     1 byte   search attributes
                        |01h     1 byte   drive letter
                        |02h-0Ch 11 bytes search template
                        |0Dh-0Eh 2 bytes  entry count within directory
                        |0Fh-12h 4 bytes  reserved
                        |13h-14h 2 bytes  cluster number of parent
                                          directory

                         15h   1 byte   file attribute
                         16h   2 bytes  file time, bit mask:
                                        0-4     half-seconds
                                        5-10    minute
                                        11-15   hour
                         18h   2 bytes  file date, bit mask:
                                        0-4     day
                                        5-8     month
                                        9-15    years since 1980
                         1Ah   2 bytes  low word of file size
                         1Ch   2 bytes  high word of file size
                         1Eh  13 bytes  name and extension of file
                                        found, plus 1 byte of 0s.  All
                                        blanks are removed from the name
                                        and extension, and if an
                                        extension is present it is
                                        preceded by a period.
note 1) This function does not support network operations.
     2) Wildcards are allowed in the filespec.
     3) If the attribute is zero, only ordinary files are found. If the
        volume label bit is set, only volume labels will be found.  Any
        other attribute will return that attribute and all normal files
        together.
     4) To look for everything except the volume label, set the hidden,
        system, and subdirectory bits all to 1.
     5) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     6) TOS: $4E SFIRST.
     7) There is a reported anomaly in MS-DOS 2.11.  Odd things happen
        when you give it "C:\" as a file name.  FindFirst reports it as
        a valid file and Open returns a handle.  If you read it twice
        DOS reports an FAT error and aborts.  You can demonstrate this
        by:
                FIND "whatever" c:\    [twice]
        Int 21h/AH=044 AL=0 returns the handle as a NUL device which
        makes a certain amount of sense since C:\ is a null terminated
        string.  The programming solution is to trap 'C:\' before it's
        used.
     8) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX. ES:EBX
        is a pointer to a 43-byte data area used instead of the DTA.
        Format:
                0-20    reserved for MOS
                21      attribute
                22-23   time of last update
                24-25   date of last update
                26-29   file size in bytes
                30-42   found filename, followed by 00h


Function  4Fh   Find Next Matching File                   (FIND NEXT)
                Find next ASCIIZ file
entry   AH      4Fh
return  CF      clear   data block written at current DTA
                set     AX      error code (02h, 12h)
note 1) If file found, DTA is formatted as in call 4Eh.
     2) Volume label searches using 4Eh/4Fh reportedly aren't 100%
        reliable under DOS 2.x.  The calls sometime report there's a
        volume label and point to a garbage DTA, and if the volume
        label is the only item they sometimes won't find it.  Most
        references recommend the use of the older FCB calls for dealing
        with the volume labels.
     3) This function does not support network operations.
     4) Use of this call assumes that the original filespec contained
        wildcards.
     5) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     6) TOS: $4F SNEXT.
     7) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX. ES:EBX
        is a pointer to a 43-byte data area used instead of the DTA.
        Format:
                0-20    reserved for MOS
                21      attribute
                22-23   time of last update
                24-25   date of last update
                26-29   file size in bytes
                30-42   found filename, followed by 00h


Function  50h   "Used Internally by DOS" - Set PSP or SetPID
 *              Set new Program Segment Prefix (current Process ID)
entry   AH      50h
        BX      segment address of new PSP
return  none - swaps PSPs regarded as current by DOS
note 1) By putting the PSP segment value into BX and issuing call 50h
        DOS stores that value into a variable and uses that value
        whenever a file call is made.
     2) Note that in the PSP (or PDB) is a table of 20 (decimal) open
        file handles.  The table starts at offset 18h into the PSP.  If
        there is an 0FFh in a byte then that handle is not in use.  A
        number in one of the bytes is an index into an internal FB
        table for that handle.  For instance the byte at offset 18h is
        for handle 0, at offset 19h handle 1, etc. up to 13h.  If the
        high bit is set then the file associated by the handle is not
        shared by child processes EXEC'd with call 4Bh.
     3) Function 50h is dangerous in background operations prior to DOS
        3.x as it uses the wrong stack for saving registers.  (same as
        functions 0..0Ch in DOS 2.x).  It doesn't use stack in 3.0+,
        so it is safe to use in later versions of DOS.
     4) Under DOS 2.x, this function cannot be invoked inside an int
        28h handler without setting the Critical Error flag.
     5) Open file information, etc. is stored in the PSP DOS views as
        current.  If a program (eg. a resident program) creates a need
        for a second PSP, then the second PSP should be set as current
        to make sure DOS closes that as opposed to the first when the
        second application finishes.
     6) See PC Mag Vol.5, No 9, p.314 for discussion, also used in
        their BCOPY.ASM utility.
     7) Used by DOS 3.3 PRINT & DEBUG, DesQview 2.01, Windows 1.03,
        SYMDEB from MASM 4.0.
     8) This call is available in the OS/2 1.x DOS Compatibility Box.
     9) DOS stores the PID in one location, but the actual address is
        version dependent.  The strategy is to find the PID and thereby
        find its address.  If you have its address, you can swap PIDs
        from the TSR pop-up code by peeking and poking.  In the
        initialization code, use function  51hto get the PID.  You
        know that DOS lives between the interrupt vectors 0:0 - 0:100
        and that PID.  Other programs might be in there, but DOS is
        too.  Search that memory for a copy of the PID.  When you find
        one, use this function to set a phony PID.  If the location
        where you found the original PID changes to the phony PID,
        you have found the address of the PID.  Don't forget to reset
        the PID with SetPID after each test.  DOS 2.0 and 2.1 maintain
        the PID in two locations, not one.


Function  51h   Get Program Segment Prefix   (GetPID)
                Returns the PSP address of currently executing program
entry   AH      51h
return  BX      address of currently executing program (process ID)
note    format of PSP:
                offset  size           description
                00h   2 bytes   program exit point
                02h     word    memory size in paragraphs
                04h     byte    unused (0)
                05h   5 bytes   CP/M style entry point (far call to DOS)
                0Ah     word    terminate address (old int 22h)
                0Ch     word    terminate segment
                0Eh     word    break address (old int 23h)
                10h     word    break segment
                12h     word    error address (old int 24h)
                14h     word    error segment
                16h     word    parent PSP segment
                18h  20 bytes   DOS 2.0+ open files, 0FFh = unused
                2Ch     word    DOS 2.0+ environment segment
                2Eh    dword    far pointer to process's SS:SP
                32h     word    DOS 3.x+ max open files
                34h             DOS 3.x+ open file table address
                36h    dword    DOS 3.x+ open file table segment
                38h 24 bytes    unused by DOS versions before 3.3
                50h  3 bytes    DOS function dispatcher (FAR routine)
                53h  9 bytes    unused
                55h             FCB #1 extension
                5Ch 16 bytes    FCB #1, filled in from first command
                                line argument
                6Ch 20 bytes    FCB #2, filled in from second command
                                line argument
                80h128 bytes    command tail / default DTA buffer
note 1) Used in DOS 2.x, 3.x uses 62h.
     2) Function 51h is dangerous in background operations prior to DOS
        3.x as it uses the wrong stack for saving registers.  (same as
        functions 0..0Ch in DOS 2.x).  Doesn't use any DOS stacks in
        DOS 3.0+.
     3) 50h and 51h might be used if you have more than one process in
        a PC.  For instance if you have a resident program that needs
        to open a file you could first call 50h to get the current ID
        and then call 50h to set the ID to your PSP.
     4) Under DOS 2.x, this function cannot be invoked inside an int
        28h handler without setting the Critical Error flag.
     5) Formerly "undocumented", now described in MS 5.0 TR.
     6) This call is available in the OS/2 1.x DOS Compatibility Box.


