MOV Memory to Register

Part of the Fundamentals section of Coddy's Assembly journey. Lesson 17 of 45.

So far, you have moved numbers directly into registers (immediate) and copied values between registers. Now you will learn how to load data from memory into a register.

Why this matters:

Remember from Chapter 0: the CPU cannot work directly with memory. Data must be loaded into registers first. This is how you do it.

Syntax:

mov register, [address]

The brackets [ ] mean: "go to this memory address and get the value stored there."

Think of it like this:

Imagine at memory address number there is a box. Inside that box is a value, for example the number 5. When you write:

mov al, [number]    ; Load the value from memory address "number" into al

You are telling the CPU: "Go to the box called number, take whatever value is inside (in this case 5), and copy it into the al register."

After this instruction is executed, the value 5 is now stored in al. The original value inside the box at address number stays there. It is not removed.

Important rules:

Rule	Explanation
The source must be a memory address	`[label]` or `[address]`
The brackets are required	`mov al, myNumber` loads the address, not the value
Size must match	If memory stores a byte, use `al` (not `rax`)

Common mistake:

; Wrong: loads the ADDRESS of myNumber, not the value
mov al, myNumber

; Correct: loads the VALUE stored at myNumber
mov al, [myNumber]

Challenge

Write one line of assembly to load the value from memory address value into the al register.

Try it yourself

section .data
    msg db 'al = '
    msg_len equ $ - msg

section .bss
    digit resb 1

section .text
    global _start

_start:
    ; The student's code runs first (loads value into al)
    
    %include "solution.asm"
    
    ; Save the value from al
    mov r12, rax
    
    ; Print "al = "
    mov rax, 1
    mov rdi, 1
    mov rsi, msg
    mov rdx, msg_len
    syscall
    
    ; Print the value in al
    mov rax, r12
    add rax, 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