Microsoft Extensible Firmware Initiative FAT32 File System Specification
Hardware
White Paper
Designing Hardware for Microsoft® Operating Systems
Microsoft Extensible Firmware
Initiative FAT32 File System Specification
FAT: General Overview of On-Disk Format
Version 1.03, December 6, 2000
Microsoft Corporation
Microsoft Corporation
The FAT (File Allocation Table) file
system has its origins in the late 1970s and early1980s and was the file system
supported by the Microsoft® MS-DOS® operating system. It was originally
developed as a simple file system suitable for floppy disk drives less than 500K
in size. Over time it has been enhanced to support larger and larger media.
Currently there are three FAT file system types: FAT12, FAT16 and FAT32. The
basic difference in these FAT sub types, and the reason for the names, is the
size, in bits, of the entries in the actual FAT structure on the disk. There
are 12 bits in a FAT12 FAT entry, 16 bits in a FAT16 FAT entry and 32 bits in a
FAT32 FAT entry.
Contents
Microsoft,
MS_DOS, Windows, and Windows NT are trademarks or registered trademarks of
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© 2000 Microsoft Corporation.
All rights reserved.
Microsoft Extensible Firmware Initiative
FAT32 File System Specification
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Notational Conventions in this
Document
Numbers that have the characters “0x” at
the beginning of them are hexadecimal (base 16) numbers.
Any numbers that do not have the characters
“0x” at the beginning are decimal (base 10) numbers.
The code fragments in this document are
written in the ‘C’ programming language. Strict typing and syntax are not
adhered to.
There are several code fragments in this
document that freely mix 32-bit and 16-bit data elements. It is assumed that
you are a programmer who understands how to properly type such operations so
that data is not lost due to truncation of 32-bit values to 16-bit values. Also
take note that all data types are UNSIGNED. Do not do FAT computations with
signed integer types, because the computations will be wrong on some FAT
volumes.
General Comments (Applicable to
FAT File System All Types)
All of the FAT file systems were originally
developed for the IBM PC machine architecture. The importance of this is that
FAT file system on disk data structure is all “little endian.” If we look at
one 32-bit FAT entry stored on disk as a series of four 8-bit bytes—the first
being byte[0] and the last being byte[4]—here is where the 32 bits numbered 00
through 31 are (00 being the least significant bit):
byte[3] 3
3 2 2 2 2 2 2
1
0 9 8 7 6 5 4
byte[2] 2
2 2 2 1 1 1 1
3
2 1 0 9 8 7 6
byte[1] 1
1 1 1 1 1 0 0
5
4 3 2 1 0 9 8
byte[0] 0
0 0 0 0 0 0 0
7 6
5 4 3 2 1 0
This is important if your machine is a “big
endian” machine, because you will have to translate between big and little
endian as you move data to and from the disk.
A FAT file system volume is composed of
four basic regions, which are laid out in this order on the volume:
0
– Reserved Region
1
– FAT Region
2
– Root Directory Region (doesn’t exist on FAT32 volumes)
3
– File and Directory Data Region
Boot Sector and BPB
The first important data structure on a FAT
volume is called the BPB (BIOS Parameter Block), which is located in the first
sector of the volume in the Reserved Region. This sector is sometimes called
the “boot sector” or the “reserved sector” or the “0th sector,” but the
important fact is simply that it is the first sector of the volume.
This is the first thing about the FAT file
system that sometimes causes confusion. In MS-DOS version 1.x, there was not a
BPB in the boot sector. In this first version of the FAT file system, there
were only two different formats, the one for single-sided and the one for
double-sided 360K 5.25-inch floppy disks. The determination of which type was
on the disk was done by looking at the first byte of the FAT (the low 8 bits of
FAT[0]).
This type of media determination was
superseded in MS-DOS version 2.x by putting a BPB in the boot sector, and the
old style of media determination (done by looking at the first byte of the FAT)
was no longer supported. All FAT volumes must have a BPB in the boot sector.
This brings us to the second point of
confusion relating to FAT volume determination: What exactly does a BPB look
like? The BPB in the boot sector defined for MS-DOS 2.x only allowed for a FAT
volume with strictly less than 65,536 sectors (32 MB worth of 512-byte
sectors). This limitation was due to the fact that the “total sectors” field
was only a 16-bit field. This limitation was addressed by MS-DOS 3.x, where the
BPB was modified to include a new 32-bit field for the total sectors value.
The next BPB change occurred with the
Microsoft Windows 95 operating system, specifically OEM Service Release 2
(OSR2), where the FAT32 type was introduced. FAT16 was limited by the maximum
size of the FAT and the maximum valid cluster size to no more than a 2 GB
volume if the disk had 512-byte sectors. FAT32 addressed this limitation on the
amount of disk space that one FAT volume could occupy so that disks larger than
2 GB only had to have one partition defined.
The FAT32 BPB exactly matches the
FAT12/FAT16 BPB up to and including the BPB_TotSec32 field. They differ
starting at offset 36, depending on whether the media type is FAT12/FAT16 or
FAT32 (see discussion below for determining FAT type). The relevant point here
is that the BPB in the boot sector of a FAT volume should always be one that
has all of the new BPB fields for either the FAT12/FAT16 or FAT32 BPB type.
Doing it this way ensures the maximum compatibility of the FAT volume and
ensures that all FAT file system drivers will understand and support the volume
properly, because it always contains all of the currently defined fields.
NOTE:
In the following description, all the fields whose
names start with BPB_ are part of the BPB. All the fields whose names start
with BS_ are part of the boot sector and not really part of the BPB. The
following shows the start of sector 0 of a FAT volume, which contains the BPB:
Boot Sector and BPB Structure
|
Name
|
Offset (byte)
|
Size (bytes)
|
Description
|
|
BS_jmpBoot
|
0
|
3
|
Jump instruction to boot
code. This field has two allowed forms:
jmpBoot[0] = 0xEB, jmpBoot[1] = 0x??, jmpBoot[2] = 0x90
and
jmpBoot[0] = 0xE9, jmpBoot[1] = 0x??, jmpBoot[2] = 0x??
0x?? indicates that
any 8-bit value is allowed in that byte. What this forms is a three-byte
Intel x86 unconditional branch (jump) instruction that jumps to the start of
the operating system bootstrap code. This code typically occupies the rest of
sector 0 of the volume following the BPB and possibly other sectors. Either
of these forms is acceptable. JmpBoot[0] = 0xEB is the more frequently
used format.
|
|
BS_OEMName
|
3
|
8
|
“MSWIN4.1” There are many
misconceptions about this field. It is only a name string. Microsoft
operating systems don’t pay any attention to this field. Some FAT drivers do.
This is the reason that the indicated string, “MSWIN4.1”, is the recommended
setting, because it is the setting least likely to cause compatibility
problems. If you want to put something else in here, that is your option, but
the result may be that some FAT drivers might not recognize the volume.
Typically this is some indication of what system formatted the volume.
|
|
BPB_BytsPerSec
|
11
|
2
|
Count of bytes per sector.
This value may take on only the following values: 512, 1024, 2048 or 4096. If
maximum compatibility with old implementations is desired, only the value 512
should be used. There is a lot of FAT code in the world that is basically
“hard wired” to 512 bytes per sector and doesn’t bother to check this field
to make sure it is 512. Microsoft operating systems will properly support
1024, 2048, and 4096.
Note: Do not
misinterpret these statements about maximum compatibility. If the media being
recorded has a physical sector size N, you must use N and this must still be
less than or equal to 4096. Maximum compatibility is achieved by only using
media with specific sector sizes.
|
|
BPB_SecPerClus
|
13
|
1
|
Number of sectors per
allocation unit. This value must be a power of 2 that is greater than 0. The
legal values are 1, 2, 4, 8, 16, 32, 64, and 128. Note however, that a value
should never be used that results in a “bytes per cluster” value (BPB_BytsPerSec * BPB_SecPerClus) greater than
32K (32 * 1024). There is a misconception that values greater than this are
OK. Values that cause a cluster size greater than 32K bytes do not work properly;
do not try to define one. Some versions of some systems allow 64K bytes per
cluster value. Many application setup programs will not work correctly on
such a FAT volume.
|
BPB_RsvdSecCnt
|
14
|
2
|
Number of reserved sectors
in the Reserved region of the volume starting at the first sector of the
volume. This field must not be 0. For FAT12 and FAT16 volumes, this value
should never be anything other than 1. For FAT32 volumes, this value is
typically 32. There is a lot of FAT code in the world “hard wired” to 1
reserved sector for FAT12 and FAT16 volumes and that doesn’t bother to check
this field to make sure it is 1. Microsoft operating systems will properly
support any non-zero value in this field.
|
|
BPB_NumFATs
|
16
|
1
|
The count of FAT data
structures on the volume. This field should always contain the value 2 for
any FAT volume of any type. Although any value greater than or equal to 1 is
perfectly valid, many software programs and a few operating systems’ FAT file
system drivers may not function properly if the value is something other than
2. All Microsoft file system drivers will support a value other than 2, but
it is still highly recommended that no value other than 2 be used in this
field.
The reason the standard
value for this field is 2 is to provide redundancy for the FAT data
structure so that if a sector goes bad in one of the FATs, that data is not lost because it is duplicated in the other
FAT. On non-disk-based media, such as FLASH memory cards, where such
redundancy is a useless feature, a value of 1 may be used to save the space
that a second copy of the FAT uses, but some FAT file system drivers might
not recognize such a volume properly.
|
|
BPB_RootEntCnt
|
17
|
2
|
For FAT12 and FAT16
volumes, this field contains the count of 32-byte directory entries in the
root directory. For FAT32 volumes, this field must be set to 0. For FAT12 and
FAT16 volumes, this value should always specify a count that when multiplied
by 32 results in an even multiple of BPB_BytsPerSec.
