汇编cmp指令

Question

The following cmp instruction is confusing me:

%eax,0x18(%esp,%ebx,4)

I know it's comparing arg1 to arg2, but what exactly is going on in arg2 with the stuff in parenthesis?

Thanks

Answer 1

It calculates the address of a memory location based on the values of esp and ebx. You didn't specify the assembly notation used, so I can just guess it's esp + 4*ebx + 0x18.

Answer 2

That's an effective address in at&t syntax. The general form is displacement(base, index, scale) where displacement is a number (address), base and index are registers, and scale is a factor of 1 , 2 , 4 or 8 . The resulting address is calculated as displacement + base + index * scale . Components may be omitted.

By the way, you can switch gnu tools into intel syntax mode if you like that better.