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xiii THE 8086 INSTRUCTION SET Before I go through the instructions I need to say something about the structure of the instructions. For the majority of instructions, two bits in the first instruction byte and the whole second byte determine whether it is a byte or word instruction and whether it is: 1. register, register 2. register, memory 3. memory, register These refer to the 8 arithmetic registers, not the segment registers. "Memory" is any allowable addressing mode. Therefore, it is simpler to abbreviate this as: reg/mem, reg/mem This will always mean: 1. either a byte or word operation is allowed 2. any of the above 3 possibilities is allowed unless specifically stated otherwise. Another possibility is the operations with constants: add ax, 7 xor sign_flag, 80h These follow the same form. There are two possibilities: 1. register, constant 2. memory, constant where this may be a byte or a word, "register" is any arithmetic register, and "memory" is any allowable addressing mode. These will be abbreviated: reg/mem, constant If we are taliking about a segment register, they will be abbreviated: segreg If anything does not follow this pattern, it will be explicitly stated. DESTINATION,SOURCE The standard way of writing instructions is with the destination on the left and the source on the right: ______________________ The PC Assembler Tutor - Copyright (C) 1989 Chuck Nelson The PC Assembler Tutor xiv ______________________ add destination, source The register or memory location on the left ALWAYS gets the result of the operation. The thing on the right is the second operand (if any). ADDRESSING MODES These are the natural (default) segments of all addressing modes: (1) DS variable + (constant) [bx] + (constant) [si] + (constant) [di] + (constant) [bx+si] + (constant) [bx+di] + (constant) (2) SS [bp] + (constant) [bp+si] + (constant) [bp+di] + (constant) Where the constant is optional. Segment overrides may be used. The segment overrides are: SEGMENT OVERRIDE MACHINE CODE (hex) CS: 2E DS: 3E ES: 26 SS: 36 These default segments apply to all instructions except the string instructions, which will be explained individually. THE INSTRUCTION SET AAA (ascii adjust for addition) adjusts AL, assuming that it contains the result of a legitimate unpacked BCD addition. If the lower half-byte has generated a result 10 or over, it subtracts 10, carries 1 to AH, and sets the carry flag. If the result is 9 or less, it clears CF. In either case it zeroes the upper half-byte of AL. AAD (ascii adjust for division) PREPARES AL and AH for division. It assumes that AH contains the 10's digit and AL contains the 1's digit of a two byte unpacked BCD number. It multiplies AH by 10 and adds it to AL, thus making a single integer between 0 and 99. It zeroes AH in preparation for unsigned division. Appendix II - The 8086 Instruction Set xv ______________________________________ AAM (ascii adjust for multiplication) adjusts AL, assuming that it contains the result of a legitimate BCD multiplication. It divides the result by 10, putting the quotient in AH and the remainder in AL. If AL contains 73 before AAM, then afterwards AH will contain 7 and AL will contain 3 AAS (ascii adjust for subtraction) adjusts AL, assuming that it contains the result of a legitimate unpacked BCD subtraction. If the lower half-byte has generated a result -1 or less, it borrows 1 from AH, adds 10 to AL, and sets the carry flag. If the result is 0 or more, it clears CF. In either case it zeroes the upper half-byte of AL. ADC (add with carry) adds two integers, either signed or unsigned, and adds in the carry from the previous arithmetic operation. It is used for multiple word (byte) addition. adc reg/mem, reg/mem adc reg/mem, constant ADD adds two integers, either signed or unsigned. add reg/mem, reg/mem add reg/mem, constant AND ands two bytes or words. A bit remains set only if both its respective source and destination bits were set. and reg/mem, reg/mem and reg/mem, constant CALL calls a procedure. The two forms we have used are NEAR and FAR calls. call set_regs call FAR PTR calculate_array There are two other forms where the addresses of the subprograms are in memory and changable. You should only use these forms if you are writing an operating