multiply_test/tests/mult35.a at main · TobyLobster/multiply_test · GitHub

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; mult35.a
; from 'Atari Roots' by Mark Andrews (1984) p173: https://archive.org/details/ataribooks-atari-roots/page/n173/mode/2up
; also available at https://www.atariarchives.org/roots/chapter_10.php
;
; 8 bit x 8 bit unsigned multiply, 16 bit result
; Average cycles: 188.00
; 20 bytes


mpr   = $02
mpd   = $03
prodl = $04
; prodh = $05

*=$0200

; ***************************************************************************************
; On Entry:
;   mpr: multiplier
;   mpd: multiplicand
; On Exit:
;   prodl: low byte of product
;   A: high byte of product
values
    lda #0
    sta prodl
    ldx #8
loop
    lsr mpr
    bcc noadd
    clc
    adc mpd
noadd
    ror
    ror prodl
    dex
    bne loop
;    sta prodh
    rts