Question

PLATFORM: Pentium 4 processor (i really need to upgrade 🙂 running Windows XP. I am currently assembling with MASM32.

I am an absolute beginner with assembly and I am trying to figure out how to add variables. I understand how to define variables, place data in a register, and how to place registers in the stack, but I don’t know how to combine the values of two variables (by adding).

Also, could anyone give me a link or something to an easy to understand tutorial on x86 assembly, all the ones that I’ve found so far are really high-level stuff and they don’t explain much. That would be really appreciated.

Thanks in advance,
Progrmr

Answer 1

Simple addition:

add eax, ebx   ; Adds eax and ebx and stores the result in eax

Load from memory, then add, then store in memory:

mov eax, DWORD PTR [esi]    ; Load a double word from memory into eax
mov ebx, DWORD PTR [edi]    ; Load a double word from memory into ebx
add eax, ebx                ; Adds eax and ebx and stores the result in eax
mov DWROD PTR[esi], eax     ; Store a double word in eax into memory

In the examples above, DWORD PTR isn’t strictly needed, but is a good habit to practice because it remove any ambiguity and makes code easier to read.

It’s important to remember that you can only add registers of the same size (DWORD in the examples above). If you want to add two registers of different sizes:

mov al, BYTE PTR [esi]    ; Loads a single byte from memory into al
mov bx, WORD PTR [edi]    ; Loads a word from memory into bx
movzx eax, al             ; Zero extends al into the entire eax register
movzx ebx, bx             ; Zero extends bx into the entire ebx register
add eax, ebx              ; Adds eax and ebx and stores the result in eax

This isn’t a terribly good example because there are easier ways to do the same thing, but hopefully it’ll show some of the tricks you can use.

You can also add a memory value to a registers:

mov eax, DWORD PTR [esi]     ; Load a double word from memory into eax
add eax, DWORD PTR [edi]     ; Add a double word in memory to eax

Here’s a good site to use as a reference: http://ref.x86asm.net/