The .data Section

Part of the Fundamentals section of Coddy's Assembly journey — lesson 23 of 45.

In Chapter 2, you loaded values from memory and stored values to memory. But where did that memory come from? Who created num1, num2, and result?

The answer is the .data section. This is where you create your own data in memory.

What is the .data section?

The .data section is a part of your assembly program where you define data that will be stored in memory before your program runs.

Syntax:

section .data
    ; Your data definitions go here

Why you need it:

Without .data section	With .data section
You have no data to work with	You can create numbers, strings, and variables
Memory addresses are unknown	You can use labels like `myNumber` or `message`
You cannot test your code	You can load and store data you created

A simple example:

section .data
    age db 25           ; Create a byte called "age" with value 25
    price dw 1000       ; Create a word called "price" with value 1000
    name db 'Bob'       ; Create a string called "name" with letters B, o, b

Where it goes in your files:

Remember the two-file pattern from Chapter 0:

File	Content
`data.asm`	`section .data` with your data definitions
`main.asm`	`%include "data.asm"` followed by `section .text` with your code

What happens when your program runs:

The operating system loads your program into memory
It reads the .data section and places your data at specific memory addresses

Your labels (age, price, name) become addresses
Your code can now access these addresses using [age], [price], [name]

Important rules:

Rule	Explanation
One `.data` section per program	Put all your data in one place
Labels must be unique	You cannot have two `age` labels
Data is created before your code runs	It is ready to use when `_start` begins

In the next lessons, you will learn how to define different types of data using db, dw, dd, and dq

Challenge

Write the code in the data.asm file to:

Create the .data section
Create a byte called count with the value 5

Try it yourself

%include "data.asm"

section .text
    global _start

_start:
    ; Load the value from count into al
    mov al, [count]
    
    ; Save it for printing
    mov r12, rax
    
    ; Convert to ASCII and print (0-9 only)
    add al, 48
    mov [digit], al
    
    mov rax, 1
    mov rdi, 1
    mov rsi, digit
    mov rdx, 1
    syscall
    
    ; Exit
    mov rax, 60
    mov rdi, 0
    syscall

section .bss
    digit resb 1