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USING VFAT

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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

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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          -- UTF8 is the filesystem safe version of Unicode that

                 is used by the console.  It can be enabled for the

                 filesystem with this option. If 'uni_xlate' gets set,

                 UTF8 gets disabled.

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.

Explanation of Native Language Support in the VFAT Filesystem

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There are two different character sets are needed by the vfat

filesystem.  The first is the codepage character set.  The codepage is

the character set that is used to store short filenames on disk.  Its

mount option is 'codepage=437' which 437 is the codepage number.

Long filenames are stored in Unicode, but since the Linux filesystem

doesn't deal with 16 bit characters, we need some way of converting

characters.  There are a couple options of how to do this.  One is to

use the 'utf8' mount option and I will cover that a bit later.  The

other is to use the 'iocharset=iso8859-1' mount option where the

iso8859-1 tells the filesystem which character set is used for input

and output.  If you are in Russia, you might specify koi8-r here.

If a Unicode character on disk cannot be mapped to anything in the

iocharset, it is replaced with a '?'.

The iocharset is used to convert long filenames to and from Unicode.

It is currently implemented.  The codepage is used to convert short

filenames to and from the iocharset.  This translation is not currently

implemented.

If no iocharset is specified and the default is unable to be loaded,

the mount will succeed while falling back to doing no conversions at

all.  If a charset is explicity specified and the charset cannot be

loaded, the mount will fail.

For the codepage, the default mount option is 'codepage=437'.  If a

codepage is explicitly asked for and the load of the character set

fails, the mount will fail.  Is no codepage is explicitly asked for

and the load of the character set fails, the load will still succeed.

UTF8 is an 8 bit, filesystem safe representation of Unicode.  It does

not lose any information in the conversion.  However, you need to have

a terminal or a program that knows how to deal with UTF8.  The Linux

console can be put into a mode where it will correctly display UTF8

characters.  I don't know if there is a similar mode for xterms, but

I don't believe there is.  More information about UTF8 can be found

at http://www.unicode.com

TODO

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* 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 add dcache_lookup code msdos filesystem.  This means the

  directories need to be versioned like the vfat filesystem.

* 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

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* 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

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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

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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

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(This documentation was provided by Galen C. Hunt 

 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":

           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.