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Compare and Branching

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• Compare
  • The CMP compares two values without changing either, then sets or clears the processor flags according to the results of the comparison.

  • Example:

    cmp ax, bx ;; computes ax - bx ;; Set the flags according to result

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• Branch instructions
  • The opcode for the branch instructions are:

    je jump when equal jne jump when not equal jl jump when less than jle jump when less than or equal jg jump when greater than jge jump when greater than or equal jmp unconditional jump

  • How to use compare and branch:

    cmp eax, ebx ;; computes EAX - EBX jg next ;; jump to "next" when EAX > EBX

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• Example Program
  • Consider this program:

    int x, y; if ( x >= y ) System.out.print( x + " >= " + y ); else System.out.print( x + " < " + y );

    Intel Assembler code:

    .data Str1 db "%d >= %d", 13, 10, 0 Str2 db "%d < %d", 13, 10, 0 x dd 40 y dd 6 .code start: mov eax, x mov edx, y cmp eax, edx ;; x - y jl Else1 ;; jump if x < y ;; Then part: x >= y push y push x push offset Str1 call crt_printf jmp Done ;; Else part: x < y Else1: push y push x push offset Str2 call crt_printf Done:

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  • Example Program: (Demo above code)                                                
    • Prog file: click here