Computers

Complete 8086 instruction set

Quick reference:

AAA
AAD
AAM
AAS
ADC
ADD
AND
CALL
CBW
CLC
CLD
CLI
CMC
CMP
CMPSB
CMPSW
CWD
DAA
DAS
DEC
DIV
HLT
IDIV
IMUL
IN
INC
INT
INTO
IRET
JA
JAE
JB
JBE
JC
JCXZ
JE
JG
JGE
JL
JLE
JMP
JNA
JNAE
JNB
JNBE
JNC
JNE
JNG
JNGE
JNL
JNLE
JNO
JNP
JNS
JNZ
JO
JP
JPE
JPO
JS
JZ
LAHF
LDS
LEA
LES
LODSB
LODSW
LOOP
LOOPE
LOOPNE
LOOPNZ
LOOPZ
MOV
MOVSB
MOVSW
MUL
NEG
NOP
NOT
OR
OUT
POP
POPA
POPF
PUSH
PUSHA
PUSHF
RCL
RCR
REP
REPE
REPNE
REPNZ
REPZ
RET
RETF
ROL
ROR
SAHF
SAL
SAR
SBB
SCASB
SCASW
SHL
SHR
STC
STD
STI
STOSB
STOSW
SUB
TEST
XCHG
XLATB
XOR

Operand types:

REG: AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.

SREG: DS, ES, SS, and only as second operand: CS.

memory: [BX], [BX+SI+7], variable, etc...(see Memory Access).

immediate: 5, -24, 3Fh, 10001101b, etc...

Notes:

• When two operands are required for an instruction they are separated by comma. For example:
  
  REG, memory
  
  
• When there are two operands, both operands must have the same size (except shift and rotate instructions). For example:
  
  AL, DL
  DX, AX
  m1 DB ?
  AL, m1
  m2 DW ?
  AX, m2
  
  
• Some instructions allow several operand combinations. For example:
  
  memory, immediate
  REG, immediate
  
  memory, REG
  REG, SREG
  
  
• Some examples contain macros, so it is advisable to use Shift + F8 hot key to Step Over (to make macro code execute at maximum speed set step delay to zero), otherwise emulator will step through each instruction of a macro. Here is an example that uses PRINTN macro: include 'emu8086.inc' ORG 100h MOV AL, 1 MOV BL, 2 PRINTN 'Hello World!' ; macro. MOV CL, 3 PRINTN 'Welcome!' ; macro. RET

These marks are used to show the state of the flags:

1 - instruction sets this flag to 1.
0 - instruction sets this flag to 0.
r - flag value depends on result of the instruction.
? - flag value is undefined (maybe 1 or 0).

Some instructions generate exactly the same machine code, so disassembler may have a problem decoding to your original code. This is especially important for Conditional Jump instructions (see "Program Flow Control" in Tutorials for more information).

Instructions in alphabetical order:

Instruction Operands Description AAA No operands ASCII Adjust after Addition.
Corrects result in AH and AL after addition when working with BCD values.

It works according to the following Algorithm:

if low nibble of AL > 9 or AF = 1 then:

• AL = AL + 6
• AH = AH + 1
• AF = 1
• CF = 1

else

• AF = 0
• CF = 0

in both cases:
clear the high nibble of AL.

Example: MOV AX, 15 ; AH = 00, AL = 0Fh AAA ; AH = 01, AL = 05 RET C Z S O P A r ? ? ? ? r AAD No operands ASCII Adjust before Division.
Prepares two BCD values for division.

Algorithm:

• AL = (AH * 10) + AL
• AH = 0

Example: MOV AX, 0105h ; AH = 01, AL = 05 AAD ; AH = 00, AL = 0Fh (15) RET C Z S O P A ? r r ? r ? AAM No operands ASCII Adjust after Multiplication.
Corrects the result of multiplication of two BCD values.

Algorithm:

• AH = AL / 10
• AL = remainder

Example: MOV AL, 15 ; AL = 0Fh AAM ; AH = 01, AL = 05 RET C Z S O P A ? r r ? r ? AAS No operands ASCII Adjust after Subtraction.
Corrects result in AH and AL after subtraction when working with BCD values.

Algorithm:

if low nibble of AL > 9 or AF = 1 then:

• AL = AL - 6
• AH = AH - 1
• AF = 1
• CF = 1

else

• AF = 0
• CF = 0

in both cases:
clear the high nibble of AL.

Example: MOV AX, 02FFh ; AH = 02, AL = 0FFh AAS ; AH = 01, AL = 09 RET C Z S O P A r ? ? ? ? r ADC REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Add with Carry.


Algorithm:

operand1 = operand1 + operand2 + CF

Example: STC ; set CF = 1 MOV AL, 5 ; AL = 5 ADC AL, 1 ; AL = 7 RET C Z S O P A r r r r r r ADD REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Add.


Algorithm:

operand1 = operand1 + operand2

Example: MOV AL, 5 ; AL = 5 ADD AL, -3 ; AL = 2 RET C Z S O P A r r r r r r AND REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Logical AND between all bits of two operands. Result is stored in operand1.

These rules apply:

1 AND 1 = 1
1 AND 0 = 0
0 AND 1 = 0
0 AND 0 = 0


Example: MOV AL, 'a' ; AL = 01100001b AND AL, 11011111b ; AL = 01000001b ('A') RET C Z S O P 0 r r 0 r CALL procedure name
label
4-byte address
Transfers control to procedure, return address is (IP) is pushed to stack. 4-byte address may be entered in this form: 1234h:5678h, first value is a segment second value is an offset (this is a far call, so CS is also pushed to stack).


