Instructions That May Be Used for Assignment 1

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Instructions That May Be Used for Assignment 1

• Department of Computer Science
• National Tsing Hua University

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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 lt source
• Example destination gt source

mov al,4 cmp al,5 Carry flag set
mov al,6 cmp 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 gt source
• Example destination lt source

mov al,5 cmp al,-2 Sign flag Overflow flag
mov al,-1 cmp al,5 Sign flag ! Overflow flag
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CMP and Jcond Instruction

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

CMP X, op1 JNG EndIf ltgt EndIf
If (X gt op1) Then ltgt End If
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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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More Frequently Used Jcond

• JE (Equal)
• JNE (Not Equal)
• JG or JGE (Greater Than or Equal)
• JL or JLE (Less Than or Equal)
• Note JGJNLE, JGEJNL, etc.

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Simple IF

• If (op1op2) then ltgt end if
• Two different approaches

CMP op1, op2 JE True JMP EndIf True ltgt End
If
CMP op1, op2 JNE False ltgt False