program multiplication
        
include	'/home/dbarker/182/examples/Driver'

;
; Lab 4 - simulate multiplication
;
; 
;
; Patrick Tuckey
; Grant Palin
; 2003-01-22
;


BIT15		equ		0b1	; used for testing if bit one is on for any binary number
sumAddrOffset	equ		-4 	; distance back from FP
paramAOffset	equ		-6 	; distance back from FP
paramBOffset	equ		-8 	; distance back from FP

        data
ENTER	db		LF, CR, 0	; short for creating a new line
RESULT	db		?
        code

multiplication				; Program entry point
	stack	60;
	
	call	Driver
		
	sys	EXIT
		
multiply
	push    fp	  		; save caller's FP
        mov     sp, fp          	; copy sp (preamble)
        
        push	r5
        push	r6
        push	r8
        push	r9
        
	lodw	paramAOffset, fp, r5	; load value of first number from stack
	lodw	paramBOffset, fp, r6	; load value of second number from stack
	lodw	sumAddrOffset, fp, r9	; load address to store result from stack
		
	mov		r0, r8		; r8 set SUM to zero
		
NUMBERCRUNCH
	cmp		r6, 0
	be		OUT

;test rightmost bit in r6 to see if we add NUMA to SUM
	test	r6, BIT15
	bz		NOADD
		
	add		r5, r8, r8
		
	bc		NOSTORE
		
NOADD
	shr		r6, 1, r6
	bz		OUT
		
	shl		r5, 1, r5
	bc		NOSTORE
		
	br		NUMBERCRUNCH
	
OUT
	stow	0, r9, r8
		
NOSTORE
	pop		r9
	pop		r8
	pop		r6
	pop		r5
		
	pop		fp
	ret
        
        end