For maximum compatibility, FAT16 volumes should use the value 512.
|
|
BPB_TotSec16
|
19
|
2
|
This field is the old
16-bit total count of sectors on the volume. This count includes the count of
all sectors in all four regions of the volume. This field can be 0; if it is
0, then BPB_TotSec32 must be non-zero. For FAT32 volumes, this field must be
0. For FAT12 and FAT16 volumes, this field contains the sector count, and
BPB_TotSec32 is 0 if the total sector count “fits” (is less than 0x10000).
|
|
BPB_Media
|
21
|
1
|
0xF8 is the standard value
for “fixed” (non-removable) media. For removable media, 0xF0 is frequently
used. The legal values for this field are 0xF0, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC,
0xFD, 0xFE, and 0xFF. The only other important point is that whatever value
is put in here must also be put in the low byte of the FAT[0] entry. This
dates back to the old MS-DOS 1.x media determination noted earlier and is no
longer usually used for anything.
|
|
BPB_FATSz16
|
22
|
2
|
This field is the
FAT12/FAT16 16-bit count of sectors occupied by ONE FAT. On FAT32 volumes
this field must be 0, and BPB_FATSz32 contains the FAT size count.
|
|
BPB_SecPerTrk
|
24
|
2
|
Sectors per track for
interrupt 0x13. This field is only relevant for media that have a geometry
(volume is broken down into tracks by multiple heads and cylinders) and are visible
on interrupt 0x13. This field contains the “sectors per track” geometry
value.
|
|
BPB_NumHeads
|
26
|
2
|
Number of heads for
interrupt 0x13. This field is relevant as discussed earlier for BPB_SecPerTrk. This field contains the one
based “count of heads”. For example, on a 1.44 MB 3.5-inch floppy drive this
value is 2.
|
|
BPB_HiddSec
|
28
|
4
|
Count of hidden sectors
preceding the partition that contains this FAT volume. This field is
generally only relevant for media visible on interrupt 0x13. This field
should always be zero on media that are not partitioned. Exactly what value
is appropriate is operating system specific.
|
|
BPB_TotSec32
|
32
|
4
|
This field is the new
32-bit total count of sectors on the volume. This count includes the count of
all sectors in all four regions of the volume. This field can be 0; if it is
0, then BPB_TotSec16 must be non-zero. For FAT32 volumes, this field must be
non-zero. For FAT12/FAT16 volumes, this field contains the sector count if
BPB_TotSec16 is 0 (count is greater than or equal to 0x10000).
|
At this point, the BPB/boot sector for
FAT12 and FAT16 differs from the BPB/boot sector for FAT32. The first table
shows the structure for FAT12 and FAT16 starting at offset 36 of the boot
sector.
Fat12 and Fat16 Structure Starting at Offset 36
|
Name
|
Offset (byte)
|
Size (bytes)
|
Description
|
|
BS_DrvNum
|
36
|
1
|
Int 0x13 drive number (e.g.
0x80). This field supports MS-DOS bootstrap and is set to the INT 0x13 drive
number of the media (0x00 for floppy disks, 0x80 for hard disks).
NOTE: This field is actually operating system specific. |
|
BS_Reserved1
|
37
|
1
|
Reserved (used by Windows
NT). Code that formats FAT volumes should always set this byte to 0.
|
|
BS_BootSig
|
38
|
1
|
Extended boot signature
(0x29). This is a signature byte that indicates that the following three
fields in the boot sector are present.
|
|
BS_VolID
|
39
|
4
|
Volume serial number. This
field, together with BS_VolLab,
supports volume tracking on removable media. These values allow FAT file
system drivers to detect that the wrong disk is inserted in a removable
drive. This ID is usually generated by simply combining the current date and
time into a 32-bit value.
|
|
BS_VolLab
|
43
|
11
|
Volume label. This field
matches the 11-byte volume label recorded in the root directory.
NOTE: FAT file system drivers should make sure that they update this field when the volume label file in the root directory has its name changed or created. The setting for this field when there is no volume label is the string “NO NAME ”. |
|
BS_FilSysType
|
54
|
8
|
One of the strings “FAT12 ”, “FAT16 ”, or “FAT ”. NOTE:
Many people think that the string in this field has something to do with the
determination of what type of FAT—FAT12, FAT16, or FAT32—that the volume has.
This is not true. You will note from its name that this field is not actually
part of the BPB. This string is informational only and is not used by
Microsoft file system drivers to determine FAT typ,e because it is frequently
not set correctly or is not present. See the FAT Type Determination section
of this document. This string should be set based on the FAT type though,
because some non-Microsoft FAT file system drivers do look at it.
|
Here is the structure for FAT32 starting at
offset 36 of the boot sector.
FAT32 Structure Starting at Offset 36
|
Name
|
Offset (byte)
|
Size (bytes)
|
Description
|
|
BPB_FATSz32
|
36
|
4
|
This field is only defined for FAT32 media and does not exist on
FAT12 and FAT16 media. This field is the FAT32 32-bit count of sectors
occupied by ONE FAT. BPB_FATSz16 must be 0.
|
|
BPB_ExtFlags
|
40
|
2
|
This field is only defined for FAT32 media and does not exist on
FAT12 and FAT16 media.
Bits
0-3 -- Zero-based number of active FAT.
Only valid if mirroring is disabled.
Bits
4-6 -- Reserved.
Bit 7 --
0 means the FAT is mirrored at runtime into all FATs.
-- 1 means only one FAT is
active; it is the one referenced in bits 0-3.
Bits
8-15 -- Reserved.
|
|
BPB_FSVer
|
42
|
2
|
This field is only defined
for FAT32 media and does not exist on FAT12 and FAT16 media. High byte is
major revision number. Low byte is minor revision number. This is the version
number of the FAT32 volume. This supports the ability to extend the FAT32
media type in the future without worrying about old FAT32 drivers mounting
the volume. This document defines the version to 0:0. If this field is non-zero, back-level
Windows versions will not mount the volume.
NOTE: Disk utilities should
respect this field and not operate on volumes with a higher major or minor
version number than that for which they were designed. FAT32 file system
drivers must check this field and not mount the volume if it does not contain
a version number that was defined at the time the driver was written.
|
|
BPB_RootClus
|
44
|
4
|
This field is only defined
for FAT32 media and does not exist on FAT12 and FAT16 media. This is set to
the cluster number of the first cluster of the root directory, usually 2 but
not required to be 2.
NOTE: Disk utilities that change
the location of the root directory should make every effort to place the
first cluster of the root directory in the first non-bad cluster on the drive
(i.e., in cluster 2, unless it’s marked bad). This is specified so that disk
repair utilities can easily find the root directory if this field
accidentally gets zeroed.
|
|
BPB_FSInfo
|
48
|
2
|
This field is only defined for FAT32 media and does not exist on
FAT12 and FAT16 media. Sector number of FSINFO structure in the reserved area
of the FAT32 volume. Usually 1.
NOTE: There will be a
copy of the FSINFO structure in BackupBoot,
but only the copy pointed to by this field will be kept up to date (i.e.,
both the primary and backup boot record will point to the same FSINFO
sector).
|
|
BPB_BkBootSec
|
50
|
2
|
This field is only defined
for FAT32 media and does not exist on FAT12 and FAT16 media. If non-zero,
indicates the sector number in the reserved area of the volume of a copy of
the boot record. Usually 6. No value other than 6 is recommended.
|
|
BPB_Reserved
|
52
|
12
|
This field is only defined
for FAT32 media and does not exist on FAT12 and FAT16 media. Reserved for
future expansion. Code that formats FAT32 volumes should always set all of
the bytes of this field to 0.
|
|
BS_DrvNum
|
64
|
1
|
This field has the same
definition as it does for FAT12 and FAT16 media. The only difference for
FAT32 media is that the field is at a different offset in the boot sector.
|
|
BS_Reserved1
|
65
|
1
|
This field has the same
definition as it does for FAT12 and FAT16 media. The only difference for
FAT32 media is that the field is at a different offset in the boot sector.
|
|
BS_BootSig
|
66
|
1
|
This field has the same
definition as it does for FAT12 and FAT16 media. The only difference for
FAT32 media is that the field is at a different offset in the boot sector.
|
|
BS_VolID
|
67
|
4
|
This field has the same
definition as it does for FAT12 and FAT16 media. The only difference for
FAT32 media is that the field is at a different offset in the boot sector.
|
|
BS_VolLab
|
71
|
11
|
This field has the same
definition as it does for FAT12 and FAT16 media. The only difference for
FAT32 media is that the field is at a different offset in the boot sector.
|
|
BS_FilSysType
|
82
|
8
|
Always set to the string
”FAT32 ”. Please see the note for this field in the
FAT12/FAT16 section earlier. This field has nothing to do with FAT type
determination.
|
There is one other important note about
Sector 0 of a FAT volume. If we consider the contents of the sector as a byte
array, it must be true that sector[510] equals 0x55, and sector[511] equals
0xAA.
NOTE: Many FAT documents mistakenly say that this 0xAA55 signature
occupies the “last 2 bytes of the boot sector”. This statement is correct if —
and only if — BPB_BytsPerSec is 512. If BPB_BytsPerSec is greater than 512, the offsets
of these signature bytes do not change (although it is perfectly OK for the
last two bytes at the end of the boot sector to also contain this signature).
Check your assumptions about the value in
the BPB_TotSec16/32 field. Assume we have a disk or partition of size in
sectors DskSz. If the BPB TotSec field (either BPB_TotSec16 or
BPB_TotSec32 — whichever is non-zero) is less
than or equal to DskSz, there is
nothing whatsoever wrong with the FAT volume. In fact, it is not at all unusual
to have a BPB_TotSec16/32 value that is slightly smaller than DskSz. It is also perfectly OK for the
BPB_TotSec16/32 value to be considerably smaller than DskSz.
All this means is that disk space is being
wasted. It does not by itself mean that the FAT volume is damaged in some way.