Example: ORG 100h ; directive to make simple .com file. CALL p1 ADD AX, 1 RET ; return to OS. p1 PROC ; procedure declaration. MOV AX, 1234h RET ; return to caller. p1 ENDP C Z S O P A unchanged CBW No operands Convert byte into word.

Algorithm:

if high bit of AL = 1 then:

• AH = 255 (0FFh)

else

• AH = 0

Example: MOV AX, 0 ; AH = 0, AL = 0 MOV AL, -5 ; AX = 000FBh (251) CBW ; AX = 0FFFBh (-5) RET C Z S O P A unchanged CLC No operands Clear Carry flag.

Algorithm:

CF = 0

C 0 CLD No operands Clear Direction flag. SI and DI will be incremented by chain instructions: CMPSB, CMPSW, LODSB, LODSW, MOVSB, MOVSW, STOSB, STOSW.

Algorithm:

DF = 0

D 0 CLI No operands Clear Interrupt enable flag. This disables hardware interrupts.

Algorithm:

IF = 0

I 0 CMC No operands Complement Carry flag. Inverts value of CF.

Algorithm:

if CF = 1 then CF = 0
if CF = 0 then CF = 1


C r CMP REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Compare.

Algorithm:

operand1 - operand2

result is not stored anywhere, flags are set (OF, SF, ZF, AF, PF, CF) according to result.

Example: MOV AL, 5 MOV BL, 5 CMP AL, BL ; AL = 5, ZF = 1 (so equal!) RET C Z S O P A r r r r r r CMPSB No operands Compare bytes: ES:[DI] from DS:[SI].

Algorithm:

• DS:[SI] - ES:[DI]
  
• set flags according to result:
  OF, SF, ZF, AF, PF, CF
  
• if DF = 0 then
  • SI = SI + 1
  • DI = DI + 1
  else
  • SI = SI - 1
  • DI = DI - 1

Example:
see cmpsb.asm in c:\emu8086\examples\.

C Z S O P A r r r r r r CMPSW No operands Compare words: ES:[DI] from DS:[SI].

Algorithm:

• DS:[SI] - ES:[DI]
  
• set flags according to result:
  OF, SF, ZF, AF, PF, CF
  
• if DF = 0 then
  • SI = SI + 2
  • DI = DI + 2
  else
  • SI = SI - 2
  • DI = DI - 2

Example:
see cmpsw.asm in c:\emu8086\examples\.

C Z S O P A r r r r r r CWD No operands Convert Word to Double word.

Algorithm:

if high bit of AX = 1 then:

• DX = 65535 (0FFFFh)

else

• DX = 0

Example: MOV DX, 0 ; DX = 0 MOV AX, 0 ; AX = 0 MOV AX, -5 ; DX AX = 00000h:0FFFBh CWD ; DX AX = 0FFFFh:0FFFBh RET C Z S O P A unchanged DAA No operands Decimal adjust After Addition.
Corrects the result of addition of two packed BCD values.

Algorithm:

if low nibble of AL > 9 or AF = 1 then:

• AL = AL + 6
• AF = 1

if AL > 9Fh or CF = 1 then:

• AL = AL + 60h
• CF = 1

Example: MOV AL, 0Fh ; AL = 0Fh (15) DAA ; AL = 15h RET C Z S O P A r r r r r r DAS No operands Decimal adjust After Subtraction.
Corrects the result of subtraction of two packed BCD values.

Algorithm:

if low nibble of AL > 9 or AF = 1 then:

• AL = AL - 6
• AF = 1

if AL > 9Fh or CF = 1 then:

• AL = AL - 60h
• CF = 1

Example: MOV AL, 0FFh ; AL = 0FFh (-1) DAS ; AL = 99h, CF = 1 RET C Z S O P A r r r r r r DEC REG
memory
Decrement.

Algorithm:

operand = operand - 1


Example: MOV AL, 255 ; AL = 0FFh (255 or -1) DEC AL ; AL = 0FEh (254 or -2) RET Z S O P A r r r r r CF - unchanged! DIV REG
memory
Unsigned divide.

Algorithm:

when operand is a byte:
AL = AX / operand
AH = remainder (modulus) when operand is a word:
AX = (DX AX) / operand
DX = remainder (modulus) Example: MOV AX, 203 ; AX = 00CBh MOV BL, 4 DIV BL ; AL = 50 (32h), AH = 3 RET C Z S O P A ? ? ? ? ? ? HLT No operands Halt the System.

Example: MOV AX, 5 HLT C Z S O P A unchanged IDIV REG
memory
Signed divide.

Algorithm:

when operand is a byte:
AL = AX / operand
AH = remainder (modulus) when operand is a word:
AX = (DX AX) / operand
DX = remainder (modulus) Example: MOV AX, -203 ; AX = 0FF35h MOV BL, 4 IDIV BL ; AL = -50 (0CEh), AH = -3 (0FDh) RET C Z S O P A ? ? ? ? ? ? IMUL REG
memory
Signed multiply.

