The register shuffle in Assembly

Fibonacci in three registers, no memory at all.

; Fibonacci in three registers: the classic register shuffle.
        global _start

        section .text
_start:
        xor rax, rax            ; fib(0)
        mov rbx, 1              ; fib(1)
        mov rcx, 10             ; steps to take
.step:
        mov rdx, rax
        add rdx, rbx            ; next = a + b
        mov rax, rbx            ; a = b
        mov rbx, rdx            ; b = next
        dec rcx
        jnz .step

        mov rdi, rax            ; fib(10) = 55
        mov rax, 60             ; exit code 55
        syscall

How it works

1. Two registers hold the pair; a third stages the sum.
2. The rotation a=b, b=next advances the sequence.
3. Ten steps land on fib(10) = 55.

Keywords and builtins used here

• add
• dec
• global
• jnz
• mov
• rax
• rbx
• rcx
• rdi
• rdx
• section
• syscall
• xor

The run, in numbers

Lines

19

Characters to type

435

Tokens

56

Three-star pace

60 tpm

At the three-star pace of 60 tokens a minute, this run takes about 56 seconds.

Type this snippet

Step 4 of 4 in Memory & strings, step 25 of 31 in Language basics.

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