;;; exports
	
	global gcd
	

;;; imports
	extern getnum
	extern putnum
	extern usage


	

	section .text		


;;;  gcd --> expects arguments in rbx and rcx; returns answer in rbx

gcd:
	push rax 		; save the registers we are modifying and aren't used for
	push rcx		; for a return value
	push rdx
	
	
;;; compare rbx to rcx
	
	cmp rbx,rcx

;;; check to see if these are equal
	
	je finish

;;; check to see if rbx > rcx

	jg again

;;; if not, swap rbx and rcx so that rbx > rcx

	xchg rbx,rcx

again:

	;; shuffle rbx and rcx up to rax and rbx
	mov rax,rbx
	mov rbx,rcx

	;; zero out rdx in preparation for division rdx:rax by rcx
	xor rdx,rdx

	;; divide by rcx, remainder is in rdx
	div rcx

	cmp rdx,0
	je finish

	mov rbx,rcx
	mov rcx,rdx

	jmp again

finish:

	pop rdx
	pop rcx
	pop rax

	ret