Actual Assembly-- NASM

CS 301 Lecture, Dr. Lawlor

So check out "Assembly-NASM" mode in NetRun. Here's the cheat sheet for x86 assembly.

A typical line of assembly code looks like this:

	mov eax,1234 ;  I'm returning 1234, like the homework says...

There are several parts to this line:

"mov" is the "opcode mnemonic". It corresponds to the first byte or so that tells the CPU what to do.
"eax" is the destination of the move, also known as the "destination operand". It's a register, and it happens to be 32 bits wide, so this is a 32-bit move.
1234 is the source of the moved data, also known as the "source operand". It's a constant, so you could use an expression (like "2+5*8") or a label (like "foo") instead.
The semicolon indicates the end of the line, and the start of a comment.

Unlike C/C++, assembly is line-oriented, so the following WILL NOT WORK:

	mov eax,
	         1234 ;  I'm returning 1234, like the homework says...

Yup, line-oriented stuff is indeed annoying. Be careful that your editor doesn't mistakenly add newlines!

Registers

The really important registers:

eax. This is the register that stores a function's return value.
eax, ecx, edx, esi, edi. "Scratch" registers you can always overwrite with any value.
rdi, rsi, rdx, rcx, ... In 64-bit mode, these registers contain subroutine arguments, in left-to-right order.
esp, ebp. Registers used to run the stack. Be careful with these!

All the other registers:

Size	Register names	Meaning (note: not the official meanings!)	Introduced in
8-bit	al,ah, bl,bh, cl,ch, dl,dh	"Low" and "High" parts of bigger registers	1972, Intel 8008
16-bit	ax, bx, cx, dx, si, di, sp, bp	"eXtended" versions of the original 8-bit registers	1978, Intel 8086/8088
32-bit	eax, ebx, ecx, edx, esi, edi, esp, ebp	"Extended eXtended" registers	1985, Intel 80386
64-bit	rax, rbx, rcx, rdx, rsi, rdi, rsp, rbp, r8, r9, r10, r11, r12, r13, r14, r15	"Really eXtended" registers	2003, AMD Opteron / Athlon64 2004, Intel EM64T CPUs

x86 is rather unique in that all the smaller registers from bygone eras are still right there as *part* of the new, longer registers. So for example, this routine returns 0x0000AB00, because 0xAB is put into byte 1 of eax:

	xor eax,eax ; Clear eax, by xor'ing it with itself.
	mov ah,0xAB ;  eax is return result register.

Opcodes

A list of all possible mnemonics can be found in:

Roger Jegerlehner's CodeTable, in categorized form.
Gary Burt's HTML table, just the basics, listed in alphabetical order, but maybe too terse.
Giant Intel PDF reference manual (section 3.2), impossibly complete; but nearly impossible to understand.

The really important opcodes are listed in my cheat sheet. Most programs can be writen with mov, add, sub, mul, call, ret, cmp, jmp, jl/je/jg/..., push, and pop.