Documentation/filesystems/vfat.rst - linux - Git at Google

 ====
 VFAT
 ====

 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
 ==================

 **uid=###**
 	Set the owner of all files on this filesystem.
 	The default is the uid of current process.

 **gid=###**
 	Set the group of all files on this filesystem.
 	The default is the gid of current process.

 **umask=###**
 	The permission mask (for files and directories, see *umask(1)*).
 	The default is the umask of current process.

 **dmask=###**
 	The permission mask for the directory.
 	The default is the umask of current process.

 **fmask=###**
 	The permission mask for files.
 	The default is the umask of current process.

 **allow_utime=###**
 	This option controls the permission check of mtime/atime.

 		**-20**: If current process is in group of file's group ID,
                 you can change timestamp.

 		**-2**: Other users can change timestamp.

 	The default is set from dmask option. If the directory is
 	writable, utime(2) is also allowed. i.e. ~dmask & 022.

 	Normally utime(2) checks current process is owner of
 	the file, or it has CAP_FOWNER capability. But FAT
 	filesystem doesn't have uid/gid on disk, so normal
 	check is too inflexible. With this option you can
 	relax it.

 **codepage=###**
 	Sets the codepage number for converting to shortname
 	characters on FAT filesystem.
 	By default, FAT_DEFAULT_CODEPAGE setting is used.

 **iocharset=<name>**
 	Character set to use for converting between the
 	encoding is used for user visible filename 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.
 	By default, FAT_DEFAULT_IOCHARSET setting is used.

 	There is also an option of doing UTF-8 translations
 	with the utf8 option.

 .. note:: ``iocharset=utf8`` is not recommended. If unsure, you should consider
 	  the utf8 option instead.

 **utf8=<bool>**
 	UTF-8 is the filesystem safe version of Unicode that
 	is used by the console. It can be enabled or disabled
 	for the filesystem with this option.
 	If 'uni_xlate' gets set, UTF-8 gets disabled.
 	By default, FAT_DEFAULT_UTF8 setting is used.

 **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 is ':' and the four digits of hexadecimal
 	unicode.

 **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.

 **usefree**
 	Use the "free clusters" value stored on FSINFO. It will
 	be used to determine number of free clusters without
 	scanning disk. But it's not used by default, because
 	recent Windows don't update it correctly in some
 	case. If you are sure the "free clusters" on FSINFO is
 	correct, by this option you can avoid scanning disk.

 **quiet**
 	Stops printing certain warning messages.

 **check=s|r|n**
 	Case sensitivity checking setting.

 	**s**: strict, case sensitive

 	**r**: relaxed, case insensitive

 	**n**: normal, default setting, currently case insensitive

 **nocase**
 	This was deprecated for vfat. Use ``shortname=win95`` instead.

 **shortname=lower|win95|winnt|mixed**
 	Shortname display/create setting.

 	**lower**: convert to lowercase for display,
 	emulate the Windows 95 rule for create.

 	**win95**: emulate the Windows 95 rule for display/create.

 	**winnt**: emulate the Windows NT rule for display/create.

 	**mixed**: emulate the Windows NT rule for display,
 	emulate the Windows 95 rule for create.

 	Default setting is `mixed`.

 **tz=UTC**
 	Interpret timestamps as UTC rather than local time.
 	This option disables the conversion of timestamps
 	between local time (as used by Windows on FAT) and UTC
 	(which Linux uses internally).  This is particularly
 	useful when mounting devices (like digital cameras)
 	that are set to UTC in order to avoid the pitfalls of
 	local time.

 **time_offset=minutes**
 	Set offset for conversion of timestamps from local time
 	used by FAT to UTC. I.e. <minutes> minutes will be subtracted
 	from each timestamp to convert it to UTC used internally by
 	Linux. This is useful when time zone set in ``sys_tz`` is
 	not the time zone used by the filesystem. Note that this
 	option still does not provide correct time stamps in all
 	cases in presence of DST - time stamps in a different DST
 	setting will be off by one hour.

 **showexec**
 	If set, the execute permission bits of the file will be
 	allowed only if the extension part of the name is .EXE,
 	.COM, or .BAT. Not set by default.

 **debug**
 	Can be set, but unused by the current implementation.

 **sys_immutable**
 	If set, ATTR_SYS attribute on FAT is handled as
 	IMMUTABLE flag on Linux. Not set by default.

 **flush**
 	If set, the filesystem will try to flush to disk more
 	early than normal. Not set by default.

 **rodir**
 	FAT has the ATTR_RO (read-only) attribute. On Windows,
 	the ATTR_RO of the directory will just be ignored,
 	and is used only by applications as a flag (e.g. it's set
 	for the customized folder).