FUNCTION 52h - see file 52h
        provided with full version of DOSREF


Function  53h   "Used Internally by DOS" - Translate BPB
 *              Translates BPB (BIOS Parameter Block, see below) into
                a DOS Disk Block (see function call 32h).
entry   AH      53h
        DS:SI   pointer to BPB (BIOS Parameter Block)
        ES:BP   pointer to buffer area for DOS Disk Block
                Layout of BPB:
                offset  size         description
                00h-01h word  bytes per sector, get from DDB bytes
                              02h-03h.
                02h     byte  sectors per cluster, get from (DDB byte
                              4) + 1
                03h-04h word  reserved sectors, get from DDB bytes
                              06h-07h
                05h     byte  number of FATs, get from DDB byte 08h
                06h-07h word  # of root dir entries, get from DDB bytes
                              09h-0Ah
   for DOS 3.x: 08h-09h word  total number of sectors, get from:
                              ((DDB bytes 0Dh-0Eh) - 1) * (sectors per
                              cluster (BPB byte 2)) + (DDB bytes 0Bh-0Ch)
   for DOS 4.x:               set to zero if partition is larger than
                              32Mb, set dword at 15h to actual number
                              of sectors
                0Ah     word  media descriptor byte, get from DDB byte
                              16h
                0Bh-0Ch word  number of sectors per FAT, get from DDB
                              byte 0Fh
   for DOS 3.x: 0Dh     word  number of sectors per track
                0Fh     word  number of heads
                11h    dword  number of hidden sectors
                15h 11 bytes  reserved
   for DOS 4.x: 15h    dword  total number of sectors if word at 08h
                              contains zero
return  unknown
note    This function is documented as 'SetDPB' in the Zenith MS-DOS
        3.05 TRM.


Function  54h   Get Verify Setting
                Get verify flag status
entry   AH      54h
return  AL      00h if flag off
                01h if flag on
note 1) Flag can be set with function 2Eh.
     2) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     3) The verify state is off by default.


Function  55h   "Used Internally by DOS" - Create "Child" PSP
 *              Create PSP: similar to function 26h (which creates a
                new Program Segment Prefix at segment in DX) except
                creates a "child" PSP rather than copying the existing
                one.
entry   AH      55h
        DX      segment number at which to create new PSP.
return  unknown
note 1) This call is similar to call 26h which creates a PSP except that
        unlike call 26h the segment address of the parent process is
        obtained from the current process ID rather than from the CS
        value on the stack (from the INT 21h call).  DX has the new PSP
        value and SI contains the value to be placed into PSP:2 (top of
        memory).
     2) Function 55 is merely a substitute for function 26h.  It will
        copy the current PSP to the segment address DX with the
        addition that SI is assumed to hold the new memory top segment.
        This means that function 26h sets SI to the segment found in the
        current PSP and then calls function 55h.


Function  56h   Rename a File
entry   AH      56h
        DS:DX   pointer to ASCIIZ old pathname
        ES:DI   pointer to ASCIIZ new pathname
return  CF      clear   successful rename
                set     AX      error code (02h, 03h, 05h, 11h)
note 1) Works with files in same logical drive only.
     2) Wildcard characters not allowed in filename.
     3) The name of a file is its full pathname.  The file's full
        pathname can be changed while leaving the actual FILENAME.EXT
        unchanged.  Changing the pathname allows the file to be
        "moved" from subdirectory to subdirectory on a logical drive
        without actually copying the file.
     4) DOS 3.x allows renaming of directories.
     5) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     6) TOS: $56 RENAME. Similar to MS-DOS 2.x.
     7) Under DOS 3.1+ networks, the user must have Read and Create
        access to the affected directories.
     8) For PC-MOS/386 native mode, use DS:EDX and ES:EDI instead of
        DS:DX and ES:DI.


Function  57h  Get/Set a File's Date and Time
               Read or modify time and date stamp on a file's directory
               entry
entry   AH      57h
        AL      function code
                00h     Get Date and Time
                01h     Set Date and Time
                        CX      time to be set
                        DX      date to be set
                02h     unknown (DOS 4.0+)
                03h     unknown
                04h     unknown (DOS 4.0+)
        BX      file handle
return  CF      clear   CX      time of last write (if AL = 0)
                        DX      date of last write (if AL = 0)
                set     AX      error code (01h, 06h)
note 1) Date/time formats are:
        CX bits 0Bh-0Fh hours (0-23)
                05h-0Ah minutes (0-59)
                00h-04h #2 sec. incr. (0-29)
        DX bits 09h-0Fh year (relative to 1980)
                05h-08h month (0-12)
                00h-04h day of the month (0-31)
     2) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     3) TOS: $57 GSDTOF.


Function  58h   Get/Set Allocation Strategy   (DOS 3.0+)
entry   AH      58h
        AL      00h     Get Current Strategy (see 01h)
                        Used to obtain the current allocation strategy.
                        First Fit is the normal default.

                01h     Set New Current Strategy
                        BL      new strategy
                        00h     First Fit - chooses the lowest block in
                                memory which will fit (this is the
                                default). (use first memory block large
                                enough)
                        01h     Best Fit - chooses the smallest block
                                which will fill the request.
                        02h     Last Fit - chooses the highest block
                                which will fit.
        (DOS 5.0)       40h     FIRST_FIT_HIGHONLY - search upper memory
                                area for the lowest available block
        (DOS 5.0)       41h     BEST_FIT_HIGHONLY -  search upper memory
                                area for the smallest block that fits
                                the request
        (DOS 5.0)       42h     LAST_FIT_HIGHONLY -  search upper memory
                                area for the highest available block
        (DOS 5.0)       80h     FIRST_FIT_HIGH - search upper memory
                                area for the lowest available block.  If
                                no block found, load in conventional
                                memory
        (DOS 5.0)       81h     BEST_FIT_HIGH - seach the upper memory
                                area for the closest match to the
                                requested size.  If no match is found,
                                load in convenional memory.
        (DOS 5.0)       82h     LAST_FIT_HIGH - search the upper memory
                                area for the available block at the
                                highest address.  If no block is found,
                                load in conventional memory.
                02h     Get UMB Link State
                        Indicates if high DOS memory arenas are
                        currently part  of the conventional DOS memory
                        arena.
                03h     Set UMB Link State
                        BX      00h     Unlink High DOS memory blocks
                                01h     Link High DOS memory blocks
                        note:   Adds or removes high memory blocks from
                                the DOS arena.  Returns an error if no
                                high arenas exist.
return  CF      clear   successful
          (AL=00)       AX      strategy code (see 01h)  For DOS 5.0+,
          (AL=02)       AL      00h   if upper memory area is not linked
                                01h   if the upper memory area is linked
                set     error
                        AX      error code (01h)
          (AL=03)       AX      0001h   invalid function if DOS was
                                        loaded without DOS=UMB
                                0007h   memory arena trashed
note 1) A program that changes the allocation strategy or the high DOS
        link state should ALWAYS restore it to its original condition
        before exiting, or subsequent programs may fail.
     2) The set subfunction accepts any value in BL; 2 or greater means
        last fit.  The get subfunction returns the last value set, so
        programs should check whether the value is greater than or equal
        to 2.
     3) For discussion of best fit vs. first fit allocation strategies,
        see Knuth, *Fundamental Algorithms.*  Very briefly, Knuth finds
        that first fit methods are far superior to best fit, which
        increases both allocation overhead and memory fragmentation.
        The "last" fit that DOS offers is nothing other than a first
        fit, starting the search at the other end of the chain (the
        algorithm still takes the first fitting block that is
        encountered in the search).
     4) For DOS 5+, the default scheme is "allocate high first".  This
        is actually a variant of the previous first/best/last
        allocation options.  When enabled, the high-first strategy
        causes DOS to begin its free block search at the first UMB. If
        DOS can't find enough memory in the UMB, it searches from the
        beginning of conventional memory.  As a result, high-first may
        result in a worse fit than best fit alone if there is a better
        fit low than high.


