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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
----------------------------------------------------------------------
codepage=### -- Sets the codepage for converting to shortname characters
on FAT and VFAT filesystems. By default, codepage 437
is used. This is the default for the U.S. and some
European countries.
iocharset=name -- Character set to use for converting between 8 bit characters
and 16 bit Unicode characters. Long filenames are stored on
disk in Unicode format, but Unix for the most part doesn't
know how to deal with Unicode. There is also an option of
doing UTF8 translations with the utf8 option.
utf8=<bool> -- UTF8 is the filesystem safe version of Unicode that
is used by the console. It can be be enabled for the
filesystem with this option. If 'uni_xlate' gets set,
UTF8 gets disabled.
uni_xlate=<bool> -- 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=<bool> -- 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=<bool> -- 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.
<bool>: 0,1,yes,no,true,false
TODO
----------------------------------------------------------------------
* When only shortnames exist, translate them from the codepage character
set to the iocharset. Currently, translations only occur when longnames
exist. To translate, first convert from codepage to Unicode and then
to the output character set.
* 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.
* 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.
* autoconv option does not work correctly.
BUG REPORTS
----------------------------------------------------------------------
If you have trouble with the VFAT filesystem, mail bug reports to
chaffee@bmrc.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://bmrc.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.
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