8-bit Multiply

The following code multiplies two 8-bit integers (range 0...255) and outputs a 16-bit result using only real calculation, no lockup table so the size of the code is very small.

<source lang="6502tasm">

8-bit multiply

by Bregalad

Enter with A,Y, numbers to multiply

Output with YA = 16-bit result (X is unchanged)

Multiply 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

       pha
       lda Res2

clc adc Mult sta Res2 ;If so add number to result

       pla

+ lsr Res2 ;Shift result right ror Res dey bne -

       lda Res
       ldy Res2

rts </source>

Russian Peasant Algorithm

This is the Russian peasant algorithm which was suggested by Bob Rost in his NES Development class PDFs. With 8 bit values the maximum number of iterations would be 7. Try to have value2ptr point to the lesser of the two values to reduce iterations. <source lang="6502tasm">

Multiply two bytes in memory using Russian peasant algorithm

by frantik

Accepts

value1ptr and value2ptr, pointers to bytes in memory

value2ptr should point to the lesser of the two values

for increased efficiency

Uses

$00, $01, $02 for temporary variables

Returns

16 bit value in $00 and $01

.macro multiply value1ptr, value2ptr

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

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

-loop: asl value1ptr ; double first value rol temp ; using 16bit precision lsr value2ptr ; halve second vale start: lda value2ptr ; and #01 ; is new 2nd value an odd number? beq -loop: ; clc ; if so, add new 1st value to running total lda ret ; adc value1ptr ; sta ret ; lda ret+1 ; adc temp ; sta ret+1 ; lda value2ptr ; cmp #01 ; is 2nd value 1? if so, we're done bne -loop: ; otherwise, loop .endm </source>