Function  59h   Get Extended Error Code (DOS 3.0+)
       The Get Extended Error function call (59h) is intended to provide
     a common set of error codes and to supply more extensive
     information about the error to the application.  The information
     returned from function call 59h, in addition to the error code,
     is the error class, the locus, and the recommended action.  The
     error class provides information about the error type (hardware,
     internal, system, etc.).  The locus provides information about
     the area involved in the failure (serial device, block device,
     network, or memory).  The recommended action provides a default
     action for programs that do not understand the specific error code.

       Newly written programs should use the extended error support
     both from interrupt 24h hard error handlers and after any int
     21h function calls.  FCB function calls report an error by
     returning 0FFh in AL.  Handle function calls report an error by
     setting the carry flag and returning the error code in AX.  Int
     21h handle function calls for DOS 2.x continue to return error
     codes 0-18.  Int 24h handle function calls continue to return
     error codes 0-12.  But the application can obtain any of the
     error codes used in the extended error codes table by issuing
     function call 59h.  Handle function calls for DOS 3.x can return
     any of the error codes.  However, it is recommended that the
     function call be followed by function call 59h to obtain the
     error class, the locus, and the recommended action.

       The Get Extended Error function (59h) can always be called,
     regardless of whether the previous DOS call was old style (error
     code in AL) or new style (carry bit).  It can also be used inside
     an int 24h handler.

       You can either check AL or the carry bit to see if there was no
     error, and call function 59h only if there was an error, or take
     the simple approach of always calling 59h and letting it tell you
     if there was an error or not.  When you call function 59h it will
     return with AX=0 if the previous DOS call was successful.

       Various versions of IBM's DOS, MSDOS, OEM customized versions
     of MSDOS, Digital's DRDOS, and other DOS emulating environments
     sometimes return different error codes for the same function.
     Be careful if you are testing for one specific error condition.

entry   AH      59h
        BX      version code (0000 for DOS 3.0 and 3.1)
return  AX      extended error code:
                01h     Invalid function number                     2.0+
                02h     File not found                              2.0+
                03h     Path not found                              2.0+
                04h     Too many open files, no file handles left   2.0+
                05h     Access denied                               2.0+
                06h     Invalid handle                              2.0+
                07h     Memory control blocks destroyed             2.0+
                08h     Insufficient memory                         2.0+
                09h     Invalid memory block address                2.0+
                0Ah     Invalid environment                         2.0+
                0Bh     Invalid format                              2.0+
                0Ch     Invalid access code                         2.0+
                0Dh     Invalid data                                2.0+
                0Eh     Reserved                                    2.0+
                0Fh     Invalid drive was specified                 2.0+
                10h     Attempt to remove the current directory     2.0+
                11h     Not same device                             2.0+
                12h     No more files                               2.0+
                13h     Tried to access write-protected diskette    2.0+
                14h     Unknown unit                                2.0+
                15h     Drive not ready                             2.0+
                16h     Unknown command                             2.0+
                17h     Bad CRC check                               2.0+
                18h     Bad request structure length                2.0+
                19h     Seek error                                  2.0+
                1Ah     Unknown media type                          2.0+
                1Bh     Sector not found                            2.0+
                1Ch     Printer out of paper                        2.0+
                1Dh     Write fault                                 2.0+
                1Eh     Read fault                                  2.0+
                1Fh     General failure                             2.0+
                20h     Sharing violation                           3.0+
                21h     Lock violation                              3.0+
                22h     Invalid disk change                         3.0+
                23h     FCB unavailable                             3.0+
                24h     Sharing buffer overflow                     3.3+
                25h     Bad code page                               4.0+
                26h     Handle EOF                                  4.0+
                27h     Handle disk full                            4.0+
                28h     Reserved
                29h        "
                2Ah        "
                2Bh        "
                2Ch        "
                2Dh        "
                2Eh        "
                2Fh        "
                30h        "
                31h     Reserved
                32h     Network: request not supported              3.1+
                33h     Network: remote computer not listening      3.1+
                34h     Network: duplicate name on network          3.3+
                35h     Network: name not found                     3.3+
                36h     Network: busy                               3.3+
                37h     Network: device no longer exists            3.3+
                38h     Network: NETBIOS command limit exceeded     3.3+
                39h     Network: adapter hardware error             3.3+
                3Ah     Network: incorrect response from network    3.3+
                3Bh     Network: unexpected network error           3.3+
                3Ch     Network: incompatible remote adapter        3.3+
                3Dh     Network: print queue full                   3.3+
                3Eh     Network: not enough space for print file    3.3+
                3Fh     Network: print file was deleted             3.3+
                40h     Network: name was deleted                   3.3+
                41h     Network: access denied                      3.3+
                42h     Network: device type incorrect              3.3+
                43h     Network: name not found                     3.3+
                44h     Network: name limit exceeded                3.3+
                45h     Network: NETBIOS session limit exceeded     3.3+
                46h     Network: sharing temporarily paused         3.3+
                47h     Network: request not accepted               3.3+
                48h     Network: print or disk redirection paused   3.1+
                49h     Reserved
                4Ah        "
                4Bh        "
                4Ch        "
                4Dh        "
                4Eh        "
                4Fh     Reserved
                50h     File exists                                 3.3+
                51h     Reserved                                    3.3+
                52h     Cannot make directory entry                 4.0+
                53h     Fail on interrupt 24h                       3.3+
                54h     Network: too many redirections              3.3+
                55h     Network: duplicate redirection              3.3+
                56h     Invalid password                            3.3+
                57h     Invalid parameter                           3.3+
                58h     Network: data fault (write error)           3.3+
                59h     Reserved
                5Ah     Comp not loaded                             5.0+
        BH      class of error:                          (DOS 5.0 name)
                01h     Out of resource                 ERRCLASS_OUTRES
                        (not enough disk space, etc)
                02h     Temporary situation             ERRCLASS_TEMPSIT
                        (not an error, but a temporary situation that is
                         expected to end, such as a locked region in a
                         file)
                03h     Authorization                   ERRCLASS_AUTH
                        (denied access - sharing or network)
                04h     Internal                        ERRCLASS_INTRN
                        (DOS internal error)
                05h     Hardware failure                ERRCLASS_HRDFAIL
                        (bad floppy or HD controller, etc.)
                06h     System failure                  ERRCLASS_SYSFAIL
                        (error not due to executing program, such as
                         missing configuration or data files)
                07h     Application program error       ERRCLASS_APPERR
                        (executing program bombed)
                08h     Not found                       ERRCLASS_NOTFND
                        (file or device not found)
                09h     Bad format                      ERRCLASS_BADFMT
                        (file or item invalid format or type)
                0Ah     Locked                          ERRCLASS_LOCKED
                        (network or SHARE lock)
                0Bh     Media error                     ERRCLASS_MEDIA
                        (wrong volume ID, disk failure)
                0Ch     Already exists                  ERRCLASS_ALREADY
                        (file or device already exists)
                0Dh     Unknown                         ERRCLASS_UNK
                        (Smurfs infesting system board)
        BL      suggested action code:
                01h     Retry                           ERRACT_RETRY
                02h     Delayed retry                   ERRACT_DLYRET
                03h     Prompt user                     ERRACT_USER
                04h     Abort after cleanup             ERRACT_ABORT
                05h     Immediate abort                 ERRACT_PANIC
                06h     Ignore                          ERRACT_IGNORE
                07h     Retry after user intervention   ERRACT_INTRET
        CH      locus (where error occurred):
                01h     Unknown or not appropriate      ERRLOC_UNK
                02h     Block device                    ERRLOC_DISK
                03h     Network related                 ERRLOC_NET
                04h     Serial device                   ERRLOC_SERDEV
                05h     Memory related                  ERRLOC_MEM
note 1) Not all DOS functions use the carry flag to indicate an error.
        Carry should be tested only on those functions which are
        documented to use it.
     2) None of the DOS functions which existed before 2.0 use the
        carry indicator.  Many of them use register AL as an error
        indication instead, usually by putting 0FFh in AL on an error.
        Most, but not all, the "new" (2.x, 3.x) functions do use carry,
       and most, but not all, of the "old" (1.x) functions use AL.
     3) On return, CL, DI, DS, DX, ES, BP, and SI are destroyed - save
        before calling this function if required.
     4) DOS 2.x Error Codes:  If you are using function calls 38h-57h
        with DOS 2.x, to check if an error has occurred, check for the
        following error codes in the AX register:
        call| error code    call| error code     call| error code
        ----|-------------------|--------------------|----------------
        38h | 2             41h | 2,3,5          4Ah | 7,8,9
        39h | 3,5           42h | 1,6            4Bh | 1,2,3,5,8,10,11
        3Ah | 3,5,15        43h | 1,2,3,5        4Eh | 2,3,18
        3Bh | 3             44h | 1,3,5,6        4Fh | 18
        3Ch | 3,4,5         45h | 4,6            56h | 2,3,5,17
        3Dh | 2,3,4,5,12    46h | 4,6            57h | 1,6
        3Eh | 6             47h | 15
        3Fh | 5,6           48h | 7,8
        40h | 5,6           49h | 7,9
     5) Note that extended error codes 13h through 1Fh correspond to
        error codes 00h through 0Ch returned by int 24h.
     6) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.


