sdcc/device/lib/mos6502/_muluchar.s

   1 ;-------------------------------------------------------------------------
   2 ;   _muluchar.s - routine for multiplication of 8 bit (unsigned char)
   3 ;
   4 ;   Copyright (C) 2009, Ullrich von Bassewitz
   5 ;   Copyright (C) 2022-2023, Gabriele Gorla
   6 ;
   7 ;   This library is free software; you can redistribute it and/or modify it
   8 ;   under the terms of the GNU General Public License as published by the
   9 ;   Free Software Foundation; either version 2, or (at your option) any
  10 ;   later version.
  11 ;
  12 ;   This library is distributed in the hope that it will be useful,
  13 ;   but WITHOUT ANY WARRANTY; without even the implied warranty of
  14 ;   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15 ;   GNU General Public License for more details.
  16 ;
  17 ;   You should have received a copy of the GNU General Public License
  18 ;   along with this library; see the file COPYING. If not, write to the
  19 ;   Free Software Foundation, 51 Franklin Street, Fifth Floor, Boston,
  20 ;   MA 02110-1301, USA.
  21 ;
  22 ;   As a special exception, if you link this library with other files,
  23 ;   some of which are compiled with SDCC, to produce an executable,
  24 ;   this library does not by itself cause the resulting executable to
  25 ;   be covered by the GNU General Public License. This exception does
  26 ;   not however invalidate any other reasons why the executable file
  27 ;   might be covered by the GNU General Public License.
  28 ;-------------------------------------------------------------------------
  29
  30         .module _muluchar
  31
  32 ;--------------------------------------------------------
  33 ; exported symbols
  34 ;--------------------------------------------------------
  35         .globl __muluchar   ; arguments in A and X, result in AX
  36         .globl ___umul8     ; arguments in ret0 and ret1, result in AX
  37
  38 ;--------------------------------------------------------
  39 ; overlayable function parameters in zero page
  40 ;--------------------------------------------------------
  41         .area   OSEG    (PAG, OVR)
  42
  43 ;--------------------------------------------------------
  44 ; local aliases
  45 ;--------------------------------------------------------
  46         .define arg1 "___SDCC_m6502_ret0"
  47         .define arg2 "___SDCC_m6502_ret2"
  48
  49 ;--------------------------------------------------------
  50 ; code
  51 ;--------------------------------------------------------
  52         .area CODE
  53
  54 __muluchar:
  55         sta     *arg1
  56         stx     *arg2
  57 ___umul8:
  58         lda     #0              ; Clear byte 1
  59         ldy     #8              ; Number of bits
  60         lsr     *arg2           ; Get first bit of RHS into carry
  61 L0:     bcc     L1
  62         clc
  63         adc     *arg1
  64 L1:     ror
  65         ror     *arg2
  66         dey
  67         bne     L0
  68         tax                     ; Load the result MSB
  69         lda     *arg2           ; Load the result LSB
  70         rts                     ; Done