Algorithm:

when operand is a byte:
AX = AL * operand. when operand is a word:
(DX AX) = AX * operand. Example: MOV AL, -2 MOV BL, -4 IMUL BL ; AX = 8 RET C Z S O P A r ? ? r ? ? CF=OF=0 when result fits into operand of IMUL. IN AL, im.byte
AL, DX
AX, im.byte
AX, DX Input from port into AL or AX.
Second operand is a port number. If required to access port number over 255 - DX register should be used.
Example: IN AX, 4 ; get status of traffic lights. IN AL, 7 ; get status of stepper-motor. C Z S O P A unchanged INC REG
memory
Increment.

Algorithm:

operand = operand + 1

Example: MOV AL, 4 INC AL ; AL = 5 RET Z S O P A r r r r r CF - unchanged! INT immediate byte Interrupt numbered by immediate byte (0..255).

Algorithm:

• Push to stack:
  • flags register
  • CS
  • IP
• IF = 0
• Transfer control to interrupt procedure

Example: MOV AH, 0Eh ; teletype. MOV AL, 'A' INT 10h ; BIOS interrupt. RET C Z S O P A I unchanged 0 INTO No operands Interrupt 4 if Overflow flag is 1.

Algorithm:

if OF = 1 then INT 4

Example: ; -5 - 127 = -132 (not in -128..127) ; the result of SUB is wrong (124), ; so OF = 1 is set: MOV AL, -5 SUB AL, 127 ; AL = 7Ch (124) INTO ; process error. RET IRET No operands Interrupt Return.

Algorithm:

Pop from stack:

• IP
• CS
• flags register

C Z S O P A popped JA label Short Jump if first operand is Above second operand (as set by CMP instruction). Unsigned.

Algorithm:

if (CF = 0) and (ZF = 0) then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 250 CMP AL, 5 JA label1 PRINT 'AL is not above 5' JMP exit label1: PRINT 'AL is above 5' exit: RET C Z S O P A unchanged JAE label Short Jump if first operand is Above or Equal to second operand (as set by CMP instruction). Unsigned.

Algorithm:

if CF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 5 CMP AL, 5 JAE label1 PRINT 'AL is not above or equal to 5' JMP exit label1: PRINT 'AL is above or equal to 5' exit: RET C Z S O P A unchanged JB label Short Jump if first operand is Below second operand (as set by CMP instruction). Unsigned.

Algorithm:

if CF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 1 CMP AL, 5 JB label1 PRINT 'AL is not below 5' JMP exit label1: PRINT 'AL is below 5' exit: RET C Z S O P A unchanged JBE label Short Jump if first operand is Below or Equal to second operand (as set by CMP instruction). Unsigned.

Algorithm:

if CF = 1 or ZF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 5 CMP AL, 5 JBE label1 PRINT 'AL is not below or equal to 5' JMP exit label1: PRINT 'AL is below or equal to 5' exit: RET C Z S O P A unchanged JC label Short Jump if Carry flag is set to 1.

Algorithm:

if CF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 255 ADD AL, 1 JC label1 PRINT 'no carry.' JMP exit label1: PRINT 'has carry.' exit: RET C Z S O P A unchanged JCXZ label Short Jump if CX register is 0.

Algorithm:

if CX = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV CX, 0 JCXZ label1 PRINT 'CX is not zero.' JMP exit label1: PRINT 'CX is zero.' exit: RET C Z S O P A unchanged JE label Short Jump if first operand is Equal to second operand (as set by CMP instruction). Signed/Unsigned.

Algorithm:

if ZF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 5 CMP AL, 5 JE label1 PRINT 'AL is not equal to 5.' JMP exit label1: PRINT 'AL is equal to 5.' exit: RET C Z S O P A unchanged JG label Short Jump if first operand is Greater then second operand (as set by CMP instruction). Signed.

Algorithm:

if (ZF = 0) and (SF = OF) then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 5 CMP AL, -5 JG label1 PRINT 'AL is not greater -5.' JMP exit label1: PRINT 'AL is greater -5.' exit: RET C Z S O P A unchanged JGE label Short Jump if first operand is Greater or Equal to second operand (as set by CMP instruction). Signed.

Algorithm:

if SF = OF then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, -5 JGE label1 PRINT 'AL < -5' JMP exit label1: PRINT 'AL >= -5' exit: RET C Z S O P A unchanged JL label Short Jump if first operand is Less then second operand (as set by CMP instruction). Signed.

Algorithm:

if SF <> OF then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, -2 CMP AL, 5 JL label1 PRINT 'AL >= 5.' JMP exit label1: PRINT 'AL < 5.' exit: RET C Z S O P A unchanged JLE label Short Jump if first operand is Less or Equal to second operand (as set by CMP instruction). Signed.

Algorithm:

if SF <> OF or ZF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, -2 CMP AL, 5 JLE label1 PRINT 'AL > 5.' JMP exit label1: PRINT 'AL <= 5.' exit: RET C Z S O P A unchanged JMP label
4-byte address
Unconditional Jump. Transfers control to another part of the program. 4-byte address may be entered in this form: 1234h:5678h, first value is a segment second value is an offset.


Algorithm:

always jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 5 JMP label1 ; jump over 2 lines! PRINT 'Not Jumped!' MOV AL, 0 label1: PRINT 'Got Here!' RET C Z S O P A unchanged JNA label Short Jump if first operand is Not Above second operand (as set by CMP instruction). Unsigned.

