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1 USING VFAT
2 ----------------------------------------------------------------------
3 To use the vfat filesystem, use the filesystem type 'vfat'. i.e.
4 mount -t vfat /dev/fd0 /mnt
6 No special partition formatter is required. mkdosfs will work fine
7 if you want to format from within Linux.
9 VFAT MOUNT OPTIONS
10 ----------------------------------------------------------------------
11 codepage=### -- Sets the codepage for converting to shortname characters
12 on FAT and VFAT filesystems. By default, codepage 437
13 is used. This is the default for the U.S. and some
14 European countries.
15 iocharset=name -- Character set to use for converting between 8 bit characters
16 and 16 bit Unicode characters. Long filenames are stored on
17 disk in Unicode format, but Unix for the most part doesn't
18 know how to deal with Unicode. There is also an option of
19 doing UTF8 translations with the utf8 option.
20 utf8=<bool> -- UTF8 is the filesystem safe version of Unicode that
21 is used by the console. It can be be enabled for the
22 filesystem with this option. If 'uni_xlate' gets set,
23 UTF8 gets disabled.
24 uni_xlate=<bool> -- Translate unhandled Unicode characters to special
25 escaped sequences. This would let you backup and
26 restore filenames that are created with any Unicode
27 characters. Until Linux supports Unicode for real,
28 this gives you an alternative. Without this option,
29 a '?' is used when no translation is possible. The
30 escape character is ':' because it is otherwise
31 illegal on the vfat filesystem. The escape sequence
32 that gets used, where u is the unicode character, is:
33 ':', (u & 0x3f), ((u>>6) & 0x3f), (u>>12),
34 posix=<bool> -- Allow names of same letters, different case such as
35 'LongFileName' and 'longfilename' to coexist. This has some
36 problems currently because 8.3 conflicts are not handled
37 correctly for POSIX filesystem compliance.
38 nonumtail=<bool> -- When creating 8.3 aliases, normally the alias will
39 end in '~1' or tilde followed by some number. If this
40 option is set, then if the filename is
41 "longfilename.txt" and "longfile.txt" does not
42 currently exist in the directory, 'longfile.txt' will
43 be the short alias instead of 'longfi~1.txt'.
45 quiet -- Stops printing certain warning messages.
46 check=s|r|n -- Case sensitivity checking setting.
47 s: strict, case sensitive
48 r: relaxed, case insensitive
49 n: normal, default setting, currently case insensitive
51 <bool>: 0,1,yes,no,true,false
53 TODO
54 ----------------------------------------------------------------------
55 * When only shortnames exist, translate them from the codepage character
56 set to the iocharset. Currently, translations only occur when longnames
57 exist. To translate, first convert from codepage to Unicode and then
58 to the output character set.
60 * Need to get rid of the raw scanning stuff. Instead, always use
61 a get next directory entry approach. The only thing left that uses
62 raw scanning is the directory renaming code.
64 * Fix the POSIX filesystem support to work in 8.3 space. This involves
65 renaming aliases if a conflict occurs between a new filename and
66 an old alias. This is quite a mess.
69 POSSIBLE PROBLEMS
70 ----------------------------------------------------------------------
71 * vfat_valid_longname does not properly checked reserved names.
72 * When a volume name is the same as a directory name in the root
73 directory of the filesystem, the directory name sometimes shows
74 up as an empty file.
75 * autoconv option does not work correctly.
77 BUG REPORTS
78 ----------------------------------------------------------------------
79 If you have trouble with the VFAT filesystem, mail bug reports to
80 chaffee@bmrc.cs.berkeley.edu. Please specify the filename
81 and the operation that gave you trouble.
83 TEST SUITE
84 ----------------------------------------------------------------------
85 If you plan to make any modifications to the vfat filesystem, please
86 get the test suite that comes with the vfat distribution at
88 http://bmrc.berkeley.edu/people/chaffee/vfat.html
90 This tests quite a few parts of the vfat filesystem and additional
91 tests for new features or untested features would be appreciated.
93 NOTES ON THE STRUCTURE OF THE VFAT FILESYSTEM
94 ----------------------------------------------------------------------
95 (This documentation was provided by Galen C. Hunt <gchunt@cs.rochester.edu>
96 and lightly annotated by Gordon Chaffee).
98 This document presents a very rough, technical overview of my
99 knowledge of the extended FAT file system used in Windows NT 3.5 and
100 Windows 95. I don't guarantee that any of the following is correct,
101 but it appears to be so.
103 The extended FAT file system is almost identical to the FAT
104 file system used in DOS versions up to and including 6.223410239847
105 :-). The significant change has been the addition of long file names.
