_TEXT	SEGMENT
a$ = 48
b$ = 56
gcd	PROC

; 5    : U64 gcd( U64 a, U64 b ) {                      ; a in rcx, b in rdx

$LN5:
	sub	rsp, 40					; reserve some stack space
	mov	r8, rdx                                 ; put a copy of b in r8

; 6    :     return !b ? a : gcd( b, a % b );

	mov	rax, rcx                                ; put a in rax (result register)
	test	rdx, rdx                                ; test b
	je	SHORT $LN4@gcd                          ; jump to return if it is 0; ie. if b=0 return a
	xor	edx, edx                                ; empty rdx ready for div
	mov	rcx, r8                                 ; move b into rcx ( ie. b becomes a for recursive call )
	div	r8                                      ; divide rdx:rax (0:a) by r8 (b); leaves quotient in rax, the remainder (modulus) in rdx (thus b in the recursive call)
	call	gcd                                     ; recurse
$LN4@gcd:
                                                        ; result is in rax.
; 7    : }

	add	rsp, 40					; clean up the stack
	ret	0                                       ; return popping 0 bytes from the stack
gcd	ENDP
_TEXT	ENDS