Algorithm:

if CF = 1 or ZF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, 5 JNA label1 PRINT 'AL is above 5.' JMP exit label1: PRINT 'AL is not above 5.' exit: RET C Z S O P A unchanged JNAE label Short Jump if first operand is Not Above and Not Equal to second operand (as set by CMP instruction). Unsigned.

Algorithm:

if CF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, 5 JNAE label1 PRINT 'AL >= 5.' JMP exit label1: PRINT 'AL < 5.' exit: RET C Z S O P A unchanged JNB label Short Jump if first operand is Not Below second operand (as set by CMP instruction). Unsigned.

Algorithm:

if CF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 7 CMP AL, 5 JNB label1 PRINT 'AL < 5.' JMP exit label1: PRINT 'AL >= 5.' exit: RET C Z S O P A unchanged JNBE label Short Jump if first operand is Not Below and Not Equal to second operand (as set by CMP instruction). Unsigned.

Algorithm:

if (CF = 0) and (ZF = 0) then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 7 CMP AL, 5 JNBE label1 PRINT 'AL <= 5.' JMP exit label1: PRINT 'AL > 5.' exit: RET C Z S O P A unchanged JNC label Short Jump if Carry flag is set to 0.

Algorithm:

if CF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 ADD AL, 3 JNC label1 PRINT 'has carry.' JMP exit label1: PRINT 'no carry.' exit: RET C Z S O P A unchanged JNE label Short Jump if first operand is Not Equal to second operand (as set by CMP instruction). Signed/Unsigned.

Algorithm:

if ZF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, 3 JNE label1 PRINT 'AL = 3.' JMP exit label1: PRINT 'Al <> 3.' exit: RET C Z S O P A unchanged JNG label Short Jump if first operand is Not Greater then second operand (as set by CMP instruction). Signed.

Algorithm:

if (ZF = 1) and (SF <> OF) then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, 3 JNG label1 PRINT 'AL > 3.' JMP exit label1: PRINT 'Al <= 3.' exit: RET C Z S O P A unchanged JNGE label Short Jump if first operand is Not Greater and Not Equal to second operand (as set by CMP instruction). Signed.

Algorithm:

if SF <> OF then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, 3 JNGE label1 PRINT 'AL >= 3.' JMP exit label1: PRINT 'Al < 3.' exit: RET C Z S O P A unchanged JNL label Short Jump if first operand is Not Less then second operand (as set by CMP instruction). Signed.

Algorithm:

if SF = OF then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, -3 JNL label1 PRINT 'AL < -3.' JMP exit label1: PRINT 'Al >= -3.' exit: RET C Z S O P A unchanged JNLE label Short Jump if first operand is Not Less and Not Equal to second operand (as set by CMP instruction). Signed.

Algorithm:

if (SF = OF) and (ZF = 0) then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 2 CMP AL, -3 JNLE label1 PRINT 'AL <= -3.' JMP exit label1: PRINT 'Al > -3.' exit: RET C Z S O P A unchanged JNO label Short Jump if Not Overflow.

Algorithm:

if OF = 0 then jump

Example: ; -5 - 2 = -7 (inside -128..127) ; the result of SUB is correct, ; so OF = 0: include 'emu8086.inc' ORG 100h MOV AL, -5 SUB AL, 2 ; AL = 0F9h (-7) JNO label1 PRINT 'overflow!' JMP exit label1: PRINT 'no overflow.' exit: RET C Z S O P A unchanged JNP label Short Jump if No Parity (odd). Only 8 low bits of result are checked. Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if PF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 00000111b ; AL = 7 OR AL, 0 ; just set flags. JNP label1 PRINT 'parity even.' JMP exit label1: PRINT 'parity odd.' exit: RET C Z S O P A unchanged JNS label Short Jump if Not Signed (if positive). Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if SF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 00000111b ; AL = 7 OR AL, 0 ; just set flags. JNS label1 PRINT 'signed.' JMP exit label1: PRINT 'not signed.' exit: RET C Z S O P A unchanged JNZ label Short Jump if Not Zero (not equal). Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if ZF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 00000111b ; AL = 7 OR AL, 0 ; just set flags. JNZ label1 PRINT 'zero.' JMP exit label1: PRINT 'not zero.' exit: RET C Z S O P A unchanged JO label Short Jump if Overflow.

Algorithm:

if OF = 1 then jump

Example: ; -5 - 127 = -132 (not in -128..127) ; the result of SUB is wrong (124), ; so OF = 1 is set: include 'emu8086.inc' org 100h MOV AL, -5 SUB AL, 127 ; AL = 7Ch (124) JO label1 PRINT 'no overflow.' JMP exit label1: PRINT 'overflow!' exit: RET C Z S O P A unchanged JP label Short Jump if Parity (even). Only 8 low bits of result are checked. Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if PF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 00000101b ; AL = 5 OR AL, 0 ; just set flags. JP label1 PRINT 'parity odd.' JMP exit label1: PRINT 'parity even.' exit: RET C Z S O P A unchanged JPE label Short Jump if Parity Even. Only 8 low bits of result are checked. Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if PF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 00000101b ; AL = 5 OR AL, 0 ; just set flags. JPE label1 PRINT 'parity odd.' JMP exit label1: PRINT 'parity even.' exit: RET C Z S O P A unchanged JPO label Short Jump if Parity Odd. Only 8 low bits of result are checked. Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if PF = 0 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 00000111b ; AL = 7 OR AL, 0 ; just set flags. JPO label1 PRINT 'parity even.' JMP exit label1: PRINT 'parity odd.' exit: RET C Z S O P A unchanged JS label Short Jump if Signed (if negative). Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if SF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 10000000b ; AL = -128 OR AL, 0 ; just set flags. JS label1 PRINT 'not signed.' JMP exit label1: PRINT 'signed.' exit: RET C Z S O P A unchanged JZ label Short Jump if Zero (equal). Set by CMP, SUB, ADD, TEST, AND, OR, XOR instructions.