system or a compiler, since they are an invitation to disaster. CBW sign extends the signed byte in AL to AH:AL in preparation for signed division. It is used alone without any register specification. cbw The PC Assembler Tutor xvi ______________________ CLC clears the carry flag (CF = 0). CLD clears the direction flag (increments string pointers). CLI clears the interrupt flag (disables interrupts). All maskable interrupts must wait till the flag is set again before they will be processed. CMC changes the value of the carry flag. If CF =1, then CF = 0; if CF = 0, then CF = 1. CMP compares two values. It is the same as SUB except it does not alter the "destination" variable. Its job is to set the flags as if a subtraction had been performed, in preparation for a conditional jump instruction. cmp reg/mem, reg/mem cmp reg/mem, constant CMPS compares the byte (or word) at DS:[si] with the one at ES:[di], (calculating [si] - [di]) and sets the flags accordingly. It increments (or decrements) SI and DI depending on the setting of DF, the direction flag (by 1 for bytes and by 2 for words). You may use CS:[si], SS:[si] or ES:[si], but you MAY NOT OVERRIDE ES:[di]. Notice that SI and DI are reversed from their position in the other string instructions. cmpsb cmpsw cmps BYTE PTR SS:[si], ES:[di] ;or CS, DS, ES:[si] cmps WORD PTR SS:[si], ES:[di] ;or CS, DS, ES:[si] CWD sign extends the signed integer in AX to DX:AX in preparation for signed word division. No register specification is used. cwd DAA (decimal adjust for addition) adjusts AL, assuming that it contains the result of a legitimate packed BCD addition. It treats AL as two independent half-bytes. If the result of the lower half-byte is 10 or over, it subtracts 10 from the lower half-byte and adds the carry to the upper half byte. It then looks at the upper half byte. If its result is 10 or over, DAA subtracts 10 from the upper half byte and sets the carry flag. Appendix II - The 8086 Instruction Set xvii ______________________________________ Otherwise the carry flag is cleared. DAS (decimal adjust for subtraction) adjusts AL, assuming that it contains the result of a legitimate packed BCD subtraction.It treats AL as two independent half-bytes. If the result of the lower half-byte is -1 or less, it adds 10 to the lower half-byte and borrows 1 from the upper half byte. It then looks at the upper half byte. If its result is -1 or less, DAA adds 10 to the upper half byte and sets the carry flag to indicate a borrow. Otherwise the carry flag is cleared. DEC decrements the byte or word by 1. It does not alter CF. dec reg/mem DIV performs unsigned division. If byte division, the unsigned double byte must be in AH:AL. Afterwards, AL has the quotient and AH has the remainder. If word division, the unsigned word must be in DX:AX. Afterwards, AX has the quotient, and DX the remainder. Division by a constant is not allowed. The DIV instruction does not mention DX:AX or AH:AL. They are understood. div reg/mem ESC (escape) signals to the 8086 that the bytes starting with the ESC byte are a coprocessor instruction. The 8086 will calculate a memory address (if appropriate) and give it to the coprocessor. The 8086 will then go on to the next instruction. HLT (halt) halts the machine. It can be restarted with a reset, a non-maskable interrupt, or (if IEF is set) with a maskable interrupt. IDIV performs signed integer division. For byte division, the value in AL must be sign extended to AH (with CBW), giving the double signed byte AH:AL, after division, AL contains the quotient and AH contains the remainder. For word division, the value in AX must be sign extended to DX (with CWD), giving the double signed word DX:AX, after division, AX contains the quotient and DX contains the remainder. Division by a constant is not allowed. IDIV cannot perform multiple word division. To do that, you need to make the numbers positive and use DIV, adjusting the signs afterwards. IDIV does not mention the AL or AX register, it is understood. The PC Assembler Tutor xviii ______________________ idiv reg/mem IMUL performs signed integer multiplication. The multiplicand must be in AX (or AL for bytes). After the multiplication, the result is in DX:AX for words and AH:AL for bytes. If DX (or AH for bytes) contains significant