Stack Memory

00000H

00001H

FFFFFH

FFFFEH

SS0105

SP0008

TOS

BOS

BOS = 01050+FFFF

= 1104FH

01058H

1104FH

PUSH AX

TOS

1234HAX

1058H

XXH

XXH

XXH

XXH

XXH

XXH

XXH

1057H

1056H

1055H

Before execution

1058H

XXH

XXH

XXH

34H

12H

XXH

XXH

1057H

1056H

1055H

TOS

After execution

POP BX

1234HBX

Before execution

1058H

XXH

XXH

XXH

34H

12H

XXH

XXH

1057H

1056H

1055H

TOS

After execution

1058H

XXH

XXH

XXH

34H

12H

XXH

XXH

1057H

1056H

1055H

XXXXHBX

TOS

I/O address space

open

open

reserved00F8H

00FFH

0000H

FFFFH

A separate 64 Kbyte address space is provided where I/O interfaces are implemented.

Certain I/O instructions can only perform operations to I/O devices located in this I/O address space.

Addressing modes of the 8086 / 8088 μP

Register addressing modeMOV AX, BX

MOV destination operand, source operand

XXXXHAX

1234HBX

Before the execution

1234HAX

1234HBX

After the execution

Coding in machine language

opcode D W

Operation code

D= 1 direction is to the register specified in byte 2

D=0 direction is from the register specified in byte 2

Byte 1 Byte 2

mod reg R/M

W=0 8 bit data w=1 16 bit data

00 memory mode no displacement

01 Memory mode, 8 bit displacement

10 Memory mode 16 bit displacement

11 Register mode no displacement

mod

Except when R/M =110, 16 bit displacement follows

MOD = 11 Effective address ( offset ) calculation

000 AL AX 000 [BX]+[SI] [BX]+[SI]+D8 [BX]+[SI]+D16

001 CL CX 001 [BX]+[DI] [BX]+[DI]+D8 [BX]+[DI]+D16

010 DL DX 010 [BP]+[SI] [BP]+[SI]+D8 [BP]+[SI]+D16

011 BL BX 011 [BP]+[DI] [BP]+[DI]+D8 [BP]+[DI]+D16

100 AH SP 100 [SI] [SI]+D8 [SI]+D16

101 CH BP 101 [DI] [DI]+D8 [DI]+D16

110 DH SI 110 Direct address [BP]+D8 [BP]+D16

111 BH DI 111 [BX] [BX]+D8 [BX]+D16

R/M W=0 W=1 R/M MOD=00 MOD=01 MOD=10

General Instruction Format

Byte 1 Byte 2 Byte 3 Byte 4 Byte 5 Byte 6

MOV BL,AL

Register / memory to/ from register

100010dw Mod reg r/m Displ-lo Displ-hi

Machine code

10001000 11 000 011

8 8 c 3 H

Assembly code

Immediate addressing mode

MOV AL, 15H

MOV destination operand, immediate source operand

XXXXHAX

Before the execution

XXA5HAX

After the execution

Immediate operand must begin with 0-9

MOV AL, 0A5H

Immediate to register 1011 w reg data Data if w=1

MOV AL, 15H 10110000 00010101

Assembly code Machine code

B015H

Direct addressing mode

The memory locations following the instruction op code hold an effective memory address

MOV AX, [0004]

Register / memory to / from register

100010dw Mod reg r/m Disp-lo Disp-hi

10001011 00000110 00000100 00000000

8 b 0 6 04 00

The logical address is [DS] : [ 0004]

μP

0000IP

0100

0200

XXXX

CS

DS

SS

ES

AX

μP

0004IP

0100

0200

BEED

CS

DS

SS

ES

AX

8B

06

04

00

ED

BE

8B

06

0400

ED

BE

01000

0100101002

01003

02004

02005

01000

0100101002

01003

02004

02005

XX

MOV AX, [0004]

Before execution After execution

Direct addressing mode

Register indirect addressing mode

MOV AX, [SI]

The logical address is [DS] : [BX]

or [DS] : [SI]

or [DS] : [DI]

μP0000IP

0100

0200

1234

XXXX

CS

DS

SI

AX

8B

04

XX

XX

ED

BE

01000

0100101002

01003

03234

03235

μP0002IP

0100

0200

1234

BEED

CS

DS

SI

AX

8B

04

XX

XX

ED

BE

01000

0100101002

01003

03234

03235

MOV AX, [SI]Register / memory to/ from register

100010dw Mod reg r/m Displ-lo Displ-hi

Machine code

10001011 00 000 100

8 B 0 4 H

Assembly code

Indexed addressing mode

Logical address = [ DS] : [SI] + direct or indirect displacement

[DS] : [DI] + direct or indirect displacementor

MOV AL,[SI]+1234H

Register / memory to / from register

100010dw Mod reg r/m Disp-lo Disp-hi

10001010 10000100 00110100 00010010

8 A 8 4 34 12

Based addressing mode

Logical address = [ DS] : [BX] + direct or indirect displacement

[SS] : [BP] + direct or indirect displacementor

MOV [BX]+1234H,AL

Register / memory to / from register

100010dw Mod reg r/m Disp-lo Disp-hi

10001000 10000111 00110100 00010010

8 8 8 7 34 12

Based addressing mode

MOV [BX]+1234H, AL

μP0000IP

0100

0200

1234

BEED

CS

DS

SI

AX

88

87

34

12

XX

XX

01000

0100101002

01003

04234

04235

μP0004IP

0100

0200

1234

BEED

CS

DS

SI

AX

8B

04

XX

XX

ED

XX

01000

0100101002

01003

04234

04235

1000BX

Indexed addressing mode

MOV AL,[SI]+1234H

μP0000IP

0100

0200

2000

XXXX

CS

DS

SI

AX

8A

84

34

12

BE

01000

0100101002

01003

05234

μP0004IP

0100

0200

1234

XXBE

CS

DS

SI

AX

8A

84

34

12

BE

01000

0100101002

01003

05234