Algorithm:

if ZF = 1 then jump

Example: include 'emu8086.inc' ORG 100h MOV AL, 5 CMP AL, 5 JZ label1 PRINT 'AL is not equal to 5.' JMP exit label1: PRINT 'AL is equal to 5.' exit: RET C Z S O P A unchanged LAHF No operands Load AH from 8 low bits of Flags register.

Algorithm:

AH = flags register

AH bit: 7 6 5 4 3 2 1 0 [SF] [ZF] [0] [AF] [0] [PF] [1] [CF] bits 1, 3, 5 are reserved.

C Z S O P A unchanged LDS REG, memory Load memory double word into word register and DS.

Algorithm:

• REG = first word
• DS = second word

Example: ORG 100h LDS AX, m RET m DW 1234h DW 5678h END AX is set to 1234h, DS is set to 5678h.

C Z S O P A unchanged LEA REG, memory Load Effective Address.

Algorithm:

• REG = address of memory (offset)

Example: MOV BX, 35h MOV DI, 12h LEA SI, [BX+DI] ; SI = 35h + 12h = 47h Note: The integrated 8086 assembler automatically replaces LEA with a more efficient MOV where possible. For example: org 100h LEA AX, m ; AX = offset of m RET m dw 1234h END

C Z S O P A unchanged LES REG, memory Load memory double word into word register and ES.

Algorithm:

• REG = first word
• ES = second word

Example: ORG 100h LES AX, m RET m DW 1234h DW 5678h END AX is set to 1234h, ES is set to 5678h.

C Z S O P A unchanged LODSB No operands Load byte at DS:[SI] into AL. Update SI.

Algorithm:

• AL = DS:[SI]
  
• if DF = 0 then
  • SI = SI + 1
  else
  • SI = SI - 1

Example: ORG 100h LEA SI, a1 MOV CX, 5 MOV AH, 0Eh m: LODSB INT 10h LOOP m RET a1 DB 'H', 'e', 'l', 'l', 'o' C Z S O P A unchanged LODSW No operands Load word at DS:[SI] into AX. Update SI.

Algorithm:

• AX = DS:[SI]
  
• if DF = 0 then
  • SI = SI + 2
  else
  • SI = SI - 2

Example: ORG 100h LEA SI, a1 MOV CX, 5 REP LODSW ; finally there will be 555h in AX. RET a1 dw 111h, 222h, 333h, 444h, 555h C Z S O P A unchanged LOOP label Decrease CX, jump to label if CX not zero.

Algorithm:

• CX = CX - 1
  
• if CX <> 0 then
  • jump
  else
  • no jump, continue

Example: include 'emu8086.inc' ORG 100h MOV CX, 5 label1: PRINTN 'loop!' LOOP label1 RET C Z S O P A unchanged LOOPE label Decrease CX, jump to label if CX not zero and Equal (ZF = 1).

Algorithm:

• CX = CX - 1
  
• if (CX <> 0) and (ZF = 1) then
  • jump
  else
  • no jump, continue

Example: ; Loop until result fits into AL alone, ; or 5 times. The result will be over 255 ; on third loop (100+100+100), ; so loop will exit. include 'emu8086.inc' ORG 100h MOV AX, 0 MOV CX, 5 label1: PUTC '*' ADD AX, 100 CMP AH, 0 LOOPE label1 RET C Z S O P A unchanged LOOPNE label Decrease CX, jump to label if CX not zero and Not Equal (ZF = 0).

Algorithm:

• CX = CX - 1
  
• if (CX <> 0) and (ZF = 0) then
  • jump
  else
  • no jump, continue

Example: ; Loop until '7' is found, ; or 5 times. include 'emu8086.inc' ORG 100h MOV SI, 0 MOV CX, 5 label1: PUTC '*' MOV AL, v1[SI] INC SI ; next byte (SI=SI+1). CMP AL, 7 LOOPNE label1 RET v1 db 9, 8, 7, 6, 5 C Z S O P A unchanged LOOPNZ label Decrease CX, jump to label if CX not zero and ZF = 0.

Algorithm:

• CX = CX - 1
  
• if (CX <> 0) and (ZF = 0) then
  • jump
  else
  • no jump, continue

Example: ; Loop until '7' is found, ; or 5 times. include 'emu8086.inc' ORG 100h MOV SI, 0 MOV CX, 5 label1: PUTC '*' MOV AL, v1[SI] INC SI ; next byte (SI=SI+1). CMP AL, 7 LOOPNZ label1 RET v1 db 9, 8, 7, 6, 5 C Z S O P A unchanged LOOPZ label Decrease CX, jump to label if CX not zero and ZF = 1.