Function  5Ah   Create Temporary File
                Create unique filename (for temporary use) (DOS 3.0+)
entry   AH      5Ah
        DS:DX   pointer to buffer containing an ASCIIZ directory
                pathname ending with a backslash (\).  The buffer must
                have at least 13 bytes free following the backslash,
                as the file name and extension will be written there.
        CX      file attribute (00h, 01h, 02h, 04h, 20h only)
return  CF      clear   AX      handle
                        DS:DX   new ASCIIZ pathname
                set     AX      error code (03h, 04h, 05h)
note 1) The file created is not truly "temporary".  It must be removed
        by the user.
     2) If the filename created already exists in the current
        directory, this function will call itself again with another
        unique filename until a truly unique filename is found.
     3) The temporary filename usually consists of mixed letters and
        numbers.  No file extension appears to be generated.
     4) DOS 3.0-4.01 used mixed letters and numbers.  MS DOS 5.0 appears
        to use letters only.  DR DOS 5.0 appears to use numbers only.
     5) Under DOS 3.1+ and NETBIOS compatible networks, DOS opens the
        file in compatibility mode.
     6) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     7) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  5Bh   Create a New File   (DOS 3.0+)
entry   AH      5Bh
        DS:DX   segment/offset pointer to an ASCIIZ pathname
        CX      file attribute (00h, 01h, 02h, 04h, 20h only)
return  CF      clear   AX      file handle
                        DS:DX   new ASCIIZ pathname
                set     AX      error code (03h, 04h, 05h, 50h)
note 1) Unlike function 3Ch, function 5Bh will fail if the file already
        exists.  This is useful on networks, since another application
        might create a file of the same name with 3Ch first, causing
        the first 3Ch to fail.  Use of this call instead of 3Ch is good
        practice for code that will not need to run under DOS 2.x.
     2) Under DOS 3.1+ networks, DOS opens the file in read/write mode.
        This call will fail if the user does not have Create access.
     3) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     4) The MSDOS Encyclopedia suggests this call may be used to
        implement semaphores in LANs or multitasking environments.  A
        zero-byte marker file could be created to indicate semaphore set.
     5) For PC-MOS/386 native mode, use DS:EDX instead of DS:DX.


Function  5Ch   Lock/Unlock File Access   (DOS 3.0+)
entry   AH      5Ch
        AL      00h     to lock file region
                01h     to unlock file region
        BX      file handle
        CX:DX   4-byte starting offset from beginning of file of region
                to lock
        SI:DI   4-byte integer, high/low size of region to lock
                (in bytes)
return  CF      clear   successful
                set     AX      error code (01h, 06h, 21h, 24h)
note 1) Unlock all files before exiting or undefined results may occur.
        Programs using file locking should trap int 23h (Control-C
        Handler Address) and int 24h (Critical Error Handler Address)
        and unlock files before returning to the caller.
     2) Programs spawned with EXEC inherit all the parent's file
        handles but not the file locks.
     3) This call is explicitly supported in the OS/2 1.x DOS
        Compatibility Box.
     4) You may lock an entire file, any part of a file, or several
        parts of the same file.  For example, it would be more
        efficient to lock an area in a database containing a single
        record than to lock the entire file.  If two adjacent sections
        of a file are locked separately, they  must be unlocked
        separately - you cannot change the lock pointers and use a
        single unlock call.
     5) You should lock only as much of a file as you need and keep
        the lock only for as long as necessary.  Should a file need to
        be shared and updated often, continual locking and unlocking
        can slow file access detectably.
     6) This call returns error 01h if SHARE is not loaded.
     7) Locked files must be unlocked before the program terminates,
        or the result is undefined.
     8) A transaction-oriented algorithm for using locking is
        recommended.  In effect, assert lock, read data, change data,
        remove lock.  An application should release its lock when a
        transaction is complete.
     9) Locking past end-of-file does not cause an error.
    10) Locking a portion of a file with fn 5Ch denies all other
        processes both read and write access to the locked region.
    11) For PC-MOS/386 native mode, use ECX and EDX instead of CX:DX.


