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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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Endnd
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