gbdk-2020 | GameBoy Development Kit |
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gbdk-lib/libc/asm/mos6502/_mulint.s
1 ;-------------------------------------------------------------------------
2 ; _mulint.s - routine for multiplication of 16 bit (unsigned) int
3 ;
4 ; Copyright (C) 2009, Ullrich von Bassewitz
5 ; Copyright (C) 2022, 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 _mulint
31
32 ;--------------------------------------------------------
33 ; exported symbols
34 ;--------------------------------------------------------
35 .globl __mulint_PARM_2
36 .globl __mulint
37
38 ;--------------------------------------------------------
39 ; overlayable function parameters in zero page
40 ;--------------------------------------------------------
41 .area OSEG (PAG, OVR)
42 __mulint_PARM_2:
43 .ds 2
44
45 ;--------------------------------------------------------
46 ; local aliases
47 ;--------------------------------------------------------
48 .define tmp "___SDCC_m6502_ret2"
49
50 ;--------------------------------------------------------
51 ; code
52 ;--------------------------------------------------------
53 .area _CODE
54
55 __mulint:
56 sta *___SDCC_m6502_ret0
57 stx *___SDCC_m6502_ret1
58 lda #0
59 sta *tmp
60 ldy #16
61 lsr *___SDCC_m6502_ret1
62 ror *___SDCC_m6502_ret0
63 next_bit:
64 bcc skip
65 clc
66 adc *__mulint_PARM_2+0
67 tax
68 lda *__mulint_PARM_2+1
69 adc *tmp
70 sta *tmp
71 txa
72 skip:
73 ror *tmp
74 ror a
75 ror *___SDCC_m6502_ret1
76 ror *___SDCC_m6502_ret0
77 dey
78 bne next_bit
79
80 lda *___SDCC_m6502_ret0
81 ldx *___SDCC_m6502_ret1
82 rts
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