problem2.asm

;
; Problem 2
;
; Each new term in the Fibonacci sequence is generated by adding the previous
; two terms. By starting with 1 and 2, the first 10 terms will be:
;
; 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
;
; By considering the terms in the Fibonacci sequence whose values do not exceed
; four million, find the sum of the even-valued terms.
;


section .text
global _start

_start:
    mov r8, 1           ; initial number
    mov r9, 1           ; initial number

.loop
    xor ebx, ebx        ; zero out ebx for later

    bt r8, 0            ; if bit 1 is set, it's an odd number
    cmovnc rbx, r8      ; copy number to add if
    ; setc bl

    add r10, rbx        ; add the even number or zero

    xadd r8, r9         ; xchg and add

    cmp r8, 0x3d0900    ; see if we're less than 4,000,000
    jl .loop

.done
    mov eax, 1          ; syscall for exit
    mov ebx, 0          ; exit with status zeo

    int 80h             ; syscall