However, if BPB_TotSec16/32 is larger than
DskSz, the volume is seriously damaged or
malformed because it extends past the end of the media or overlaps data that
follows it on the disk. Treating a volume for which the BPB_TotSec16/32 value
is “too large” for the media or partition as valid can lead to catastrophic
data loss.
FAT Data Structure
The next data structure that is important
is the FAT itself. What this data structure does is define a singly linked list
of the “extents” (clusters) of a file. Note at this point that a FAT directory
or file container is nothing but a regular file that has a special attribute
indicating it is a directory. The only other special thing about a directory is
that the data or contents of the “file” is a series of 32=byte FAT directory
entries (see discussion below). In all other respects, a directory is just like
a file. The FAT maps the data region of the volume by cluster number. The first
data cluster is cluster 2.
The first sector of cluster 2 (the data
region of the disk) is computed using the BPB fields for the volume as follows.
First, we determine the count of sectors occupied by the root directory:
RootDirSectors = ((BPB_RootEntCnt * 32) +
(BPB_BytsPerSec – 1)) / BPB_BytsPerSec;
Note that on a FAT32 volume the BPB_RootEntCnt value is always 0, so on a FAT32
volume RootDirSectors is always 0. The 32
in the above is the size of one FAT directory entry in bytes. Note also
that this computation rounds up.
The start of the data region, the first
sector of cluster 2, is computed as follows:
If(BPB_FATSz16 != 0)
FATSz =
BPB_FATSz16;
Else
FATSz =
BPB_FATSz32;
FirstDataSector = BPB_ResvdSecCnt + (BPB_NumFATs *
FATSz) + RootDirSectors;
NOTE:
This sector number is relative to the first sector
of the volume that contains the BPB (the sector that contains the BPB is sector
number 0). This does not necessarily map directly onto the drive, because sector
0 of the volume is not necessarily sector 0 of the drive due to partitioning.
Given any valid data cluster number N, the sector number of the first
sector of that cluster (again relative to sector 0 of the FAT volume) is
computed as follows:
FirstSectorofCluster = ((N – 2) * BPB_SecPerClus)
+ FirstDataSector;
NOTE:
Because BPB_SecPerClus
is restricted to powers of 2 (1,2,4,8,16,32….), this means that division and
multiplication by BPB_SecPerClus can
actually be performed via SHIFT operations on 2s complement architectures that
are usually faster instructions than MULT and DIV instructions. On current
Intel X86 processors, this is largely irrelevant though because the MULT and
DIV machine instructions are heavily optimized for multiplication and division
by powers of 2.
FAT Type Determination
There is considerable confusion over
exactly how this works, which leads to many “off by 1”, “off by 2”, “off by
10”, and “massively off” errors. It is really quite simple how this works. The
FAT type—one of FAT12, FAT16, or FAT32—is determined by the count of clusters
on the volume and nothing else.
Please read everything in this section
carefully, all of the words are important. For example, note that the statement
was “count of clusters.” This is not the same thing as “maximum valid cluster
number,” because the first data cluster is 2 and not 0 or 1.
To begin, let’s discuss exactly how the
“count of clusters” value is determined. This is all done using the BPB fields
for the volume. First, we determine the count of sectors occupied by the root
directory as noted earlier.
RootDirSectors = ((BPB_RootEntCnt * 32) +
(BPB_BytsPerSec – 1)) / BPB_BytsPerSec;
Note that on a FAT32 volume, the BPB_RootEntCnt value is always 0; so on a FAT32
volume, RootDirSectors is always 0.
Next, we
determine the count of sectors in the data region of the volume:
If(BPB_FATSz16 != 0)
FATSz =
BPB_FATSz16;
Else
FATSz =
BPB_FATSz32;
If(BPB_TotSec16 != 0)
TotSec =
BPB_TotSec16;
Else
TotSec =
BPB_TotSec32;
DataSec = TotSec – (BPB_ResvdSecCnt + (BPB_NumFATs
* FATSz) + RootDirSectors);
Now we
determine the count of clusters:
CountofClusters = DataSec / BPB_SecPerClus;
Please note that this computation rounds down.
Now we can determine the FAT type. Please note carefully or you will commit an
off-by-one error!
In the following example, when it says
<, it does not mean <=. Note also that the numbers are correct. The first
number for FAT12 is 4085; the second number for FAT16 is 65525. These numbers
and the ‘<’ signs are not wrong.
If(CountofClusters < 4085) {
/* Volume is FAT12 */
} else if(CountofClusters < 65525) {
/*
Volume is FAT16 */
} else {
/*
Volume is FAT32 */
}
This is the one and only way that FAT type
is determined. There is no such thing as a FAT12 volume that has more than 4084
clusters. There is no such thing as a FAT16 volume that has less than 4085
clusters or more than 65,524 clusters. There is no such thing as a FAT32 volume
that has less than 65,525 clusters. If you try to make a FAT volume that
violates this rule, Microsoft operating systems will not handle them correctly
because they will think the volume has a different type of FAT than what you
think it does.
NOTE:
As is noted numerous times earlier, the world is
full of FAT code that is wrong. There is a lot of FAT type code that is off by
1 or 2 or 8 or 10 or 16. For this reason, it is highly recommended that if you
are formatting a FAT volume which has maximum compatibility with all existing
FAT code, then you should you avoid making volumes of any type that have close
to 4,085 or 65,525 clusters. Stay at least 16 clusters on each side away from
these cut-over cluster counts.
Note also that the CountofClusters value is exactly that—the count of data clusters starting at cluster 2. The maximum valid
cluster number for the volume is CountofClusters
+ 1, and the “count of clusters including the two reserved clusters” is CountofClusters + 2.
There is one more important computation
related to the FAT. Given any valid cluster number N, where in the FAT(s) is the entry for that cluster number? The
only FAT type for which this is complex is FAT12. For FAT16 and FAT32, the
computation is simple:
If(BPB_FATSz16 != 0)
FATSz =
BPB_FATSz16;
Else
FATSz =
BPB_FATSz32;
If(FATType == FAT16)
FATOffset = N * 2;
Else if (FATType == FAT32)
FATOffset = N * 4;
ThisFATSecNum = BPB_ResvdSecCnt + (FATOffset /
BPB_BytsPerSec);
ThisFATEntOffset = REM(FATOffset /
BPB_BytsPerSec);
REM(…) is the remainder operator. That
means the remainder after division of FATOffset
by BPB_BytsPerSec. ThisFATSecNum is the sector number of the FAT
sector that contains the entry for cluster N in the first FAT. If you want the
sector number in the second FAT, you add FATSz
to ThisFATSecNum; for the third FAT, you
add 2*FATSz, and so on.
You now read sector number ThisFATSecNum (remember this is a sector number
relative to sector 0 of the FAT volume). Assume this is read into an 8-bit byte
array named SecBuff. Also assume that the
type WORD is a 16-bit unsigned and that the type DWORD is a 32-bit unsigned.
If(FATType == FAT16)
FAT16ClusEntryVal = *((WORD *) &SecBuff[ThisFATEntOffset]);
Else
FAT32ClusEntryVal = (*((DWORD *) &SecBuff[ThisFATEntOffset])) &
0x0FFFFFFF;
Fetches the contents of that cluster. To
set the contents of this same cluster you do the following:
If(FATType == FAT16)
*((WORD
*) &SecBuff[ThisFATEntOffset]) = FAT16ClusEntryVal;
Else {
FAT32ClusEntryVal = FAT32ClusEntryVal & 0x0FFFFFFF;
*((DWORD
*) &SecBuff[ThisFATEntOffset]) =
(*((DWORD *) &SecBuff[ThisFATEntOffset])) & 0xF0000000;
*((DWORD
*) &SecBuff[ThisFATEntOffset]) =
(*((DWORD *) &SecBuff[ThisFATEntOffset])) | FAT32ClusEntryVal;
}
Note how the FAT32 code above works. A
FAT32 FAT entry is actually only a 28-bit entry. The high 4 bits of a FAT32 FAT
entry are reserved. The only time that the high 4 bits of FAT32 FAT entries
should ever be changed is when the volume is formatted, at which time the whole
32-bit FAT entry should be zeroed, including the high 4 bits.
A bit more explanation is in order here,
because this point about FAT32 FAT entries seems to cause a great deal of
confusion. Basically 32-bit FAT entries are not really 32-bit values; they are
only 28-bit values. For example, all of these 32-bit cluster entry values:
0x10000000, 0xF0000000, and 0x00000000 all indicate that the cluster is FREE,
because you ignore the high 4 bits when you read the cluster entry value. If
the 32-bit free cluster value is currently 0x30000000 and you want to mark this
cluster as bad by storing the value 0x0FFFFFF7 in it. Then the 32-bit entry
will contain the value 0x3FFFFFF7 when you are done, because you must preserve
the high 4 bits when you write in the 0x0FFFFFF7 bad cluster mark.
Take note that because the BPB_BytsPerSec value is always divisible by 2
and 4, you never have to worry about a FAT16 or FAT32 FAT entry spanning over a
sector boundary (this is not true of FAT12).
The code for FAT12 is more complicated
because there are 1.5 bytes (12-bits) per FAT entry.
if
(FATType == FAT12)
FATOffset = N + (N / 2);
/* Multiply by 1.5 without using floating point,
the divide by 2 rounds DOWN */
ThisFATSecNum = BPB_ResvdSecCnt + (FATOffset / BPB_BytsPerSec);
ThisFATEntOffset = REM(FATOffset / BPB_BytsPerSec);
We now have to check for the sector
boundary case:
If(ThisFATEntOffset == (BPB_BytsPerSec – 1)) {
/* This
cluster access spans a sector boundary in the FAT */
/* There
are a number of strategies to handling this. The */
/*
easiest is to always load FAT sectors into memory */
/* in
pairs if the volume is FAT12 (if you want to load */
/* FAT
sector N, you also load FAT sector N+1 immediately */
/*
following it in memory unless sector N is the last FAT */
/*
sector). It is assumed that this is the strategy used here */
/* which
makes this if test for a sector boundary span */
/*
unnecessary.
