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�Instruction Set of 8086

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Why study instruction set?

  • Study of instruction set is required to write instructions in machine code that can be recognized and executed by a central processing unit.
  • The knowledge will help you write lines of code or program by which you will be able to tell the processor, the sequence of operations to be performed.

​

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What are we going to study?

Instruction Set

    • Different types of instructions with examples
    • How to use instructions in assembly language programming

​

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Types of Instructions in 8086

  1. Data Copy/Transfer instructions
  2. Arithmetic
  3. Logical instructions
  4. Shift & Rotate instructions
  5. Branch instructions
  6. Loop instructions
  7. Machine Control instructions
  8. Flag Manipulation instructions
  9. String instructions

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1: Data Copy/Transfer instructions

  • There will be transfer of data from source to destination.

MOV Destination, Source

  • Source can be register, memory location or immediate data.
  • Destination can be register or memory operand.
  • Both Source and Destination cannot be memory location or segment registers at the same time.

MOV AX,BX

​

​

AH AL

AX

FF 33

BX

BH AL

​

​

10 AB

Eg:

10 AB

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1: Data Copy/Transfer instructions

  • Pushes the 16 bit content specified by source in the instruction on to the stack

PUSH Source

  • Pushing operation decrements stack pointer.
  • Stack pointer is a 16-bit register, contains the address of the data item currently on top of the stack.

​

PUSH BX

SP

BX

BH BL

​

​

10 AB

Eg:

10 AB

​

​

​

​

​

​

FF

33

5000

5001

5002

5003

​

​

50

10

AB

02

01

00

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1: Data Copy/Transfer instructions

  • Pops the 16 bit content from stack to destination specified in instruction.

POP Destination

  • Popping operation increments stack pointer stack pointer.

​

POP DS

SP

DS

​

​

22

Eg:

​

​

​

​

​

​

FF

33

5000

5001

5002

5003

​

​

50

02

01

00

10

AB

33

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1: Data Copy/Transfer instructions

  • This instruction exchanges contents of Source with destination.

XCHG Destination, Source

  • It cannot exchange two memory locations directly.

XCHG BX,AX

Eg:

AX

BX

​

​

​

​

FF 33

10 AB

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1: Data Copy/Transfer instructions

  • It copies data to accumulator from a port with 8-bit or 16-bit address.

IN AL/AX, 8-bit/16-bit port address�

  • DX is the only register is allowed to carry port address.

IN AL, 80H

Eg:

AH AL

AX

​

​

12

IN AX, DX

​

​

80

PORT

AH AL

AX

​

​

PORT

Eg:

​

​

DX

80 00

12 34

​

​

80 00

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1: Data Copy/Transfer instructions

XLAT�

  • It translates code of the key pressed to the corresponding 7-segment code.
  • After execution this instruction contents of AL register always gets replaced.

MOV AX, TABLE SEGMENT ADDRESS

MOV DS, AX

MOV AL, DISPLAY CODE

MOV BX, OFFSET TABLE

XLAT

Eg:

AH AL

AX

​

​

12

​

​

50 00

BX

  • Translate instruction is used to find out codes in case of code conversion.

​

​

​

​

​

​

3000:5000

3000:5001

3000:5002

3000:5003

7E

30

6D

79

12

​

​

30 00

DS

6D

30 00

X 02

X

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2: Arithmetic Instructions

  • Addition
  • Subtraction
  • Increment
  • Decrement
  • Multiply
  • Divide

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2: Arithmetic Instructions

ADD Destination, Source�

  • The source may be immediate data, memory location or register.
  • The destination may be memory location or register.
  • AX is the default destination register.

​

ADD AX,BX

Eg:

AH AL

AX

​

​

12

​

​

50 00

BX

  • This instruction adds the contents of source operand with the contents of destination operand. The result is stored in destination operand.

BH BL

+

AH AL

AX

​

​

20 00

70 00

XX XX

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2: Arithmetic Instructions

ADC Destination, Source

  • The source may be immediate data, memory location or register.
  • The destination may be memory location or register.
  • The result is stored in destination operand.
  • AX is the default destination register.

​

ADC AX,BX

Eg:

AH AL

AX

​

​

12

​

​

50 00

BX

  • This instruction adds the contents of source operand with the contents of destination operand with carry flag bit.

BH BL

+

AH AL

AX

​

​

20 00

70 01

XX XX

+

​

1

CY

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2: Arithmetic Instructions

INC source�

  • The source may be memory location or register.
  • The source can not be immediate data.
  • The result is stored in the same place.

​

​

​

INC AX

Eg:

AH AL

AX

​

​

  • This instruction increases the contents of source operand by 1.

+

AH AL

AX

​

​

70 00

70 01

70 00

​

1

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2: Arithmetic Instructions

MUL operand�

  • Operand may be general purpose register or memory location.
  • If operand is of 8-bit then multiply it with contents of AL.
  • If operand is of 16-bit then multiply it with contents of AX.
  • Result is stored in accumulator AX in 8 bit operation and DX-AX in 16bit operation.

​

MUL BH

Eg:

AH AL

AX

​

​

  • This instruction will multiple unsigned operand 8-bit/16-bit with AL/AX and store the result in AX/DX-AX.

X

AH AL

AX

​

​

XX 04

00 08

XX XX

12

​

​

02 XX

BX

BH BL

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2: Arithmetic Instructions

DIV Operand

  • Operand may be general purpose register or memory location.
  • AL=AX/Operand (8-bit)
  • AL= Quotient, AH=Remainder.
  • AX=DX-AX/Operand (16-bit)
  • AX= Quotient, DX=Remainder.

