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Name : C. S. Manjula, Grad IETE Designation : Lab Mechanic Branch : Computer Engineering Institute : S.P.W. Polytechnic, Tirupati, Semester : IV semester Subject : Microprocessors Subject Code : CM 405 Topic : Instruction Set Duration : 100 Minutes Sub Topic : Addressing Modes of 8086 Teaching Aids : PPT Revised by : K.Srikanth,Lecturer,GPT,

Nizamabad

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Addressing Modes of 8086

Definition Addressing mode indicates the method through which an

operand is specified or indicated in an instruction

Types of Addressing Modes: 11

1. Immediate Addressing mode

2. Direct Addressing mode

3. Register Addressing mode

4. Register Indirect Addressing mode

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Addressing Modes of 8086 (Contd..)

5. Register Relative Addressing mode

6. Based Indexed Addressing mode

7. Relative Based Indexed Addressing mode

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Branch Related Addressing Modes

8. Intra-segment Direct Addressing mode

9. Intra-segment Indirect Addressing mode

10. Inter-segment Direct Addressing mode

11. Inter-segment Indirect Addressing mode

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1. Immediate Addressing mode

Operand is indicated in the instruction itself

Ex: MOV AX, 0007H

0007H is the immediate data here.

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2. Direct Addressing mode

• Offset Address of the operand is specified directly in the

instruction

EX: MOV AX , [5000H]

NOTE:1.[ ] indicates offset or effective address

2. Data resides in the Data Segment (DS),

whose Physical address is computed as

PA = DS x 10H + 5000H

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3. Register Addressing mode

The register which holds the data is mentioned in the

instruction

Ex: MOV BX, AX

All the registers, except IP and CS may be used

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4. Register Indirect Addressing

• Offset address is stored in a register• That register is indicated in the instruction

EX: MOV AX, [BX]

• The offset address of the data is in BX.• The Physical Address (PA) is given as: • PA = [DS] x 10H + [BX]

• Permitted Registers: SI, DI, BX

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5. Register Relative Addressing mode

• Offset address is stored in a register • Permitted Registers: SI, DI, BX, BP• That register, and an 8-bit or 16-bit displacement are

given within the instruction• The Offset Address of the data is computed by adding

the displacement with the memory offset address present in the given register

EX: MOV AX, 50H[BX]

Offset address = 50H + [BX]

PA = 10H * DS + 8 or 16 bit relative addr.

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6. Based Indexed Addressing• A base register (BX/BP), and an index register (SI/DI)

are given within the instruction

• The Effective Address of the data is computed by adding the addresses present in the base and index regs.

EX: MOV AX, [BX] [SI]

EA = [BX] + [SI]

PA = 10H * DS + EA

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7. Relative Based Indexed Addressing Mode

• A base register (BX/BP), an index register (SI/DI), and an 8-bit or 16-bit displacement are given within the instruction

• The Effective Address of the data is computed by adding the addresses present in the given registers with the 8-bit or 16-bit displacement

EX: MOV AX, 50H [BX][SI]

EA = [BX] + [SI] + 50H

PA = 10H * DS + EA

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Branch Related Addressing Modes

Instructions under this category doesn’t try to access data, instead they alter normal sequence of program execution (By branching to an instruction somewhere else)

• Inter segment branch – Branching within the same code segment

• Intra segment branch – Branching into another code segment

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8. Intra-segment Direct Addressing mode

• An 8-bit or 16-bit displacement is given within the

instruction

• The effective branch address (code segment offset

address) is given as the sum of 8-bit or 16-bit

displacement and the contents of Instruction Pointer (IP)

and lies within the same segment

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9. Intra-segment Indirect Addressing mode

• The address of a register or a memory location, where

the 16-bit code segment offset address is located, is

given within the instruction

• A branch is affected by copying this offset address into

IP register

• This branch is to a location within the code segment

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10. Inter-segment Direct Addressing Mode

• Two 16-bit values, specifying the base and offset

addresses of the code segment, are given within the

instruction

• A branch is affected by copying the contents of the

above mentioned addresses into CS and IP registers.

This branch can be to a location outside the code

segment

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11. Inter-segment Indirect Addressing Mode

• The starting address of a memory block, containing the base and offset addresses of the code segment, is specified using any of the data related addressing modes

• A branch is affected by copying the contents of the above mentioned memory block into CS and IP registers. This branch can be to a location outside the code segment

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Summary

We have discussed about

• Types of addressing modes

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Quiz

1.What do you mean by addressing mode ?

a) Locating ALU

b) Locating operand

c) Locating memory

d) Locating register

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Quiz (Contd.)

2.The length of an instruction of 8086 can be ?

a) 1 to 4 bytes

b) 2 to 4 bytes

c) 1 to 6 bytes

d) 2 to 6 bytes

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Quiz (Contd.)

3.What is meant by inter segment addressing mode ?

a) Branch address is available in the same segment

b) Branch address is available outside the segment

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Frequently Asked Questions

1. List out addressing modes supported by 8086 ?

2. Explain each addressing mode by using suitable examples ?

3. Compare direct and indirect addressing modes and explain the similarities and differences with suitable examples