Array in x86 Assembly

In this article at OpenGenus, we have explained how to implement an Array in x86 Assembly Programming Language. This gives a strong insight on how array function behind the high level programming languages.

Table of contents:

1. How to implement Array in Assembly?
2. Array in x86 Assembly Language
3. Compile and run the code

How to implement Array in Assembly?

We define an array of 5 bytes using DB directive in .data section and initialized the first 5 elements as 10, 20, 30, 40 and 50.

myArray db 10, 20, 30, 40, 50 ;

We can access an element by loading it to a register. For example, in this assembly code snippet, we loaded the second element into a register using MOV:

mov bl, [myArray + 1]   ;

We can also, modify an array element using MOV by moving the value of a register to a specific array element.

mov [myArray + 3], dl   ;

We used a loop to iteration over the element of the array, loaded each element to a register by incrementing the loop counter. ECX is used as the loop counter which is initialized to 0 and the base address of the array is EBX.

mov ebx, 0 

At the end, exit the program using (understand this part using this article):

; Exit the program
mov eax, 1              ; Set EAX to 1 (system call for exit)
xor ebx, ebx            ; Set EBX to 0 (exit code)
int 0x80                ; Invoke the system call

Array in x86 Assembly Language

Following is the complete assembly code for implementing an array of 5 elements:

section .data
    myArray db 10, 20, 30, 40, 50 ; Define an array of bytes with 5 elements

section .text
    global _start

_start:
    ; Accessing array elements
    mov al, [myArray]       ; Load the first element of the array into the AL register
    mov bl, [myArray + 1]   ; Load the second element of the array into the BL register
    mov cl, [myArray + 2]   ; Load the third element of the array into the CL register

    ; Modifying array elements
    mov [myArray + 3], dl   ; Store the contents of DL into the fourth element of the array

    ; Accessing array elements using a loop
    mov ecx, 0              ; Set ECX to 0
    mov ebx, 0              ; Set EBX to the starting address of the array

loop_start:
    cmp ecx, 5              ; Compare ECX with the length of the array
    je loop_end             ; If ECX equals 5, jump to loop_end

    mov al, [ebx + ecx]     ; Load the current element of the array into AL
    inc ecx                 ; Increment ECX
    jmp loop_start          ; Jump to loop_start

loop_end:
    ; Rest of the program
    ; ...

    ; Exit the program
    mov eax, 1              ; Set EAX to 1 (system call for exit)
    xor ebx, ebx            ; Set EBX to 0 (exit code)
    int 0x80                ; Invoke the system call

Compile and run the code

Save the above code in a file named as array.asm.

To compile the above code in NASM, use the following command:

nasm -f elf64 -o array.o array.asm
ld -o array array.o

The executable is named as array. To run the executable, use the following command:

./array

With this article at OpenGenus, you must have the complete idea of how to implement an array in x86 Assembly language.