X86 cmpl and jne

Question

I am tracking some x86 code for assignment, and I was wondering what exactly cmpl does and how to predict whether jne will execute.

80484bf:    83 7d f0 07             cmpl   $0x7,-0x10(%ebp)
80484c3:    75 16                   jne    80484db

In addition, Intel syntax is used for this.

Thank.

+5

Richarizard Mar 08 '13 at 7:34

1 answer

Omar MEBARKI · Answer 1 · 2013-03-08T08:04:34+0000

cmpl subtracts -0x10 (% ebp) from $ 0x7 and changes the flags: AF CF OF PF SF ZF .

if -0x10 (% ebp), which is one of your function arguments, is 0x7, then the ZF flag is set .
jne 80484db means that if the two numbers being compared are different (ZF = 0), go to 80484db

To summerize, your code is equivalent:

compare A to B
jump into 80484db if they are different.