8086 Instruction Set

8086 INSTRUCTION SET

•Data transfer instruction•Arithmetic instruction•Bit manipulated instruction•String instruction•Iteration control instruction•Branch instruction•Data size conversion instruction•Processor control instruction

MOV Instruction

• Syntax:• Action: Copies the data from src to dest• Src: Immediate data/reg/mem location• Dest: reg/mem location• Flags: No flags are affected

MOV destination, source

• MOV AX, BX• MOV AX, [BX]• MOV AL,[1234H]• MOV AL,BL• MOV AL,A• MOV SP, BP

• MOV A,B• MOV [BX],[SI]• MOV 1234H,BX• MOV IP, AX• MOV CS, AX• MOV DS, ES• MOV DS, 1234H• MOV AL,BX

Valid instruction

Invalid instruction

MOV AX, [BX]

BEFORE AFTER

BX = 9ABCHDS = 5678HAX = 1234H

BX = 9ABCHDS = 5678HAX = 4433H

PA = BX + (DS)*10H = 9ABC + 5678*10H = 6023CH

11H

22H

33H

44H

55H

………60239H6023AH

6023BH6023CH6023DH

6023EH

DS MemoryDS Memory

………60239H6023AH

6023BH6023CH6023DH

6023EH

Memory read

11H

22H

33H

44H

55H

MOV 1234H[BP],SI

BEFORE AFTER

SI = 5678HBP = AB12HSS = 9000H

SI = 5678HBP = AB12HSS = 9000H

PA = 1234 + BP + (SS)*10H = 1234 + AB12 + 9000*10H = 9BD46H

11H

22H

33H

44H

55H

11H

22H

78H

56H

55H

………9BD43H9BD44H

9BD45H9BD46H9BD47H

9BD48H

SS Memory SS Memory

………9BD43H9BD44H

9BD45H9BD46H9BD47H

9BD48H

Memory write

MOV CS:[BX][DI],53H

BEFORE AFTER

DI = 1234HBX = AB12HCS = 9000H

DI = 1234HBX = AB12HCS = 9000H

PA = DI + BX + (CS)*10H = 1234 + AB12 + 9000*10H = 9BD46H

11H

22H

33H

44H

55H

11H

22H

53H

44H

55H

………9BD43H9BD44H

9BD45H9BD46H9BD47H

9BD48H

CS Memory CS Memory

………9BD43H9BD44H

9BD45H9BD46H9BD47H

9BD48H

Memory write

MOV [BX],AL

BEFORE AFTER

AL= 34HBX = 5678HDS = 9000H

AL = 34HBX = 5678HDS = 9000H

PA = BX + (DS)*10H = 5678 + 9000*10H = 95678H

99H 34H95678H

Memory Memory

95678H

Memory write

LEA -Load Effective Address(EA)

• Syntax: reg - BX,SI,DI,BP,SP src - memory location

• Action: 1) instr copies the EA of the memory & it is transferred to the specified reg.

2) instr is reqd for initializing the memory pointers (BX,SI,DI,BP,SP)

3) instr does not transfer data from memory.

LEA reg, src

• LEA BX, DS:1234H• LEA BX, [1234H]• LEA BX, MSG• LEA CX, MSG• LEA BP, MSG• LEA SP, MSG

• LEA DS, 56H[SI]• LEA BX, SI• LEA BX, 1234H• LEA IP, MSG

Valid instruction

Invalid instruction

LEA BX, 56H[SI]

BEFORE AFTER

BX = 1000HSI = 7894HFLAG = 0234H

BX = 78EAHSI = 7894HFLAG = 0234H

LEA BX, 56H[SI]

EA = 56H + 7894H = 78EAH

LEA BP, 05H[BX+DI]BEFORE AFTER

BX = A1B2HDI = 5678HBP = 3239H

BX = A1B2HDI = 5678HBP = F82FH

LEA BP, 05H[BX+DI]

EA = 05H + A1B2H + 5678H = F82FH

LDS-Load Register And Data Segment

• Syntax: reg – 16 bit reg except IP src - memory location

• Action: 1) instr transfers 32-bit data from memory.

2) 1st 16-bits transferred to reg specified 3) 2nd 16-bits transferred to DS reg .

LDS reg, src

LDS AX, [1234H]

BEFORE AFTER

DS = 2000HAX = 0000H

DS = 7856HAX = 3412H

LDS AX, [1234H]

PA = 1234H + (DS)*10H = 1234 + 20000H = 21234H

12H

34H

56H

78H

12H

34H

56H

78H

2123421235

2123621237

2123421235

2123621237

Memory Memory

2nd word1st word

LES-Load Register And Extra Segment

• Syntax: reg – 16 bit reg except IP src - memory location

• Action: 1) instr transfers 32-bit data from memory.

