x86 Assembly

CS 301 Lecture, Dr. Lawlor, 2005/09/23

Last lecture we saw what basic assembly looked like. Where do you find the rest of the registers and instructions?

As usual, by reading a manual! The Intel S oftware Developer's Manuals are incredibly long, boring, and complete--they give all the nitty-gritty details. Volume 1 lists the processor registers in section 3.4.1. and is what you need for all x86 instructions. Volume 2 lists all the instructions.

In summary, the basic registers are:

eax

ecx

edx

ebx

esi

edi

ebp

esp

Be careful using ebp and esp, though--they're used for subroutine calls (as we'll talk about later).

Some useful instructions are:

Mnemonic	Purpose	Examples
mov src,dest	Move data between registers, load immediate data into registers, move data between registers and memory.	mov $4,%eax # Load constant into eax mov %eax,%ebx # Copy eax into ebx mov %ebx,123 # Copy ebx to memory address 123
add src,dest	dest=dest+src	add %ebx,%eax # Add ebx to eax
mul src	Multiply eax and src as unsigned integers, and put the result in eax. High 32 bits of product go into eax.	mul %ebx #Multiply eax by ebx
jmp label	Goto the instruction label:. Skips anything else in the way.	jmp post_mem mov %eax,0 # Write to NULL! post_mem: # OK here...
cmp a,b	Compare two values. Sets flags that are used by the conditional jumps (below). WARNING: compare is relative to last argument, so "jl" jumps if b<a!	cmp $10,%eax
jl label	Goto label if previous comparison came out as less-than. Other conditionals available are: jle (<=), je (==), jge (>=), jg (>), jne (!=), and many others.	jl loop_start # Jump if eax<10

You've now seen enough instructions to be able to do everything you could do in UEMU on the real machine!