Algorithm:

• CX = CX - 1
  
• if (CX <> 0) and (ZF = 1) then
  • jump
  else
  • no jump, continue

Example: ; Loop until result fits into AL alone, ; or 5 times. The result will be over 255 ; on third loop (100+100+100), ; so loop will exit. include 'emu8086.inc' ORG 100h MOV AX, 0 MOV CX, 5 label1: PUTC '*' ADD AX, 100 CMP AH, 0 LOOPZ label1 RET C Z S O P A unchanged MOV REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate

SREG, memory
memory, SREG
REG, SREG
SREG, REG Copy operand2 to operand1.

The MOV instruction cannot:

• set the value of the CS and IP registers.
• copy value of one segment register to another segment register (should copy to general register first).
• copy immediate value to segment register (should copy to general register first).

Algorithm:

operand1 = operand2 Example: ORG 100h MOV AX, 0B800h ; set AX = B800h (VGA memory). MOV DS, AX ; copy value of AX to DS. MOV CL, 'A' ; CL = 41h (ASCII code). MOV CH, 01011111b ; CL = color attribute. MOV BX, 15Eh ; BX = position on screen. MOV [BX], CX ; w.[0B800h:015Eh] = CX. RET ; returns to operating system. C Z S O P A unchanged MOVSB No operands Copy byte at DS:[SI] to ES:[DI]. Update SI and DI.

Algorithm:

• ES:[DI] = DS:[SI]
  
• if DF = 0 then
  • SI = SI + 1
  • DI = DI + 1
  else
  • SI = SI - 1
  • DI = DI - 1

Example: ORG 100h CLD LEA SI, a1 LEA DI, a2 MOV CX, 5 REP MOVSB RET a1 DB 1,2,3,4,5 a2 DB 5 DUP(0) C Z S O P A unchanged MOVSW No operands Copy word at DS:[SI] to ES:[DI]. Update SI and DI.

Algorithm:

• ES:[DI] = DS:[SI]
  
• if DF = 0 then
  • SI = SI + 2
  • DI = DI + 2
  else
  • SI = SI - 2
  • DI = DI - 2

Example: ORG 100h CLD LEA SI, a1 LEA DI, a2 MOV CX, 5 REP MOVSW RET a1 DW 1,2,3,4,5 a2 DW 5 DUP(0) C Z S O P A unchanged MUL REG
memory
Unsigned multiply.

Algorithm:

when operand is a byte:
AX = AL * operand. when operand is a word:
(DX AX) = AX * operand. Example: MOV AL, 200 ; AL = 0C8h MOV BL, 4 MUL BL ; AX = 0320h (800) RET C Z S O P A r ? ? r ? ? CF=OF=0 when high section of the result is zero. NEG REG
memory
Negate. Makes operand negative (two's complement).

Algorithm:

• Invert all bits of the operand
• Add 1 to inverted operand

Example: MOV AL, 5 ; AL = 05h NEG AL ; AL = 0FBh (-5) NEG AL ; AL = 05h (5) RET C Z S O P A r r r r r r NOP No operands No Operation.

Algorithm:

• Do nothing
  

Example: ; do nothing, 3 times: NOP NOP NOP RET C Z S O P A unchanged NOT REG
memory
Invert each bit of the operand.

Algorithm:

• if bit is 1 turn it to 0.
  
• if bit is 0 turn it to 1.
  

Example: MOV AL, 00011011b NOT AL ; AL = 11100100b RET C Z S O P A unchanged OR REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Logical OR between all bits of two operands. Result is stored in first operand.

These rules apply:

1 OR 1 = 1
1 OR 0 = 1
0 OR 1 = 1
0 OR 0 = 0


Example: MOV AL, 'A' ; AL = 01000001b OR AL, 00100000b ; AL = 01100001b ('a') RET C Z S O P A 0 r r 0 r ? OUT im.byte, AL
im.byte, AX
DX, AL
DX, AX Output from AL or AX to port.
First operand is a port number. If required to access port number over 255 - DX register should be used.

Example: MOV AX, 0FFFh ; Turn on all OUT 4, AX ; traffic lights. MOV AL, 100b ; Turn on the third OUT 7, AL ; magnet of the stepper-motor. C Z S O P A unchanged POP REG
SREG
memory Get 16 bit value from the stack.

Algorithm:

• operand = SS:[SP] (top of the stack)
• SP = SP + 2

Example: MOV AX, 1234h PUSH AX POP DX ; DX = 1234h RET C Z S O P A unchanged POPA No operands Pop all general purpose registers DI, SI, BP, SP, BX, DX, CX, AX from the stack.
SP value is ignored, it is Popped but not set to SP register).

Note: this instruction works only on 80186 CPU and later!

Algorithm:

• POP DI
• POP SI
• POP BP
• POP xx (SP value ignored)
• POP BX
• POP DX
• POP CX
• POP AX

C Z S O P A unchanged POPF No operands Get flags register from the stack.

Algorithm:

• flags = SS:[SP] (top of the stack)
• SP = SP + 2

C Z S O P A popped PUSH REG
SREG
memory
immediate Store 16 bit value in the stack.

Note: PUSH immediate works only on 80186 CPU and later!

Algorithm:

• SP = SP - 2
• SS:[SP] (top of the stack) = operand

Example: MOV AX, 1234h PUSH AX POP DX ; DX = 1234h RET C Z S O P A unchanged PUSHA No operands Push all general purpose registers AX, CX, DX, BX, SP, BP, SI, DI in the stack.
Original value of SP register (before PUSHA) is used.

Note: this instruction works only on 80186 CPU and later!