2) 1st 16-bits transferred to reg specified 3) 2nd 16-bits transferred to ES reg .

LES reg, src

LES BX, [1234H]

BEFORE AFTERDS = 2000HBX = 0000HES = 9000H

DS = 2000HBX = 3412HES = 7856H

LES BX, [1234H]

PA = 1234H + (DS)*10H = 1234 + 20000H = 21234H

12H

34H

56H

78H

12H

34H

56H

78H

2123421235

2123621237

2123421235

2123621237

Memory Memory

2nd word1st word

XLAT-Translate Instruction

• Action: 1) instr is useful to convert from one code to another

using lookup table 2) has no operands & is implied mode of addressing. So

prior to execution of this instr, BX should be initialized to contain the start address of the lookup table & AL should contain the code to be converted.

3) After executing the instr, 1 byte data stored in offset address AL+BX in the DS memory is copied into AL ie

(AL)Ds:[(BX) + (AL)]4) AL contains the converted code.

XLAT

BEFORE AFTERAL = 0AHBX = 0100HDS = 9000H

AL = 41HBX = 0100HDS = 9000H

PA = (AL) + (BX) +(DS)*10H = 0AH + 0100H + 90000H = 9010AH

41H

90000H ------90100 H

9010AH

DS Memory

LOOKUP TABLE

Read from table

Starting addr of DS

Starting addr of lookup table

• LAHF (Load AH with lower byte of flag reg) -- no operands

STCMOV AX,46H ; AX=0046HLAHF ; AX=0346H

• SAHF (Store AH contents in lower byte of flag reg) -- no operands

• XCHG (Exchange operation) Syntax: XCHG op1,op2XCHG BX, 1234[BX+SI]XCHG [BX],[SI]XCHG CS,BX

Invalid Instruction

SF ZF 0 AF 0 PF 1 CF

7 6 5 4 3 2 1 0

STACK

PUSH BX

BEFORE AFTER

SP = 1234HSS = 5000HBX = 5678H

SP = 1232HSS = 5000HBX = 5678H

PA = 1234H + (SS)*10H = 1234 + 50000H = 51234H

C3H

78H

56H

C3H

…….. 5122FH

51232H

51234H

SS Memory SS Memory

Top of stack

…….. 5122FH

51232H

51234H

Top

POP BX

BEFORE AFTER

SP = 1232HSS = 5000HBX = 1234H

SP = 1234HSS = 5000HBX = 5678H

PA = 1232H + (SS)*10H = 1234 + 50000H = 51232H

78H

56H

C3H C3H

…….. 5122FH

51232H

51234H

SS Memory SS Memory

Top of stack

…….. 5122FH

51232H

51234HTop

STRING INSTRUCTIONS

• An array (a sequence) of characters is called a STRING• STR DB “SVIT”

……..

………

53

56

49

54

…….

…….

STR[0]STR[1]STR[2]STR[3]

2000=2000+02000=2000+12000=2000+22000=2000+3

Base address of array STR

•Data segment register (used as source segment)•Extra segment register (used as destination segment)

•Source index register(SI) should be associated with DS•Destination index register(DI) should be associated with ES

STRING INSTRUCTIONS IN 8086

1. MOVSB/ MOVSW2. CMPSB/ CMPSW3. SCASB/ SCASW4. LODSB/ LODSW5. STOSB/ STOSW6. REP/REPE/REPZ/REPNE/REPNZ

• During string manipulation, SI register should contain offset address of the source byte or word with respect to data segment.

LEA SI, STR1 or MOV SI, OFFSET STR1

• During string manipulation, DI register should contain offset address of the destination byte or word with respect to extra segment.

LEA DI, STR2 or MOV DI, OFFSET STR2

STOSB -store byte from AL into string element addressed by DI in ES & updates DI (ES:[DI])

• ES:[(DI)] [AL]• No flags affected• If DF=0 then DI=DI+1• If DF=1 then DI=DI-1• Segment for memory access : ES

MOV AL,12HMOV CX,5REP STOSB

ES=4F57

12 12 12 12 12

DI=02 DI=03 DI=04 DI=05 DI=06

STOSW -store word from AX into string element addressed by DI in ES & updates DI (ES:[DI])

• ES:[(DI)] [AX]• No flags affected• If DF=0 then DI=DI+2• If DF=1 then DI=DI-2• Segment for memory access : ES

MOV AX,1234HMOV CX,5REP STOSW

ES=4F57

1234 1234 1234 1234 1234

DI=02 DI=04 DI=06 DI=08 DI=0A

LODSB –load byte at DS:[SI] into AL & updates SI

• DS:[SI] AL• No flags affected• If DF=0 then SI=SI+1• If DF=1 then SI=SI-1• Segment for memory access : DS

LEA SI, xMOV CX,5REP LODSB

AL=41h

41 42 43 44 45

SI=02 SI=03 SI=04 SI=05 SI=06

x db ‘A’, ’B’ , ’C’ , ’D’ , ’E’

