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Diffstat (limited to 'firmware/asm/m68k/ffs.S')
-rw-r--r-- | firmware/asm/m68k/ffs.S | 78 |
1 files changed, 78 insertions, 0 deletions
diff --git a/firmware/asm/m68k/ffs.S b/firmware/asm/m68k/ffs.S new file mode 100644 index 0000000000..2a39ae5bcd --- /dev/null +++ b/firmware/asm/m68k/ffs.S | |||
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1 | /*************************************************************************** | ||
2 | * __________ __ ___. | ||
3 | * Open \______ \ ____ ____ | | _\_ |__ _______ ___ | ||
4 | * Source | _// _ \_/ ___\| |/ /| __ \ / _ \ \/ / | ||
5 | * Jukebox | | ( <_> ) \___| < | \_\ ( <_> > < < | ||
6 | * Firmware |____|_ /\____/ \___ >__|_ \|___ /\____/__/\_ \ | ||
7 | * \/ \/ \/ \/ \/ | ||
8 | * $Id$ | ||
9 | * | ||
10 | * Copyright (C) 2008 by Michael Sevakis | ||
11 | * | ||
12 | * This program is free software; you can redistribute it and/or | ||
13 | * modify it under the terms of the GNU General Public License | ||
14 | * as published by the Free Software Foundation; either version 2 | ||
15 | * of the License, or (at your option) any later version. | ||
16 | * | ||
17 | * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY | ||
18 | * KIND, either express or implied. | ||
19 | * | ||
20 | ****************************************************************************/ | ||
21 | #include "config.h" | ||
22 | |||
23 | /**************************************************************************** | ||
24 | * int find_first_set_bit(uint32_t val); | ||
25 | * | ||
26 | * Find the index of the least significant set bit in the 32-bit word. | ||
27 | * | ||
28 | * return values: | ||
29 | * 0 - bit 0 is set | ||
30 | * 1 - bit 1 is set | ||
31 | * ... | ||
32 | * 31 - bit 31 is set | ||
33 | * 32 - no bits set | ||
34 | ****************************************************************************/ | ||
35 | .text | ||
36 | .align 2 | ||
37 | .global find_first_set_bit | ||
38 | .type find_first_set_bit,@function | ||
39 | find_first_set_bit: | ||
40 | | this is a coldfire version of the ffs algorithm devised by D.Seal | ||
41 | | and posted to comp.sys.arm on 16 Feb 1994. | ||
42 | | | ||
43 | | Output modified to suit rockbox purposes. | ||
44 | |||
45 | | Standard trick to isolate bottom bit in r0 or 0 if r0 = 0 on entry | ||
46 | move.l 4(%sp), %d1 | %d1 = %d1 & -%d1 | ||
47 | lea.l L_ffs_table, %a0 | %a0 = table address | ||
48 | move.l %d1, %d0 | | ||
49 | neg.l %d1 | | ||
50 | and.l %d0, %d1 | | ||
51 | |||
52 | | now %d1 has at most one set bit, call this X | ||
53 | |||
54 | move.l #0x0450fbaf, %d0 | %d0 = multiplier | ||
55 | mulu.l %d0, %d1 | %d1 = X * 0x0450fbaf | ||
56 | |||
57 | | now lookup in table indexed on top 6 bits of %d0 | ||
58 | moveq.l #26, %d0 | %d0 = final shift count | ||
59 | lsr.l %d0, %d1 | | ||
60 | |||
61 | | the top 24 bits of d0 are 0 so just load the value over it | ||
62 | move.b (%a0, %d1.l), %d0 | | ||
63 | rts | | ||
64 | |||
65 | .section .irodata | ||
66 | L_ffs_table: | ||
67 | | 0 1 2 3 4 5 6 7 | ||
68 | |---------------------------------------------- | ||
69 | .byte 32, 0, 1, 12, 2, 6, 0, 13 | 0- 7 | ||
70 | .byte 3, 0, 7, 0, 0, 0, 0, 14 | 8-15 | ||
71 | .byte 10, 4, 0, 0, 8, 0, 0, 25 | 16-23 | ||
72 | .byte 0, 0, 0, 0, 0, 21, 27, 15 | 24-31 | ||
73 | .byte 31, 11, 5, 0, 0, 0, 0, 0 | 32-39 | ||
74 | .byte 9, 0, 0, 24, 0, 0, 20, 26 | 40-47 | ||
75 | .byte 30, 0, 0, 0, 0, 23, 0, 19 | 48-55 | ||
76 | .byte 29, 0, 22, 18, 28, 17, 16, 0 | 56-63 | ||
77 | |||
78 | .size find_first_set_bit, .-find_first_set_bit | ||