Algorithm:

• PUSH AX
• PUSH CX
• PUSH DX
• PUSH BX
• PUSH SP
• PUSH BP
• PUSH SI
• PUSH DI

C Z S O P A unchanged PUSHF No operands Store flags register in the stack.

Algorithm:

• SP = SP - 2
• SS:[SP] (top of the stack) = flags

C Z S O P A unchanged RCL memory, immediate
REG, immediate

memory, CL
REG, CL Rotate operand1 left through Carry Flag. The number of rotates is set by operand2.
When immediate is greater then 1, assembler generates several RCL xx, 1 instructions because 8086 has machine code only for this instruction (the same principle works for all other shift/rotate instructions).

Algorithm:

shift all bits left, the bit that goes off is set to CF and previous value of CF is inserted to the right-most position.

Example: STC ; set carry (CF=1). MOV AL, 1Ch ; AL = 00011100b RCL AL, 1 ; AL = 00111001b, CF=0. RET C O r r OF=0 if first operand keeps original sign. RCR memory, immediate
REG, immediate

memory, CL
REG, CL Rotate operand1 right through Carry Flag. The number of rotates is set by operand2.

Algorithm:

shift all bits right, the bit that goes off is set to CF and previous value of CF is inserted to the left-most position.

Example: STC ; set carry (CF=1). MOV AL, 1Ch ; AL = 00011100b RCR AL, 1 ; AL = 10001110b, CF=0. RET C O r r OF=0 if first operand keeps original sign. REP chain instruction
Repeat following MOVSB, MOVSW, LODSB, LODSW, STOSB, STOSW instructions CX times.

Algorithm:

check_cx:

if CX <> 0 then

• do following chain instruction
• CX = CX - 1
• go back to check_cx

else

• exit from REP cycle

Z r REPE chain instruction
Repeat following CMPSB, CMPSW, SCASB, SCASW instructions while ZF = 1 (result is Equal), maximum CX times.

Algorithm:

check_cx:

if CX <> 0 then

• do following chain instruction
• CX = CX - 1
• if ZF = 1 then:
  • go back to check_cx
  else
  • exit from REPE cycle

else

• exit from REPE cycle

Example:
see cmpsb.asm in c:\emu8086\examples\.

Z r REPNE chain instruction
Repeat following CMPSB, CMPSW, SCASB, SCASW instructions while ZF = 0 (result is Not Equal), maximum CX times.

Algorithm:

check_cx:

if CX <> 0 then

• do following chain instruction
• CX = CX - 1
• if ZF = 0 then:
  • go back to check_cx
  else
  • exit from REPNE cycle

else

• exit from REPNE cycle

Z r REPNZ chain instruction
Repeat following CMPSB, CMPSW, SCASB, SCASW instructions while ZF = 0 (result is Not Zero), maximum CX times.

Algorithm:

check_cx:

if CX <> 0 then

• do following chain instruction
• CX = CX - 1
• if ZF = 0 then:
  • go back to check_cx
  else
  • exit from REPNZ cycle

else

• exit from REPNZ cycle

Z r REPZ chain instruction
Repeat following CMPSB, CMPSW, SCASB, SCASW instructions while ZF = 1 (result is Zero), maximum CX times.

Algorithm:

check_cx:

if CX <> 0 then

• do following chain instruction
• CX = CX - 1
• if ZF = 1 then:
  • go back to check_cx
  else
  • exit from REPZ cycle

else

• exit from REPZ cycle

Z r RET No operands
or even immediate Return from near procedure.

Algorithm:

• Pop from stack:
  • IP
• if immediate operand is present: SP = SP + operand

Example: ORG 100h ; for COM file. CALL p1 ADD AX, 1 RET ; return to OS. p1 PROC ; procedure declaration. MOV AX, 1234h RET ; return to caller. p1 ENDP C Z S O P A unchanged RETF No operands
or even immediate Return from Far procedure.

Algorithm:

• Pop from stack:
  • IP
  • CS
• if immediate operand is present: SP = SP + operand

C Z S O P A unchanged ROL memory, immediate
REG, immediate

memory, CL
REG, CL Rotate operand1 left. The number of rotates is set by operand2.

Algorithm:

shift all bits left, the bit that goes off is set to CF and the same bit is inserted to the right-most position.

Example: MOV AL, 1Ch ; AL = 00011100b ROL AL, 1 ; AL = 00111000b, CF=0. RET C O r r OF=0 if first operand keeps original sign. ROR memory, immediate
REG, immediate

memory, CL
REG, CL Rotate operand1 right. The number of rotates is set by operand2.

Algorithm:

shift all bits right, the bit that goes off is set to CF and the same bit is inserted to the left-most position.

Example: MOV AL, 1Ch ; AL = 00011100b ROR AL, 1 ; AL = 00001110b, CF=0. RET C O r r OF=0 if first operand keeps original sign. SAHF No operands Store AH register into low 8 bits of Flags register.

Algorithm:

flags register = AH

AH bit: 7 6 5 4 3 2 1 0 [SF] [ZF] [0] [AF] [0] [PF] [1] [CF] bits 1, 3, 5 are reserved.

C Z S O P A r r r r r r SAL memory, immediate
REG, immediate

memory, CL
REG, CL Shift Arithmetic operand1 Left. The number of shifts is set by operand2.

Algorithm:

• Shift all bits left, the bit that goes off is set to CF.
• Zero bit is inserted to the right-most position.