DIV BL

Eg:

AH AL

AX

​

​

  • This instruction will divide unsigned operand AX/DX-AX by 8-bit/16-bit number and store the result in AX/DX-AX

/

AH AL

AX

​

​

88 88

04 04

88 88

12

​

​

XX 22

BX

BH BL

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3: Bit Manipulation Instructions (LOGICAL Instructions)

  • AND
  • OR
  • XOR
  • NOT

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3: Bit Manipulation Instructions (LOGICAL Instructions)

AND

AND BL, 0FH

Eg:

BH BL

BX

​

​

  • Especially used in clearing certain bits (masking)

X

AH AL

AX

​

​

XX 88

XX 08

XX 88

12

​

​

XX 0F

xxxx xxxx AND 0000 1111 = 0000 xxxx

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Reflection Spot

You just studied about the AND operator is used to clear certain bits. Which operator would be used to set certain bits without effecting the other bits

Pause and write your answer in your course journal

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Reflection Spot (Answer)

OR

MOV AX, 2000h

OR AX, 0008h

Eg:

xxxx xxxx OR 0000 1111 = xxxx 1111

  • Used to multiply each bit in a byte/word with the corresponding bit in another byte/word.

AH AL

AX

​

​

or

AH AL

AX

​

​

20 00

20 08

20 00

12

​

​

00 08

This sets bit 4 in the AX register

0010 0000 OR 0000 1000 = 0010 1000

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4: Instructions to perform shift operations

SHL

MOV BL,5d

SHL BL,1

Eg:

BH BL

Before

​

​

  • The SHL (shift left) instruction performs a logical left shift on the destination operand, filling the lowest bit with 0.

BH BL

After

​

​

XX 05

XX 0A

XX 05

0

0

0

0

1

0

1

0

0

0

0

0

0

1

0

1

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5: Branch instructions�

  • CALL − Used to call a procedure and save their return address to the stack.
    • Eg: CALL Label�
  • RET − Used to return from the procedure to the main program.

​

  • JMP − Used to jump to the provided address to proceed to the next instruction.
    • JMP Label

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5: Branch instructions�

  • JA/JNBE − Used to jump if above/not below/equal instruction satisfies.
  • JAE/JNB − Used to jump if above/not below instruction satisfies.
  • JBE/JNA − Used to jump if below/equal/ not above instruction satisfies.
  • JC − Used to jump if carry flag CF = 1
  • JE/JZ − Used to jump if equal/zero flag ZF = 1
  • JG/JNLE − Used to jump if greater/not less than/equal instruction satisfies.
  • JGE/JNL − Used to jump if greater than/equal/not less than instruction satisfies.
  • JL/JNGE − Used to jump if less than/not greater than/equal instruction satisfies.
  • JLE/JNG − Used to jump if less than/equal/if not greater than instruction satisfies.
  • JNC − Used to jump if no carry flag (CF = 0)
  • JNE/JNZ − Used to jump if not equal/zero flag ZF = 0
  • JNO − Used to jump if no overflow flag OF = 0
  • JNP/JPO − Used to jump if not parity/parity odd PF = 0
  • JNS − Used to jump if not sign SF = 0
  • JO − Used to jump if overflow flag OF = 1
  • JP/JPE − Used to jump if parity/parity even PF = 1
  • JS − Used to jump if sign flag SF = 1

Instructions to transfer the instruction during an execution with some conditions

​

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JMP address

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JC address

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JC

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What we have learnt

    • Studied 5 types of instruction set.
    • How different instructions can manipulate data in different ways.

Summery

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References

  • Advanced Microprocessors and Peripheral

- By K Bhurchandi, A. K. Ray 

​

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DATA TRANSFER/COPY INSTRUCTION

  • MOV
  • IN, OUT
  • LEA
  • LDS, LES
  • PUSH , POP
  • XCHG
  • XLAT

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LOGICAL INSTRUCTION:

  • NOT
  • AND
  • OR
  • XOR
  • TEST

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ROTATE & SHIFT INSTRUCTION

  • SHL, SHR
  • SAL, SAR
  • ROL, ROR
  • RCL, RCR

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ARITHEMATIC INSTRUCTION

  • ADD, SUB
  • ADC, SBB
  • INC, DEC
  • CMP
  • MUL, DIV
  • IMUL, IDIV
  • CBW, CWD

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ADJUSTMENT AFTER AIRTHEMATIC OPERATIONS

  • AAA, AAS, AAM, AAD
  • DAA, DAS

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TRANSFER INSTRUCTION

  • JMP( Unconditional Jump)

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Conditional Jump

  • JA(JNBE)
  • JAE(JNB)
  • JB(JNAE)
  • JBE(JNA)
  • JE(JZ)

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  • JG(JNLE)
  • JGE(JNL)
  • JL(JNGE)
  • JLE(JNG)
  • JC, JNC
  • JO, JNO
  • JS, JNS
  • JNP(JPO)

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LOOP CONTROL

  • LOOP
  • LOOPE (LOOPZ)
  • LOOPNE (LOOPNZ)
  • JCXZ

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SUBROUTINE & INTERRUPT INSTRUCTION

  • CALL, RET
  • INT, INTO
  • IRET

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STRING INSRUCTION

  • MOVS
  • MOVSB, MOVSW
  • CMPS
  • SCAS
  • LODS, STOS

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REPEAT INSTRUCTION (placed in front of other String Instruction)

  • REP
  • REPE, REPZ
  • REPNE, REPNZ

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PROCESSOR CONTROL INSTRUCTION:�FLAG MANIPULATION�

  • STC, CLC, CMC
  • STD, CLD
  • STI, CLI
  • LAHF, SAHF
  • PUSHF, POPF

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CO-PROCESSOR MULTIPROCESSOR INTERFACE

  • ESC
  • LOCK

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INACTIVE STATUS

  • NOP
  • WAIT
  • HLT

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