Convención de llamada de funciones (System V AMD64)

; sum3(a, b, c) -> a + b + c
; Args: rdi=a, rsi=b, rdx=c ; result in rax
section .text
global sum3
sum3:
    mov     rax, rdi
    add     rax, rsi
    add     rax, rdx
    ret

; A non-leaf function that calls printf — must save rbx (callee-saved)
extern printf
fmt: db "n=%d", 10, 0

global show_n
show_n:
    push    rbx             ; save callee-saved reg we will use
    mov     rbx, rdi        ; keep arg across the printf call

    sub     rsp, 8          ; align rsp to 16 before 'call'
    mov     rdi, fmt
    mov     esi, ebx
    xor     eax, eax        ; 0 vector args (variadic ABI)
    call    printf
    add     rsp, 8          ; restore rsp

    mov     rax, rbx        ; return n
    pop     rbx             ; restore rbx
    ret