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A FEW BASIC A FEW BASIC INSTRUCTIONSINSTRUCTIONS

Presented By

Contents

There are over a hundred instructions in the instruction set for the 8086 CPU.

Here we discuss 7 of the most useful instructions for transferring data and doing arithmetic.

The discussed instructions are…….. MOV,XCHG,ADD,SUB,INC,DEC,NEG.

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IntroductionIntroduction

MOVMOV Transfer data

◦ Between registers◦ Between register and a memory location◦ Move a no. directly to a register or a memory

location Syntax MOV destination, source

ExampleMOV AX, WORD1

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How?

XCHGExchange the contents of

◦ Two registers◦ Register and a memory location

Syntax XCHG destination, sourceExample

XCHG AH, BL

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AL

BL

00 1A

05 00AH AL

BH BL

How?

Legal Combinations of Operands for MOV Legal Combinations of Operands for MOV And XCHG And XCHG

Destination Operand

Source Operand

Legal

General Register

General Register

YES

General Register

Memory Location

YES

Memory Location

General Register

YES

Memory Location

Memory Location

NO

Destination Operand

Source Operand

Legal

General Register

General Register

YES

General Register

Memory Location

YES

General Register

Segment Register

YES

General Register

Constant YES

Memory Location

General Register

YES

Memory Location

Memory Location

NO

Memory Location

Segment Register

YES

Memory Location

Constant YES

MOVXCHG

ADD InstructionADD InstructionTo add contents of:

◦ Two registers◦ A register and a memory location◦ A number to a register◦ A number to a memory location

ExampleADD WORD1, AX

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How?

SUB InstructionSUB InstructionTo subtract the contents of:

◦ Two registers◦ A register and a memory location◦ A number from a register◦ A number from a memory location

ExampleSUB AX, DX

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How?

Legal Combinations of Operands for Legal Combinations of Operands for ADD & SUB instructions ADD & SUB instructions

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Destination Operand

Source Operand Legal

General Register General Register YES

General Register Memory Location YES

General Register Constant YES

Memory Location General Register YES

Memory Location Memory Location NO

Memory Location Constant YES

INC & DECINC & DEC INC (increment) instruction is used to add 1 to the

contents of a register or memory location.◦ Syntax: INC destination ◦ Example: INC WORD1

DEC (decrement) instruction is used to subtract 1 from the contents of a register or memory location.◦ Syntax: DEC destination ◦ Example: DEC BYTE1

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Destination can be 8-bit or 16-bits wide.Destination can be a register or a memory

location.

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INC WORD1

DEC BYTE1

INC & DECINC & DEC

How?

How?

NEGNEGUsed to negate the contents of destination.Replace the contents by its 2’s complement.Syntax:

NEG destinationExample:

NEG BX

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ExamplesExamples Consider instructions: MOV, ADD, SUB, INC, DEC,

NEG A and B are two word variables Translate statements into assembly language:

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Statement Translation

A =-( A+1)

MOV AX,A INC AX NEG AX MOV A,AX

B= 3B-7

MOV AX,B ADD AX,B ADD AX,B SUB AX,7 MOV B,AX

Example:1

Example:2

Statement

Translation

A = B – A-1

MOV AX,B SUB AX,A DEC AX MOV A,AX

Example:3

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ApplicationApplication .datamsg1 DB 'Input Any number From 1 To 9:$' ; Data segmentmsg2 DB 'The Series Summation Is:$'.code

main proc mov AX,@DATA ; Data segment mov DS,AX lea dx,msg1 ;showing msg1 mov ah,9 int 21h mov ah,1 ;taking input int 21h xor ah,ah sub al,30h XOR CX,CX ;loop same as input mov cx,ax XOR bx,bx

## A series summation of n numbers where 1<=n<=9

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sub bx,0Ah add ax,1 ;counter=ax=3 CMP bx ,39h ;bx<39h JL DISPLAY sub bx,0Ah add ax,1 ;counter=ax=4 CMP bx ,39h ;bx<39h JL DISPLAY DISPLAY: add al,30h mov cl,al lea dx,msg2 ;showing msg2 mov ah,9 int 21h mov ah,2 mov dl,cl int 21h mov dl,bl int 21h

ApplicationApplication TOP: add bx,cx ;adding LOOP TOP mov ah,2 mov dl,0Dh int 21h ;printing new line mov dl,0Ah int 21h xor ax,ax add bx,30h CMP bx ,39h ;bx<39h JL DISPLAY sub bx,0Ah add ax,1 ;counter=ax=1 CMP bx ,39h ;bx<39h JL DISPLAY sub bx,0Ah add ax,1 ;counter=ax=2 CMP bx ,39h ;bx<39h JL DISPLAY

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