mult79.a

; mult79.a
; from a NASA Report (1981) https://archive.org/details/NASA_NTRS_Archive_19820015020/page/n65/mode/2up
;
; 8 bit x 8 bit unsigned multiply, 16 bit result
; Average cycles: 399.00
; 39 bytes


mplr  = $02
mpcnd = $03
prodl = $04
prodh = $05
temp  = $06

* = $0200

; 8 bit x 8 bit unsigned multiply, 16 bit result
;
; No branches based on data are allowed in this routine, I think so it can be run
; simultaneously across multiple processors and they all finish at exactly the same time.
;
; On Entry:
;   mplr:  multiplier
;   mpcnd: multiplicand
; On Exit:
;   (prodl, prodh): 16 bit product
mult
    lda #0
    sta prodl
    sta prodh
    ldx #8
loop
    asl prodl
    rol prodh
    asl mplr
    lda #0
    sbc #0
    eor #$ff
    and mpcnd
    sta temp
    clc
    adc prodl
    sta prodl
    lda prodh
    adc #0
    sta prodh
    dex
    bne loop
end
    rts