*/
}
We now access the FAT entry as a WORD just
as we do for FAT16, but if the cluster number is EVEN, we only want the low
12-bits of the 16-bits we fetch; and if the cluster number is ODD, we only want
the high 12-bits of the 16-bits we fetch.
FAT12ClusEntryVal = *((WORD *)
&SecBuff[ThisFATEntOffset]);
If(N &
0x0001)
FAT12ClusEntryVal = FAT12ClusEntryVal >> 4; /* Cluster number is ODD */
Else
FAT12ClusEntryVal = FAT12ClusEntryVal & 0x0FFF; /* Cluster number is EVEN */
Fetches the contents of that cluster. To
set the contents of this same cluster you do the following:
If(N & 0x0001) {
FAT12ClusEntryVal = FAT12ClusEntryVal << 4; /* Cluster number is ODD */
*((WORD
*) &SecBuff[ThisFATEntOffset]) =
(*((WORD *) &SecBuff[ThisFATEntOffset])) & 0x000F;
} Else {
FAT12ClusEntryVal = FAT12ClusEntryVal & 0x0FFF; /* Cluster number is EVEN */
*((WORD
*) &SecBuff[ThisFATEntOffset]) =
(*((WORD *) &SecBuff[ThisFATEntOffset])) & 0xF000;
}
*((WORD *) &SecBuff[ThisFATEntOffset]) =
(*((WORD
*) &SecBuff[ThisFATEntOffset])) | FAT12ClusEntryVal;
NOTE:
It is assumed that the >> operator shifts a
bit value of 0 into the high 4 bits and that the << operator shifts a bit
value of 0 into the low 4 bits.
The way the data of a file is associated
with the file is as follows. In the directory entry, the cluster number of the
first cluster of the file is recorded. The first cluster (extent) of the file
is the data associated with this first cluster number, and the location of that
data on the volume is computed from the cluster number as described earlier
(computation of FirstSectorofCluster).
Note that a zero-length file—a file that
has no data allocated to it—has a first cluster number of 0 placed in its
directory entry. This cluster location in the FAT (see earlier computation of ThisFATSecNum and ThisFATEntOffset) contains either an EOC mark (End Of Clusterchain) or the cluster number of the next
cluster of the file. The EOC value is FAT type dependant (assume FATContent is the contents of the cluster entry
in the FAT being checked to see whether it is an EOC mark):
IsEOF = FALSE;
If(FATType == FAT12) {
If(FATContent >= 0x0FF8)
IsEOF = TRUE;
} else if(FATType == FAT16) {
If(FATContent >= 0xFFF8)
IsEOF = TRUE;
} else if (FATType == FAT32) {
If(FATContent >= 0x0FFFFFF8)
IsEOF = TRUE;
}
Note that the cluster number whose cluster
entry in the FAT contains the EOC mark is allocated to the file and is also the
last cluster allocated to the file. Microsoft operating system FAT drivers use
the EOC value 0x0FFF for FAT12, 0xFFFF for FAT16, and 0x0FFFFFFF for FAT32 when
they set the contents of a cluster to the EOC mark. There are various disk
utilities for Microsoft operating systems that use a different value, however.
There is also a special “BAD CLUSTER” mark.
Any cluster that contains the “BAD CLUSTER” value in its FAT entry is a cluster
that should not be placed on the free list because it is prone to disk errors.
The “BAD CLUSTER” value is 0x0FF7 for FAT12, 0xFFF7 for FAT16, and 0x0FFFFFF7
for FAT32. The other relevant note here is that these bad clusters are also
lost clusters—clusters that appear to be allocated because they contain a
non-zero value but which are not part of any files allocation chain. Disk repair
utilities must recognize lost clusters that contain this special value as bad
clusters and not change the content of the cluster entry.
NOTE: It is not possible for the bad cluster mark to be an allocatable
cluster number on FAT12 and FAT16 volumes, but it is feasible for 0x0FFFFFF7 to
be an allocatable cluster number on FAT32 volumes. To avoid possible confusion
by disk utilities, no FAT32 volume should ever be configured such that
0x0FFFFFF7 is an allocatable cluster number.
The list of free clusters in the FAT is
nothing more than the list of all clusters that contain the value 0 in their
FAT cluster entry. Note that this value must be fetched as described earlier as
for any other FAT entry that is not free. This list of free clusters is not stored
anywhere on the volume; it must be computed when the volume is mounted by
scanning the FAT for entries that contain the value 0. On FAT32 volumes, the BPB_FSInfo sector may contain a valid count of free clusters on the volume. See the
documentation of the FAT32 FSInfo sector.
What are the two reserved clusters at the
start of the FAT for? The first reserved cluster, FAT[0], contains the BPB_Media byte value in its low 8 bits, and all
other bits are set to 1. For example, if the BPB_Media
value is 0xF8, for FAT12 FAT[0] = 0x0FF8, for FAT16 FAT[0] = 0xFFF8, and for
FAT32 FAT[0] = 0x0FFFFFF8. The second reserved cluster, FAT[1], is set by
FORMAT to the EOC mark. On FAT12 volumes, it is not used and is simply always
contains an EOC mark. For FAT16 and FAT32, the file system driver may use the
high two bits of the FAT[1] entry for dirty volume flags (all other bits, are
always left set to 1). Note that the bit location is different for FAT16 and
FAT32, because they are the high 2 bits of the entry.
For FAT16:
ClnShutBitMask = 0x8000;
HrdErrBitMask = 0x4000;
For FAT32:
ClnShutBitMask = 0x08000000;
HrdErrBitMask = 0x04000000;
Bit ClnShutBitMask
– If bit is 1, volume is “clean”.
If bit is 0, volume is “dirty”. This indicates that the file system driver did not Dismount the volume properly the last time it had the volume mounted. It would be a good idea to run a Chkdsk/Scandisk disk repair utility on it, because it may be damaged.
If bit is 0, volume is “dirty”. This indicates that the file system driver did not Dismount the volume properly the last time it had the volume mounted. It would be a good idea to run a Chkdsk/Scandisk disk repair utility on it, because it may be damaged.
Bit HrdErrBitMask
– If this bit is 1, no disk
read/write errors were encountered.
If this bit is 0, the file system driver encountered a disk I/O error on the Volume the last time it was mounted, which is an indicator that some sectors may have gone bad on the volume. It would be a good idea to run a Chkdsk/Scandisk disk repair utility that does surface analysis on it to look for new bad sectors.
If this bit is 0, the file system driver encountered a disk I/O error on the Volume the last time it was mounted, which is an indicator that some sectors may have gone bad on the volume. It would be a good idea to run a Chkdsk/Scandisk disk repair utility that does surface analysis on it to look for new bad sectors.
Here are two more important notes about the
FAT region of a FAT volume:
- The last sector of the FAT is not necessarily all part of the
FAT. The FAT stops at the cluster number in the last FAT sector that
corresponds to the entry for cluster number CountofClusters + 1 (see the CountofClusters
computation earlier), and this entry is not necessarily at the end of the
last FAT sector. FAT code should not make any assumptions about what the
contents of the last FAT sector are after the CountofClusters + 1 entry. FAT format code should zero the
bytes after this entry though.
- The BPB_FATSz16 (BPB_FATSz32 for FAT32 volumes) value may be bigger than it needs to be.
In other words, there may be totally unused FAT sectors at the end of each
FAT in the FAT region of the volume. For this reason, the last sector of
the FAT is always computed using the CountofClusters
+ 1 value, never from the BPB_FATSz16/32 value. FAT code should not make
any assumptions about what the contents of these “extra” FAT sectors are.
FAT format code should zero the contents of these extra FAT sectors
though.
FAT Volume Initialization
At this point, the careful reader should
have one very interesting question. Given that the FAT type (FAT12, FAT16, or
FAT32) is dependant on the number of clusters—and that the sectors available in
the data area of a FAT volume is dependant on the size of the FAT—when handed
an unformatted volume that does not yet have a BPB, how do you determine all
this and compute the proper values to put in BPB_SecPerClus
and either BPB_FATSz16 or BPB_FATSz32? The way Microsoft operating systems do
this is with a fixed value, several tables, and a clever piece of arithmetic.
Microsoft operating systems only do FAT12
on floppy disks. Because there is a limited number of floppy formats that all
have a fixed size, this is done with a simple table:
“If it is a
floppy of this type, then the BPB looks like this.”
There is no dynamic computation for FAT12.
For the FAT12 formats, all the computation for BPB_SecPerClus
and BPB_FATSz16 was worked out by hand on a piece of paper and recorded in the
table (being careful of course that the resultant cluster count was always less
than 4085). If your media is larger than 4 MB, do not bother with FAT12. Use
smaller BPB_SecPerClus values so that the
volume will be FAT16.
The rest of this section is totally
specific to drives that have 512 bytes per sector. You cannot use these tables,
or the clever arithmetic, with drives that have a different sector size. The
“fixed value” is simply a volume size that is the “FAT16 to FAT32 cutover
value”. Any volume size smaller than this is FAT16 and any volume of this size
or larger is FAT32. For Windows, this value is 512 MB. Any FAT volume smaller
than 512 MB is FAT16, and any FAT volume of 512 MB or larger is FAT32.
Please don’t draw an incorrect conclusion
here.
There are many FAT16 volumes out there that
are larger than 512 MB. There are various ways to force the format to be FAT16
rather than the default of FAT32, and there is a great deal of code that
implements different limits. All we are talking about here is the default cutover value for MS-DOS and
Windows on volumes that have not yet been formatted. There are two tables—one
is for FAT16 and the other is for FAT32. An entry in these tables is selected
based on the size of the volume in 512 byte sectors (the value that will go in
BPB_TotSec16 or BPB_TotSec32), and the value that this table sets is the BPB_SecPerClus value.
struct
DSKSZTOSECPERCLUS {
DWORD DiskSize;
BYTE SecPerClusVal;
};
/*
*This is the table for FAT16 drives. NOTE that
this table includes
* entries for disk sizes larger than 512 MB even
though typically
* only the entries for disks < 512 MB in size
are used.
