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Diffstat (limited to 'Documentation/filesystems/vfat.txt')
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diff --git a/Documentation/filesystems/vfat.txt b/Documentation/filesystems/vfat.txt new file mode 100644 index 000000000..6f03cc799 --- /dev/null +++ b/Documentation/filesystems/vfat.txt @@ -0,0 +1,464 @@ +USING VFAT +---------------------------------------------------------------------- +To use the vfat filesystem, use the filesystem type 'vfat'. i.e. + mount -t vfat /dev/fd0 /mnt + +No special partition formatter is required. mkdosfs will work fine +if you want to format from within Linux. + +VFAT MOUNT OPTIONS +---------------------------------------------------------------------- +uni_xlate -- Translate unhandled Unicode characters to special + escaped sequences. This would let you backup and + restore filenames that are created with any Unicode + characters. Until Linux supports Unicode for real, + this gives you an alternative. Without this option, + a '?' is used when no translation is possible. The + escape character is ':' because it is otherwise + illegal on the vfat filesystem. The escape sequence + that gets used, where u is the unicode character, is: + ':', (u & 0x3f), ((u>>6) & 0x3f), (u>>12), +posix -- Allow names of same letters, different case such as + 'LongFileName' and 'longfilename' to coexist. This has some + problems currently because 8.3 conflicts are not handled + correctly for Posix filesystem compliance. +nonumtail -- When creating 8.3 aliases, normally the alias will + end in '~1' or tilde followed by some number. If this + option is set, then if the filename is + "longfilename.txt" and "longfile.txt" does not + currently exist in the directory, 'longfile.txt' will + be the short alias instead of 'longfi~1.txt'. + +quiet -- Stops printing certain warning messages. + + +TODO +---------------------------------------------------------------------- +* Need to get rid of the raw scanning stuff. Instead, always use + a get next directory entry approach. The only thing left that uses + raw scanning is the directory renaming code. + +* Need to add dcache_lookup code msdos filesystem. This means the + directories need to be versioned like the vfat filesystem. + +* Add support for different codepages. Right now, we only support + the a single English codepage. + +* Fix the Posix filesystem support to work in 8.3 space. This involves + renaming aliases if a conflict occurs between a new filename and + an old alias. This is quite a mess. + + +POSSIBLE PROBLEMS +---------------------------------------------------------------------- +* vfat_valid_longname does not properly checked reserved names. +* When a volume name is the same as a directory name in the root + directory of the filesystem, the directory name sometimes shows + up empty an empty file. + +BUG REPORTS +---------------------------------------------------------------------- +If you have trouble with the VFAT filesystem, mail bug reports to +chaffee@bugs-bunny.cs.berkeley.edu. Please specify the filename +and the operation that gave you trouble. + +TEST SUITE +---------------------------------------------------------------------- +If you plan to make any modifications to the vfat filesystem, please +get the test suite that comes with the vfat distribution at + + http://www-plateau.cs.berkeley.edu/people/chaffee/vfat.html + +This tests quite a few parts of the vfat filesystem and additional +tests for new features or untested features would be appreciated. + +NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM +---------------------------------------------------------------------- +(This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu> + and lightly annotated by Gordon Chaffee). + +This document presents a very rough, technical overview of my +knowledge of the extended FAT file system used in Windows NT 3.5 and +Windows 95. I don't guarantee that any of the following is correct, +but it appears to be so. + +The extended FAT file system is almost identical to the FAT +file system used in DOS versions up to and including 6.223410239847 +:-). The significant change has been the addition of long file names. +Theses names support up to 255 characters including spaces and lower +case characters as opposed to the traditional 8.3 short names. + +Here is the description of the traditional FAT entry in the current +Windows 95 filesystem: + + struct directory { // Short 8.3 names + unsigned char name[8]; // file name + unsigned char ext[3]; // file extension + unsigned char attr; // attribute byte + unsigned char lcase; // Case for base and extension + unsigned char ctime_ms; // Creation time, milliseconds + unsigned char ctime[2]; // Creation time + unsigned char cdate[2]; // Creation date + unsigned char adate[2]; // Last access date + unsigned char reserved[2]; // reserved values (ignored) + unsigned char time[2]; // time stamp + unsigned char date[2]; // date stamp + unsigned char start[2]; // starting cluster number + unsigned char size[4]; // size of the file + }; + +The lcase field specifies if the base and/or the extension of an 8.3 +name should be capitalized. This field does not seem to be used by +Windows 95 but it is used by Windows NT. The case of filenames is not +completely compatible from Windows NT to Windows 95. It is not completely +compatible in the reverse direction, however. Filenames that fit in +the 8.3 namespace and are written on Windows NT to be lowercase will +show up as uppercase on Windows 95. + +Note that the "start" and "size" values are actually little +endian integer values. The descriptions of the fields in this +structure are public knowledge and can be found elsewhere. + +With the extended FAT system, Microsoft has inserted extra +directory entries for any files with extended names. (Any name which +legally fits within the old 8.3 encoding scheme does not have extra +entries.) I call these extra entries slots. Basically, a slot is a +specially formatted directory entry which holds up to 13 characters of +a files extended name. Think of slots as additional labeling for the +directory entry of the file to which they correspond. Microsoft +prefers to refer to the 8.3 entry for a file as its alias and the +extended slot directory entries as the file name. + +The C structure for a slot directory entry follows: + + struct slot { // Up to 13 characters of a long name + unsigned char id; // sequence number for slot + unsigned char name0_4[10]; // first 5 characters in name + unsigned char attr; // attribute byte + unsigned char reserved; // always 0 + unsigned char alias_checksum; // checksum for 8.3 alias + unsigned char name5_10[12]; // 6 more characters in name + unsigned char start[2]; // starting cluster number + unsigned char name11_12[4]; // last 2 characters in name + }; + +If the layout of the slots looks a little odd, it's only +because of Microsoft's efforts to maintain compatibility with old +software. The slots must be disguised to prevent old software from +panicing. To this end, a number of measures are taken: + + 1) The attribute byte for a slot directory entry is always set + to 0x0f. This corresponds to an old directory entry with + attributes of "hidden", "system", "read-only", and "volume + label". Most old software will ignore any directory + entries with the "volume label" bit set. Real volume label + entries don't have the other three bits set. + + 2) The starting cluster is always set to 0, an impossible + value for a DOS file. + +Because the extended FAT system is backward compatible, it is +possible for old software to modify directory entries. Measures must +be taken to insure the validity of slots. An extended FAT system can +verify that a slot does in fact belong to an 8.3 directory entry by +the following: + + 1) Positioning. Slots for a file always immediately proceed + their corresponding 8.3 directory entry. In addition, each + slot has an id which marks its order in the extended file + name. Here is a very abbreviated view of an 8.3 directory + entry and its corresponding long name slots for the file + "My Big File.Extension which is long": + + <proceeding files...> + <slot #3, id = 0x43, characters = "h is long"> + <slot #2, id = 0x02, characters = "xtension whic"> + <slot #1, id = 0x01, characters = "My Big File.E"> + <directory entry, name = "MYBIGFIL.EXT"> + + Note that the slots are stored from last to first. Slots + are numbered from 1 to N. The Nth slot is or'ed with 0x40 + to mark it as the last one. + + 2) Checksum. Each slot has an "alias_checksum" value. The + checksum is calculated from the 8.3 name using the + following algorithm: + + for (sum = i = 0; i < 11; i++) { + sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i] + } + + 3) If there is in the final slot, a Unicode NULL (0x0000) is stored + after the final character. After that, all unused characters in + the final slot are set to Unicode 0xFFFF. + +Finally, note that the extended name is stored in Unicode. Each Unicode +character takes two bytes. + + +NOTES ON UNICODE TRANSLATION IN VFAT FILESYSTEM +---------------------------------------------------------------------- +(Information provided by Steve Searle <steve@mgu.bath.ac.uk>) + +Char used as Char(s) used Char(s) used in Entries which have +filename in shortname longname slot been corrected +0x80 (128) 0x80 0xC7 +0x81 (129) 0x9A 0xFC +0x82 (130) 0x90 0xE9 E +0x83 (131) 0xB6 0xE2 E +0x84 (132) 0x8E 0xE4 E +0x85 (133) 0xB7 0xE0 E +0x86 (134) 0x8F 0xE5 E +0x87 (135) 0x80 0xE7 E +0x88 (136) 0xD2 0xEA E +0x89 (137) 0xD3 0xEB E +0x8A (138) 0xD4 0xE8 E +0x8B (139) 0xD8 0xEF E +0x8C (140) 0xD7 0xEE E +0x8D (141) 0xDE 0xEC E +0x8E (142) 0x8E 0xC4 E +0x8F (143) 0x8F 0xC5 E +0x90 (144) 0x90 0xC9 E +0x91 (145) 0x92 0xE6 E +0x92 (146) 0x92 0xC6 E +0x93 (147) 0xE2 0xF4 E +0x94 (148) 0x99 0xF6 +0x95 (149) 0xE3 0xF2 +0x96 (150) 0xEA 0xFB +0x97 (151) 0xEB 0xF9 +0x98 (152) "_~1" 0xFF +0x99 (153) 0x99 0xD6 +0x9A (154) 0x9A 0xDC +0x9B (155) 0x9D 0xF8 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