;;; exports
	
	global	getnum

;;; imports
	extern  usage

;;; code

	section .text

	;;  expecting rax to point at the string to collect
	;;  return the answer in r15

getnum:
	push rbx		; save rbx
	push r14		; save r14

	xor r15,r15		; zero out r15

l:	
	cmp byte [rax],0
	je finish

	;; make sure that we have a legitimate character, leave value in rbx

	xor rbx,rbx
	mov bl, byte [rax]
	cmp rbx,0x30
	jl usage
	cmp rbx,0x39
	jg usage
	sub rbx,0x30
		
	;; multiply r15 by using r14

	shl r15,1		; first, double it r15 = r15 * 2
	mov r14,r15		; second, r14 = r15 * 2
	shl r15,2		; third, r15 = r15 * 4
	add r15,r14

        ;; add in character
	add r15,rbx

	inc rax
	jmp l

finish:
	pop r14			; retrieve r14
	pop rbx			; retrieve rbx

	ret			; answer in r15