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CHAPTER 6 THE 86 INSTRUCTION SET 6-1 Effective Addresses Most memory data accessing in the 86 family is accomplished via the mechanism of the effective address. Wherever an effective address specifier "eb", "ew" or "ed" appears in the list of 8086 instructions, you may use a wide variety of actual operands in that instruction. These include general registers, memory variables, and a variety of indexed memory quantities. GENERAL REGISTERS: Wherever an "ew" appears, you can use any of the 16-bit registers AX,BX,CX,DX,SI,DI,SP, or BP. Wherever an "eb" appears, you can use any of the 8-bit registers AL,BL,CL,DL,AH,BH,CH, or DH. For example, the "ADD ew,rw" form subsumes the 16-bit register-to-register adds; for example, ADD AX,BX; ADD SI,BP; ADD SP,AX. MEMORY VARIABLES: Wherever an "ew" appears, you can use a word- memory-variable. Wherever an "eb" appears, you can use a byte-memory-variable. Variables are typically declared in the DATA segment, using a DW declaration for a word-variable, or a DB declaration for a byte-variable. For example, you can declare variables: DATA_PTR DW ? ESC_CHAR DB ? Later, you can load or store these variables: MOV SI,DATA_PTR ; load DATA_PTR into SI for use LODSW ; fetch the word pointed to by DATA_PTR MOV DATA_PTR,SI ; store the value incremented by the LODSW MOV BL,ESC_CHAR ; load the byte-variable ESC_CHAR Alternatively, you can address specific unnamed memory locations by enclosing the location value in square brackets; for example, MOV AL,[02000] ; load contents of location 02000 into AL Note that A86 discerned from context (loading into AL) that a BYTE at 02000 was intended. Sometimes this is impossible, and you must specify byte or word: INC B[02000] ; increment the byte at location 02000 MOV W[02000],0 ; set the WORD at location 02000 to zero 6-2 INDEXED MEMORY: The 86 supports the use of certain registers as base pointers and index registers into memory. BX and BP are the base registers; SI and DI are the index registers. You may combine at most one base register, at most one index register, and a constant number into a run-time pointer that determines the location of the effective address memory to be used in the instruction. These can be given explicitly, by enclosing the index registers in brackets: MOV AX,[BX] MOV CX,W[SI+17] MOV AX,[BX+BP+5] MOV AX,[BX][BP]5 ; another way to write the same instr. Or, indexing can be accomplished by declaring variables in a based structure: STRUC [BP] ; NOTE based structures are unique to A86! BP_SAVE DW ? RET_ADDR DW ? PARM1 DW ? PARM2 DW ? ENDS INC PARM1 ; equivalent to INC W[BP+4] Finally, indexing can be done by mixing explicit components with declared ones: TABLE DB 4,2,1,3,5 MOV AL,TABLE[BX] Segmentation and Effective Addresses The 86 family has four segment registers, CS, DS, ES, and SS, used to address memory. Each segment register points to 64K bytes of memory within the 1-megabyte memory space of the 86. (The start of the 64K is calculated by multiplying the segment register value by 16; i.e., by shifting the value left by one hex digit.) If your program's code, data and stack areas can all fit in the same 64K bytes, you can leave all the segment registers set to the same value. In that case, you won't have to think about segment registers--no matter which one is used to address memory, you'll still get the same 64K. If your program needs more than 64K, you must point one or more segment registers to other parts of the memory space. In this case, you must take care that your memory references use the segment registers you intended. Each effective address memory access has a default segment register, to be used if you do not explicitly specify which segment register you wish. For most effective addresses, the default segment register is DS. The exceptions are those effective addresses that use the BP register for indexing. All BP-indexed memory references have a default of SS. (This is because BP is intended to be used for addressing local variables, stored on the stack.) 