multiply_test/tests/mult17.a at main · TobyLobster/multiply_test · GitHub

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; mult17.a
; from Elite: https://www.bbcelite.com/cassette/main/subroutine/mu11.html
; tweaked to handle X=0 on input
;
; 8 bit x 8 bit unsigned multiply, 16 bit result
; Average cycles: 150.47
; 28 bytes


T = $02
P = $03

* = $0200

; ***************************************************************************************
; 8 bit multiply
;
; On Entry:
;   A: multiplier
;   P: multiplicand
; On Exit:
;   A: high byte of product
;   P: low byte of product
mult
    cpx #0
    beq zero    ; Special case: handle X=0 separately

    dex
    stx T
    lda #0
    ldx #8
    lsr P
loop
    bcc +
    adc T
+
    ror
    ror P
    dex
    bne loop
    rts

zero
    txa
    sta P
    rts