gbdk-2020 | GameBoy Development Kit |
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gbdk-lib/libc/asm/z80/mul.s
1 ;--------------------------------------------------------------------------
2 ; mul.s
3 ;
4 ; Copyright (C) 2000, Michael Hope
5 ; Copyright (C) 2021, Philipp Klaus Krause
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 .area _CODE
31
32 .globl __mulint
33
34 __mulint:
35 ld c, l
36 ld b, h
37
38 ;; 16-bit multiplication
39 ;;
40 ;; Entry conditions
41 ;; bc = multiplicand
42 ;; de = multiplier
43 ;;
44 ;; Exit conditions
45 ;; de = less significant word of product
46 ;;
47 ;; Register used: AF,BC,DE,HL
48 __mul16::
49 xor a,a
50 ld l,a
51 or a,b
52 ld b,#16
53
54 ;; Optimise for the case when this side has 8 bits of data or
55 ;; less. This is often the case with support address calls.
56 jr NZ,2$
57 ld b,#8
58 ld a,c
59 1$:
60 ;; Taken from z88dk, which originally borrowed from the
61 ;; Spectrum rom.
62 add hl,hl
63 2$:
64 rl c
65 rla ;DLE 27/11/98
66 jr NC,3$
67 add hl,de
68 3$:
69 djnz 1$
70 ex de, hl
71 ret
72
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