CS2422 Assembly Language and System Programming

Conditional Processing

Department of Computer ScienceNational Tsing Hua University

CS2422 Assembly Language and System ProgrammingAssembly Language for Intel-Based Computers, 5th Edition

Chapter 6: Conditional Processing

(c) Pearson Education, 2006-2007. All rights reserved. You may modify and copy this slide show for your personal use, or for use in the classroom, as long as this copyright statement, the author's name, and the title are not changed.

Slides prepared by the author

Revision date: June 4, 2006

Kip Irvine

3

Chapter Overview

Boolean and Comparison Instructions AND, OR, XOR, NOT, TEST, CMP

Conditional Jumps Conditional Loop Instructions Conditional Structures Application: Finite-State Machines (skipped) Decision Directives Brief Overview of Chapter 7

4

AND Instruction

Perform a Boolean AND operation between each pair of matching bits in two operands

Syntax:AND destination, source

(same operand types as MOV) AND

5

OR Instruction

Performs a Boolean OR operation between each pair of matching bits in two operands

Syntax:OR destination, source

OR

6

XOR Instruction

Performs a Boolean exclusive-OR operation between each pair of matching bits in two operands

Syntax:XOR destination, source XOR

0 0 1 1 1 0 1 10 0 0 0 1 1 1 1

0 0 1 1 0 1 0 0

XOR

invertedunchanged

XOR is a useful way to toggle (invert) the bits in an operand.

7

NOT Instruction

Performs a Boolean NOT operation on a single destination operand

Syntax:NOT destination

NOT

0 0 1 1 1 0 1 1

1 1 0 0 0 1 0 0

NOT

inverted

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Application – An Example

Task: Convert character in AL to upper case Solution: Use AND instruction to clear bit 5

mov al,'a‘ ; AL = 01100001band al,11011111b ; AL = 01000001b

ASCII code of ‘A’ = 41HASCII code of ‘a’ = 61H

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TEST Instruction

Performs a nondestructive AND between each pair of matching bits in two operands

No operands are modified, but the Zero flag is affected.

Example: jump to a label if either bit 0 or bit 1 in AL is set.

test al,00000011bjnz ValueFound

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CMP Instruction (1/2)

Compare destination operand to source operand Nondestructive subtraction of source from

destination (destination operand is not changed) Syntax: CMP destination, source Example: destination < source

Example: destination > source

mov al,4cmp al,5 ; Carry flag set

mov al,6cmp al,5 ; ZF = 0, CF = 0

(both the Zero and Carry flags are clear)

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CMP Instruction (2/2)

The comparisons shown here are performed with signed integers.

Example: destination > source

• Example: destination < source

mov al,5cmp al,-2 ; Sign flag == Overflow flag

mov al,-1cmp al,5 ; Sign flag != Overflow flag

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What's Next

Boolean and Comparison Instructions Conditional Jumps Conditional Loop Instructions Conditional Structures Application: Finite-State Machines (skipped) Decision Directives Brief Overview of Chapter 7

13

CMP and Jcond Instruction

The IF statement in C and PASCAL is converted into CMP and Jcond instructions in x86 Assembly:

CMP X, op1JNG EndIf<…>

EndIf:

If (X > op1)Then

<…>End If

14

Jcond Instruction

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 JECXZ jumps to a label if ECX equals 0

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Jcond Ranges

Jump destinations usually confined within same procedure

Prior to the 386: jump must be within –128 to +127 bytes from

current location counter IA-32 processors:

32-bit offset permits jump anywhere in segment

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Jumps Based on Specific Flags

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Jumps Based on Equality

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Jumps Using Unsigned Comparison

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Jumps Using Signed Comparisons

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More Frequently Used Jcond

JE (Equal) JNE (Not Equal) JG or JGE (Greater Than or Equal) JL or JLE (Less Than or Equal)

Note: JG=JNLE, JGE=JNL, …etc.

21

Simple IF

If (op1=op2) then <…> end if Two different approaches:

CMP op1, op2JE TrueJMP EndIf

True:<…>

EndIf

CMP op1, op2JNE False<…>

False:

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IF … AND …

CMP X, op1JNG EndIfCMP Y, op2JNLE EndIfCMP ………

<…>EndIf:

If (X > op1)and(Y <=op2)and…

Then<…>

End If

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IF … OR …

CMP X, op1JG TrueCMP Y, op2JLE TrueCMP ………JMP EndIf

True:<…>

EndIf:

If (X > op1) or(Y <=op2) or…

Then<…>

End If

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Applications

Task: Jump to a label if unsigned EAX is greater than EBX

Solution: Use CMP, followed by JA

Task: Jump to a label if signed EAX is greater than EBX

Solution: Use CMP, followed by JG

cmp eax,ebxja Larger

cmp eax,ebxjg Greater

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What's Next

Boolean and Comparison Instructions Conditional Jumps Conditional Loop Instructions Conditional Structures Application: Finite-State Machines (skipped) Decision Directives Brief Overview of Chapter 7

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LOOPZ and LOOPE

Syntax: LOOPE destination ; loop if equalLOOPZ destination ; loop if zero

Destination label must be in -128 ~ +127 Logic:

ECX ECX – 1 if ECX > 0 and ZF=1, jump to destination

Useful when scanning an array for the first element that does not match a given value.

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LOOPNZ and LOOPNE

Syntax: LOOPNZ destinationLOOPNE destination

Logic: ECX ECX – 1; if ECX > 0 and ZF=0, jump to destination

Useful when scanning an array for the first element that matches a given value.

