x86 - How does the jmp instruction work in att assembly in this instance

Question

I am stepping through a function in AT&T assembly right now and can't figure out how this specific jmp command works.

    jmp    *0x804a140(,%eax,4)

How exactly is it using the %eax register and 4 with the jump instruction? I have never seen jmp used this way before.

Answer 1

If you are confused by at&t syntax, switch your tool to intel mode.

The effective address you see is not specific to jumps, you could have encountered it with any instruction that takes a memory operand.

In intel syntax this would look like: jmp [0x804a140 + 4 * eax]. It's an indirect jump that fetches the jump target from memory address 0x804a140 + 4 * eax. This is probably an item in a so-called jump table.