information, then CF is set (=1). If all the information is in AX (or AL for bytes) then CF = 0. Multiplication by a constant is not allowed. The IMUL instruction does not mention AX (or AL); it is understood. imul mem/reg IN allows you to move data from a port address to AX (for a word) or AL (for a byte). There are two forms: IN al, port_number IN al, dx Where 'port_number' is a CONSTANT that is hard coded into the machine instruction and is from 0 - 255. The port address in DX may be from 0-65535. It must be the DX register. INC increments a register or a variable by 1. It does not effect CF. inc reg/mem INT (interrupt) sends the program to a subprogram whose address is located in the interrupt vector table in low memory. For any interrupt number I, the 8086 goes to segment 0000, offset (I X 4). It loads IP with the first two bytes, then loads CS from the next two bytes. The next instruction executed will be at the new CS:IP. Before loading the new CS and IP, it pushes (1) the flags, (2) the old CS and (3) the old IP in that order. int 3 int 21h INTO checks OF. If OF = 1, INTO generates interrupt 4. Otherwise it does nothing. It is used for error handling of signed numbers. into IRET (interrupt return) is a special return instruction for interrupt routines. It pops (1) the old IP, (2) the old CS, and (3) the old flags in that order. Appendix II - The 8086 Instruction Set xix ______________________________________ iret JUMPS --------------------------------------------------------- There are two kinds of jumps. JMP is unconditional and can jump anywhere in the segment. All other jumps are conditional and are limited jumps from -128 to +127 bytes from the END of the jump instruction.{1} THESE ARE THE JUMP INSTRUCTIONS FOR SIGNED NUMBERS jg ; jump if greater jnle ; jump if not (less or equal) jl ; jump if less jnge ; jump if not (greater or equal) je ; jump if equal jz ; jump if zero jge ; jump if greater or equal jnl ; jump if not less jle ; jump if less or equal jng ; jump if not greater jne ; jump if not equal jnz ; jump if not zero THESE ARE THE JUMP INSTRUCTIONS FOR UNSIGNED NUMBERS ja ; jump if above jnbe ; jump if not (below or equal) jb ; jump if below jnae ; jump if not (above or equal) je ; jump if equal jz ; jump if zero jae ; jump if above or equal jnb ; jump if not below jbe ; jump if below or equal jna ; jump if not above ____________________ 1 There is also a long jump which can jump anywhere in memory (from 00000 to FFFFF). Except under special circumstances, using this is truly lousy programming practice. The long jump is for the Olympics, not for quality programming. The PC Assembler Tutor xx ______________________ jne ; jump if not equal jnz ; jump if not zero THESE JUMPS CHECK A SINGLE FLAG These come in opposite pairs jc ; jump if the carry flag is set jnc ; jump if the carry flag is not set jo ; jump if the overflow flag is set jno ; jump if the overflow flag is not set jp or jpe ; jump if parity flag is set (parity is even) jnp or jpo ;jump if parity flag is not set (parity is odd) js ; jump if the sign flag is set (negative ) jns ; jump if the sign flag is not set (positive or 0) THIS CHECKS THE CX REGISTER jcxz ; jump if cx is zero This is used before entry to a loop to make sure the loop counter is not set to 0. INDIRECT JUMPS are jumps where the information for the jump is stored in memory (or in a register for an in-segment jump). These forms are dangerous and should be used only when writing things like multi-tasking operating systems. Have you written one lately? END OF JUMPS - - - - - - - - - - - - - - - - - - - - - - - LAHF (load AH from flags) loads the lower half of the flags register into AH. LDS (load DS) loads the first two bytes of the memory address into the named arithmetic register and the next two bytes into DS. The register may be any arithmetic register, but this is designed for the 4 addressing registers: BX, SI, DI and BP. lds reg, memory_address LEA calculates an offset address and puts it in the named register. Appendix II - The 8086 Instruction Set xxi ______________________________________ lea ax, [bp+si]+145 LES (load ES) loads the first two bytes into the named arithmetic register and the next two bytes into ES. The register may be any arithmetic register, but this is designed for the 4 addressing registers: BX, SI, DI and BP. les reg, memory_address LOCK is for use if there is more than one 8086 operating in a computer. LOCK allows one 8086 to be the only one which can read from or write to memory during the following instruction. LODS moves a byte or word from DS:[si] to AL or AX, and increments (or decrements) SI depending on the setting of DF, the direction flag (by 1 for bytes and by 2 for words). You may use CS:[si], SS:[si] or ES:[si]. lodsb lodsw lods BYTE PTR SS:[si] ; or CS, DS, ES:[si] lods WORD PTR SS:[si] ; or CS, DS, ES:[si] LOOP/LOOPE/LOOPNE are conditional jumps (-128 to + 127 bytes). They will jump back to the start of the loop (which is identified by a label), under the following conditions: loop decrement cx ; jump back if cx is not zero loope decrement cx ; jump back if cx not zero AND zf = 1 loopz decrement cx ; jump back if cx not zero AND zf = 1 loopne decrement cx ; jump back if cx not zero AND zf = 0 loopnz decrement cx ; jump back if cx not zero AND zf = 0 Here, 'e' stands for equal, 'z' is zero and 'n' is not. loop some_label MOV is the general instruction for moving bytes or words on the 8086. The forms are: reg/mem, reg/mem reg/mem, constant segreg, reg/mem reg/mem, segreg Notice that you may not (1) move a constant to a segment register, (2) move a segment register to another segment The PC Assembler Tutor xxii ______________________ register, or (3) move from memory to memory. MOVS moves a byte (or a word) from DS:[si] to ES:[di], and increments (or decrements) SI and DI depending on the setting of DF, the direction flag (by 1 for bytes and by 2 for words). You may use CS:[si], SS:[si] or ES:[si], but you MAY NOT OVERRIDE ES:[di]. movsb movsw movs BYTE PTR ES:[di], SS:[si] ;or CS, DS, ES:[si] movs WORD PTR ES:[di], SS:[si] ;or CS, DS, ES:[si] MUL performs unsigned integer multiplication. The multiplicand must be in AX (or AL for bytes). After the multiplication, the result is in DX:AX (or AH:AX for bytes). If DX (or AH for bytes) contains significant information, then CF = 1. If all the significant information is in AX (or AL for bytes) then CF = 0. Multiplication by a constant is not allowed. The MUL instruction does not mention AX (or AL). It is understood. mul reg/mem NEG performs '0 - number' on a number and sets the flags accordingly. neg reg/mem NOP (no operation) does absolutely nothing. It can fill up space which was previously occupied by a different instruction. NOT performs bitwise logical NOT on a word or a byte. not reg/mem OR performs bitwise logical OR on a byte or word or reg/mem, reg/mem or reg/mem, constant OUT moves a byte (or word) from AL (or AX) to the named port. There are two forms: Appendix II - The 8086 Instruction Set xxiii ______________________________________ out port_number, ax out dx, ax where 'port_number' is a CONSTANT which is coded into the machine code. The constant may be from 0-255. DX (and it must be DX) may hold a port number from 0-65535. POP pops a WORD off the stack. Technically, POP takes the word at SS:SP and puts it into the named register or memory location, then ADDS 2 to SP. pop mem/reg pop segreg POPF pops a WORD off the top of the stack into the flags register. Its technical operation is the same as for POP. PUSH pushes a WORD on the stack from the named register or memory location. Technically, PUSH subtracts 2 from SP, then moves the word to SS:SP. push reg/mem push segreg PUSHF pushes the flags register on the stack. Its technical operation is the same as for PUSH. RCR (rotate through carry right) and RCL (rotate through carry left) rotate the bits of a register or of memory data right and left respectively. The bit which is shoved off the register (or data) is placed in CF and CF is placed on the other side of the register (or data). There are two fixed forms. (1) rotate 1 bit and (2) rotate by the number in CL. rcr reg/mem, 1 rcl reg/mem, cl REP. The string instructions may be prefixed by REP/REPE/REPNE which will repeat the instructions according to the following conditions: rep decrement cx ; repeat if cx is not zero repe decrement cx ; repeat if cx not zero AND zf = 1 repz decrement cx ; repeat if cx not zero AND zf = 1 repne decrement cx ; repeat if cx not zero AND zf = 0 The PC Assembler Tutor xxiv ______________________ repnz decrement cx ; repeat if cx