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LOOPNZ Example

Find the first positive value in an array:.dataarray SWORD -3,-6,-1,-10,10,30,40,4.code

mov esi,OFFSET arraymov ecx,LENGTHOF arraysub esi,TYPE array

next:add esi, TYPE arraytest WORD PTR [esi],8000h ; test sign bitloopnz next ; continue loopjnz quit ; none found… ; ESI points to value

quit:

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What's Next

Boolean and Comparison Instructions Conditional Jumps Conditional Loop Instructions Conditional Structures Application: Finite-State Machines (skipped) Decision Directives Brief overview of Chapter 7

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Block-Structured IF Statements

Assembly language programmers can easily translate logical statements written in C++/Java into assembly language:

mov eax,op1 cmp eax,op2 jne L1 mov X,1 jmp L2

L1: mov X,2L2:

if( op1 == op2 ) X = 1;else X = 2;

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WHILE

A WHILE loop is really an IF statement followed by the body of the loop, followed by an unconditional jump to the top of the loop.

While:CMP op1, op2JNL EndDo<…>JMP While

EndDo:

DO WHILE(op1<op2)<…>

END DO

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REPEAT UNTIL

repeat:<…>CMP X, op1JE EndIfCMP Y, op2JNG repeat

EndIf:

REPEAT<…>

UNTIL(X == op1) or(Y > op2)

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What's Next

Boolean and Comparison Instructions Conditional Jumps Conditional Loop Instructions Conditional Structures Application: Finite-State Machines (skipped) Decision Directives

.IF, .WHILE, and .REPEAT directives (note: not instructions)

Brief Overview of Chapter 7

34

Runtime Expressions

.IF, .ELSE, .ELSEIF, and .ENDIF can be used to instruct the assembler to create block-structured IF statements

• MASM generates "hidden" code for you, consisting of code labels, CMP and conditional jump instructions

.IF eax>ebxmov edx,1

.ELSEmov edx,2

.ENDIF

.IF eax>ebx && eax>ecxmov edx,1

.ELSEmov edx,2

.ENDIF

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Relational and Logical Operators

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MASM-Generated Code

MASM automatically generates an unsigned jump, because val1 is unsigned JBE: jump if below or equal

mov eax,6cmp eax,val1jbe @C0001 mov result,1

@C0001:

.dataval1 DWORD 5result DWORD ?.codemov eax,6.IF eax > val1mov result,1.ENDIF

Generated code:

37

MASM-Generated Code

MASM automatically generates a signed jump (JLE)

mov eax,6cmp eax,val1jle @C0001 mov result,1

@C0001:

.dataval1 SDWORD 5result SDWORD ?.codemov eax,6.IF eax > val1mov result,1.ENDIF

Generated code:

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.REPEAT Directive

Executes the loop body before testing the loop condition associated with the .UNTIL directive.

Example:

; Display integers 1 ~ 10:

mov eax,0.REPEAT

inc eaxcall WriteDeccall Crlf

.UNTIL eax == 10

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.WHILE Directive

Tests the loop condition before executing the loop body. The .ENDW directive marks the end of the loop.

Example:

; Display integers 1 ~ 10:

mov eax,0.WHILE eax < 10

inc eaxcall WriteDeccall Crlf

.ENDW

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When to Use or Not to Use Directives?

Directives make assembly language easier to write and to understand, by hiding tedious work. (Food for thought: Wouldn’t it be even better to use C language?)

Don’t use directives if you want to have total control.

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What's Next

Boolean and Comparison Instructions Conditional Jumps Conditional Loop Instructions Conditional Structures Application: Finite-State Machines (skipped) Decision Directives Brief Overview of Chapter 7

42

Logical vs Arithmetic Shifts

A logical shift fills the newly created bit position with zero:

An arithmetic shift fills the newly created bit position with the sign bit:

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SHL Instruction

Shift left: performs a logical left shift on the destination operand, filling the lowest bit with 0.

Operand types for SHL:

SHL reg,imm8SHL mem,imm8SHL reg,CLSHL mem,CL

(Same for all shift and rotate instructions)

44

SHR Instruction

Shift right: performs a logical right shift on the destination operand. The highest bit position is filled with a zero

Shifting right n bits divides the operand by 2n

mov dl,80shr dl,1 ; DL = 40shr dl,2 ; DL = 10

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SAL and SAR Instructions

Shift arithmetic left: identical to SHL Shift arithmetic right: performs a right arithmetic

shift on the destination operand

An arithmetic shift preserves the number's sign

mov dl,-80sar dl,1 ; DL = -40sar dl,2 ; DL = -10

46

MUL Instruction

The MUL (unsigned multiply) instruction multiplies an 8-, 16-, or 32-bit operand by either AL, AX, or EAX.

The instruction formats are:MUL r/m8MUL r/m16MUL r/m32 Implied operands:

47

DIV Instruction

Unsigned divide: performs 8-bit, 16-bit, and 32-bit division on unsigned integers

A single operand is supplied (register or memory operand), which is assumed to be the divisor

Instruction formats:DIV r/m8DIV r/m16DIV r/m32

Default Operands:

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Summary Bitwise inst. (AND, OR, XOR, NOT, TEST)

manipulate individual bits in operands CMP: implied sub for comparing operands

sets condition flags Conditional Jumps & Loops

equality: JE, JNE flag values: JC, JZ, JNC, JP, ... signed: JG, JL, JNG, ... unsigned: JA, JB, JNA, ... LOOPZ, LOOPNZ, LOOPE, LOOPNE

Shift: SHL, SHR, SAL, SAR Multiplication and division: MUL, DIV