6-3 If you wish your memory access to use a different segment register, you provide a segment-override byte before the instruction containing the effective address operand. In the A86 language, you code the override by giving the name of the segment register you wish before the instruction mnemonic. For example, suppose you want to load the AL register with the memory byte pointed to by BX. If you code MOV AL,[BX], the DS register will be used to determine which 64K segment BX is pointing to. If you want the byte to come from the CS-segment instead, you code CS MOV AL,[BX]. Be aware that the segment-override byte has effect only upon the single instruction that follows it. If you have a sequence of instructions requiring overrides, you must give an override byte before every instruction in the sequence. (In that case, you may wish to consider changing the value of the default segment register for the duration of the sequence.) NOTE: This method for providing segment-overrides is unique to the A86 assembler! The assemblers provided by Intel and IBM (MS- DOS) attempt to figure out segment allocation for you, and plug in segment-override bytes "behind your back". In order to do this, those assemblers require you to inform them which variables and structures are pointed to by which segment registers. That is what the ASSUME directive in those assemblers is all about. I wrote Intel's first 86 assembler, ASM86, so I have been watching the situation since day one. Over the years, I have concluded that the ASSUME-mechanism creates far, far more confusion that it solves. So I scrapped it; and the result is an assembler with far less red tape. But if your program needs more than 64K, you do have to manage those segment registers yourself; so take care! Effective Use of Effective Addresses Remember that all of the common instructions of the 86 family allow effective addresses as operands. (The only major functions that don't are the AL/AX specific ones: multiply, divide, and input/output). This means that you don't have to funnel many through AL or AX just to do something with them. You can perform all the common arithmetic, PUSH/POP, and MOVes from any general register to any general register; from any memory location (indexed if you like) to any register; and (this is most often overlooked) from any register TO memory. The only thing you can't do in general is memory-to-memory. Among the more common operations that inexperienced 86 programmers overlook are: * setting memory variables to immediate values * testing memory variables, and comparing them to constants * preserving memory variables by PUSHing and POPping them * incrementing and decrementing memory variables * adding into memory variables 6-4 Encoding of Effective Addresses Unless you are concerned with the nitty-gritty details of 86 instruction encoding, you don't need to read this section. Every instruction with an effective address has an encoded byte, known as the effective address byte, following the 1-byte opcode for the instruction. (For obscure reasons, Intel calls this byte the ModRM byte.) If the effective address is a memory variable, or an indexed memory location with a non-zero constant offset, then the effective address byte will be immediately followed by the offset amount. Amounts in the range -128 to +127 are given by a single signed byte, denoted by "d8" in the table below. Amounts requiring a 2-byte representation are denoted by "d16" in the table below. As with all 16-bit memory quantities in the 86 family, the word is stored with the least significant byte FIRST. The following table of effective address byte values is organized into 32 rows and 8 columns. The 32 rows give the possible values for the effective address operand: 8 registers and 24 memory indexing modes. A 25th indexing mode, [BP] with zero displacement, has been pre-empted by the simple-memory-variable case. If you code [BP] with no displacement, you will get [BP]+d8, with a d8-value of zero. The 8 columns of the table reflect further information given by the effective address byte. Usually, this is the identity of the other (always a register) operand of a 2-operand instruction. Those instructions are identified by a "/r" following the opcode byte in the instruction list. Sometimes, the information given supplements the opcode byte in identifying the instruction itself. Those instructions are identified by a "/" followed by a digit from 0 through 7. The digit tells which of the 8 columns you should use to find the effective address byte. For example, suppose you have a perverse wish to know the precise bytes encoded by the instruction SUB B[BX+17],100. This instruction subtracts an immediate quantity, 100, from an effective address quantity, B[BX+17]. By consulting the instruction list, you find the general form SUB eb,ib. The opcode bytes given there are 80 /5 ib. The "/5" denotes an effective-address byte, whose value will be taken from column 5 of the table below. The offset 17 decimal, which is 11 hex, will fit in a single "d8" byte, so we take our value from the "[BX] + d8" row. The table tells us that the effective address byte is 6F. Immediately following the 6F is the offset, 11 hex. Following that is the ib-value of 100 decimal, which is 64 hex. So the bytes generated by SUB B[BX+17],100 are 80 6F 11 64. 