lec_3 [compatibility mode]

8/6/2019 Lec_3 [Compatibility Mode]

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Assembly Language

Conditional Processing

Lec-3

1Rakhi Budhrani (lecturer SSCCS)


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Overview

Flags

Conditional Jumps

Conditional Loop Instructions Conditional Structures



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Status Flags - Review

The Zero flag is set when the result of an operation

equals zero.

The Carry flag is set when an instruction generates a

result that is too large for the destination operand.

The Sign flag is set if the destination operand is negative,

and it is clear if the destination operand is positive.

The Overflow flag is set when an instruction generatesan invalid signed result.



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CMP Instruction (1 of 3) Compares the destination operand to the source operand

Nondestructive subtraction of source from destination (destinationoperand is not changed)

Syntax: CMP destination, source

Example: destination == source

mo v a l , 5c mp a l , 5 ; Z e r o f l a g s e t

Example: destination < source

mo v a l , 4c mp a l , 5 ; Ca r r y f l a g s e t



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CMP Instruction (2 of 3) Example: destination > source

mo v a l , 6c mp a l , 5 ; Z F = 0 , CF = 0

(both the Zero and Carry flags are clear)



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CMP Instruction (3 of 3)

Example: destination > source

mo v a l , 5c mp a l , - 2 ; S i g n f l a g = = Ov e r f l o w f l a g

The comparisons shown here are performed with signed integers.

Example: destination < source

mo v a l , - 1

c mp a l , 5 ; S i g n f l a g ! = Ov e r f l o w f l a g



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Conditional Jumps Jumps Based On . . .

Specific flags

Equality

Unsigned comparisonsSigned Comparisons

Applications



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8

JcondInstruction A conditional jump instruction branches to

a label when specific register or flag

conditions are met

Examples:

JB, JC jump to a label if the Carry flag is set

JE, JZ jump to a label if the Zero flag is set

JS jumps to a label if the Sign flag is set

JNE, JNZ jump to a label if the Zero flag is clear

JcxZ jumps to a label if cx equals 0Rakhi Budhrani (lecturer SSCCS)


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Jumps Based on Specific Flags



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Jumps Based on Equality



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Jumps Based on Unsigned

Comparisons



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Jumps Based on Signed

Comparisons



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Conditional Structures

Block-Structured IF Statements

Compound Expressions with AND

Compound Expressions with OR WHILE Loops

Switch Selection



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Your turn . . .Implement the following pseudocode in

assembly language. All values are unsigned:

Cmp b x , c x

j a n e x tMo v a x , 5Mo v d x , 6

n e x t :

I f ( b x < = c x )

{

a x = 5 ;

d x = 6 ;

}



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Block-Structured IF StatementsAssembly language programmers can easily translate logical statements

written in C++/Java into assembly language. For example:

mo v a x , o p 1

c mp a x , o p 2 j n e L 1mo v X , 1

j mp L 2L 1 : mo v X, 2L 2 :

i f ( o p 1 = = o p 2 )

X = 1 ;

e l s e

X = 2 ;



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Your turn . . .Implement the following pseudocode in

assembly language

mo v a x , v a r 1c mp a x , v a r 2

j l e L 1mo v v a r 3 , 6

mo v v a r 4 , 7 j mp L 2

L 1 : mo v v a r 3 , 1 0L 2 :

i f ( v a r 1 < = v a r 2 )

v a r 3 = 1 0 ;

e l s e

{v a r 3 = 6 ;

v a r 4 = 7 ;

}



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Compound Expression with AND (1 of 3) When implementing the logical AND operator, consider that HLLs use

short-circuit evaluation

In the following example, if the first expression is false, the second

expression is skipped:

i f ( a l > b l ) A ND ( b l > c l )

X = 1 ;



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Compound Expression with AND (2 of 3)

c mp a l , b l ; f i r s t e x p r e s s i o n . . . j a L 1 j mp n e x t

L 1 :c mp b l , c l ; s e c o n d e x p r e s s i o n . . .

j a L 2

j mp n e x tL 2 : ; b o t h a r e t r u e

mo v X , 1 ; s e t X t o 1n e x t :

i f ( a l > b l ) A ND ( b l > c l )

X = 1 ;

This is one possible implementation . . .



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Compound Expression with AND (3 of 3)

c mp a l , b l ; f i r s t e x p r e s s i o n . . .j b e n e x t ; q u i t i f f a l s ec mp b l , c l ; s e c o n d e x p r e s s i o n . . .

j b e n e x t ; q u i t i f f a l s emo v X , 1 ; b o t h a r e t r u e

n e x t :

i f ( a l > b l ) A ND ( b l > c l )

X = 1 ;

But the following implementation uses 29% less code by reversing the

first relational operator. We allow the program to "fall through" to thesecond expression:



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Rakhi Budhrani (lecturer

SSCCS)21

Compound Expression with OR (1 of 2) When implementing the logical OR operator, consider that HLLs use short-

circuit evaluation

In the following example, if the first expression is true, the second

expression is skipped:

i f ( a l > b l ) OR ( b l > c l )

X = 1 ;


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Compound Expression with OR (1 of 2)

c mp a l , b l ; i s A L > B L ?j a L 1 ; y e sc mp b l , c l ; n o : i s B L > CL ?

j b e n e x t ; n o : s k i p n e x t s t a t e me n t

L 1 : mo v X , 1 ; s e t X t o 1n e x t :

i f ( a l > b l ) OR ( b l > c l )

X = 1 ;

We can use "fall-through" logic to keep the code as short as possible:



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Switch SelectionCMP BL, 30H ; compare input digit and 0

JL @NEGATIVE ; jump to label @NEGATIVE if digit0

@NEGATIVE: ; jump label

MOV DL, 'N

JMP @DISPLAY ; jump to label @DISPLAY@ZERO: ; jump label

MOV DL, 'Z

JMP @DISPLAY ; jump to label @DISPLAY

@POSITIVE: ; jump label

MOV DL, 'P

JMP @DISPLAY ; jump to label @DISPLAY

@DISPLAY: ; jump label

MOV AH, 2 ; print the character

INT 21H

Rakhi Budhrani (lecturer SSCCS) 23


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Your turn . . .

t o p : c mp b x , v a l 1 ; c h e c k l o o p c o n d i t i o nj a n e x t ; f a l s e ? e x i t l o o pa d d b x , 5 ; b o d y o f l o o pd e c v a l 1

j mp t o p ; r e p e a t t h e l o o pn e x t :

wh i l e ( b x < = v a l 1 )

{

b x = b x + 5 ;

v a l 1 = v a l 1 - 1

}

Implement the following loop



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Operations: Branches Conditional branches:

These must be preceded by aninstruction which sets at least

one status flag (this includes

cmp operations)

the flag tested is based onwhich branch is used

je/jne location

branch if zero flag set/clear

jg/jge/jl/jle location

jump on > (positive flag set),>=, < (negative flag set),


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DOS Function Calls (Using Int 21h)

AH = 01H : For reading a character from keyboard . Theinput value is put in AL register. (read.asm)

AH = 02 H : This prints 8 bit data that is stored in DLregister on the screen (e02.asm)

AH = 08H : This is an input function for inputting onecharacter. (dispstr.asm)

AH = 09H : This program outputs a string whose offset isstored in DX register and that is terminated using a $

character.

AH = 0AH : for input of string up to 255 characters.

(Loop.asm)


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