Indirect Register Addressing

Question

I am trying to figure out how register indirect addressing works. I have a variable that stores the value of 5 as follows:

section .data

number db 53 ; ascii code for 5

section .bss
number2 resb 1
section .text
global _start

_start:

mov eax,number
mov number2,[eax]

At the last two lines of the code what I am essentially trying to do is made eax act like a pointer to the data stored at number and then move this data into the number2 variable. I had though indirect register addressing was done via [register] but my code does not seem to work. Any help with regards to syntax would be much appreciated.

Answer 1

Labels work as addresses in nasm so your mov number2, [eax] would translate to something like mov 0x12345678, [eax] which is of course invalid because you cannot move data to immediate operand. So you would need mov [number2], [eax] but that's also invalid.

You can achieve this using some register to temporarily hold the value [eax]:

mov eax, number
mov dl, [eax]
mov [number2], dl

Answer 2

The problem here is, that number and number2 are not numbers, i.e. immediate literals. Instead they are interpreted as absolute memory addresses and the corresponding instructions, if they would exist would be e.g.

 mov eax, [0x80000100]        ;; vs
 mov [0x80000104], [eax]     ;; Invalid instruction

One has to pay attention to the instruction format as well, as answered by Mika Lammi -- is the instruction

 mov src, dst   ;; vs
 mov dst, src

In addition, one should match the register size to the variable size; i.e

.data
number   db 1;   // this is a byte
.code
mov al, number