1. Stack Memory 00000H 00001H 01058H TOS SS 0105 0008 SP 1104FH BOS
FFFFEH
BOS = FFFF
= 1104FH
FFFFFH

2. PUSH AX After execution Before execution XXH XXH 1055H 1055H 1056H 34H
TOS
After execution
TOS
1234H AX

3. POP BX After execution Before execution 1058H XXH 34H 12H 1057H 1056H
TOS
34H
12H
1058H
XXH
TOS
XXH
XXXXH BX
1234H BX

4. I/O address space
FFFFH
A separate 64 Kbyte address space is provided where I/O interfaces are implemented.
open
Certain I/O instructions can only perform operations to I/O devices located in this I/O address space.
00FFH
reserved
00F8H
open
0000H

5. Addressing modes of the 8086 / 8088 μP
Register addressing mode
MOV AX, BX
MOV destination operand, source operand
Before the execution
After the execution AX
XXXXH AX
1234H BX
1234H BX
1234H

6. Coding in machine language
mod
memory mode no displacement
Byte 1
Byte 2
01 Memory mode, 8 bit displacement
10 Memory mode 16 bit displacement
opcode D W
mod
reg
R/M
11 Register mode no displacement
Except when R/M =110, 16 bit displacement follows
W=0 8 bit data w=1 16 bit data
D=0 direction is from the register specified in byte 2
D= 1 direction is to the register specified in byte 2
Operation code

7. Effective address ( offset ) calculation
MOD = 11
Effective address ( offset ) calculation
AL AX
[BX]+[SI] [BX]+[SI]+D [BX]+[SI]+D16
CL CX
[BX]+[DI] [BX]+[DI]+D [BX]+[DI]+D16
DL DX
[BP]+[SI] [BP]+[SI]+D [BP]+[SI]+D16
BL BX
[BP]+[DI] [BP]+[DI]+D [BP]+[DI]+D16
AH SP
[SI] [SI]+D [SI]+D16
CH BP
[DI] [DI]+D [DI]+D16
DH SI
110 Direct address [BP]+D [BP]+D16
BH DI
[BX] [BX]+D [BX]+D16
R/M W=0 W=1 R/M MOD= MOD= MOD=10

8. General Instruction Format
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6

9. MOV BL,AL 10001000 11 000 011 8 c 3 H Machine code
Register / memory to/ from register
100010dw
Mod reg r/m
Displ-lo
Displ-hi
Assembly code 8 c 3 H
Machine code

10. Immediate addressing mode
MOV AL, 15H
MOV destination operand, immediate source operand
Immediate operand must begin with 0-9
MOV AL, 0A5H
Before the execution
After the execution AX
XXXXH AX
XXA5H

11. 1011 w reg Data if w=1 Immediate to register MOV AL, 15H 10110000
Assembly code
Machine code
B015H

12. The logical address is [DS] : [ 0004]
Direct addressing mode
The memory locations following the instruction op code hold an effective memory address
MOV AX, [0004]
The logical address is [DS] : [ 0004]
Register / memory to / from register
100010dw
Mod reg r/m
Disp-lo
Disp-hi 8 b 6 04 00

13. Direct addressing mode
01000 8B μP μP
0000 IP
0100
0200
XXXX CS DS SS ES AX
01000 8B
01001 06 IP
0004
01001 06
01002 04
01002 04 CS
0100
01003 00
01003 00 DS
0200 XX SS ES
02004 ED AX
BEED
02004 ED
02005 BE
02005 BE
Before execution
After execution
MOV AX, [0004]
Direct addressing mode

14. Register indirect addressing mode
MOV AX, [SI]
The logical address is [DS] : [BX]
or [DS] : [SI]
or [DS] : [DI] μP
0000 IP
0100
0200
1234
XXXX CS DS SI AX 8B 04 XX ED BE
01000
01001
01002
01003
03234
03235 μP
0002 IP
0100
0200
1234
BEED CS DS SI AX 8B 04 XX ED BE
01000
01001
01002
01003
03234
03235

15. MOV AX, [SI] 10001011 00 000 100 8 B 4 H Machine code
Register / memory to/ from register
100010dw
Mod reg r/m
Displ-lo
Displ-hi
Assembly code 8 B 4 H
Machine code

16. Register Indirect Relative
The logical address is [DS] : [BX] + 8 bit or 16 bit displacement
or [DS] : [SI] + 8 bit or 16 bit displacement
or [DS] : [DI] + 8 bit or 16 bit displacement
MOV AL,[SI]+1234H
A H
MOV CX, [BX]+1234
b f

17. Indexed Relative addressing mode
Logical address = [ DS] : [SI] + direct or indirect displacement or
[DS] : [DI] + direct or indirect displacement
MOV AL,[SI]+1234H
Register / memory to / from register
100010dw
Mod reg r/m
Disp-lo
Disp-hi 8 A 8 4 34 12

18. Based Relative addressing mode
Logical address = [ DS] : [BX] + direct or indirect displacement or
[SS] : [BP] + direct or indirect displacement
MOV [BX]+1234H,AL
Register / memory to / from register
100010dw
Mod reg r/m
Disp-lo
Disp-hi 8 8 8 7 34 12
MOV AX, [BP]+1234H
100010dw
Mod reg r/m
Disp-lo
Disp-hi 8B 86
3 4 12

19. Based Relative addressing mode
MOV [BX]+1234H, AL μP
0000 IP
0100
0200
1234
BEED CS DS SI AX 88 87 34 12 XX
01000
01001
01002
01003
04234
04235
0004 8B 04 ED
1000 BX

20. Indexed Relative addressing mode
MOV AL,[SI]+1234H
01000 8A μP
0004 IP
0100
0200
1234
XXBE CS DS SI AX 8A 84 34 12 BE
01000
01001
01002
01003
05234 μP IP
0000
01001 84
01002 34 CS
0100
01003 12 DS
0200 SI
2000
05234 BE AX
XXXX