8086 Instruction Set By Er. Swapnil V. Kaware, Lecture notes of Microprocessors

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Instruction Set of 8086

Microprocessor

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Microprocessor Notes by Er. Swapnil V. Kaware (svkaware@yahoo.co.in)

Instruction Set of 8086 Microprocessor

Presented By,

Er. Swapnil Kaware,

M.E. (Electronics & Telecommunication),

svkaware@yahoo.co.in

www.slideshare.net/svkaware

• Assembler:- it converts the instruction into sequence of

binary bits, so that this bits can be read by the

processor.

• Mnemonics:- these are the symbolic codes for either

instructions or commands to perform a particular

function.

• E.g. MOV, ADD, SUB etc.

Instruction set basics

Microprocessor Notes By, Er. Swapnil

Instruction Formats

• In 8086 microprocessor there are following six types of

instruction formats.

(1). 1-Byte instruction,

(2). Register to register,

(3). Register to/from memory with no displacement,

(4). Register to/from memory with displacement,

(5). Immediate operand to register,

(6). Immediate operand to memory with 16-Bit

displacement.

Microprocessor Notes By, Er. Swapnil

• The instruction is 2-byte long in size.
• First byte of code denotes opcode & width of operand.
• Second byte denotes register operands & R/M field.
• ‘REG’ field denotes type of register used.
• ‘R/M’ field denotes register or memory location used.
• If ‘W’ field is ‘0’ then operand is of 8 - bits, & If ‘W’ field

is ‘1’ then operand is of 16 - bits.

( 2 ). Register to Memory

OPCODE W 11 REG R/M

D 7 D 1 D0 D 7 D 5 D 2 D

D 6 D 3

Microprocessor Notes By, Er. Swapnil

(3). Register to/from memory with no

Displacement

• The instruction is 2 - byte long in size.
• First byte of code denotes opcode & width of operand.
• Second byte denotes mod,register operands & R/M

field.

• ‘ MOD ’ field denotes type of addressing mode used.

OPCODE W MOD REG R/M

D 7

D 1 D 0 D 7 D6 D 5 D 3 D2 D

Microprocessor Notes By, Er. Swapnil

• The instruction is 4-byte long in size.
• First byte of code denotes opcode.
• Second byte denotes opcode,R/M field.
• Third byte denotes lower byte of data.
• fourth byte denotes higher byte of data.

(5). Immediate operand to register

OPCODE 11 REG R/M

D 0

Lower Byte of

data

Higher Byte of data

D 7 D7 D6 D 5 D 3 D2 D0 D 7 D 0 D7 D

Microprocessor Notes By, Er. Swapnil

• The instruction is 5 or 6-byte long in size.
• First byte of code denotes opcode.
• Second byte denotes register mod,opcode & R/M field.
• Third byte denotes lower byte of displacement.

(6). Immediate operand to memory with

16 - bit displacement

OPCODE MOD OPCODE R/M

D

Lower Byte of

displacement

D 7 D 7 D6 D5 D 3 D2 D 0 D7 D

Higher Byte of

displacement

Lower Byte of

data

Higher Byte of

data

D 7 D7 D0 D 7 D D

Microprocessor Notes By, Er. Swapnil

Addressing modes & corresponding

MOD,REG & R/M field

Microprocessor Notes By, Er. Swapnil

Addressing Modes of 8086

Microprocessor

• (1). Immediate,
• (2). Direct,
• (3). Register,
• (4). Register Indirect,
• (5). Indexed,
• (6). Register Relative,
• (7). Based Indexed,
• (8). Relative Based Indexed.
• (9). Intrasegment Direct Mode.
• (10). Intrasegment Indirect Mode.
• (11). Intersegment Direct.
• (12). Intersegment Indirect.

Microprocessor Notes By, Er. Swapnil

(1). Data copy/transfer instructions.

• (1). MOV Destination, Source;
• There will be transfer of data from source to destination.
• 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.

• E.g.
• (1). MOV CX, 037A H;
• (2). MOV AL, BL;
• (3). MOV BX, [0301 H];

Microprocessor Notes By, Er. Swapnil

BX 2000H AX 2000H

BEFORE EXECUTION AFTER EXECUTION

MOV BX,AX;

AH AL

BH BL

CH CL

DH DL

AH AL

BH BL

CH CL 40

DH DL

MOV CL,M;

BEFORE EXECUTION AFTER EXECUTION

(1). Data copy/transfer instructions.

• (2). Push Source;
• Source can be register, segment register or memory.
• This instruction pushes the contents of specified source

on to the stack.

• In this stack pointer is decremented by 2.
• The higher byte data is pushed first (SP-1).
• Then lower byte data is pushed (SP-2).
• E.g.:
• (1). PUSH AX;
• (2). PUSH DS;
• (3). PUSH [5000H];

Microprocessor Notes By, Er. Swapnil

INITIAL POSITION

DECREMENTS SP & STORES HIGHER BYTE

HIGHER BYTE

DECREMENTS SP & STORES LOWER BYTE

LOWER BYTE

HIGHER BYTE

(1) STACK POINTER

(2) STACK POINTER

(3) STACK POINTER

Microprocessor Notes By, Er. Swapnil