Programming
assembly x86 comparison
Updated Sat, 21 May 2022 07:42:20 GMT

Understanding cmp instruction


I'm very new to assembly and now I'm trying to understand how cmp works. Here is what's written in wiki:

cmp arg2, arg1

Performs a comparison operation between arg1 and arg2. The comparison is performed by a (signed) subtraction of arg2 from arg1, the results of which can be called Temp. Temp is then discarded.

What does it mean "Temp is then discarded"? Where is it stored? How can I access this result of the comparison? Can someone explain it?




Solution

cmp arg2, arg1 performs the same operation as sub arg2, arg1 except that none of the operands are modified. The difference is not stored anywhere.

However, the flags register is updated and can be used in a conditional jump, like jump-if-equal (JE), most often as the next instruction after the cmp.

The advantage over other instructions is that you can compare two values without destroying any of them. If you did sub arg2, arg1 and they happen to be equal, one of them would be zero afterwards. With cmp they are both still there.







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