Question

What does this line do in the following code?

0x0804840c <+3>: mov 0x8(%ebp),%edx

I know mov %x, %y moves reg value %x to %y, but stack offset 8 has never been set as anything, so I’m not sure what is being moved to %edx. I’m really new to assembly and I’m completely lost.

(IA32 Assembly)

  0x08048409 <+0>: push %ebp 

  0x0804840a <+1>: mov %esp,%ebp 

  0x0804840c <+3>: mov 0x8(%ebp),%edx 

  0x0804840f <+6>: mov %edx,%eax 

  0x08048411 <+8>: shl $0x4,%eax 

  0x08048414 <+11>: sub %edx,%eax 

  0x08048416 <+13>: pop %ebp 

  0x08048417 <+14>: ret

Answer 1

This is the 32-bit x86 ELF ABI, looks like, so the stack slot at 8(%ebp) holds argument 1 to this function, put there by the caller.

The overall function computes (x << 4) - x.