Function  5Dh   Multifunction, DOS  Internal - partial (DOS 3.x+)
entry   AH      5Dh
        AL      subfunction
                00h     Indirect Function Call
                        DS:DX   pointer to buffer containing register
                                values AX, BX, CX, DX, SI, DI, DS, ES
                                for a call to int 21h
                        return  as appropriate for function being called
                        note    Does not check AH.  Out of range values
                                will crash the system.
                01h     SYNC?  (DOS 3.1+)
                        parameters unknown
                        note 1) Does something to each disk file in the
                                system.  File Table which has been
                                written to.
                             2) If remote file, calls int 2Fh/fn1107h.
                             3) Seems to update the time stamp of all
                                open files which have been written to.
                02h     SHARE.EXE? (DOS 3.1+)
                        note    Error unless SHARE is loaded
                                (calls fn 52h+4Ah)
                03h     SHARE.EXE? (DOS 3.1+)
                        note    Error unless SHARE is loaded
                                (calls fn 52h+4Ah)
                04h     SHARE.EXE functions? (DOS 3.1+)
                        note    Error unless SHARE is loaded
                                (calls fn 52h+4Ah)
                05h     Network functions? (DOS 3.1+)
                        DS:DX   pointer to buffer (see AX=0Ah), only
                                fields at offset 12h, 14h used
                        note    Error unless SHARE is loaded
                                (calls fn 52h+5Ah)
                06h     Get Address of Critical Error Flag
                        DS:DX   pointer to buffer (see 0Ah), only
                                fields at offset 12h, 14h used
                        return  CX      unknown value
                                DX      unknown value
                                DS:SI   pointer to critical error flag
                        notes   This call does a lot of other work in
                                addition to returning the pointer.
                                Setting the CritErr flag allows use of
                                functions 50h/51h from int 28h under
                                DOS 2.x by forcing use of correct stack.
    (LANtastic) 07h     Return Redirected Printer mode
                        return  DL      00      output is combined
                                        01h     output is separated
                        note    The current printer mode (either
                                printer output combined or separated)
                                is returned.
    (LANtastic) 08h     Set Redirected Printer Mode
                        DL      00h     set redirected output to be
                                        combined
                                01h     set redirected output to be
                                        separated.  (implicitly starts
                                        a new print job)
                        return  none
                        note 1) The current printer mode (either printer
                                output combined or separated) can be set.
                             2) may be used by COMMAND.COM
    (LANtastic) 09h     Flush Printer Output
                        return  none
                        note 1) Printer output is flushed and a new
                                print job is started.  If no output
                                exists to be flushed then this function
                                has no effect.
                             2) may be used by COMMAND.COM
                09h     unknown - may be used by COMMAND.COM
(DOS 5, doc'd)  0Ah     Set Extended Error Information
                        DS:DX   address of 11-word error information
                                table:
                                format of error information table:
                                offset  size    description
                                00h    word    value that next call to
                                               fn 59h  will return in AX
                                02h    word    value that next call to
                                               fn 59h will return in BX
                                04h    word    CX
                                06h    word    DX
                                08h    word    SI
                                0Ah    word    DI
                                0Ch    word    DS
                                0Eh    word    ES
                                10h    word    reserved (set to 0)
                                12h    word    user (computer) ID,
                                               0=local
                                14h    word    program ID, 0=local
                                               program
                return  none
return   DS:SI   (for 06h) pointer to critical error flag
note 1) Function 0Ah; DOS 3.1+.
     2) Function 06h; setting CritErr flag allows use of functions
        50h/51h from int 28h under DOS 2.x by forcing the use of the
        correct stack.
     3) Functions 07h, 08h, 09h are identical in DOS 3.1 and call int
        2Fh fn 1125h.


Function  5Eh   Network Printer
                DOS 3.1+ with Networks software
                PC-MOS/386 through 4.1 has limited support for functions
                5Eh through 5F04h.  The MOS NETBIOS driver must be
                loaded.  A device name is returned for 5E00h and an
                "end of list" status is returned for other functions.
entry   AH      5Eh
        AL      00h     Get Machine Name
                        DS:DX   pointer to 16-byte buffer for ASCIIZ name
                        return  CH      00h     if name not defined
                                        <>0     name is defined
                                CL      00h     name not set
                                        <>0     NetBIOS name number
                                DS:DX   pointer to ASCIIZ name if CH <>0
                                CF      set on error
                                        AX      error code (01h)
                        note 1) The ASCIIZ name is a 15 byte string
                                padded to length with zeroes.
                             2) The NETBIOS number in CL and name at
                                DS:DX are valid only if the value
                                returned in CH is <>0.
                01h     Set Machine Name
                        CH      00h     undefine name
                                <>0     define name
                        CL      name number
                        DS:DX   pointer to ASCIIZ name
                02h     Set Printer Control String
                        BX      redirection list index
                        CX      length of setup string (max 64 bytes)
                        DS:SI   pointer to ASCIIZ setup string buffer
                        return: CF      set on error
                                        AX      error code (01h)
                        note 1) Set printer setup sets a setup string
                                to be sent to the network printer
                                whenever a file is queued to the
                                printer.
                             2) LANtastic LANOS does not process this
                                request since printer setup strings are
                                controlled by the system administrator
                                using the NET_MGR program.  The system
                                call does not return an error, however.
                03h     Get Printer Control String
                        BX      redirection list index
                        ES:DI   pointer to 64-byte string buffer
                        return  CX      length of setup string
                                        (maximum 64 bytes)
                        return: CF      set on error
                                        AX      error code (01h)
                        note 1) This call will return the setup string
                                set with fn 02h above.
                             2) LANtastic LANOS does not process this
                                request since printer setup strings are
                                controlled by the system administer
                                using the NET_MGR program.  The system
                                call does not return an error, however,
                                and the length of the setup string (CX)
                                is 0.
                04h     DOS 3.1+ +Microsoft Networks - unknown
                        note    Calls int 2F/AX=111Fh with 5E04h on
                                stack.
                05h     DOS 3.1+ +Microsoft Networks - unknown
                        note    Calls int 2F/AX=111Fh with 5E05h on
                                stack.
                06h     DOS 3.1+ +Microsoft Networks - unknown
                        note    Calls int 2F/AX=111Fh with 5E06h on
                                stack.
return  CF      clear   successful
                set     error
                        AX      error code (01h for all listed subfns)
note 1) Used in IBM's & Microsoft's Network programs.
     2) Partial documentation in Fall 1985 Byte, in Advanced MS-DOS,
        in MS-DOS Encyclopedia, LANtastic Programmer's Manual.
     3) These services require that the network software be installed.
     4) SHARE must be loaded or results can be unpredictable on 00h,
        or fail with 02h or 03h.
     5) The redirection entry index is an index into a table that
        identifies the printer as a device on the network.


