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I am trying to figure out the behavior of conditional jumps (JE/JNE, JZ/JNZ) in the x86 instruction set familly.

Which condition flags CMP instruction sets and how, if the result is equal and if it is not? For example:

  • CMP eax, 0 (true)

  • CMP eax, 0 (false)

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Furthermore, with the CMP instruction, the destination operand doesn't change. Just the flags.

Let me illustrate. Let's say EAX = 00000005 and EBX = 00000005. If we do this arithmetic operation:

CMP EAX, EBX

What's happening, is in effect this:

EAX - EBX ----> 00000005 - 00000005

Since the result would be 0, but we don't change the destination operand in a CMP instruction, the zero flag is set to 1 (since it's true).

So, as we saw, depending on the result of the previous arithmetic operation, flags can be set accordingly:

enter image description here

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The CMP instruction does internally a SUB and sets the flags accordingly.

So all flags that are set by a SUB are also set by CMP.

Namely the flags SF, ZF, AF, PF, and CF are set.

This information is taken from the Intel manual for the processors (https://software.intel.com/en-us/articles/intel-sdm).

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