AL=42h AL=43h AL=44h AL=45h

LODSW –load word at DS:[SI] into AX & updates SI

• AXDS:[SI] • No flags affected• If DF=0 then SI=SI+2• If DF=1 then SI=SI-2• Segment for memory access : DS

LEA SI, xMOV CX,5REP LODSW

AX=0111h

11 01 22 02 33 03 44 04 55 05

SI=02 SI=04 SI=06 SI=08 SI=0A

x dw 111h, 222h, 333h, 444h, 555h

AX=0222h AX=0333h AX=0444h AX=0555h

MOVSB –Copy Byte at DS:[SI] to ES:[DI] & updates SI and DI

• ES:[DI] DS:[SI] • No flags affected• If DF=0 then SI=SI+1, DI=DI+1• If DF=1 then SI=SI-1, DI=DI-1• Segment for memory access : DS and ES

CLD ;set forward directionLEA SI, xLEA DI, yMOV CX,5REP MOVSB

01 02 03 04 05

SI=01 SI=02 SI=03 SI=04 SI=05

y db 5 dup(0)x db 1, 2, 3, 4, 5

Data Segment DS

Extra Segment ES 00 00 00 00 00

DI=06 DI=07 DI=08 DI=09 DI=0A

01 02 03 04 05

Move string from one memory location to another

.model small

.data str db “Test string” N dw $-str dest db 10 dup(?)

.code Mov ax,@data Mov ds,ax Mov es,axLea si,strLea di,dest Cld Mov cx,N

Rep movsb

Mov ah,4chInt 21h

MOVSW –Copy word at DS:[SI] to ES:[DI] & updates SI and DI

• ES:[DI] DS:[SI] • No flags affected• If DF=0 then SI=SI+2, DI=DI+2• If DF=1 then SI=SI-2, DI=DI-2• Segment for memory access : DS and ES

CLD ;set forward directionLEA SI, xLEA DI, yMOV CX,5REP MOVSW

01 00 02 00 03 00 04 00 05 00

SI=02 SI=04 SI=06 SI=08 SI=0A

y dw 5 dup(0)x dw 1, 2, 3, 4, 5

Data Segment DS

Extra Segment ES 00 00 00 00 00

DI=07 DI=09 DI=0A DI=0C DI=0E

01 00 02 00 03 00 04 00 05 00

Move string from one memory location to another

.model small

.data str db “Test string” N dw ($-str)/2 dest db 10 dup(?)

.code Mov ax,@data Mov ds,ax Mov es,axLea si,strLea di,dest Cld Mov cx,N

Rep movsw

Mov ah,4chInt 21h

SCASB –Compare bytes: ES:[DI] from AL & updates DI

• AL - ES:[DI]• If DF=0 then DI=DI+1• If DF=1 then DI=DI-1• Segment for memory access : ES• All flags are updated to reflect the relationship

of the destination operand to the source operand

SCASW –Compare words: ES:[DI] from AX & updates DI

• AX - ES:[DI] • If DF=0 then DI=DI+2• If DF=1 then DI=DI-2• Segment for memory access : ES• All flags are updated according to result:

OF,SF,ZF,AF,CF,PF

CMPSB –Compare bytes: ES:[DI] from DS:[SI] & updates DI and SI

• DS:[SI] - ES:[DI] • If DF=0 then DI=DI+1, SI=SI+1• If DF=1 then DI=DI-1, SI=SI-1• Segment for memory access : DS, ES• All flags are updated to reflect the relationship

of the destination operand to the source operand

CLDMOV AX,dataMOV DS,AXMOV ES,AXMOV SI,STR1MOV DI,STR2MOV CX, size ;set counter to string lengthREPE CMPSB ;compare until equalJNZ not_equalMOV AL,’y’JMP exit

not_equal: MOV AL,’n’

exit: MOV AH,04CH INT 21H

STR1 Db ‘test string’STR2 Db ‘test string’

CMPSW –Compare words: ES:[DI] from DS:[SI] & updates DI and SI

• DS:[SI] - ES:[DI] • If DF=0 then DI=DI+2, SI=SI+2• If DF=1 then DI=DI-2, SI=SI-2• Segment for memory access : DS, ES• All flags are updated to reflect the relationship

of the destination operand to the source operand

CLDMOV AX,dataMOV DS,AXMOV ES,AXMOV SI,STR1MOV DI,STR2MOV CX, size ;set counter to string lengthREPE CMPSW ;compare until equalJNZ not_equalMOV AL,’y’JMP exit

not_equal: MOV AL,’n’

exit: MOV AH,04CH INT 21H

STR1 Dw 1234h, 5678h, 9012h, 3456hSTR2 Dw 1234h, 5678h, 9012h, 3456h