Example: MOV AL, 0E0h ; AL = 11100000b SAL AL, 1 ; AL = 11000000b, CF=1. RET C O r r OF=0 if first operand keeps original sign. SAR memory, immediate
REG, immediate

memory, CL
REG, CL Shift Arithmetic operand1 Right. The number of shifts is set by operand2.

Algorithm:

• Shift all bits right, the bit that goes off is set to CF.
• The sign bit that is inserted to the left-most position has the same value as before shift.

Example: MOV AL, 0E0h ; AL = 11100000b SAR AL, 1 ; AL = 11110000b, CF=0. MOV BL, 4Ch ; BL = 01001100b SAR BL, 1 ; BL = 00100110b, CF=0. RET C O r r OF=0 if first operand keeps original sign. SBB REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Subtract with Borrow.

Algorithm:

operand1 = operand1 - operand2 - CF

Example: STC MOV AL, 5 SBB AL, 3 ; AL = 5 - 3 - 1 = 1 RET C Z S O P A r r r r r r SCASB No operands Compare bytes: AL from ES:[DI].

Algorithm:

• ES:[DI] - AL
  
• set flags according to result:
  OF, SF, ZF, AF, PF, CF
  
• if DF = 0 then
  • DI = DI + 1
  else
  • DI = DI - 1

C Z S O P A r r r r r r SCASW No operands Compare words: AX from ES:[DI].

Algorithm:

• ES:[DI] - AX
  
• set flags according to result:
  OF, SF, ZF, AF, PF, CF
  
• if DF = 0 then
  • DI = DI + 2
  else
  • DI = DI - 2

C Z S O P A r r r r r r SHL memory, immediate
REG, immediate

memory, CL
REG, CL Shift operand1 Left. The number of shifts is set by operand2.

Algorithm:

• Shift all bits left, the bit that goes off is set to CF.
• Zero bit is inserted to the right-most position.

Example: MOV AL, 11100000b SHL AL, 1 ; AL = 11000000b, CF=1. RET C O r r OF=0 if first operand keeps original sign. SHR memory, immediate
REG, immediate

memory, CL
REG, CL Shift operand1 Right. The number of shifts is set by operand2.

Algorithm:

• Shift all bits right, the bit that goes off is set to CF.
• Zero bit is inserted to the left-most position.

Example: MOV AL, 00000111b SHR AL, 1 ; AL = 00000011b, CF=1. RET C O r r OF=0 if first operand keeps original sign. STC No operands Set Carry flag.

Algorithm:

CF = 1

C 1 STD No operands Set Direction flag. SI and DI will be decremented by chain instructions: CMPSB, CMPSW, LODSB, LODSW, MOVSB, MOVSW, STOSB, STOSW.

Algorithm:

DF = 1

D 1 STI No operands Set Interrupt enable flag. This enables hardware interrupts.

Algorithm:

IF = 1

I 1 STOSB No operands Store byte in AL into ES:[DI]. Update DI.

Algorithm:

• ES:[DI] = AL
  
• if DF = 0 then
  • DI = DI + 1
  else
  • DI = DI - 1

Example: ORG 100h LEA DI, a1 MOV AL, 12h MOV CX, 5 REP STOSB RET a1 DB 5 dup(0) C Z S O P A unchanged STOSW No operands Store word in AX into ES:[DI]. Update DI.

Algorithm:

• ES:[DI] = AX
  
• if DF = 0 then
  • DI = DI + 2
  else
  • DI = DI - 2

Example: ORG 100h LEA DI, a1 MOV AX, 1234h MOV CX, 5 REP STOSW RET a1 DW 5 dup(0) C Z S O P A unchanged SUB REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Subtract.

Algorithm:

operand1 = operand1 - operand2

Example: MOV AL, 5 SUB AL, 1 ; AL = 4 RET C Z S O P A r r r r r r TEST REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Logical AND between all bits of two operands for flags only. These flags are effected: ZF, SF, PF. Result is not stored anywhere.

These rules apply:

1 AND 1 = 1
1 AND 0 = 0
0 AND 1 = 0
0 AND 0 = 0


Example: MOV AL, 00000101b TEST AL, 1 ; ZF = 0. TEST AL, 10b ; ZF = 1. RET C Z S O P 0 r r 0 r XCHG REG, memory
memory, REG
REG, REG Exchange values of two operands.

Algorithm:

operand1 < - > operand2

Example: MOV AL, 5 MOV AH, 2 XCHG AL, AH ; AL = 2, AH = 5 XCHG AL, AH ; AL = 5, AH = 2 RET C Z S O P A unchanged XLATB No operands Translate byte from table.
Copy value of memory byte at DS:[BX + unsigned AL] to AL register.

Algorithm:

AL = DS:[BX + unsigned AL]

Example: ORG 100h LEA BX, dat MOV AL, 2 XLATB ; AL = 33h RET dat DB 11h, 22h, 33h, 44h, 55h C Z S O P A unchanged XOR REG, memory
memory, REG
REG, REG
memory, immediate
REG, immediate Logical XOR (Exclusive OR) between all bits of two operands. Result is stored in first operand.

These rules apply:

1 XOR 1 = 0
1 XOR 0 = 1
0 XOR 1 = 1
0 XOR 0 = 0


Example: MOV AL, 00000111b XOR AL, 00000010b ; AL = 00000101b RET C Z S O P A 0 r r 0 r ?

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