6809_MUL16.s

;	Title:		16-bit multiplication
;	Name:			MUL16
;	Purpose:		Multiply two unsigned 16-bit words and return a 32-bit unsigned product.
;	Entry:			TOP OF STACK
;				High byte of return address
;				Low  byte of return address
;				High byte of multiplier
;				Low  byte of multiplier
;				High byte of multiplicand
;				Low  byte of multiplicand
;
;	Exit:			Product = multiplicand * multiplier
;
;				TOP OF STACK
;				High byte of high word of product
;				Low byte of high word of product
;				High byte of low word of product
;				Low byte of Low word of product
;
;	Time:	Approximately 200 cycles
;
;	Size:	Program	64 bytes
;		Data	2 stack bytes
;
MUL16:
;
; CLEAR PARTIAL PRODUCT IN FOUR STACK BYTES
;
	LDU	,S		; SAVE RETURN ADDRESS
	CLRA			; CLEAR PARTIAL PRODUCT ON STACK
	CLRB
	STD	,S		; USE BYTES OCCUPIED BY RETURN ADDRESS
	PSHS	D		; PLUS 2 EXTRA BYTES ON TOP OF STACK 
;
; MULTIPLY LOW BYTE OF MULTIPLIER TIMES LOW BYTE 
; OF MULTIPLICAND
;
	LDA	5,S		; GET LOW BYTE OF MULTIPLIER
	LDB	7,S		; GET LOW BYTE OF MULTIPLICAND
	MUL			; MULTIPLY BYTES
	STB	3,S		; STORE LOW BYTE OF PRODUCT
	STA	2,S		; STORE HIGH BYTE OF PRODUCT
;
; MULTIPLY LOW BYTE OF MULTIPLIER TIMES HIGH BYTE
; OF MULTIPLICAND
;
	LDA	5,S		; GET LOW BYTE OF MULTIPLIER
	LDA	6,S		; GET HIGH BYTE OF MULTIPLICAND
	MUL			; MULTIPLY BYTES
	ADDB	2,S		; ADD LOW BYTE OF PRODUCT T0 PARTIAL PRODUCT
	STB	2,S		; 
	ADCA	#0		; ADD HIGH BYTE OF PRODUCT PLUS CARRY TO PARTIAL PRODUCT
	STA	1,S		; STORE HIGH BYTE OF PRODUCT
;	
;	MULTIPLY HIGH BYTE OF MULTIPLIER TIMES LOW BYTE
;	OF MULTIPLICAND
;
	LDA	4,S		; GET HIGH BYTE OF MULTIPLIER
	LDB	7,S		; GET LOW BYTE OF MULTIPLICAND
	MUL			; MULTIPLY BYTES
	ADDB	2,S		; ADD LOW BYTE OF PRODUCT T0 PARTIAL PRODUCT
	STB	2,S		; 
	ADCA	1,		; ADD HIGH BYTE OF PRODUCT PLUS CARRY TO PARTIAL PRODUCT
	STA	1,S		; 
	BCC	MULHH		; BRANCH IF N0 CARRY ELSE INCREMENT MOST SIGNIFICANT
				; BYTE 0F PARTIAL PRODUCT
	INC	,S		; 

;	
;	MULTIPLY HIGH BYTE OF MULTIPLIER TIMES HIGH BYTE
;	OF MULTIPLICAND
;
	LDA	4,S		; GET HIGH BYTE OF MULTIPLIER
	LDB	6,S		; GET HIGH BYTE OF MULTIPLICAND
	MUL			; MULTIPLY BYTES
	ADDB	1,S		; ADD LOW BYTE OF PRODUCT T0 PARTIAL PRODUCT
	ADCA	,S		; ADD HIGH BYTE OF PRODUCT PLUS CARRY TO PARTIAL PRODUCT

;	HIGH BYTES OF PRODUCT END UP IN D
;	RETURN WITH PRODUCT AT TOP OF STACK
;	SAMPLE EXECUTION
;	GET LOWER 16 BITS OF PRODUCT FROM STACK
;	REMOVE PARAMETERS FROM STACK
;	PUT PRODUCT AT TOP OF STACK
;	EXIT T0 RETURN ADDRESS