Array Indexing in Nasm x86-64

In our previous lesson, we learnt how to use and implement macros to make our Nasm x64 code efficient and non-repetitive.

Intro to Macros

Arrays Are 0-Indexed (Like Any Real Language 🙂)

In Nasm x64, arrays (including strings) are 0-indexed, just like in C or Python i.e. the first element is accessed at index 0.

Here’s a Python example for context:

arr_string = "Hello Earthlings!"
print(arr_string[0]) # output: H

The following Nasm x64 demonstrates an array indexing similar to the python code above

%include "mymacros.inc"

section .data
    newline db 10                ; newline ascii character
    arr_string db "Hello Earthling!",0  ; initializing array

section .bss
    output resb 3                ; reserving 3 bytes for output

section .text
    global _start    

_start:
    mov rcx, 0                    ; set rcx to 0
    mov al, [arr_string + rcx]    ; [arr + 0] means arr[0] and then move it to al
    mov [output], al              ; move al value into output

    print output, 2
    print newline, 1
    exit 0

Don’t Do This ❌

One may be inclined to do the following

mov [output], [arr_string + rcx]   ; ❌ this is wrong, I repeat, very very wrong

The above is called memory-memory move. It is not allowed in Nasm x86. Hence the reason for an intermediate register, al.

The only moves allowed are

• register-register mov rax, rbx

• register-memory mov [output], al

• memory-register mov al, [output]

So always remember: use an intermediate register (like al, rax, etc.) when moving data from one memory location to another.

Conclusion

Array indexing in Nasm x64 follows the same logic as higher-level languages, but with more control and more responsibility. Remember that you control the registers and memory explicitly, and you must follow the hardware rules; no hand-holding here.

Using mov al, [arr + rcx], gives you full control to walk through a string or array one byte at a time. And that is the real beauty of low-level programming; you see everything that is happening under the hood.

Next time, we’ll look at how to loop through an entire array character by character and determine its length.

Exercise

Toggle the value in rcx with values in the range of 0 to 14 to display different characters.

mov rcx, 5

What character displays now?