Practice: Swap Registers

Part of the Fundamentals section of Coddy's Assembly journey — lesson 21 of 45.

Sometimes you need to swap the values between two registers. For example, if rax contains 5 and rbx contains 10, after swapping rax should be 10 and rbx should be 5.

You cannot do this with one or two MOV instructions. You need a third register as temporary storage.

Why you cannot do it directly:

; This does NOT work
mov rax, rbx   ; Now rax = 10, but the original 5 is gone forever
mov rbx, rax   ; Now rbx also = 10 (not 5)

The correct method — using a temporary register:

mov rcx, rax   ; Step 1: Save original rax into rcx (rcx = 5)
mov rax, rbx   ; Step 2: Copy rbx into rax (rax = 10)
mov rbx, rcx   ; Step 3: Copy saved value from rcx into rbx (rbx = 5)

Step by step:

Step	Instruction	rax	rbx	rcx
Start	(before any code)	5	10	?
1	`mov rcx, rax`	5	10	5
2	`mov rax, rbx`	10	10	5
3	`mov rbx, rcx`	10	5	5

After these three instructions, the values are swapped. The temporary register rcx still contains the original value (5), but that is fine.

The temporary register can be any register you are not using:

You can use rcx, rdx, r8, or any available register
Just make sure you do not need the original value in that register

Why this matters:

Swapping values is a common operation in sorting algorithms and data manipulation. It teaches you to think carefully about the order of MOV instructions.

Challenge

Write three lines of assembly to swap the values between r8 and r9.

Initial values (provided by test harness):

r8 starts with 6
r9 starts with 2

After your code:

r8 should be 2
r9 should be 6

Use r10 as your temporary register.

Try it yourself

section .data
    msg_r8 db 'r8 = '
    msg_r8_len equ $ - msg_r8
    msg_r9 db 'r9 = '
    msg_r9_len equ $ - msg_r9
    newline db 0xa

section .bss
    digit resb 1

section .text
    global _start

_start:
    ; Set initial values
    mov r8, 6
    mov r9, 2
    
    %include "solution.asm"       ; Student's swap code goes here
    
    ; Save values for printing
    mov r12, r8
    mov r13, r9
    
    ; Print "r8 = "
    mov rax, 1
    mov rdi, 1
    mov rsi, msg_r8
    mov rdx, msg_r8_len
    syscall
    
    ; Print r8 value
    mov rax, r12
    add rax, 48
    mov [digit], al
    mov rax, 1
    mov rdi, 1
    mov rsi, digit
    mov rdx, 1
    syscall
    
    ; Print newline
    mov rax, 1
    mov rdi, 1
    mov rsi, newline
    mov rdx, 1
    syscall
    
    ; Print "r9 = "
    mov rax, 1
    mov rdi, 1
    mov rsi, msg_r9
    mov rdx, msg_r9_len
    syscall
    
    ; Print r9 value
    mov rax, r13
    add rax, 48
    mov [digit], al
    mov rax, 1
    mov rdi, 1
    mov rsi, digit
    mov rdx, 1
    syscall
    
    ; Print newline
    mov rax, 1
    mov rdi, 1
    mov rsi, newline
    mov rdx, 1
    syscall
    
    ; Exit
    mov rax, 60
    mov rdi, 0
    syscall