Function  5Fh   Network Redirection
                (DOS 3.1+ and Microsoft Networks)
entry   AH      5Fh
        AL     *00h     Unknown
               *01h     Unknown
                02h     Get Redirection List Entry
                        BX      redirection entry index.  Index 0
                                specifies the first entry
                        DS:SI   pointer to 16 byte area which will
                                receive the ASCIIZ local device name
                        ES:DI   pointer to 128 byte buffer for the
                                ASCIIZ network device name
                        return  CF      set on error
                                        AX      error code (01h, 12h)
                                BH      device status flag
                                        (bit 0=0 if valid)
                                        (bit 0=1 if invalid)
                                        (bits 1-7 reserved)
                                BL      device type
                                        03      printer device
                                        04      drive device (file)
                                CX      value stored by fn 03h call.
                                        Should be 0 for compatiblity
                                        with LAN OS
                                DS:SI   pointer to 16 byte ASCIIZ local
                                        device name
                                ES:DI   pointer to 128 byte ASCIIZ
                                        network name
                        note 1) DX and BP are destroyed by this call.
                             2) This call returns information about a
                                single redirected device (see fn 03h).
                                This fn may be used to scan the list of
                                redirected devices.
                03h     Redirect Device - Make Assign List Entry
                        Redirects a workstation drive or device to a
                        server directory or device.
                        AX      error code if error
                        BL      device type
                                03h     printer device
                                04h     file device
                        CX      stored parameter value (0 for
                                compatibility with IBM PC Network
                                program and LANtastic)
                        DS:SI   pointer to 16-byte ASCIIZ source
                                device name for printer: PRN, LPT1,
                                LPT2, LPT3
                        ES:DI   pointer to destination 128-byte
                                ASCIIZ network path and ASCIIZ
                                password  (e.g.,'\\machine_name\path',
                                0,'password',0)
                        return  CF      set on error
                                        AX      error code (01h, 03h,
                                                05h, 08h, 0Fh, 12h)
                        note 1) Redirect device allows you to connect
                                local devices to network paths.  For
                                example you can connect your LPT1 to
                                a printer on another node.  References
                                to LPT1 are routed to the network.
                             2) If the password is omitted, the
                                pathname must be followed by two null
                                bytes.
                             3) For printer redirection, MS-Network
                                intercepts int 17h.  When redirection
                                is canceled, all printing is sent to
                                the first local printer. (LPT1)
                04h     Cancel Redirection Assignment
                        DS:SI   pointer to ASCIIZ device name or
                                network path to be canceled
                        return  CF      set on error
                                        AX      error code (01h, 03h,
                                                05h, 08h, 0Fh, 12h)
                        note 1) Cancel device redirection allows you
                                to remove a device redirection so that
                                the device is restored to its former
                                state.
return  CF      clear   successful
                set     if error
                        AX      error code
                                (fn 02h) 01h, 12h
                                (fn 03h) 01h, 03h, 05h, 08h
                                (fn 04h) 01h, 0Fh
note 1) Used in IBM's Network program and Microsoft MS-Networks.
     2) Partial documentation in Fall 1985 Byte, in Advanced MS-DOS,
        in the MS-DOS Encyclopedia, LANtastic Programmer's Manual.
     3) These services require that the network software be installed.
     4) SHARE must be loaded or the call will fail.
     5) The network device name requires a password.
     6) Only printer and disk devices are supported for redirection.
        STDAUX, STDIN, STDOUT, and STERR are not supported by the
        Microsoft/IBM specification.   Other companies have managed
        to do this with various nonstandard additions to the API.


Function  60h   Parse pathname  (TRUENAME)     (DOS 3.0+)
 *              Perform name processing on a string (internal to DOS)
entry   AH      60h
        DS:SI   pointer to ASCIIZ source string (null terminated)
        ES:DI   pointer to destination 80 byte ASCIIZ string buffer
return  ES:DI   buffer filled with qualified name in form (drive):(path)
        CF      set     error
                        AX      error code (02h, 03h)
                clear   no error
note 1) Documented in Zenith 3.05 Technical Reference.
     2) All name processing is performed on the input string; string
        substitution is performed on the components, current drive/
        directories are prepended, .  and ..  are removed.  Under most
        networking software, the drive letter is replaced with the node
        name, i.e. returns pathname like \\SERVER\UTILS\TEST.TXT
        instead of F:\UTILS\TEST.TXT.
     3) Example: If current drive/directory is C:\TEST,  MYFILE.X is
        translated to C:\TEST\MYFILE.X; ..\SOURCE\SAMPLE.ASM is
        translated to C:\SOURCE\SAMPLE.ASM.
     4) It is the caller's responsibility to make sure DS:SI does not
        point to a null string.  If it does, SI is incremented, a null
        byte is stored at ES:DI, and the routine returns.
     5) Used by CHKDSK, at least in DOS 3.3, and DOS 3.x.
     6) If path string is on a JOINed drive, the returned name is the
        one that would be needed if the drive were not JOINed;
        similarly for a SUBSTed drive letter.  Because of this, it is
        possible to get a qualified name that is not legal with the
        current combination of SUBSTs and JOINs.
     7) Used by DOS 4.0 SHELLC.EXE.
     8) This call has been discovered in DOS versions as early as
        2.11.
     9) In Novell Netware 2.1x, this call is not supported when the
        8th bit (high ASCII) is set on any letter in the file or
        pathname, and when the file being inquired about is on a
        remote drive.  This causes problems with machines using
        foreign code pages.
    10) You need DPMI services to use this call under MS Windows 3.x.
        There is an example in PC Magazine, 25 June 1991, p. 389
        called TRUENAME.C.


Function  61h   undocumented - (DOS 3.0)
 *              Internal to DOS - parameters not known
entry   AH      61h
return  AL      00h
note    Supposedly documented in Zenith DOS 3.05 Technical Reference.


Function  62h   Get Program Segment Prefix (PSP) (DOS 3.0+)
entry   AH      62h
return  BX      segment address of PSP
note    Under DOS 3.x+, this function does not use any of DOS' internal
        stacks and is thus fully reentrant.


Function  63h   Get Lead Byte Table  (MS-DOS 2.25 only)
                Added in DOS 2.25 for additional foreign character
                set support.
entry   AH      63h
        AL      subfunction
                00h     get system lead byte table address
                01h     set/clear interim console flag
                        DL      0000h   to clear interim console flag
                                0001h   to set interim console flag
                02h     get interim console flag
return  DS:SI   pointer to lead byte table (AL = 00h)
        DL      interim console flag (AL = 02h)
note 1) Function 63h destroys all registers except SS:SP on return.
        To avoid saving registers repeatedly, a process can copy the
        table or save the pointer for later use.
     2) Works ONLY in MS-DOS 2.25!
     3) Note fn 63h does not return errors in AL or CF.
     4) Original support was for Kanji (Japanese) and Hanegul (Korean)
        only.
     5) The lead byte table contains pairs of bytes that represent the
        inclusive boundary values for the lead bytes of the specified
        alphabet.  Because of the way bytes are ordered by the 8086
        microprocessor family, the values must be read as byte values,
        not as word values.
     6) If the interim console flag is set (DL=0001h) by a program
        through a call to fn 63h, the following int 21h functions
        return interim character information on request: 07h, 08h,
        0Bh, 0Ch.