* The way this table is accessed is to look for
the first entry
* in the table for which the disk size is less
than or equal
* to the DiskSize field in that table entry. For this table to
* work properly BPB_RsvdSecCnt must be 1,
BPB_NumFATs
* must be 2, and BPB_RootEntCnt must be 512. Any
of these values
* being different may require the first table
entries DiskSize value
* to be changed
otherwise the cluster count may be to low for FAT16.
*/
DSKSZTOSECPERCLUS DskTableFAT16 [] = {
{
8400, 0}, /* disks up to 4.1 MB, the 0 value for SecPerClusVal trips
an error */
{ 32680, 2},
/* disks up to 16 MB, 1k cluster */
{ 262144, 4},
/* disks up to 128 MB, 2k cluster
*/
{ 524288,
8}, /* disks up to 256 MB, 4k cluster */
{
1048576, 16}, /* disks up to 512 MB, 8k cluster */
/*
The entries after this point are not used unless FAT16 is forced */
{
2097152, 32}, /* disks up to 1 GB, 16k cluster */
{
4194304, 64}, /* disks up to 2 GB, 32k cluster */
{ 0xFFFFFFFF, 0} /* any disk greater
than 2GB, 0 value for SecPerClusVal trips an error */
};
/*
* This is the table for FAT32 drives. NOTE that
this table includes
* entries for disk sizes smaller than 512 MB even
though typically
* only the entries for disks >= 512 MB in size
are used.
* The way this table is accessed is to look for
the first entry
* in the table for which the disk size is less
than or equal
* to the DiskSize field in that table entry. For
this table to
* work properly BPB_RsvdSecCnt must be 32, and
BPB_NumFATs
* must be 2. Any of these values being different
may require the first
* table entries DiskSize value to be changed
otherwise the cluster count
* may be to low for FAT32.
*/
DSKSZTOSECPERCLUS DskTableFAT32 [] = {
{
66600, 0}, /* disks up to 32.5 MB, the 0 value for
SecPerClusVal trips an error */
{ 532480, 1},
/* disks up to 260 MB, .5k cluster
*/
{
16777216, 8}, /* disks up to 8 GB,
4k cluster */
{
33554432, 16}, /* disks up to 16 GB,
8k cluster */
{
67108864, 32}, /* disks up to 32 GB,
16k cluster */
{
0xFFFFFFFF, 64}/* disks greater than 32GB, 32k cluster */
};
So given a disk size and a FAT type of
FAT16 or FAT32, we now have a BPB_SecPerClus
value. The only thing we have left is do is to compute how many sectors the FAT
takes up so that we can set BPB_FATSz16 or BPB_FATSz32. Note that at this point
we assume that BPB_RootEntCnt, BPB_RsvdSecCnt, and BPB_NumFATs are appropriately set. We also assume that DskSize is the size of the volume that we are
either going to put in BPB_TotSec32 or BPB_TotSec16.
RootDirSectors = ((BPB_RootEntCnt * 32) +
(BPB_BytsPerSec – 1)) / BPB_BytsPerSec;
TmpVal1 = DskSize – (BPB_ResvdSecCnt +
RootDirSectors);
TmpVal2 = (256 * BPB_SecPerClus) + BPB_NumFATs;
If(FATType == FAT32)
TmpVal2
= TmpVal2 / 2;
FATSz = (TMPVal1 + (TmpVal2 – 1)) / TmpVal2;
If(FATType == FAT32) {
BPB_FATSz16 = 0;
BPB_FATSz32 = FATSz;
} else {
BPB_FATSz16 = LOWORD(FATSz);
/* there
is no BPB_FATSz32 in a FAT16 BPB */
}
Do not spend too much time trying to figure
out why this math works. The basis for the computation is complicated; the
important point is that this is how Microsoft operating systems do it, and it
works. Note, however, that this math does not work perfectly. It will
occasionally set a FATSz that is up to
2 sectors too large for FAT16, and occasionally up to 8 sectors too large
for FAT32. It will never compute a FATSz
value that is too small, however. Because it is OK to have a FATSz that is too large, at the expense of
wasting a few sectors, the fact that this computation is surprisingly simple
more than makes up for it being off in a safe way in some cases.
FAT32 FSInfo Sector Structure
and Backup Boot Sector
On
a FAT32 volume, the FAT can be a large data structure, unlike on FAT16 where it
is limited to a maximum of 128K worth of sectors and FAT12 where it is limited
to a maximum of 6K worth of sectors. For this reason, a provision is made to
store the “last known” free cluster count on the FAT32 volume so that it does
not have to be computed as soon as an API call is made to ask how much free space
there is on the volume (like at the end of a directory listing). The FSInfo sector number is the value in the BPB_FSInfo field; for Microsoft operating
systems it is always set to 1. Here is the structure of the FSInfo sector:
FAT32 FSInfo Sector Structure and Backup Boot Sector
|
Name
|
Offset (byte)
|
Size (bytes)
|
Description
|
|
FSI_LeadSig
|
0
|
4
|
Value 0x41615252. This lead signature is used to validate that this
is in fact an FSInfo sector.
|
|
FSI_Reserved1
|
4
|
480
|
This field is currently reserved for future expansion. FAT32 format
code should always initialize all bytes of this field to 0. Bytes in this
field must currently never be used.
|
|
FSI_StrucSig
|
484
|
4
|
Value 0x61417272. Another signature that is more localized in the
sector to the location of the fields that are used.
|
|
FSI_Free_Count
|
488
|
4
|
Contains the last known free cluster count on the volume. If the
value is 0xFFFFFFFF, then the free count is unknown and must be computed. Any
other value can be used, but is not necessarily correct. It should be range
checked at least to make sure it is <= volume cluster count.
|
|
FSI_Nxt_Free
|
492
|
4
|
This is a hint for the FAT driver. It indicates the cluster number at
which the driver should start looking for free clusters. Because a FAT32 FAT
is large, it can be rather time consuming if there are a lot of allocated
clusters at the start of the FAT and the driver starts looking for a free
cluster starting at cluster 2. Typically this value is set to the last
cluster number that the driver allocated. If the value is 0xFFFFFFFF, then
there is no hint and the driver should start looking at cluster 2. Any other
value can be used, but should be checked first to make sure it is a valid
cluster number for the volume.
|
|
FSI_Reserved2
|
496
|
12
|
This field is currently reserved for future expansion. FAT32 format
code should always initialize all bytes of this field to 0. Bytes in this
field must currently never be used.
|
|
FSI_TrailSig
|
508
|
4
|
Value 0xAA550000. This trail signature is used to validate that this
is in fact an FSInfo sector. Note that
the high 2 bytes of this value—which go into the bytes at offsets 510 and
511—match the signature bytes used at the same offsets in sector 0.
|
Another
feature on FAT32 volumes that is not present on FAT16/FAT12 is the BPB_BkBootSec field. FAT16/FAT12 volumes can be
totally lost if the contents of sector 0 of the volume are overwritten or
sector 0 goes bad and cannot be read. This is a “single point of failure” for
FAT16 and FAT12 volumes. The BPB_BkBootSec
field reduces the severity of this problem for FAT32 volumes, because starting
at that sector number on the volume—6—there is a backup copy of the boot sector
information including the volume’s BPB.
In
the case where the sector 0 information has been accidentally overwritten, all
a disk repair utility has to do is restore the boot sector(s) from the backup
copy. In the case where sector 0 goes bad, this allows the volume to be mounted
so that the user can access data before replacing the disk.
This
second case—sector 0 goes bad—is the reason why no value other than 6 should
ever be placed in the BPB_BkBootSec
field. If sector 0 is unreadable, various operating systems are “hard wired” to
check for backup boot sector(s) starting at sector 6 of the FAT32 volume. Note
that starting at the BPB_BkBootSec sector
is a complete boot record. The
Microsoft FAT32 “boot sector” is actually three 512-byte sectors long. There is
a copy of all three of these sectors starting at the BPB_BkBootSec sector. A copy of the FSInfo
sector is also there, even though the BPB_FSInfo
field in this backup boot sector is set to the same value as is stored in the
sector 0 BPB.
NOTE: All 3 of these sectors have the 0xAA55 signature in sector offsets
510 and 511, just like the first boot sector does (see the earlier discussion
at the end of the BPB structure description).
FAT Directory Structure
We will first talk about short directory
entries and ignore long directory entries for the moment.
A FAT directory is nothing but a “file”
composed of a linear list of 32-byte structures. The only special directory,
which must always be present, is the root directory. For FAT12 and FAT16 media,
the root directory is located in a fixed location on the disk immediately
following the last FAT and is of a fixed size in sectors computed from the BPB_RootEntCnt value (see computations for RootDirSectors earlier in this document). For
FAT12 and FAT16 media, the first sector of the root directory is sector number
relative to the first sector of the FAT volume:
FirstRootDirSecNum = BPB_ResvdSecCnt +
(BPB_NumFATs * BPB_FATSz16);
For FAT32, the root directory can be of
variable size and is a cluster chain, just like any other directory is. The
first cluster of the root directory on a FAT32 volume is stored in BPB_RootClus. Unlike other directories, the
root directory itself on any FAT type does not have any date or time stamps,
does not have a file name (other than the implied file name “\”), and does not
contain “.” and “..” files as the first two directory entries in the directory.
The only other special aspect of the root directory is that it is the only
directory on the FAT volume for which it is valid to have a file that has only
the ATTR_VOLUME_ID attribute bit set (see below).