 	If you want to use ATTR_RO as read-only flag even for
 	the directory, set this option.

 **errors=panic|continue|remount-ro**
 	specify FAT behavior on critical errors: panic, continue
 	without doing anything or remount the partition in
 	read-only mode (default behavior).

 **discard**
 	If set, issues discard/TRIM commands to the block
 	device when blocks are freed. This is useful for SSD devices
 	and sparse/thinly-provisioned LUNs.

 **nfs=stale_rw|nostale_ro**
 	Enable this only if you want to export the FAT filesystem
 	over NFS.

 		**stale_rw**: This option maintains an index (cache) of directory
 		*inodes* by *i_logstart* which is used by the nfs-related code to
 		improve look-ups. Full file operations (read/write) over NFS is
 		supported but with cache eviction at NFS server, this could
 		result in ESTALE issues.

 		**nostale_ro**: This option bases the *inode* number and filehandle
 		on the on-disk location of a file in the MS-DOS directory entry.
 		This ensures that ESTALE will not be returned after a file is
 		evicted from the inode cache. However, it means that operations
 		such as rename, create and unlink could cause filehandles that
 		previously pointed at one file to point at a different file,
 		potentially causing data corruption. For this reason, this
 		option also mounts the filesystem readonly.

 	To maintain backward compatibility, ``'-o nfs'`` is also accepted,
 	defaulting to "stale_rw".

 **dos1xfloppy  <bool>: 0,1,yes,no,true,false**
 	If set, use a fallback default BIOS Parameter Block
 	configuration, determined by backing device size. These static
 	parameters match defaults assumed by DOS 1.x for 160 kiB,
 	180 kiB, 320 kiB, and 360 kiB floppies and floppy images.


 LIMITATION
 ==========

 The fallocated region of file is discarded at umount/evict time
 when using fallocate with FALLOC_FL_KEEP_SIZE.
 So, User should assume that fallocated region can be discarded at
 last close if there is memory pressure resulting in eviction of
 the inode from the memory. As a result, for any dependency on
 the fallocated region, user should make sure to recheck fallocate
 after reopening the file.

 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.


 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 as an empty file.
 - autoconv option does not work correctly.


 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://web.archive.org/web/*/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.
 These 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:: 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 file's 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
 panicking.  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 ensure 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:: 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 free space 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 either two or four bytes, UTF-16LE encoded.
	====
	VFAT
	====

	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
	==================

	uid=###
	Set the owner of all files on this filesystem.
	The default is the uid of current process.

	gid=###
	Set the group of all files on this filesystem.
	The default is the gid of current process.

	umask=###
	The permission mask (for files and directories, see umask(1)).
	The default is the umask of current process.

	dmask=###
	The permission mask for the directory.
	The default is the umask of current process.

	fmask=###
	The permission mask for files.
	The default is the umask of current process.

	allow_utime=###
	This option controls the permission check of mtime/atime.

	-20: If current process is in group of file's group ID,
	you can change timestamp.

	-2: Other users can change timestamp.

	The default is set from dmask option. If the directory is
	writable, utime(2) is also allowed. i.e. ~dmask & 022.

	Normally utime(2) checks current process is owner of
	the file, or it has CAP_FOWNER capability. But FAT
	filesystem doesn't have uid/gid on disk, so normal
	check is too inflexible. With this option you can
	relax it.

	codepage=###
	Sets the codepage number for converting to shortname
	characters on FAT filesystem.
	By default, FAT_DEFAULT_CODEPAGE setting is used.

	iocharset=<name>
	Character set to use for converting between the
	encoding is used for user visible filename 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.
	By default, FAT_DEFAULT_IOCHARSET setting is used.

	There is also an option of doing UTF-8 translations
	with the utf8 option.

	.. note:: ``iocharset=utf8`` is not recommended. If unsure, you should consider
	the utf8 option instead.

	utf8=<bool>
	UTF-8 is the filesystem safe version of Unicode that
	is used by the console. It can be enabled or disabled
	for the filesystem with this option.
	If 'uni_xlate' gets set, UTF-8 gets disabled.
	By default, FAT_DEFAULT_UTF8 setting is used.

	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 is ':' and the four digits of hexadecimal
	unicode.

