8-bit Multiply

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Since the 6502 CPU has no multiplication instruction, this feature has to be written in software.


This 8x8 multiply routine from Thwaite does binary long multiplication (a.k.a. "the Russian Peasant method").

; Multiplies two 8-bit factors to produce a 16-bit product
; in about 153 cycles.
; @param A one factor
; @param Y another factor
; @return high 8 bits in A; low 8 bits in $0000
;         Y and $0001 are trashed; X is untouched
.proc mul8
prodlo  = $0000
factor2 = $0001

  ; Factor 1 is stored in the lower bits of prodlo; the low byte of
  ; the product is stored in the upper bits.
  lsr a  ; prime the carry bit for the loop
  sta prodlo
  sty factor2
  lda #0
  ldy #8
  ; At the start of the loop, one bit of prodlo has already been
  ; shifted out into the carry.
  bcc noadd
  adc factor2
  ror a
  ror prodlo  ; pull another bit out for the next iteration
  dey         ; inc/dec don't modify carry; only shifts and adds do
  bne loop

This is a similar technique to Bregalad's method below, but is further optimized to keep part of the running total in the accumulator.

Assuming no page crossings and zero page, this routine takes 137-169 cycles, not counting the JSR to call it. (Each non-zero bit in A adds 4 cycles.)

tepples unrolled

The above code can be made to run slightly faster by both unrolling the loop and pre-decrementing factor2 so that CLC isn't required. Note that the low byte is now returned in A, the high byte in Y, and that CA65 syntax is used.

; @param A one factor
; @param Y another factor
; @return low 8 bits in A; high 8 bits in Y
    sta prodlo
    beq mul8_early_return
    sty factor2
    lda #0
.repeat 8, i
    .if i > 0
        ror prodlo
    bcc :+
    adc factor2
    lda prodlo

The code takes up 70 bytes and runs in 120 cycles or less.


This routine by Bregalad is similar to tepples' method above, but is less optimized.

;8-bit multiply
;by Bregalad
;Enter with A,Y, numbers to multiply
;Output with YA = 16-bit result (X is unchanged)
	sty Factor  ;Store input factor
	ldy #$00
	sty Res
	sty Res2    ;Clear result
	ldy #$08    ;Number of shifts needed

-	lsr A       ;Shift right input number
	bcc +       ;Check if bit is set
	lda Res2
	adc Factor
	sta Res2    ;If so add number to result
+	lsr Res2    ;Shift result right
	ror Res
	bne -
	lda Res
	ldy Res2

An optimization for efficiency is made here; binary long multiplication requires adding one multiplicand to the result at various bit-shifts (i.e. multiply by each power of 2). The naive approach might maintain the value to add as a 16-bit value, left shifting it once each iteration to reach the next power of 2. This one, however, takes advantage of the input being only 8-bits wide, and instead pre-multiplies the result by 256 (8 bits), and each iteration instead right-shifts the result. After 8 iterations the pre-multiply is undone, and the advantage gained is that only the shift is 16-bit; adding the multiplicand remains an efficient 8-bit add.

Assuming no page crossings and zero page, this routine takes 184-320 cycles, not counting the JSR to call it. (Each non-zero bit in A adds 17 cycles.)


This routine by frantik is another binary long multiplication.

; Multiply two bytes in memory using Russian peasant algorithm
; by frantik

; Accepts: value1 and value2, labels for bytes in memory
;   value2 should ideally be the lesser of the two input values 
;   for increased efficiency
; Uses: $00, $01, $02 for temporary variables
; Returns: 16 bit value in $00 and $01

.macro multiply value1, value2

ret  = $00              ; return value
temp = $02              ; temp storage

	lda #$00        ; clear temporary variables
	sta ret
	sta ret+1
	sta temp
	lda value2
	bne +end"
	jmp +start:

	asl value1      ; double first value
	rol temp        ; using 16bit precision
	lsr value2      ; halve second vale
	lda value2      ;
	and #01         ; is new 2nd value an odd number?
	beq -loop:      ; 
	clc             ; if so, add new 1st value to running total
	lda ret         ;
	adc value1      ;
	sta ret         ;
	lda ret+1       ;
	adc temp        ;
	sta ret+1       ;
	lda value2      ;
	cmp #01         ; is 2nd value 1?  if so, we're done
	bne -loop:      ; otherwise, loop

Assuming no page crossings and zero page, this routine takes 17-403 cycles, though it is an in-place macro generating 46 bytes of new code each time it is used.


The MMC5 contains an 8x8-bit multiplier at $5205-$5206.

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