FAT 32 Byte Directory Entry Structure
|
Name
|
Offset (byte)
|
Size (bytes)
|
Description
|
|
DIR_Name
|
0
|
11
|
Short name.
|
|
DIR_Attr
|
11
|
1
|
File attributes:
ATTR_READ_ONLY 0x01
ATTR_HIDDEN 0x02
ATTR_SYSTEM 0x04
ATTR_VOLUME_ID 0x08
ATTR_DIRECTORY 0x10
ATTR_ARCHIVE 0x20
ATTR_LONG_NAME ATTR_READ_ONLY | ATTR_HIDDEN | ATTR_SYSTEM | ATTR_VOLUME_ID
The upper two bits of the
attribute byte are reserved and should always be set to 0 when a file is
created and never modified or looked at after that.
|
|
DIR_NTRes
|
12
|
1
|
Reserved for use by Windows
NT. Set value to 0 when a file is created and never modify or look at it
after that.
|
|
DIR_CrtTimeTenth
|
13
|
1
|
Millisecond stamp at file
creation time. This field actually contains a count of tenths of a second.
The granularity of the seconds part of DIR_CrtTime
is 2 seconds so this field is a count of tenths of a second and its valid
value range is 0-199 inclusive.
|
|
DIR_CrtTime
|
14
|
2
|
Time
file was created.
|
|
DIR_CrtDate
|
16
|
2
|
Date
file was created.
|
|
DIR_LstAccDate
|
18
|
2
|
Last
access date. Note that there is no last access time, only a date. This is the
date of last read or write. In the case of a write, this should be set to the
same date as DIR_WrtDate.
|
|
DIR_FstClusHI
|
20
|
2
|
High word of this entry’s
first cluster number (always 0 for a FAT12 or FAT16 volume).
|
|
DIR_WrtTime
|
22
|
2
|
Time of last write. Note
that file creation is considered a write.
|
|
DIR_WrtDate
|
24
|
2
|
Date of last write. Note
that file creation is considered a write.
|
|
DIR_FstClusLO
|
26
|
2
|
Low word of this entry’s
first cluster number.
|
|
DIR_FileSize
|
28
|
4
|
32-bit DWORD holding this
file’s size in bytes.
|
DIR_Name[0]
Special notes about the first byte (DIR_Name[0]) of a FAT directory entry:
·
If DIR_Name[0] == 0xE5, then the directory entry is free (there is no
file or directory name in this entry).
·
If DIR_Name[0] == 0x00, then the directory entry is free (same as for
0xE5), and there are no allocated directory entries after this one (all of the DIR_Name[0] bytes in all of the entries after
this one are also set to 0).
The special 0
value, rather than the 0xE5 value, indicates to FAT file system driver code
that the rest of the entries in this directory do not need to be examined
because they are all free.
·
If DIR_Name[0] == 0x05, then the actual file name character for this
byte is 0xE5. 0xE5 is actually a valid KANJI lead byte value for the character
set used in Japan. The special 0x05 value is used so that this special file
name case for Japan can be handled properly and not cause FAT file system code
to think that the entry is free.
The DIR_Name
field is actually broken into two parts+ the 8-character main part of the name,
and the 3-character extension. These two parts are “trailing space padded” with
bytes of 0x20.
DIR_Name[0] may not equal 0x20. There is an implied ‘.’ character between
the main part of the name and the extension part of the name that is not
present in DIR_Name. Lower case
characters are not allowed in DIR_Name
(what these characters are is country specific).
The following characters are not legal in
any bytes of DIR_Name:
·
Values less than 0x20 except
for the special case of 0x05 in DIR_Name[0]
described above.
·
0x22, 0x2A, 0x2B, 0x2C, 0x2E,
0x2F, 0x3A, 0x3B, 0x3C, 0x3D, 0x3E, 0x3F, 0x5B, 0x5C, 0x5D, and 0x7C.
Here are some examples of how a
user-entered name maps into DIR_Name:
“foo.bar” -> “FOO BAR”
“FOO.BAR” -> “FOO BAR”
“Foo.Bar” -> “FOO BAR”
“foo” -> “FOO “
“foo.” -> “FOO “
“PICKLE.A” -> “PICKLE A “
“prettybg.big” -> “PRETTYBGBIG”
“.big” -> illegal, DIR_Name[0]
cannot be 0x20
DIR_Attr specifies attributes of the file:
ATTR_READ_ONLY Indicates
that writes to the file should fail.
ATTR_HIDDEN Indicates
that normal directory listings should not show this file.
ATTR_SYSTEM Indicates
that this is an operating system file.
ATTR_VOLUME_ID There should
only be one “file” on the volume that has this attribute set, and that file
must be in the root directory. This name of this file is actually the label for
the volume. DIR_FstClusHI and DIR_FstClusLO must always be 0 for the volume
label (no data clusters are allocated to the volume label file).
ATTR_DIRECTORY Indicates
that this file is actually a container for other files.
ATTR_ARCHIVE This attribute supports backup
utilities. This bit is set by the FAT file system driver when a file is
created, renamed, or written to. Backup utilities may use this attribute to
indicate which files on the volume have been modified since the last time that
a backup was performed.
Note that the ATTR_LONG_NAME attribute bit
combination indicates that the “file” is actually part of the long name entry
for some other file. See the next section for more information on this
attribute combination.
When a directory is created, a file with
the ATTR_DIRECTORY bit set in its DIR_Attr
field, you set its DIR_FileSize to 0. DIR_FileSize is not used and is always 0 on a
file with the ATTR_DIRECTORY attribute (directories are sized by simply
following their cluster chains to the EOC mark). One cluster is allocated to
the directory (unless it is the root directory on a FAT16/FAT12 volume), and
you set DIR_FstClusLO and DIR_FstClusHI to that cluster number and place
an EOC mark in that clusters entry in the FAT. Next, you initialize all bytes
of that cluster to 0. If the directory is the root directory, you are done
(there are no dot or dotdot entries in the root directory). If
the directory is not the root directory, you need to create two special entries
in the first two 32-byte directory entries of the directory (the first two 32
byte entries in the data region of the cluster you just allocated).
The first directory entry has DIR_Name set to:
“.
”
The second has DIR_Name set to:
“..
”
These are called the dot and dotdot entries. The DIR_FileSize field on both entries is set to 0, and all of the date
and time fields in both of these entries are set to the same values as they
were in the directory entry for the directory that you just created. You now
set DIR_FstClusLO and DIR_FstClusHI for the dot entry (the first entry) to the same values you put in those
fields for the directories directory entry (the cluster number of the cluster
that contains the dot and dotdot
entries).
Finally, you set DIR_FstClusLO and DIR_FstClusHI
for the dotdot
entry (the second entry) to the first cluster number of the directory in which
you just created the directory (value is 0 if this directory is the root
directory even for FAT32 volumes).
Here is the summary for the dot and dotdot entries:
·
The dot entry is a directory that points to itself.
·
The dotdot entry points to the
starting cluster of the parent of this directory (which is 0 if this
directories parent is the root directory).
Date and Time Formats
Many FAT file systems do not support
Date/Time other than DIR_WrtTime and DIR_WrtDate. For this reason, DIR_CrtTimeMil, DIR_CrtTime,
DIR_CrtDate, and DIR_LstAccDate are actually optional fields. DIR_WrtTime and
DIR_WrtDate must be supported,
however. If the other date and time fields are not supported, they should be
set to 0 on file create and ignored on other file operations.
Date
Format. A FAT directory entry date stamp is a
16-bit field that is basically a date relative to the MS-DOS epoch of
01/01/1980. Here is the format (bit 0 is the LSB of the 16-bit word, bit 15 is
the MSB of the 16-bit word):
Bits 0–4: Day of
month, valid value range 1-31 inclusive.
Bits 5–8: Month
of year, 1 = January, valid value range 1–12 inclusive.
Bits 9–15: Count
of years from 1980, valid value range 0–127 inclusive (1980–2107).
Time
Format. A FAT directory entry time stamp is a
16-bit field that has a granularity of 2 seconds. Here is the format (bit 0 is
the LSB of the 16-bit word, bit 15 is the MSB of the 16-bit word).
Bits 0–4:
2-second count, valid value range 0–29 inclusive (0 – 58 seconds).
Bits 5–10:
Minutes, valid value range 0–59 inclusive.
Bits 11–15:
Hours, valid value range 0–23 inclusive.
The valid time range is from Midnight
00:00:00 to 23:59:58.
FAT Long Directory Entries
In adding long directory entries to the FAT
file system it was crucial that their addition to the FAT file system's
existing design:
Be essentially transparent
on earlier versions of MS-DOS. The
primary goal being that existing MS-DOS APIs on previous versions of
MS-DOS/Windows do not easily "find" long directory entries. The only MS-DOS APIs that can
"find" long directory entries are the FCB-based-find APIs when used
with a full meta-character matching pattern (i.e. *.*) and full attribute
matching bits (i.e. matching attributes are FFh). On post-Windows 95 versions of MS-DOS/Windows,
no MS-DOS API can accidentally "find" a single long directory entry.
Be located in close
physical proximity, on the media, to the short directory entries they are
associated with. As will be evident,
long directory entries are immediately contiguous to the short directory entry
they are associated with and their existence imposes an unnoticeable
performance impact on the file system.
If detected by disk
maintenance utilities, they do not jeopardize the integrity of existing file
data. Disk maintenance utilities
typically do not use MS-DOS APIs to access on-media file-system-specific data
structures. Rather they read physical or
logical sector information from the disk and judge for themselves what the
directory entries contain. Based on the
heuristics employed in the utilities, the utility may take various steps to
"repair" what it perceives to be "damaged" file-system-specific
data structures. Long directory entries
were added to the FAT file system in such a way as to not cause the loss of
file data if a disk containing long directory entries was "repaired"
by a pre-Windows 95-compatible disk utility on a previous version of
MS-DOS/Windows.
In order to meet the goals of
locality-of-access and transparency, the long directory entry is defined as a
short directory entry with a special attribute.
