omult1.a

; omult1.a
; from Programming the 65816 by David Eyes (1986), https://archive.org/details/0893037893ProgrammingThe65816/page/n295/mode/2up?q=multiply
;
; Only returns the lower 16 bits of the result.
;
; 16 bit x 16 bit unsigned multiply, lower 16 bit result
; Average cycles: 649.00
; 33 bytes


mcand1 = $02
mcand2 = $04

* = $0200

mult
    ldx #0
    ldy #0
mult1
    lda mcand1
    ora mcand1+1
    beq done
    lsr mcand1+1
    ror mcand1
    bcc mult2
    clc
    tya
    adc mcand2
    tay
    txa
    adc mcand2+1
    tax
mult2
    asl mcand2
    rol mcand2+1
    jmp mult1
done
    rts