Addressing Modes

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Addressing Modes

• When the 8088 executes an instruction, it
  performs the specified function on data
• These data, called operands,
• May be a part of the instruction
• May reside in one of the internal registers
• May be stored in memory
• To access these operands, 8088 is provided with
  various addressing modes
• An addressing mode is a method of specifying an
  operand

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• Register Operand Addressing
• MOV AX, BX
• Immediate Operand Addressing Mode
• MOV AL,15h
• Memory Addressing Modes
• One must calculate the PA (physical address)

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Register Operand Addressing
MOV AX, BX
C38B16
Before execution
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After execution
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Immediate Addressing Mode

• In immediate addressing, source operand is a
  constant
• Used to load information to some registers
• MOV AX,2550h
• MOV CX, 625 decimal 625
• MOV BL,40h

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MOV AL, 15H15B016
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Direct Addressing ModeMOV CX, 1234h12340E8B16
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Register Indirect Addressing ModeMOV AX, SI
048B16
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Based Addressing Mode MOV CX,BX10 PA
(DS)0 (BX) 10 MOV AL,BP5 PA (SS)0
(BP) 5
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MOV BX1234H,AL1234878816
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Indexed Addressing Mode MOV DX,SI5 PA (DS)
0 (SI) 5 MOV CL,DI20 PA (DS) 0 DI 20
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MOV AL, SI 1234H 1234848A16
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Based Indexed Addressing Mode

• MOV CX,BXDI8h PA (DS)0 (BX) (DI) 8h
• MOV AH,BPSI29 PA (SS)0 (BP) (SI) 2
• Other notations possible MOV AH,BPSI29
• or MOV AH, SIBP29
• HoweverMOV AX, SIDI displacement is illegal

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Example Based-Indexed Addressing ModesMOV AH,
BXSI 1234H1234A08A16
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Segment Override
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The PTR Operator

• MOV AL,20h 8 bit data copied into AL
• MOV AX,20h 16 bit data copied into AX
• INC 20h 8 bit or 16 bits incremented?
• Byte or word or doubleword?
• To clarify we use the PTR operator
• INC BYTE PTR 20h
• INC WORD PTR 20h
• INC DWORD PTR 20h

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Example

• Copy the contents of a block of memory (16 bytes)
  starting at location 20100H to another block of
  memory starting at 20120H
• MOV AX, 2000H
• MOV DS, AX
• MOV SI, 100h
• MOV DI, 120h
• MOV CX, 10h
• NXTPT MOV, AH, SI
• MOV DI, AH
• INC SI
• INC DI
• DEC CX
• JNZ NXTPT