106 These names support up to 255 characters including spaces and lower
107 case characters as opposed to the traditional 8.3 short names.
109 Here is the description of the traditional FAT entry in the current
110 Windows 95 filesystem:
112 struct directory { // Short 8.3 names
113 unsigned char name[8]; // file name
114 unsigned char ext[3]; // file extension
115 unsigned char attr; // attribute byte
116 unsigned char lcase; // Case for base and extension
117 unsigned char ctime_ms; // Creation time, milliseconds
118 unsigned char ctime[2]; // Creation time
119 unsigned char cdate[2]; // Creation date
120 unsigned char adate[2]; // Last access date
121 unsigned char reserved[2]; // reserved values (ignored)
122 unsigned char time[2]; // time stamp
123 unsigned char date[2]; // date stamp
124 unsigned char start[2]; // starting cluster number
125 unsigned char size[4]; // size of the file
126 };
128 The lcase field specifies if the base and/or the extension of an 8.3
129 name should be capitalized. This field does not seem to be used by
130 Windows 95 but it is used by Windows NT. The case of filenames is not
131 completely compatible from Windows NT to Windows 95. It is not completely
132 compatible in the reverse direction, however. Filenames that fit in
133 the 8.3 namespace and are written on Windows NT to be lowercase will
134 show up as uppercase on Windows 95.
136 Note that the "start" and "size" values are actually little
137 endian integer values. The descriptions of the fields in this
138 structure are public knowledge and can be found elsewhere.
140 With the extended FAT system, Microsoft has inserted extra
141 directory entries for any files with extended names. (Any name which
142 legally fits within the old 8.3 encoding scheme does not have extra
143 entries.) I call these extra entries slots. Basically, a slot is a
144 specially formatted directory entry which holds up to 13 characters of
145 a file's extended name. Think of slots as additional labeling for the
146 directory entry of the file to which they correspond. Microsoft
147 prefers to refer to the 8.3 entry for a file as its alias and the
148 extended slot directory entries as the file name.
150 The C structure for a slot directory entry follows:
152 struct slot { // Up to 13 characters of a long name
153 unsigned char id; // sequence number for slot
154 unsigned char name0_4[10]; // first 5 characters in name
155 unsigned char attr; // attribute byte
156 unsigned char reserved; // always 0
157 unsigned char alias_checksum; // checksum for 8.3 alias
158 unsigned char name5_10[12]; // 6 more characters in name
159 unsigned char start[2]; // starting cluster number
160 unsigned char name11_12[4]; // last 2 characters in name
161 };
163 If the layout of the slots looks a little odd, it's only
164 because of Microsoft's efforts to maintain compatibility with old
165 software. The slots must be disguised to prevent old software from
166 panicking. To this end, a number of measures are taken:
168 1) The attribute byte for a slot directory entry is always set
169 to 0x0f. This corresponds to an old directory entry with
170 attributes of "hidden", "system", "read-only", and "volume
171 label". Most old software will ignore any directory
172 entries with the "volume label" bit set. Real volume label
173 entries don't have the other three bits set.
175 2) The starting cluster is always set to 0, an impossible
176 value for a DOS file.
178 Because the extended FAT system is backward compatible, it is
179 possible for old software to modify directory entries. Measures must
180 be taken to ensure the validity of slots. An extended FAT system can
181 verify that a slot does in fact belong to an 8.3 directory entry by
182 the following:
184 1) Positioning. Slots for a file always immediately proceed
185 their corresponding 8.3 directory entry. In addition, each
186 slot has an id which marks its order in the extended file
187 name. Here is a very abbreviated view of an 8.3 directory
188 entry and its corresponding long name slots for the file
189 "My Big File.Extension which is long":
191 <proceeding files...>
192 <slot #3, id = 0x43, characters = "h is long">
193 <slot #2, id = 0x02, characters = "xtension whic">
194 <slot #1, id = 0x01, characters = "My Big File.E">
195 <directory entry, name = "MYBIGFIL.EXT">
197 Note that the slots are stored from last to first. Slots
198 are numbered from 1 to N. The Nth slot is or'ed with 0x40
199 to mark it as the last one.
201 2) Checksum. Each slot has an "alias_checksum" value. The
202 checksum is calculated from the 8.3 name using the
203 following algorithm:
205 for (sum = i = 0; i < 11; i++) {
206 sum = (((sum&1)<<7)|((sum&0xfe)>>1)) + name[i]
207 }
209 3) If there is free space in the final slot, a Unicode NULL (0x0000)
210 is stored after the final character. After that, all unused
211 characters in the final slot are set to Unicode 0xFFFF.
213 Finally, note that the extended name is stored in Unicode. Each Unicode
214 character takes two bytes.