Assembly language je jump function

Question

Hello everyone! I am trying to find online the usage of the assembly language function "je". I read that je means jump if equal and that is exactly what I want. What is the actual usage of this function, or in other words, how to I type this function to check a value and jump if it is equal to something?

Please let me know.

BTW, I am using NASM if that makes a difference.

Thanks

Answer 1

Well, I finally found my answer. :P Basically you call je label_to_jump_to after a cmp call.

If cmp shows that the two values are equal, je will jump to the specified label. If not, it will keep execution flowing.

Answer 2

You'll precede the je with a cmp (or test or equivalent) usually, which sets a flag in the EFLAGS register. Here's a link to a simple echo server in NASM that might help in general. Ignore the annoying Google ads.

An example usage for je might be:

    cmp eax, ebx
    je  RET_FAIL
    jmp RET_SUCCESS

RET_FAIL:
    push 1
    pop  eax
    ret

RET_SUCCESS:
    push 0
    pop eax
    ret

Answer 3

This will jump if the "equal flag" (also known as the "zero flag") in the FLAGS register is set. This gets set as a result of arithmetic operations, or instructions like TEST and CMP.

For example: (if memory serves me right this is correct :-)

cmp eax, ebx    ; Subtract EBX from EAX -- the result is discarded
                ; but the FLAGS register is set according to the result.
je .SomeLabel   ; Jump to some label if the result is zero (ie. they are equal).
                ; This is also the same instruction as "jz".

Answer 4

Let's say you want to check if EAX is equal to 5, and perform different actions based on the result of that comparison. An if-statement, in other words.

  ; ... some code ...

  cmp eax, 5
  je .if_true
  ; Code to run if comparison is false goes here.
  jmp short .end_if
.if_true:
  ; Code to run if comparison is true goes here.
.end_if:

  ; ... some code ...