mult49.a

; mult49.a
; from Micro 6502 Journal Issue 31 (Dec 1980) p71-74, https://archive.org/details/micro-6502-journal-31/page/n73/mode/2up
; bug fixed
; removed shortcut to 8x16 multiply (resulting in slightly faster and smaller code)
;
; 16 bit x 16 bit unsigned multiply, 32 bit result
; Average cycles: 703
; 43 bytes


* = $0200

acl     = $02
ach     = $03
xtndl   = $04
xtndh   = $05
auxl    = $06
auxh    = $07

; 16 x 16 bit unsigned multiply
;
; On Entry:
;   auxl/h: 16 bit multiplier
;   X (low),Y (high): 16 bit multiplicand
; On Exit:
;   acl, ach, xtndl, xtndh: 32 bit product
mult16x
    lda #$10
multi
    sty auxl
    stx auxh
    tay
    lda #0
    sta xtndl
    sta xtndh
mul3x
    lda acl
    lsr
    bcc mul4x       ; Bug fix - original listing was missing this line!
    lda xtndl
    clc
    adc auxl
    sta xtndl
    lda xtndh
    adc auxh
    sta xtndh
mul4x
    ror xtndh
    ror xtndl
    ror ach
    ror acl
    dey
    bne mul3x
    rts