Chap 3_2.ppt

Instruction Set of 8086
An instruction is a binary pattern designed
inside a microprocessor to perform a specific
function.
The entire group of instructions that a
microprocessor supports is called
Instruction Set.
8086 has more than 20,000 instructions.
2

Classification of Instruction Set
1. Arithmetic Instructions and Logical Instruction
2. Data Transfer Instructions
3. Branch and Loop Instruction
4. M/C control Instruction
5. Flag Manipulation Instructions
6. Shift and Rotate Instruction
7. String Instructions
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ohmshankar.ece@act.edu.in

1. Arithmetic And Logical
Instructions

Arithmetic Instructions
ADD Des, Src:
It adds a byte to byte or a word to word.
It effects AF, CF, OF, PF, SF, ZF flags.
E.g.:
 ADD AL, 74H
 ADD DX, AX
 ADD AX, [BX]
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ADC Des, Src:
It adds the two operands with CF.
E.g.:
 ADC AL, 74H
 ADC DX, AX
 ADC AX, [BX]
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SUB Des, Src:
 It subtracts a byte from byte or a word from word.
 It effects AF, CF, OF, PF, SF, ZF flags.
 For subtraction, CF acts as borrow flag.
 E.g.:
 SUB AL, 74H
 SUB DX, AX
 SUB AX, [BX]
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SBB Des, Src:
It subtracts the two operands and also the
borrow from the result.
E.g.:
 SBB AL, 74H
 SBB DX, AX
 SBB AX, [BX]
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 MUL Src:
 It is an unsigned multiplication instruction.
 It multiplies two bytes to produce a word or two words to
produce a double word.
 AX = AL * Src
 DX : AX = AX * Src
 This instruction assumes one of the operand in AL or AX.
 Src can be a register or memory location. And Flags-OF, CF
 Unused bits of destination register is always filled with sign bit
 IMUL Src:
 It is a signed multiplication instruction.
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DIV Src:
It is an unsigned division instruction.
It divides word by byte or double word by word.
The operand is stored in AX, divisor is Src and
the result is stored as:
 AH = remainder, AL = quotient (for word/byte)
 DX=remainder, AX=quotient (for D-word/word)
IDIV Src:
It is a signed division instruction.
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 CBW (Convert Byte to Word):
 This instruction converts byte in AL to word in AX.
 The conversion is done by extending the sign bit of AL
throughout AH.
 CWD (Convert Word to Double Word):
 This instruction converts word in AX to double word in
DX : AX.
 The conversion is done by extending the sign bit of AX
throughout DX.
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INC Src:
It increments the byte or word by one.
The operand can be a register or memory
location.
E.g.: INC AX
INC [SI]
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DEC Src:
It decrements the byte or word by one.
The operand can be a register or memory
location.
E.g.: DEC AX
DEC [SI]
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 CMP Des, Src:
 It compares two specified bytes or words.
 The Src and Des can be a constant, register or memory
location.
 Both operands cannot be a memory location at the same
time.
 The comparison is done simply by internally subtracting
the source from destination.
 The value of source and destination does not change, but
the flags CF, ZF, SF are modified to indicate the result.
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NEG Src:
It creates 2’s complement of a given
number.
That means, it changes the sign of a
number.
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 DAA (Decimal Adjust after Addition)
 It is used to make sure that the result of adding two BCD
numbers is adjusted to be a correct BCD number.
 It only works on AL register.
 For Subtraction : DAS (Decimal Adjust after
Subtraction)
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 AAA (ASCII Adjust after Addition):
 This Instruction Can be used to convert the contents of the
AL register to unpacked BCD result
 i.e. If lower nibble of AL>9 then
1. AL=AL+6 2. AH=AH+1
3. AL=AL AND 0FH
 This instruction does not have any operand.
 Other ASCII Instructions:
 AAS (ASCII Adjust after Subtraction)
 AAM (ASCII Adjust after Multiplication)
 AAD (ASCII Adjust Before Division)
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 NOT Src:
 It complements each bit of Src to produce 1’s
complement of the specified operand.
 The operand can be a register or memory location.
 e,.g NOT AX
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Logical Instructions

