Question about intel assembly

I am trying to wo开发者_开发技巧rk my way through an Intel assembly file and I ran into a bit of code I don't understand.

# Save current stack pointer to old thread's stack, if any.
movl SWITCH_CUR(%esp), %eax
movl %esp, (%eax,%edx,1)

I understand that the first line moves the value of 12 offset from register %esp into the register %eax.

But I don't understand what the second line does it moves %esp into what.

Is (%eax,%edx,1) a logical and operation? Or addition?

Any help would be great.

Thanks

The meaning is

mov %esp, (%eax + %edx * 1)

The number can be 1, 2, 4 or 8.

See http://en.wikibooks.org/wiki/X86_Assembly/GAS_Syntax

(%eax,%edx,1) refers to the location in memory whose address is formed by adding the value in %eax to the value in %edx. The 1 is the scale factor which doesn't actually "matter" because it is 1.

ADDENDUM

In detail (in case anyone else lands on this page):

mov %esp, %edx              # move contents of esp into edx

Assuming edx contains 100 and eax contains 20:

mov %esp, (%edx)            # move contents of esp in address 100
mov %esp, (%edx,%eax)       # ... into address 100 + 20 = 120
mov %esp, (%edx,%eax,4)     # ... into address 100 + (20*4) = 180