As described previously, a long directory entry is just a regular
directory entry in which the attribute field has a value of:
ATTR_LONG_NAME ATTR_READ_ONLY |
ATTR_HIDDEN |
ATTR_SYSTEM |
ATTR_VOLUME_ID
ATTR_HIDDEN |
ATTR_SYSTEM |
ATTR_VOLUME_ID
A mask for determining whether an entry is
a long-name sub-component should also be defined:
ATTR_LONG_NAME_MASK ATTR_READ_ONLY |
ATTR_HIDDEN |
ATTR_SYSTEM |
ATTR_VOLUME_ID |
ATTR_DIRECTORY |
ATTR_ARCHIVE
ATTR_HIDDEN |
ATTR_SYSTEM |
ATTR_VOLUME_ID |
ATTR_DIRECTORY |
ATTR_ARCHIVE
When such a directory entry is encountered
it is given special treatment by the file system. It is treated as part of a set of directory
entries that are associated with a single short directory entry. Each long directory
entry has the following structure:
FAT Long Directory Entry Structure
|
Name
|
Offset
(byte)
|
Size
(bytes)
|
Description
|
|
LDIR_Ord
|
0
|
1
|
The order of this entry in the sequence of long dir entries
associated with the short dir entry at the end of the long dir set.
If masked with 0x40 (LAST_LONG_ENTRY), this indicates the entry is the last long dir entry in a set of long dir entries. All valid sets of long dir entries must begin with an entry having this mask. |
|
LDIR_Name1
|
1
|
10
|
Characters 1-5 of the long-name sub-component in this dir entry.
|
|
LDIR_Attr
|
11
|
1
|
Attributes - must be ATTR_LONG_NAME
|
|
LDIR_Type
|
12
|
1
|
If zero, indicates a directory entry that is
a sub-component of a long name. NOTE:
Other values reserved for future extensions.
Non-zero implies other dirent types.
|
|
LDIR_Chksum
|
13
|
1
|
Checksum of name in the short dir entry at the end of the long dir
set.
|
|
LDIR_Name2
|
14
|
12
|
Characters 6-11 of the long-name sub-component in this dir entry.
|
|
LDIR_FstClusLO
|
26
|
2
|
Must be ZERO. This is an artifact of the FAT "first cluster"
and must be zero for compatibility with existing disk utilities. It's meaningless in the context of a long
dir entry.
|
|
LDIR_Name3
|
28
|
4
|
Characters 12-13 of the long-name sub-component in this dir entry.
|
Organization and Association of Short & Long Directory
Entries
A set of long entries is always associated
with a short entry that they always immediately precede. Long entries are paired with short entries
for one reason: only short directory entries are visible to previous versions
of MS-DOS/Windows. Without a short entry
to accompany it, a long directory entry would be completely invisible on
previous versions of MS-DOS/Windows. A
long entry never legally exists all by itself.
If long entries are found without being paired with a valid short entry,
they are termed orphans. The following figure depicts a set of n long
directory entries associated with it's single short entry.
Long entries always immediately precede and
are physically contiguous with, the short entry they are associated with. The file system makes a few other checks to
ensure that a set of long entries is actually associated with a short entry.
Sequence Of Long
Directory Entries
|
Entry
|
Ordinal
|
|
Nth Long entry
|
LAST_LONG_ENTRY (0x40) | N
|
|
… Additional Long Entries
|
…
|
|
1st Long entry
|
1
|
|
Short Entry Associated With Preceding Long Entries
|
(not applicable)
|
First, every member of a set of long
entries is uniquely numbered and the last member of the set is or'd with a flag
indicating that it is, in fact, the last member of the set. The LDIR_Ord field is used to make this
determination. The first member of a set
has an LDIR_Ord value of one. The nth
long member of the set has a value of (n OR LAST_LONG_ENTRY). Note that the LDIR_Ord field cannot have
values of 0xE5 or 0x00. These values have
always been used by the file system to indicate a "free" directory
entry, or the "last" directory entry in a cluster. Values for LDIR_Ord do not take on these two
values over their range. Values for
LDIR_Ord must run from 1 to (n OR LAST_LONG_ENTRY). If they do not, the long entries are
"damaged" and are treated as orphans by the file system.
Second, an 8-bit checksum is computed on
the name contained in the short directory entry at the time the short and long
directory entries are created. All 11
characters of the name in the short entry are used in the checksum
calculation. The check sum is placed in
every long entry. If any of the check
sums in the set of long entries do not agree with the computed checksum of the
name contained in the short entry, then the long entries are treated as
orphans. This can occur if a disk
containing long and short entries is taken to a previous version of
MS-DOS/Windows and only the short name of a file or directory with a long
entries is renamed.
The algorithm, implemented
in C, for computing the checksum is:
//-----------------------------------------------------------------------------
// ChkSum()
// Returns an unsigned byte checksum computed on an unsigned byte
// array. The array must be 11 bytes long and is assumed to contain
// a name stored in the format of a MS-DOS directory entry.
// Passed: pFcbName Pointer to an unsigned byte array assumed to be
// ChkSum()
// Returns an unsigned byte checksum computed on an unsigned byte
// array. The array must be 11 bytes long and is assumed to contain
// a name stored in the format of a MS-DOS directory entry.
// Passed: pFcbName Pointer to an unsigned byte array assumed to be
// 11 bytes long.
// Returns: Sum An 8-bit unsigned checksum of the array pointed
// Returns: Sum An 8-bit unsigned checksum of the array pointed
// to by pFcbName.
//------------------------------------------------------------------------------
unsigned char ChkSum (unsigned char *pFcbName)
{
short FcbNameLen;
unsigned char Sum;
Sum = 0;
for (FcbNameLen=11; FcbNameLen!=0; FcbNameLen--) {
// NOTE: The operation is an unsigned char rotate right
Sum = ((Sum & 1) ? 0x80 : 0) + (Sum >> 1) + *pFcbName++;
}
return (Sum);
}
//------------------------------------------------------------------------------
unsigned char ChkSum (unsigned char *pFcbName)
{
short FcbNameLen;
unsigned char Sum;
Sum = 0;
for (FcbNameLen=11; FcbNameLen!=0; FcbNameLen--) {
// NOTE: The operation is an unsigned char rotate right
Sum = ((Sum & 1) ? 0x80 : 0) + (Sum >> 1) + *pFcbName++;
}
return (Sum);
}
As a consequence of this pairing, the short
directory entry serves as the structure that contains fields like: last access
date, creation time, creation date, first cluster, and size. It also holds a name that is visible on
previous versions of MS-DOS/Windows. The
long directory entries are free to contain new information and need not
replicate information already available in the short entry. Principally, the long entries contain the
long name of a file. The name contained
in a short entry which is associated with a set of long entries is termed the alias name, or simply alias, of the file.
Storage of a Long-Name Within Long Directory Entries
A long name can consist of more characters
than can fit in a single long directory entry.
When this occurs the name is stored in more than one long entry. In any event, the name fields themselves
within the long entries are disjoint.
The following example is provided to illustrate how a long name is
stored across several long directory entries.
Names are also NUL terminated and padded with 0xFFFF characters in order
to detect corruption of long name fields by errant disk utilities. A name that fits exactly in a n long
directory entries (i.e. is an integer multiple of 13) is not NUL terminated and
not padded with 0xFFFFs.
Suppose a file is created with the name:
"The quick brown.fox". The
following example illustrates how the name is packed into long and short
directory entries. Most fields in the
directory entries are also filled in as well.
The heuristics used to
"auto-generate" a short name from a long name are explained in a
later section.
Name Limits and Character Sets
Short Directory Entries
Short names are limited to 8 characters
followed by an optional period (.) and extension of up to 3 characters. The total path length of a short name cannot
exceed 80 characters (64 char path + 3 drive letter + 12 for 8.3 name + NUL)
including the trailing NUL. The
characters may be any combination of letters, digits, or characters with code
point values greater than 127. The
following special characters are also allowed:
$ %
' - _
@ ~ `
! ( )
{ } ^
# &
Names are stored in a short directory entry
in the OEM code page that the system is configured for at the time the
directory entry is created. Short
directory entries remain in OEM for compatibility with previous versions of
MS-DOS/Windows. OEM characters are
single 8-bit characters or can be DBCS character pairs for certain code pages.
Short names passed to the file system are
always converted to upper case and their original case value is lost. One problem that is generally true of most
OEM code pages is that they map lower to upper case extended characters in a
non-unique fashion. That is, they map
multiple extended characters to a single upper case character. This creates problems because it does not
preserve the information that the extended character provides. This mapping also prevents the creation of
some file names that would normally differ, but because of the mapping to upper
case they become the same file name.
Long Directory Entries
Long names are limited to 255 characters,
not including the trailing NUL. The
total path length of a long name cannot exceed 260 characters, including the
trailing NUL. The characters may be any
combination of those defined for short names with the addition of the period
(.) character used multiple times within the long name. A space is also a valid character in a long
name as it always has been for a short name.
However, in short names it typically is not used. The following six special characters are now
allowed in a long name. They are not
legal in a short name.
+ ,
; = [ ]
Embedded spaces within a long name are
allowed. Leading and trailing spaces in
a long name are ignored.
Leading and embedded periods are allowed in
a name and are stored in the long name.
Trailing periods are ignored.
Long names are stored in long directory
entries in UNICODE. UNICODE characters
are 16-bit characters. It is not be
possible to store UNICODE in short directory entries since the names stored
there are 8-bit characters or DBCS characters.
Long names passed to the file system are
not converted to upper case and their original case value is preserved. UNICODE solves the case mapping problem
prevalent in some OEM code pages by always providing a translation for lower
case characters to a single, unique upper case character.
Name Matching In Short & Long
Names
The names contained in the set of all short
directory entries are termed the "short name space". The names contained in the set of all long
directory entries are termed the "long name space". Together, they form a single unified name
space in which no duplicate names can exist.
