.ORIG	x3000
	JSR	MULTIPLY
	HALT

; MULTIPLY SUBROUTINE
; Takes the input at X and Y and stores the product X*Y
; at RESULT.
; INPUTS: X, Y
; OUTPUTS: RESULT
X	.FILL 0
Y	.FILL 0
RESULT .FILL 0

MULTIPLY
	; Insert code here.
	ST	R0, TMP_R0
	ST	R1, TMP_R1
	ST	R2, TMP_R2
	ST	R3, TMP_R3
	ST	R7, TMP_R7

	AND	R2, R2, 0	; R2 = result
	LD	R0, X		; R0 = a
	LD	R1, Y		; R1 = b

MUL_LP	BRz	MUL_SKIP	; while (b!= 0)
	AND	R3, R1, x01	; if (b AND 1) != 0
	BRz	MUL_SH
	ADD	R2, R2, R0	; res = res + a
MUL_SH	ADD	R0, R0, R0	; a <<= 1
	ST	R1, RSH_IN
	JSR	RSH		
	LD	R1, RSH_RES	; b >>= 1
	BRnp	MUL_LP		
MUL_SKIP
	ST	R2, RESULT

	LD	R7, TMP_R7
	LD	R3, TMP_R3
	LD	R2, TMP_R2
	LD	R1, TMP_R1
	LD	R0, TMP_R0	
MUL_RET RET

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; DO NOT MODIFY 
;
; RSH SUBROUTINE
; Effectively performs a "right shift" by 
; left-shifting the input at RSH_IN by 15 bits
; and storing the result at RSH_RES
;
; INPUT: RSH_IN
; OUTPUT: RSH_RES
RSH_IN		.FILL 0
RSH_RES		.FILL 0

RSH	
	ST	R0, TMPTMP_R0	; Save registers we intend to use
	ST	R1, TMPTMP_R1
	ST	R3, TMPTMP_R3
	AND	R3, R3, 0
	ADD	R3, R3, 15
	LD	R0, RSH_IN
	AND	R1, R1, 0
RLOOP	ADD	R1, R1, R1	; Left shift R1
	ADD	R0, R0, 0
	BRzp	RSKIP		
	ADD	R1, R1, 1
RSKIP	ADD	R0, R0, R0	; Left shift R0
	ADD	R3, R3, -1
	BRp	RLOOP
	ST	R1, RSH_RES
	LD	R3, TMPTMP_R3	; Restore registers
	LD	R1, TMPTMP_R1
	LD	R0, TMPTMP_R0
RSH_RET RET

; First level tmp registers
TMP_R0:	.FILL 0
TMP_R1:	.FILL 0
TMP_R2:	.FILL 0
TMP_R3: .FILL 0
TMP_R4: .FILL 0
TMP_R5: .FILL 0
TMP_R6: .FILL 0
TMP_R7: .FILL 0

; Second level tmp registers
TMPTMP_R0:	.FILL 0
TMPTMP_R1:	.FILL 0
TMPTMP_R2:	.FILL 0
TMPTMP_R3:	.FILL 0
TMPTMP_R4:	.FILL 0
TMPTMP_R5:	.FILL 0
TMPTMP_R6: 	.FILL 0
TMPTMP_R7: 	.FILL 0
	

	.END