	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.

	usefree
	Use the "free clusters" value stored on FSINFO. It will
	be used to determine number of free clusters without
	scanning disk. But it's not used by default, because
	recent Windows don't update it correctly in some
	case. If you are sure the "free clusters" on FSINFO is
	correct, by this option you can avoid scanning disk.

	quiet
	Stops printing certain warning messages.

	check=s\|r\|n
	Case sensitivity checking setting.

	s: strict, case sensitive

	r: relaxed, case insensitive

	n: normal, default setting, currently case insensitive

	nocase
	This was deprecated for vfat. Use ``shortname=win95`` instead.

	shortname=lower\|win95\|winnt\|mixed
	Shortname display/create setting.

	lower: convert to lowercase for display,
	emulate the Windows 95 rule for create.

	win95: emulate the Windows 95 rule for display/create.

	winnt: emulate the Windows NT rule for display/create.

	mixed: emulate the Windows NT rule for display,
	emulate the Windows 95 rule for create.

	Default setting is `mixed`.

	tz=UTC
	Interpret timestamps as UTC rather than local time.
	This option disables the conversion of timestamps
	between local time (as used by Windows on FAT) and UTC
	(which Linux uses internally). This is particularly
	useful when mounting devices (like digital cameras)
	that are set to UTC in order to avoid the pitfalls of
	local time.

	time_offset=minutes
	Set offset for conversion of timestamps from local time
	used by FAT to UTC. I.e. <minutes> minutes will be subtracted
	from each timestamp to convert it to UTC used internally by
	Linux. This is useful when time zone set in ``sys_tz`` is
	not the time zone used by the filesystem. Note that this
	option still does not provide correct time stamps in all
	cases in presence of DST - time stamps in a different DST
	setting will be off by one hour.

	showexec
	If set, the execute permission bits of the file will be
	allowed only if the extension part of the name is .EXE,
	.COM, or .BAT. Not set by default.

	debug
	Can be set, but unused by the current implementation.

	sys_immutable
	If set, ATTR_SYS attribute on FAT is handled as
	IMMUTABLE flag on Linux. Not set by default.

	flush
	If set, the filesystem will try to flush to disk more
	early than normal. Not set by default.

	rodir
	FAT has the ATTR_RO (read-only) attribute. On Windows,
	the ATTR_RO of the directory will just be ignored,
	and is used only by applications as a flag (e.g. it's set
	for the customized folder).

	If you want to use ATTR_RO as read-only flag even for
	the directory, set this option.

	errors=panic\|continue\|remount-ro
	specify FAT behavior on critical errors: panic, continue
	without doing anything or remount the partition in
	read-only mode (default behavior).

	discard
	If set, issues discard/TRIM commands to the block
	device when blocks are freed. This is useful for SSD devices
	and sparse/thinly-provisioned LUNs.

	nfs=stale_rw\|nostale_ro
	Enable this only if you want to export the FAT filesystem
	over NFS.

	stale_rw: This option maintains an index (cache) of directory
	inodes by i_logstart which is used by the nfs-related code to
	improve look-ups. Full file operations (read/write) over NFS is
	supported but with cache eviction at NFS server, this could
	result in ESTALE issues.

	nostale_ro: This option bases the inode number and filehandle
	on the on-disk location of a file in the MS-DOS directory entry.
	This ensures that ESTALE will not be returned after a file is
	evicted from the inode cache. However, it means that operations
	such as rename, create and unlink could cause filehandles that
	previously pointed at one file to point at a different file,
	potentially causing data corruption. For this reason, this
	option also mounts the filesystem readonly.

	To maintain backward compatibility, ``'-o nfs'`` is also accepted,
	defaulting to "stale_rw".

	dos1xfloppy <bool>: 0,1,yes,no,true,false
	If set, use a fallback default BIOS Parameter Block
	configuration, determined by backing device size. These static
	parameters match defaults assumed by DOS 1.x for 160 kiB,
	180 kiB, 320 kiB, and 360 kiB floppies and floppy images.



	LIMITATION
	==========

	The fallocated region of file is discarded at umount/evict time
	when using fallocate with FALLOC_FL_KEEP_SIZE.
	So, User should assume that fallocated region can be discarded at
	last close if there is memory pressure resulting in eviction of
	the inode from the memory. As a result, for any dependency on
	the fallocated region, user should make sure to recheck fallocate
	after reopening the file.

	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.


	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 as an empty file.
	- autoconv option does not work correctly.


	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://web.archive.org/web/*/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.
	These 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:: 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 file's 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
	panicking. 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 ensure 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:: 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 free space 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 either two or four bytes, UTF-16LE encoded.