diff options
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 | |||
| @@ -0,0 +1,78 @@ | |||
| 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 | ||