1. Addressing Modes of
8086
Prepared & Presented by-
Rajvir Singh

2. • The different ways in which a
processor can access data are called
addressing modes

3. • 8086 accesses code bytes using
contents of CS & IP register
• 8086 accesses stack using contents
of SS & SP register

4. How 8086 accesses data?
• 8086 assembly language instructions
can be used to illustrate the
addressing modes
• Format of MOV instruction
MOV destination, source

5. • Source of data can be
– Immediate data
– A specified register
– A memory location specified in 1 of 24
different ways
• Destination of data can be
– A specified register
– A memory location specified in 1 of 24
different ways

6. Classification of
Addressing Modes
1. Register addressing mode
2. Immediate addressing mode
3. Memory addressing mode
4. Port addressing mode
5. Relative addressing mode
6. Implied addressing mode

7. 1. Register addressing mode
• Source/destination can be one of the
8086 registers
• MOV AX, BX ; 16-bit data transfer
• MOV AL, BL ; 8-bit data transfer

8. 2. Immediate addressing mode
• 8 or 16-bit data can be specified as
part of the instruction
• E.g. MOV AL, 45H
MOV BX, 5062H

9. 3. Memory addressing mode
• One of the operands is in memory
• 8086 must use
– a segment register
– A 16-bit offset (effective address)
• By default, DS register is used for
accessing data from memory
• 16-bit offset can be specified in
many ways

10. Different ways of
specifying offset
• Direct memory addressing
• Register Indirect addressing
• Based addresing
• Indexed addressing
• Based Indexed addressing
• String addressing

11. a. Direct Memory addressing
• Effective address is specified
directly in the instruction
• E.g. MOV BX, [5062]
• 20-bit physical address of memory
location is calculated using DS and
offset value 5062

12. Accessing memory
Segment register
+ = 20-bit physical address
of operand
16-bit offset

13. b. Register indirect addressing
• The effective address of memory
operand is present in one of the base
or index registers [BX, BP, SI, DI]
• E.g. MOV CX, [BX]
• If DS = 2000H, BX = 0004H
• THEN contents of memory location
20004H are copied to register CX

14. c. Based addressing
• Effective address is sum of
– 16-bit offset given in instruction
– Contents of base register BX or BP
• Segment register is DS or SS
• E.g. MOV AX, 4[BX]
• E.G. PUSH CX

15. d. Indexed addressing
• Effective address is sum of
– 16-bit offset given in instruction
– Contents of index register SI or DI
• Segment register is DS
• E.g. MOV AX, 4[SI]

16. e. Based Indexed addressing
• Effective address is sum of
– 16-bit offset given in instruction
– Contents of index register SI or DI
– Contents of base register BX or BP
• Segment register is DS
• E.g. MOV AX, 4[BX][SI]

17. f. String addressing
• Used when string related instructions
are executed
• Uses index registers
• SI register points to the first byte or
word of the source string
• DI register points to the first byte or
word of the destination

18. Classification of
Addressing Modes
1. Register addressing mode
2. Immediate addressing mode
3. Memory addressing mode
4. Port addressing mode
5. Relative addressing mode
6. Implied addressing mode

19. 4. Port addressing
• Two I/O port addressing modes can
be used
– Direct (e.g. IN AL, 02H)
– Indirect (e.g. IN AL, DXH)

20. 5. Relative addressing mode
• Specifies the operand as a 8-bit
signed displacement relative to PC
• E.g. JNC 08H
• IF carry = 0, THEN PC is loaded with
current PC contents + 8-bit signed
value

21. 6. Implied addressing mode
• No operands are specified
• E.g. CLC
• Means clear carry flag