mult19.a

; mult19.a
; from Australian Personal Computer, Aug 1984: https://archive.org/details/apc_1984_08/page/115/mode/2up?q=6502+multiplication
; also from Neil Parker: https://www.llx.com/Neil/a2/mult.html
;
; 8 bit x 8 bit unsigned multiply, 16 bit result
; Average cycles: 185
; 18 bytes


NUM1    = $02
NUM2    = $03
RESULT  = $04

* = $0200

; calculate NUM1 * NUM2
; Result in A (high byte) and RESULT (low byte)
mult
    lda #0       ; Initialize RESULT to 0
    ldx #8       ; There are 8 bits in NUM2
L1
    lsr NUM2     ; Get low bit of NUM2
    bcc L2       ; 0 or 1?
    clc          ; If 1, add NUM1
    adc NUM1
L2
    ror          ; "Stairstep" shift (catching carry from add)
    ror RESULT
    dex
    bne L1
    rts