Logical Instructions
 AND Des, Src:
 It performs AND operation of Des and Src.
 Src can be immediate number, register or memory
location.
 Des can be register or memory location.
 Both operands cannot be memory locations at the same
time.
 CF and OF become zero after the operation.
 PF, SF and ZF are updated.
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Bit Manipulation Instructions
 OR Des, Src:
 It performs OR operation of Des and Src.
location.
time.
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 XOR Des, Src:
 It performs XOR operation of Des and Src.
location.
time.
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 TEST Des, Src:
 It performs AND operation of Des and Src.
 Src can be immediate number, and src/Des can be
register or memory location.
 It is Non-Destructive And means Dest is not modified
only flags are affected.
time.
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2. Data Transfer Instructions
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Data Transfer Instructions
 MOV Des, Src:
 It is used to copy the content of Src to Des
 Src operand can be register, memory location or immediate
operand.
 Des can be register or memory operand.
 Both Src and Des cannot be memory location at the same
time.
 E.g.:
 MOV CX, 037A H
 MOV AL, BL
 MOV BX, [0301 H]
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 PUSH Operand:
 It pushes the operand into top of stack.
 E.g.: PUSH BX
 POP Des:
 It pops the operand from top of stack to Des.
 Des can be a general purpose register, segment register
(except CS) or memory location.
 E.g.: POP AX
25

 XCHG Des, Src:
 This instruction exchanges Src with Des.
 It cannot exchange two memory locations directly.
 E.g.: XCHG DX, AX
26

 IN Accumulator, Port Address:
 It transfers the operand from specified port to accumulator
register.
 E.g.: IN AX, 0028 H
 OUT Port Address, Accumulator:
 It transfers the operand from accumulator to specified port.
 E.g.: OUT 0028 H, AX
27

LEA Register, Src:
It loads a 16-bit register with the offset
address of the data specified by the Src.
E.g.: LEA BX, [DI]
This instruction loads the contents of DI
(offset) into the BX register.
28

 LDS Des, Src:
 It loads 32-bit pointer from memory source to
destination register and DS.
 The word is placed in the destination register and the
segment is placed in DS.
 This instruction Copies the word at the lower memory
address to the Des reg and the word at the higher
address to the segment reg i.e. DS.
 E.g.: LDS BX, [0301 H]
29

 LES Des, Src:
 It loads 32-bit pointer from memory source to
destination register and ES.
 The Word is placed in the destination register and the
segment is placed in ES.
 This instruction is very similar to LDS except that it
initializes ES instead of DS.
 E.g.: LES BX, [0301 H]
30

 LAHF:
 It copies the lower byte of flag register to AH.
 SAHF:
 It copies the contents of AH to lower byte of flag register.
 PUSHF:
 Pushes flag register to top of stack.
 POPF:
 Pops the stack top to flag register.
31

3. Branch/Program Execution Transfer
Instructions
 These instructions cause change in the sequence of the
execution of instruction.
 This change can be a conditional or sometimes
unconditional.
 The conditions are represented by flags.
32

Branch Instructions
 CALL Des:
 This instruction is used to call a subroutine or function
or procedure.
 The address of next instruction after CALL is saved onto
stack.
 RET:
 It returns the control from procedure to calling program.
 Every CALL instruction should have a RET.
33

Branch Instructions
 JMP Des:
 This instruction is used for unconditional jump from
one place to another.
 Jxx Des (Conditional Jump):
 All the conditional jumps follow some conditional
statements or any instruction that affects the flag.
34

Conditional Jump Table
Mnemonic Meaning Jump Condition
JA Jump if Above CF = 0 and ZF = 0
JAE Jump if Above or Equal CF = 0
JB Jump if Below CF = 1
JBE Jump if Below or Equal CF = 1 or ZF = 1
JC Jump if Carry CF = 1
JE Jump if Equal ZF = 1
JNC Jump if Not Carry CF = 0
JNE Jump if Not Equal ZF = 0
JNZ Jump if Not Zero ZF = 0
JPE Jump if Parity Even PF = 1
35

Loop Instructions
 Loop Des:
 This is a looping instruction.
 The number of times looping is required is placed in the
CX register.
 With each iteration, the contents of CX are
decremented.
 ZF is checked whether to loop again or not.
36

Program Execution Transfer Instructions
 INTO (Interrupt on overflow):
 This instruction generates type 4 interrupt (i.e.
interrupt for overflow) and causes the 8086 to do an
indirect far call a procedure which is written by the
user to handle the overflow condition.
 IRET
 To return the execution to the interrupted program
37

4. Machine Control
Instructions

Machine Control Instructions
HLT (Halt) :- It causes the processor to enter in to the halt
state. It can be stop by INTR,NMI or RESET pin
NOP (No Opration) :- It causes the processor to enter in
to the wait state for 3 Clock cycles.
WAIT :- It causes the processor to enter in to the ideal state.
Can be stop by TEST, INTR OR NMI pin
LOCK :- This instruction prevents other processors to
take the control of shared resources. For e.g LOCK IN
AL,80H
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5. Flag Manipulation
Instructions