not zero AND zf = 0 Here, 'e' stands for equal, 'z' is zero and 'n' is not. These repeat instructions should NEVER be used with a segment override, since the 8086 will forget the override if a hardware interrupt occurs in the middle of the REP loop. rep movsb repe scasb repne cmpsw RET returns from the subroutine to the calling program. The return will be either NEAR or FAR depending on the type of procedure it is in. It may take the parameters off the stack (for Pascal) or leave them on the stack (for C). ret (6) ; Pascal - take 6 bytes off the stack ret ; C - do nothing ROR (rotate right) and ROL (rotate left) rotate the bits of a register or memory data right and left respectively. The bit which is shoved off one end is moved to the other end. CF indicates whether the last bit moved from one end to the other was a 1 or a 0. There are two fixed forms. (1) rotate 1 bit and (2) rotate by the number in CL. ror reg/mem, 1 rol reg/mem, cl SAHF (store AH into flags) puts AH into the low byte of the flags register. SAL (shift arithmetic left) and SHL (shift logical left) are exactly the same instruction. They move bits left. 0s are placed in the low bit. Bits are shoved off the register or memory data on the left side, and CF indicates whether the last bit shoved off was a 1 or a 0. It is used for multiplying an unsigned number by powers of 2. There are two fixed forms. (1) shift 1 bit and (2) shift by the number in CL. shl reg/mem, 1 sal reg/mem, cl SAR (shift arithmetic right) shifts bits right. The high (sign) bit stays the same throughout the operation. Bits are shoved off the register or memory location on the right side. CF indicates whether the last bit shoved off was a 1 or a 0. It is used (with Appendix II - The 8086 Instruction Set xxv ______________________________________ difficulty) for dividing a signed number by powers of 2. There are two fixed forms. (1) shift 1 bit and (2) shift by the number in CL. sar reg/mem, 1 sar reg/mem, cl SBB (subtract with borrow) subtracts one integer from another and then subtracts 1 more if CF is set. It works with both signed and unsigned numbers and is used for multiple word arithmetic. sbb reg/mem, reg/mem sbb reg/mem, constant SCAS compares AL (or AX) to the byte (or word) pointed to by ES:[di], and increments (or decrements) DI depending on the setting of DF, the direction flag (by 1 for bytes and by 2 for words). NO OVERRIDES ARE ALLOWED. scasb scasw scas BYTE PTR ES:[di] ; no override allowed scas WORD PTR ES:[di] ; no override allowed SHR (shift logical right) does the same thing as SHL but in the opposite direction. Bits are shifted right. 0s are placed in the high bit. Bits are shoved off the register or memory location on the right side and CF indicates whether the last bit shoved off was a 0 or a 1. It is used for dividing an unsigned number by powers of 2. There are two fixed forms. (1) shift 1 bit and (2) shift by the number in CL. shr reg/mem, 1 shr reg/mem, cl STC sets the carry flag (CF = 1). STD sets the direction flag (DF = 1, which decrements). STI sets the interrupt flag (IEF = 1; interrupts enabled). STOS (store to string) moves a byte (or a word) from AL (or AX) to ES:[di], and increments (or decrements) DI depending on the setting of DF, the direction flag (by 1 for bytes and by 2 for words). NO SEGMENT OVERRIDES ARE ALLOWED. stosb The PC Assembler Tutor xxvi ______________________ stosw stos BYTE PTR ES:[di] ; no override allowed stos WORD PTR ES:[di] ; no override allowed SUB subtracts one integer from another. It works for both signed and unsigned numbers. sub reg/mem, reg/mem sub reg/mem, constant TEST performs logical AND, but does not alter the value of the "destination" variable. Its purpose is to set the flags for conditional jumps. test reg/mem, reg/mem test reg/mem, constant WAIT forces the 8086 to wait until the coprocessor is finished with its instruction before the 8086 can go on to the next instruction. XCHG exchanges the values in two registers or memory locations. xchg reg/mem, reg/mem XLAT. If BX contains the address of a 256 byte translation table, then XLAT goes to (BX + AL), takes the byte there and puts it into AL. DS is the normal segment, but segment overrides are allowed. XOR performs logical exclusive OR on two numbers. xor reg/mem, reg/mem xor reg/mem, constant