How the CMP instruction uses condition flags?

Question

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:

You could get that information from the first results from DuckDuckGo (or Google (or Bing)). Did you tried that? — Paweł Łukasik, Mar 10 '19 at 09:36
Yes I did. (Google). Found a lot, what jumps read, but not exactly what CMP sets. — PaHa, Mar 10 '19 at 09:47
CMP sets all the flags based on the intermediate results of SUB that it performs. read here: http://www.godevtool.com/GoasmHelp/usflags.htm — Paweł Łukasik, Mar 10 '19 at 09:56
(CMP EAX,EDX;set zero flag if eax=edx.) So only the zero flag and if true 1 and false 0. Tanks. — PaHa, Mar 10 '19 at 10:03
read cmp documentation first: The comparison is performed by subtracting the second operand from the first operand and then setting the status flags in the same manner as the SUB instruction.. But that's not a good way to check whether a register is zero or not — phuclv, Mar 11 '19 at 16:13

she_roar · Accepted Answer · 2019-03-23T18:42:29.523

9

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:

edited Mar 23 '19 at 18:42

answered Mar 16 '19 at 16:56

she_roar

Presumably, where values are signed and of opposite signs (because the CPU doesn't have a way to distinguish signed from unsigned) that use of the C flag isn't true - so 0xff > 0x01 makes it also look like -1 > 1. I expect in that case you'd check the S flag which is true if the result would be negative. – Jim Driscoll Feb 17 '24 at 11:30

Uwe Plonus · Answer 2 · 2019-03-10T09:57:38.270

4

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).

edited Mar 10 '19 at 09:57

answered Mar 10 '19 at 09:47

Uwe Plonus

2 Answers2