 STC:
 It sets the carry flag to 1.
 CLC:
 It clears the carry flag to 0.
 CMC:
 It complements the carry flag.
41
Flag Manipulation Instructions

 STD:
 It sets the direction flag to 1.
 If it is set, string bytes are accessed from higher memory
address to lower memory address.
 CLD:
 It clears the direction flag to 0.
 If it is reset, the string bytes are accessed from lower
memory address to higher memory address.
42

 STI:
 It sets the Interrupt flag to 1.
 CLI:
 It clears the Interrupt flag to 0.
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6. Shift And Rotate Instructions

 SHL/SAL Des, Count:
 It shift bits of byte or word left, by count.
 It puts zero(s) in LSBs.
 MSB is shifted into carry flag.
 If the number of bits desired to be shifted is 1, then the
immediate number 1 can be written in Count.
 However, if the number of bits to be shifted is more than
1, then the count is put in CL register. And recent bit to
the CF (Carry flag)
45
Shift And Rotate Instructions

 SHR/SAR Des, Count:
 It shift bits of byte or word right, by count.
 It puts zero(s)(for SHL) and Sign bit (for SAL) in MSBs.
 LSB is shifted into carry flag.
the CF (Carry flag)
46

 ROL Des, Count:
 It rotates bits of byte or word left, by count.
 LSB is transferred to MSB and also to CF.
the CF (Carry flag)
47

 ROR Des, Count:
 It rotates bits of byte or word right, by count.
 MSB is transferred to LSB and also to CF.
the CF (Carry flag)
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 RCL Des, Count:
 It rotates bits of byte or word right, by count.
 LSB to MSB then MSB is transferred to CF and CF to
LSB.
the CF (Carry flag)
49

 RCR Des, Count:
 It rotates bits of byte or word left, by count.
 MSB to LSB then LSB is transferred to CF and CF to
MSB.
the CF (Carry flag)
50

7. String Manipulation
Instructions

String Manipulation
Instructions
 String in assembly language is just a sequentially
stored bytes or words.
 There are very strong set of string instructions in 8086.
 By using these string instructions, the size of the
program is considerably reduced.
52

String Manipulation Instructions
53
8086 Microprocessor
 String : Sequence of bytes or words
 8086 instruction set includes instruction for string movement, comparison, scan, load and
store.
 REP instruction prefix : used to repeat execution of string instructions
 String instructions end with S or SB or SW. S represents string, SB string byte and
SW string word.
 Offset or effective address of the source operand is stored in SI register and that of the
destination operand is stored in DI register.
 Depending on the status of DF, SI and DI registers are automatically updated.
 DF = 0  SI and DI are incremented by 1 for byte and 2 for word.
 DF = 1  SI and DI are decremented by 1 for byte and 2 for word.

String Manipulation
Instructions
 MOVS / MOVSB / MOVSW:
 It causes moving of byte or word from one string to
another.
 In this instruction, the source string is in Data Segment
referred by DS:SI and destination string is in Extra
Segment referred by ES:DI.
 For e.g. movs str1,str2
 Movsb
 Movsw
54

String Manipulation
Instructions
 LODS / LODSB / LODSW:
 It causes TRANSFER of byte or word from one string to
another.
 In this instruction, the source string is in Data Segment
referred by DS:SI transferred to Accumulator.
 For e.g. lods string
 lodsb
 lodsw
55

String Manipulation
Instructions
 STOS / STOSB / STOSW:
It causes TRANSFER of byte or word from one string to
another.
In this instruction, the string is in Extra Segment referred by
ES:DI transferred to Accumulator.
 For e.g. stos string
 stosb
 stosw
56

String Manipulation
Instructions
 CMPS Des, Src:
 It compares the string bytes or words.
 SCAS String:
 It scans a string.
 It compares the String with byte in AL or with word in
AX.
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String Manipulation
Instructions
 REP (Repeat):
 This is an instruction prefix.
 It causes the repetition of the instruction until CX
becomes zero.
 E.g.: REP MOVSB
 It copies byte by byte contents.
 REP repeats the operation MOVSB until CX becomes zero.
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59
8086 Microprocessor
REP
REPZ/ REPE
(Repeat CMPS or SCAS
until ZF = 0)
REPNZ/ REPNE
(Repeat CMPS or SCAS
until ZF = 1)
While CX  0 and ZF = 1, repeat execution of
string instruction and
(CX)  (CX) – 1
While CX  0 and ZF = 0, repeat execution of
string instruction and
(CX)  (CX) - 1
String Manipulation
Instructions

Chap 3_2.ppt

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