Function  64h   Undocumented - Used internally by DOS
entry   AH      64h
return  unknown
note 1) DOS 3.2+ internal function of some type? May be a network
        function.
     2) In DOS 3.31 it seems that when you load AL with a nonzero
        number, int 28h will get called more often.  AL=0 resets back
        to the default "boot up" state.


Function  65h   Get Extended Country Information (DOS 3.3+)
                Returns information about the selected country
                formats, code pages, and conversion tables
entry   AH      65h
        AL      information ID code
                01h     get general internationalization info
                02h     get pointer to uppercase table (130 bytes max)
                        (maps chars 80h-0FFh to their uppercase
                         equivalents, if any; used mainly to map
                         accented or other vowels to the corresponding
                         plain vowels)
                03h     unknown
                04h     get pointer to filename uppercase table (130
                        bytes max) (similar to table for AL = 02h)
   (DOS 5.0+)   05h     get filename character table - specifies which
                        chars must not be used in filenames.
                06h     get pointer to collating sequence table (258
                        bytes max)  (maps uppercase, lower, and
                        accented chars together for sorting)
                07h     get pointer to double-byte character set table
   (DOS 5.0+)   20h     convert character - converts character in DL to
                        uppercase using the current uppercase table
   (DOS 5.0+)   21h     convert string - converts string in DS:DX,
                        length CX to uppercase using current uppercase
                        table, returns in DS:DX
   (DOS 5.0+)   22h     convert ASCIIZ string - converts string in
                        DS:DX to uppercase using current uppercase
                        table, returns in DS:DX

        BX      code page (0FFFFh = current code page)
        CX      amount of information to be returned (minimum 5)
        DX      target country ID (0FFFFh = default current country)
        ES:DI   segment/offset pointer to country information buffer
return  CF      set on error
                AX      error code (02h)
                        otherwise:
        CX      size of country information returned
        ES:DI   pointer to country information:
              offset   length   description
                00h   1 byte    info ID  (for all following buffers)
        If information ID code <> 1:
                01h     dword  pointer to information
        If information ID code = 1:
                01h     word    length of remainder of buffer (<= 38)
                03h     word    country ID
                05h     word    code page number
                07h  34 bytes   same as for int 21h function 38h
        If information ID code = 2:
                01h     word    table size
                05h     dword   pointer to uppercase table
            128 bytes uppercase equivalents (if any) of chars 80h-0FFh
        If information ID code = 4:
                01h     word    table size
                05h     dword   pointer to collating table
            256 bytes   values used to sort characters 00h-0FFh
        If information ID code = 6:
                01h     word    table size
                05h     dword   pointer to filename uppercase table.
                                This table starts with a 2-byte length
                                field, then 256 ASCII values placed in
                                order
            128 bytes   uppercase equivalents (if any) of chars
                        80h-0FFh
        If information ID code = 7: (DOS 4.0)
                unknown
note 1) For AL=02h, 04h, or 06h, the first two bytes of the table
        give its length and then a 128 byte table of uppercase ASCII
        characters for 02h or 04h and a pointer to the collating
        sequence for 06h.
     2) The country code and code page must match.  If not, error 02h
        is in AX.
     3) If more information is provided than was requested by CX, it
        is truncated without generating an error.
     4) Country information:
        bytes   description
        01h     value of AL (01h)
        02h,03h size (max = 38)
        04h,05h country code
        06h,07h code page
        08h,09h date format
        0Ah,0Eh currency symbol
        0Fh,10h thousands separator
        11h,12h decimal separator
        13h,14h date separator
        15h,16h time separator
        17h     currency format flags
        18h     digits in currency
        19h     time format
        20h-22h monocase routine entry point
        23h-24h data list separator
        25h-29h zeros


Function  66h   Get/Set Global Code Page Table (DOS 3.3+)
                Query/reset code page defaults
entry   AH      66h
        AL      00h     Get Global Code Page
                01h     Set Global Page
                        BX      active code page
                        DX      system code page
                                (active page at boot time)
return  CF      clear  successful
                set    AX       error code (unknown)
        if 00h         BX       active code page
                       DX       system code page
                                (active page at boot time)
note 1) BX = active code page: 437 = US, 860 = Portugal, 863 = Canada
                               (French) 865 = Norway/Denmark, 850 =
                               multilingual
     2) MS 5.0 TR shows fns as 01h and 02h.


Function  67h   Set Handle Count  (DOS 3.3+)
                Supports more than 20 open files per process
entry   AH      67h
        BX      desired number of handles (20 to 65,535)
return  CF      clear   if OK
                set     if error
                AX      error code (unknown)
note 1) This function changes the 20-byte handle table pointer in the
        PSP to point to a new, larger handle table elsewhere in memory.
     2) The memory the 67h call allocates is taken from the normal
        DOS pool, and is reclaimed on process termination, so nothing
        is lost.
     3) When calling this function you must release enough memory for
        DOS to contain the extended handle list.
     4) If the requested number of handles is less than 20 nothing
        is done.
     5) Early versions of PC-MOS/386 v4.1 did not properly deallocate
        memory when the number of handles was reduced.
     6) The error value returned in AX is not documented in the IBM
        4.0 DOS Technical Reference, the MSDOS Encyclopedia or the MS
        5.0 TR.
     7) For PC-MOS/386 4.x, the only error code is 08h (insufficient
        memory).


Function  68h   Commit File (DOS 3.3+)
                Write all buffered data to disk
entry   AH      68h
        BX      file handle of previously opened file
return  CF      clear   successful
                set     on error
                        AX      error code (unknown)
note 1) Faster and more secure method of closing a file in a network
        than current close commands.
     2) This is effectively the same as DUPing the handle for a file
        and then closing the new one, except that this call won't fail
        if the system is out of handles.
     3) The file's buffers are flushed and its directory and FAT
        entries are updated.
     4) The error value returned in AX is not documented in the IBM
        4.0 DOS Technical Reference, the MSDOS Encyclopedia or the MS
        5.0 TR.
     5) For PC-MOS/386 4.x, the error codes are 06h (invalid handle)
        and 22h (wrong disk).


