multiply_test/tests/mult71.a at main · TobyLobster/multiply_test · GitHub

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; mult71.a
; from https://archive.org/details/Compute_s_Machine_Language_Routines_for_the_Commodore_64_and_128/page/359/mode/2up
;
; 8 bit x 8 bit unsigned multiply, 16 bit result
; Average cycles: 1572.91
; 41 bytes

b1      = $02       ; must be adjacent to each other
b2      = $03       ; must be adjacent to each other
total   = $04       ; 2 bytes

* = $0200

mulad2
    lda #0
    sta total
    sta total+1

    tay
    ldx b2
    beq mulend
    cpx b1
    bcc goahead
    ldx b1
    beq mulend
    ldy #1

goahead
    lda b1,y
loop
    dex
    beq mulstr
    clc
    adc b1,y
    bcc loop
    inc total+1
    jmp loop
mulstr
    sta total
mulend
    rts