Assembly language je jump function

Question

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.

Answer 1

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 ... 

Answer 2

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".