6-5 Table of Effective Address byte values s = ES CS SS DS rb = AL CL DL BL AH CH DH BH rw = AX CX DX BX SP BP SI DI digit= 0 1 2 3 4 5 6 7 Effective EA byte address: values: 00 08 10 18 20 28 30 38 [BX + SI] 01 09 11 19 21 29 31 39 [BX + DI] 02 0A 12 1A 22 2A 32 3A [BP + SI] 03 0B 13 1B 23 2B 33 3B [BP + DI] 04 0C 14 1C 24 2C 34 3C [SI] 05 0D 15 1D 25 2D 35 3D [DI] 06 0E 16 1E 26 2E 36 3E d16 (simple var) 07 0F 17 1F 27 2F 37 3F [BX] 40 48 50 58 60 68 70 78 [BX + SI] + d8 41 49 51 59 61 69 71 79 [BX + DI] + d8 42 4A 52 5A 62 6A 72 7A [BP + SI] + d8 43 4B 53 5B 63 6B 73 7B [BP + DI] + d8 44 4C 54 5C 64 6C 74 7C [SI] + d8 45 4D 55 5D 65 6D 75 7D [DI] + d8 46 4E 56 5E 66 6E 76 7E [BP] + d8 47 4F 57 5F 67 6F 77 7F [BX] + d8 80 88 90 98 A0 A8 B0 B8 [BX + SI] + d16 81 89 91 99 A1 A9 B1 B9 [BX + DI] + d16 82 8A 92 9A A2 AA B2 BA [BP + SI] + d16 83 8B 93 9B A3 AB B3 BB [BP + DI] + d16 84 8C 94 9C A4 AC B4 BC [SI] + d16 85 8D 95 9D A5 AD B5 BD [DI] + d16 86 8E 96 9E A6 AE B6 BE [BP] + d16 87 8F 97 9F A7 AF B7 BF [BX] + d16 C0 C8 D0 D8 E0 E8 F0 F8 ew=AX eb=AL C1 C9 D1 D9 E1 E9 F1 F9 ew=CX eb=CL C2 CA D2 DA E2 EA F2 FA ew=DX eb=DL C3 CB D3 DB E3 EB F3 FB ew=BX eb=BL C4 CC D4 DC E4 EC F4 FC ew=SP eb=AH C5 CD D5 DD E5 ED F5 FD ew=BP eb=CH C6 CE D6 DE E6 EE F6 FE ew=SI eb=DH C7 CF D7 DF E7 EF F7 FF ew=DI eb=BH d8 denotes an 8-bit displacement following the EA byte, to be sign-extended and added to the index. d16 denotes a 16-bit displacement following the EA byte, to be added to the index. Default segment register is SS for effective addresses containing a BP index; DS for other memory effective addresses. 6-6 How to Read the Instruction Set Chart The chart below summarizes the machine instructions you can program with A86. In order to use the chart, you need to learn the meanings of the specifiers (each given by 2 lower-case letters), that follow most of the instruction mnemonics. Each specifier indicates the type of operand (register byte, immediate word, etc.) that follows the mnemonic to produce the given opcodes. "c" means the operand is a code label, pointing to a part of the program to be jumped to or called. A86 will also accept a constant offset in this place (or a constant segment-offset pair in the case of "cd"). "cb" is a label within about 128 bytes (in either direction) of the current location. "cw" is a label within the same code segment as this program; "cd" is a pair of constants separated by a colon-- the segment value to the left of the colon, and the offset to the right. Note that in both the cb and cw cases, the object code generated is the offset from the location following the current instruction, not the absolute location of the label operand. In some assemblers (most notably for the Z-80 processor) you have to code this offset explicitly by putting "$-" before every relative jump operand in your source code. You do NOT need to, and should not do so with A86. "e" means the operand is an Effective Address. The concept of an Effective Address is central to the 86 machine architecture, and thus to 86 assembly language programming. It is described in detail at the start of Chapter 8. We summarize here by saying that an Effective Address is either a general- purpose register, a memory variable, or an indexed memory quantity. For example, the instruction "ADD rb,eb" includes the instructions: ADD AL,BL, or ADD CH,BYTEVAR, or ADD DL,B[BX+17]. "i" means the operand is an immediate constant, provided as part of the instruction itself. "ib" is a byte-sized constant; "iw" is a constant occupying a full 16-bit word. The operand can also be a label, defined with a colon. In that case, the immediate constant which is the location of the label is used. Examples: "MOV rw,iw" includes the instructions: MOV AX,17, or MOV SI,VAR_ARRAY, where "VAR_ARRAY:" appears somewhere in the program, defined with a colon. NOTE that if VAR_ARRAY were defined without a colon, e.g., "VAR_ARRAY DW 1,2,3", then "MOV SI,VAR_ARRAY" would be a "MOV rw,ew" NOT a "MOV rw,iw". The MOV would move the contents of memory at VAR_ARRAY (in this case 1) into SI, instead of the location of the memory. To load the location, you can code "MOV SI,OFFSET VAR_ARRAY". "m" means a memory variable or an indexed memory quantity; i.e., any Effective Address EXCEPT a register. "r" means the operand is a general-purpose register. The 8 "rb" registers are AL,BL,CL,DL,AH,BH,CH,DH; the 8 "rw" registers are AX,BX,CX,DX,SI, DI,BP,SP.