Function  69h   Disk Serial Number  DOS 4.0+ (US versions)
                Handles "Volume Serial Number" on disks formatted with
                DOS 4.0+
entry   AH      69h     Get Volume Serial Number
        AL      00h     get serial number
                01h     set serial number
        BL      drive (0=default, 1=A, etc)
        DS:DX   pointer to disk information table
return  CF      set     on error
                AX      error code
                clear   if successful
                AH      destroyed
                AL      (fn 00h) buffer filled with appropriate values
                                 from extended BPB
                        (fn 01h) extended BPB on disk set to values
                                 from buffer
        DS:DX   disk information table. Format:
                offset  size    description
                00h     word    unknown (zeroes on my system)
                02h    dword    disk serial number (binary)
                06h 11 bytes    volume label or "NO NAME    " if none
                11h  8 bytes    FAT type - string "FAT12   " or
                                "FAT16   "
note 1) The FAT type field refers to the number of bits per directory
        entry.
     2) Does not generate a critical error; all errors are returned
        in AX.
     3) Error 0005h given if no extended BPB on disk.
     4) Does not work on network drives (error 0001h).
     5) Buffer after first two bytes is exact copy of bytes 27h thru
        3Dh of extended BPB on disk.
     6) FORMAT (in DOS 5.0) does not call int 21h/fn 69h. (Get/Set
        Volume Serial Number)  For floppy disks, it calls int 21h/fn
        440Dh, CH=08, CL=46 [Set Media ID], and passes the serial
        number.   (from dsparks, BIX)


Function  6Ah   Unknown  (DOS 4.0?)


Function  6Bh   Unknown  (DOS 4.0?)


Function  6Ch   Extended Open/Create  DOS 4.0+ (US)
                Combines functions available with Open, Create, Create
                New, and Commit File
entry   AH      6Ch
        AL      00h  reserved  [which means there might be other
                                subfunctions?]
        BX      mode    format  0WF0 0000 ISSS 0AAA
                                AAA is access code (read, write,
                                                    read/write)
                                SSS is sharing mode
                                I       0       pass handle to child
                                        1       no inherit [interesting!]
                                F       0       use int 24h for errors
                                        1       disable int 24h for all
                                                I/O on this handle; use
                                                own error routine
                                W       0       no commit
                                        1       auto commit on all writes
        CX      create attribute
           bits 0       read only
                1       hidden
                2       system
                3       volume label
                4       reserved
                5       archive
                6-15    reserved
        DH      00h (reserved)
        DL      action if file exists/does not exists
           bits 0-3     action if file exists
                        0000    fail
                        0001    open
                        0010    replace/open
                4-7     action if file does not exist
                        0000    fail
                        0001    create
        DS:SI   pointer to ASCIIZ file name
return  CF      set on error
                AX      error code (unknown)
                clear
                AX      file handle
                CX      action taken
                        01h     file opened
                        02h     file created/opened
                        03h     file replaced/opened
note    When APPEND is installed, if DX=xx1x it looks only in current
        directory, if DX=xx0x it will search the full append path.
        DX is called the open flag and gives what action to take if
        the file exists or does not exist.


Function  89h   undocumented - DOS_Sleep
 *              Not documented by Microsoft
entry   AH      89h
return  unknown
note 1) Function included in Microsoft C 4.0 startup code MSDOS.INC.
     2) Debugging shows that the first instruction on entry to DOS
        compares AH with 64h (at least in DOS 3.2) and aborts the
        call if AH > 64.
     3) Possibly used in European MSDOS 4.0?



Aftermarket Application Installed Function Calls:

Novell Netware 2.11:
        Novell no longer recommends the int 21h method for invoking
        the Netware functions.  Int 21h will be supported
        indefinitely, but the new net API calls for addressing the
        software through the Multiplex Interrupt (2Fh).  You may
        address the API through int 2Fh in the same manner as int 21h;
        only the interrupt number is different.

        Novell API calls are referenced in Chapter 13.  Most functions
        from 0B6h through 0F9h are preempted by NetWare; if your
        software uses any of these calls for another purpose it will
        likely not run under NetWare.

NOTE:   Novell (and most others') network software and SoftLogic's
        DoubleDOS conflict on the following int 21h functions
        0EAh-0EEh.  Netware must use int 2Fh functions instead of 21h
        functions if DoubleDOS will be used on the network.


Functions 0E0h - 0F6h: used by AI Architects/Ergo Computing
          DOS extender


Functions 0E4h - 0EEh: used by DoubleDOS task switcher


Function  0FEh  Enable Reader v4.0 API
                Enable Reader is a speech synthesizer interface for
                the blind.  It will save all registers except the
                flags, where the zero flag will be set if the call was
                not one of the available Enable Reader 4.0 functions.

entry   AH      0FEh
        AL      01h     Driver Output
        DL      Character To Output
return  none
note    The Driver Output call will output the character in DL through
        the Enable Reader driver, without turning off the Help
        Functions.  It will be filtered by the ASCII sort routine in
        the Enable Reader Driver, and if it is punctuation it will be
        sent according to the level of punctuation selected.

entry   AH      0FEh
        AL      02h     Direct Output
        DL      ASCII character to output
return  none
note    The Direct Output call will send the byte in DL directly to
        the synthesizer, bypassing the Enable Reader driver and ASCII
        filter sort routine.

entry   AH      0FEh
        AL      03h     Message Output
        DL      character to output
return  none
note    The Message Output call allows character filtration and
        punctuation, but turns off the Help functions in the driver.

entry   AH      0FEh
        AL      41h (ASCII A) Auto
return  none
note    The AUTO call turns the Spell mode on or off.

entry   AH      0FEh
        AL      4Eh (ASCII N) Punctuation Levels
return  none
note    This call selects one of five Punctuation Character Sets that
        can be accessed.  The fifth level is "Output Punctuation",
        which allows all ASCII characters to be sent to the synthesizer
        without being filtered or translated into the Enable Reader 4.0
        Punctuation Words.

entry   AH      0FEh
        AL      4Fh (ASCII O) Help Word Levels
return  none
note    The Help call selects one of four levels of help. By selecting
        "No Help" at the start of your program, you can use the other
        Help Functions without having them speak their prompts.

entry   AH      0FEh
        AL      51h (ASCII Q) Quit
return  none
note    This call will stop the synthesizer from speaking.

entry   AH      0FEh
        AL      5Ah (ASCII Z) Letter-to-Word Translator
return  none
note    This call turns the Letter-To-Word Translator on or off.
  For a discussion on the TRANSLATOR Function, read

entry   AH      0FEh
        AL      54h (ASCII T) Upper Case Identification
return  none
note    This call toggles identification of uppercase letters.

entry   AH      0FEh
        AL      56h (ASCII V) Video Output
return  none
note    The Video Output call toggles the output to the synthesizer
        from data that is printed on the video screen.


Function  0FFh  CED   (CJ Dunford's DOS macro and command-line editor)
                CED installable commands
entry   AH      0FFh
        AL      00h     Add Installable Command
                01h     Remove Installable Command
                02h     Reserved, may be used to test for CED
                        installation
        BL      mode byte
            bit 0       callable from DOS prompt
                1       callable from application
                2-7     not used in public domain CED
        DS:SI   pointer to CR-terminated command name
        ES:DI   pointer to far routine entry point
return  CF      set on error
        AX      01h     invalid function
                02h     command not found (subfunction 1 only)
                08h     insufficient memory (subfunction 0 only)
                0Eh     bad data (subfunction 0 only)
        AH      0FFh    if CED not installed
note 1) When PCED returns, AX and the flags are changed.  Other
        registers are preserved, except as noted.