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assemblyx86x86-64attinstructions

The difference between cmpl and cmp

I am trying to understand assembly to be able to solve a puzzle. However I encountered the following instructions:

0x0000000000401136 <+44>:    cmpl   $0x7,0x14(%rsp)
0x000000000040113b <+49>:    ja     0x401230 <phase_3+294>

What I think it's doing is: The value of 0x14(%rsp) is -7380. According to my understanding cmpl compares unsigned. Also the jump is performed.

So can it be that (unsigned)-7380 > 7 --> jump

I actually don't want it to jump. But is this the correct explanation or not? Am I flipping arguments?

Also if you have any advice about how to manipulate this jump!

Solution

  • According to my understanding cmpl compares unsigned.

    It does both, in a way.

    The difference in signed vs. unsigned is here the usage of the jump instructions.

    For >, there is ja for unsigned and jg for signed (jump if above and jump if greater).

    For <, there is jb for unsigned and jl for signed (jump if below and jump if less).

    To be exact, here is the meaning of several jump commands:

    For unsigned comparisons:

    JB/JNAE (CF = 1)           : Jump if below/not above or equal
JAE/JNB (CF = 0)           : Jump if above or equal/not below
JBE/JNA (CF = 1 or ZF = 1) : Jump if below or equal/not above
JA/JNBE (CF = 0 and ZF = 0): Jump if above/not below or equal

    For signed comparisons:

    JL/JNGE (SF <> OF)          : Jump if less/not greater or equal
JGE/JNL (SF = OF)           : Jump if greater or equal/not less
JLE/JNG (ZF = 1 or SF <> OF): Jump if less or equal/not greater
JG/JNLE (ZF = 0 and SF = OF): Jump if greater/not less or equal