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I’ve come across a specific instruction sequence that I need help understanding, particularly the comparison (`cmp`) operation and how to break at this point in GDB on an Intel Core i7-11700K Rocket Lake processor . It’s low level assembly debugging in C and assembly mix
0x0000000000001410 <+241>: mov eax,DWORD PTR [rbp-0x74]
0x0000000000001413 <+244>: cmp DWORD PTR [rbp-0x70],eax
0x0000000000001416 <+247>: jne 0x149d <main+382>
0x000000000000141c <+253>: lea rsi,[rip+0xbf7] # 0x201aI’m particularly interested in the `cmp` instruction at `0x0000000000001413`. From what I understand, it compares the value stored at `[rbp-0x70]` with the value currently in the `eax` register.
What exactly is this `cmp` operation checking tho?
What happens if the values are not equal?
And how can I set a breakpoint at this comparison line in GDB to inspect the values before the comparison happens?
I tried to break at the memory address `0x0000000000001413` using `break *0x0000000000001413`, but I’m not sure if that’s the correct approach
The `cmp`, often integer comparison, compare the values then sets a condition flag which jump instructions rely on to make jump decisions. `cmp` works by subtraction (cmp a, b == b-a)
Use `info registers` in gdb to peek at registers.
So if cmp is setting the condition flags based on eax – [rbp-0x70], it must be setting the zero flag (ZF) in case they’re equal, which the jne instruction relies on to decide whether to jump or not, correct?
Correct.
Thanks 👍
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