; -----------------------------------------------------------------------------
; A 64-bit Linux application that writes the first 90 Fibonacci numbers. To
; assemble and run:
;
;     nasm -felf64 fib.asm && gcc fib.o && ./a.out
; -----------------------------------------------------------------------------

        global  main
        extern  printf

        section .text
main:
        push    rbx                     ; we have to save this since we use it

        mov     ecx, 90                 ; ecx will countdown to 0
        xor     rax, rax                ; rax will hold the current number
        xor     rbx, rbx                ; rbx will hold the next number
        inc     rbx                     ; rbx is originally 1
print:
        ; We need to call printf, but we are using rax, rbx, and rcx.  printf
        ; may destroy rax and rcx so we will save these before the call and
        ; restore them afterwards.

        push    rax                     ; caller-save register
        push    rcx                     ; caller-save register

        mov     rdi, format             ; set 1st parameter (format)
        mov     rsi, rax                ; set 2nd parameter (current_number)
        xor     rax, rax                ; because printf is varargs

        ; Stack is already aligned because we pushed three 8 byte registers
        call    printf                  ; printf(format, current_number)

        pop     rcx                     ; restore caller-save register
        pop     rax                     ; restore caller-save register

        mov     rdx, rax                ; save the current number
        mov     rax, rbx                ; next number is now current
        add     rbx, rdx                ; get the new next number
        dec     ecx                     ; count down
        jnz     print                   ; if not done counting, do some more

        pop     rbx                     ; restore rbx before returning
        ret
format:
        db  "%20ld", 10, 0