omult27.a

; omult27.a
; from Starship Command at 0x0fa8 (and $10be): http://www.level7.org.uk/miscellany/starship-command-disassembly.txt
;
; 16 bit x 8 bit unsigned multiply, 16 bit result (high bytes)
; Average cycles: 444.00
; 22 bytes

input_fraction  = $02       ;
input_pixels    = $03       ;
multiplier      = $04       ;
output_low      = $05       ;
output_high     = $06       ;

* = $0200

; 16 bit x 8 bit unsigned multiply, 16 bit result (high bytes)
;
; On Entry:
;   (input_fraction, input_pixels): 16 bit multiplicand
;   multiplier: 8 bits
; On Exit:
;   (output_fraction, output_pixels): 16 bit product
mult
    lda #0
    ldx #16                 ; 16-bit multiplication of input by multiplier
loop_over_bits_of_input
    lsr input_pixels
    ror input_fraction
    bcc input_bit_unset
    clc
    adc multiplier
input_bit_unset
    ror
    ror output_low
    dex
    bne loop_over_bits_of_input
    sta output_high
    rts