gbdk-2020 | GameBoy Development Kit |
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gbdk-lib/libc/asm/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
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