Program Organization

There are three parts to an x86 program.

section .data

Contains initialized variables and constants (such as int x = 5;).
Consider the following:

section .data
    bVar db  0 ; initializes a byte
    wVar dw  0 ; initializes a word
    dVar dd  0 ; initializes a dword (i.e.t. int dVar = 0)
    qVar dq  0 ; initializes a qword
    NULL equ 0 ; initializes a constant (i.e.t. const int NULL = 0)

The names on the left (ex: bVar) are the names of your variables. These can be whatever you would like. However, it is standard to name constants in ALL_UPPERCASE and normal variables in lowerCamelCase.

The letters in the second column (ex: db) are the data type. The “d” stands for “define” and the letter after corosponds to the type.

The values in the third column are what the initial values of your variables are, and can be changed throughout the program (unless a constant).

Also, you may notice we use ; instead of // to comment.

section .bss

Contains uninitilized data (such as int x;).
Consider the following:

bVar resb  1 ; reserves 1 byte of data
bVar resb  4 ; reserves 4 bytes of data
wVar resw  1 ; reserves 1 word of data
wVar resw 10 ; reserves 10 words of data

The names on the left (ex: bVar) are the names of your variables. These can be whatever you would like.

The letters in the second column (ex: resb) are the data type. The “res” stands for “reserve” and the letter after corosponds to the type.

The values in the third column are how many of that type you would like to reserve. To determine how much data is being used, multiply the data type and how many spots you are reserving.

section .text

Contains code.

In C++, we do:

int main(){
    /* some code*/
    return 0;
}

In x86, we do:

global _start
_start:
    ; some code
mov rax, SYS_exit
mov rdi, EXIT_GOOD
syscall

Running your program

Unlike C++, you do not “compile” our code, we “assemble” it.
You will be provided a make file, so you can run

make ast01

Then, to run your program:

./ast01