8. Addressing Modes

8/9/2019 8. Addressing Modes

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Engr. Nio E. Merencilla


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Real Mode Operation

It allows the microprocessor to

address only the first 1 M byte ofmemory space.

It also allows application softwarewritten for 8086/8088 and above

to function in 80286 with out

changing the software.

Real memory it is also known as

conventional memory system.


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Segments and Offsets

The combination of a segment

address and an offset address isused to access a memory location

in the real mode.

Segment address located

within one of the segment

registers, defines the beginning

address of any 64 K byte memory

segment.


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Offset address selects any

location within 64K byte memorysegment. which sometimes called

displacement segmentin thereal mode and always have a

length of 64K byte.


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Segment and Offset

relationship

Segment Offset Special Purpose

CS IP Instruction address

SS SP or BP Stack address

DS BX, DI, SI Data address

ES DI for stringinstructions

String destinationaddress


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Addressing Modes

A structure of the different

instruction used by themicroprocessor to execute the

fetched instruction


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Register addressing

Transfer a copy of a byte or word

from the source register to thedestination register.

EX:

MOV CX, DX

The instruction copies the word-size contents

of register DX to CX.


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Immediate Addressing

Transfers the source-immediate

byte or word of data in thedestination register or memory

location.EX:

MOV AL, 22H


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Direct Addressing

Moves a byte or word between a

memory location and a register.

EX:

MOV AL, VAR1

The instruction copies the word-sized contents

of memory location VAR1 into register.


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Register Indirect Addressing

Transfers a byte or word betweena register and a memory location

addressed by an index or baseregister.

The index and base registers are BP, BX, DI andSI

EX:

MOV AX, [BX]The instruction copies the word sized data

from the data segment offset address indexedby the BX into the register AX.


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Base-plus-index Addressing

Transfers a byte or word between

a register and a memory locationaddressed by a base register (BP

or BX) plus index register (DI or SI)

EX:

MOV [BX + DI], CL

It copies the byte size content of register CL to

the Data segment location addressed by BX plus

DI.


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Register Relative Addressing

Moves a byte or word between a

register and a memory locationaddressed by an index or base

register plus displacement.EX:

MOV AX, [BX + 4]Loads AX from the data segment address

formed by BX plus 4.


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Base relative plus index

Addressing Transfers a byte or word between

a register and a memory locationaddressed by a base register andindex register plus a displacement

EX:MOV AX, [BX + DI + Displacement]

Similar to the base-plus-index addressing modebut it adds a displacement besides using a baseregister and index register


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8. Addressing Modes