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assemblyx86att

What cmpl $0x1, -0x18(ebp)?

So I have a line that does logical compare like: cmpl $0x1, -0x18(ebp) . After this it jumps if this is equal, but what exactly does that -0x18(ebp) do?

Solution

  • Let's break it down a bit.

    -0x18(ebp) takes the value stored in the ebp register, subtracts 0x18 from it, and uses the result as a memory address to read from.

    Therefore, cmpl $0x1, -0x18(ebp) reads the value stored at the memory address calculated from -0x18(ebp) and subtracts $0x1 from it, setting various flags based on that calculation (e.g. setting the zero flag ZF to 1 if the subtraction results in 0). The various jmp instructions then determine whether to jump or not based on the flags that were set.