That is: any name within a specific directory, whether it is a short
name or a long name, can occur only once in the name space. Furthermore, although the case of a name is
preserved in a long name, no two names can have the same name although the
names on the media actually differ by case.
That is names like "foobar" cannot be created if there is
already a short entry with a name of "FOOBAR" or a long name with a
name of "FooBar".
All types of search operations within the
file system (i.e. find, open, create, delete, rename) are
case-insensitive. An open of
"FOOBAR" will open either "FooBar" or "foobar" if
one or the other exists. A find using
"FOOBAR" as a pattern will find the same files mentioned. The same rules are also true for extended
characters that are accented.
A short name search operation checks only
the names of the short directory entries for a match. A long name search operation checks both the
long and short directory entries. As the
file system traverses a directory, it caches the long-name sub-components
contained in long directory entries. As
soon as a short directory entry is encountered that is associated with the
cached long name, the long name search operation will check the cached long
name first and then the short name for a match.
When a character on the media, whether it
is stored in the OEM character set or in UNICODE, cannot be translated into the
appropriate character in the OEM or ANSI code page, it is always
"translated" to the "_" (underscore) character as it is
returned to the user – it is NOT modified on the disk. This character is the same in all OEM code
pages and ANSI.
Naming Conventions and Long Names
An API allows the caller to specify the
long name to be assigned to a file or directory. They do not
allow the caller to independently specify the short name. The reason for this prohibition is that the
short and long names are considered to be a single unified name space. As should be obvious the file system's name
space does not support duplicate names.
In other words, a long name for a file may not contain the same name,
ignoring case, as the short name in a different file. This restriction is intended to prevent
confusion among users, and applications, regarding the proper name of a file or
directory. To make this restriction
transparent, whenever a long name is created and the no matching long name
exists, the short name is automatically generated from the long name in such a
way that it does not collide with an existing short name.
The technique chosen to auto-generate short
names from long names is modeled after Windows NT. Auto-generated short names are composed of
the basis-name and an optional numeric-tail.
The Basis-Name Generation Algorithm
The basis-name
generation algorithm is outlined below.
This is a
sample algorithm and serves to illustrate how short names can
be auto-generated from long names. An implementation should follow
this basic sequence of steps.
1. The
UNICODE name passed to the file system is converted to upper case.
2. The
upper cased UNICODE name is converted to OEM.
if (the uppercased UNICODE glyph does not exist as an OEM glyph in the OEM code page)
or (the OEM glyph is invalid in an 8.3 name)
{
Replace the glyph to an OEM '_' (underscore) character.
Set a "lossy conversion" flag.
}
if (the uppercased UNICODE glyph does not exist as an OEM glyph in the OEM code page)
or (the OEM glyph is invalid in an 8.3 name)
{
Replace the glyph to an OEM '_' (underscore) character.
Set a "lossy conversion" flag.
}
3. Strip
all leading and embedded spaces from the long name.
4. Strip
all leading periods from the long name.
5. While (not at end of the long name)
and (char is not a period)
and (total chars copied < 8)
{
Copy characters into primary portion of the basis name
}
and (char is not a period)
and (total chars copied < 8)
{
Copy characters into primary portion of the basis name
}
6. Insert
a dot at the end of the primary components of the basis-name iff the basis name has an extension after the last
period in the name.
7. Scan
for the last embedded period in the long name.
If (the last embedded period was found)
{
While (not at end of the long name)
and (total chars copied < 3)
{
Copy characters into extension portion of the basis name
}
}
If (the last embedded period was found)
{
While (not at end of the long name)
and (total chars copied < 3)
{
Copy characters into extension portion of the basis name
}
}
Proceed to numeric-tail generation.
The Numeric-Tail Generation Algorithm
If (a "lossy conversion"
was not flagged)
and (the long name fits within the 8.3 naming conventions)
and (the basis-name does not collide with any existing short name)
{
The short name is only the basis-name without the numeric tail.
}
else
{
Insert a numeric-tail "~n" to the end of the primary name such that the value of the "~n" is chosen so that the
name thus formed does not collide with any existing short name and that the primary name does not exceed eight characters in length.
}
and (the long name fits within the 8.3 naming conventions)
and (the basis-name does not collide with any existing short name)
{
The short name is only the basis-name without the numeric tail.
}
else
{
Insert a numeric-tail "~n" to the end of the primary name such that the value of the "~n" is chosen so that the
name thus formed does not collide with any existing short name and that the primary name does not exceed eight characters in length.
}
The "~n" string can range from
"~1" to "~999999".
The number "n" is chosen so that it is the next number in a
sequence of files with similar basis-names.
For example, assume the following short names existed: LETTER~1.DOC and
LETTER~2.DOC. As expected, the next
auto-generated name of name of this type would be LETTER~3.DOC. Assume the following short names existed:
LETTER~1.DOC, LETTER~3.DOC. Again, the
next auto-generated name of name of this type would be LETTER~2.DOC. However, one absolutely cannot count on this behavior. In a directory with a very large mix of names
of this type, the selection algorithm is optimized for speed and may select
another "n" based on the characteristics of short names that end in
"~n" and have similar
leading name patterns.
Effect of Long Directory Entries
on Down Level Versions of FAT
The support of long names is most important
on the hard disk, however it will be supported on removable media as well. The implementation provides support for long
names without breaking compatibility with the existing FAT format. A disk can be read by a down level system
without any compatibility problems. An
existing disk does not go through a conversion process before it can start
using long names. All of the current
files remain unmodified. The long name
directory entries are added when a long name is created. The addition of a long name to an existing
file may require the 8.3 directory entry to be moved if the required adjacent
directory entries are not available.
The long name entries are as hidden as
hidden or system files are on a down level system. This is enough to keep the casual user from
causing problems. The user can copy the
files off using the 8.3 name, and put new files on without any side effects
The interesting part of this is what
happens when the disk is taken to a down level FAT system and the directory is
changed. This can affect the long name
entries since the down level system ignores these long names and will not
ensure they are properly associated with the 8.3 names.
A down level system will only see the long
name entries when searching for a label.
On a down level system, the volume label will be incorrectly reported if
the true volume label does not come before all of the long name entries in the
root directory. This is because the long
name entries also have the volume label bit set. This is unfortunate, but is not a critical
problem.
If an attempt is made to remove the volume
label, one of the long name directory entries may be deleted. This would be a rare occurrence. It is easily detected on an aware system. The long name entry will no longer be a valid
file entry, since one or more of the long entries is marked as deleted. If the deleted entry is reused, then the
attribute byte will not have the proper value for a long name entry.
If a file is renamed on a down level
system, then only the short name will be renamed. The long name will not be affected. Since the long and short names must be kept
consistent across the name space, it is desirable to have the long name become
invalid as a result of this rename. The
checksum of the 8.3 name that is kept in the long name directory provides the
ability to detect this type of change.
This checksum will be checked to validate the long name before it is
used. Rename will cause problems only if
the renamed 8.3 file name happens to have the same checksum. The checksum algorithm chosen has a
relatively flat distribution across the short name space.
This rename of the 8.3 name must also not
conflict with any of the long names.
Otherwise a down level system could create a short name in one file that
matches a long name, when case is ignored, in a different file. To prevent this, the automatic creation of an
8.3 name from a long name, that has an 8.3 format, will directly map the long
name to the 8.3 name by converting the characters to upper case.
If the file is deleted, then the long name
is simply orphaned. If a new file is
created, the long name may be incorrectly associated with the new file
name. As in the case of a rename the
checksum of the 8.3 name will help prevent this incorrect association.
Validating The Contents of a
Directory
These guidelines are provided so that disk
maintenance utilities can verify individual directory entries for 'correctness'
while maintaining compatibility with future enhancements to the directory
structure.
1. DO NOT look at the content of directory entry fields marked
'reserved' and assume that, if they are any value other than zero, that they
are 'bad'.
2. DO NOT reset the content of directory entry fields marked reserved to zero when they contain
non-zero values (under the assumption that they are "bad"). Directory entry fields are designated reserved, rather than must-be-zero. They should be ignored by your
application.. These fields are intended
for future extensions of the file system.
By ignoring them an utility can continue to run on future versions of
the operating system.
3. DO use the A_LONG attribute first
when determining whether a directory entry is a long directory entry or a short
directory entry. The following algorithm
is the correct algorithm for making this determination:
if
(((LDIR_attr & ATTR_LONG_NAME_MASK) == ATTR_LONG_NAME) &&
(LDIR_Ord != 0xE5))
{
/* Found an active long name sub-component. */
}
{
/* Found an active long name sub-component. */
}
4. DO use bits 4 and 3 of a short entry together when determining what type of short directory entry is
being inspected. The following
algorithm is the correct algorithm for making this determination:
if (((LDIR_attr & ATTR_LONG_NAME_MASK) !=
ATTR_LONG_NAME) && (LDIR_Ord != 0xE5))
{
if ((DIR_Attr & (ATTR_DIRECTORY | ATTR_VOLUME_ID)) == 0x00)
/* Found a file. */
else if ((DIR_Attr & (ATTR_DIRECTORY | ATTR_VOLUME_ID)) == ATTR_DIRECTORY)
/* Found a directory. */
else if ((DIR_Attr & (ATTR_DIRECTORY | ATTR_VOLUME_ID)) == ATTR_VOLUME_ID)
/* Found a volume label. */
else
/* Found an invalid directory entry. */
}
{
if ((DIR_Attr & (ATTR_DIRECTORY | ATTR_VOLUME_ID)) == 0x00)
/* Found a file. */
else if ((DIR_Attr & (ATTR_DIRECTORY | ATTR_VOLUME_ID)) == ATTR_DIRECTORY)
/* Found a directory. */
else if ((DIR_Attr & (ATTR_DIRECTORY | ATTR_VOLUME_ID)) == ATTR_VOLUME_ID)
/* Found a volume label. */
else
/* Found an invalid directory entry. */
}
5. DO NOT assume that a non-zero value in the "type" field
indicates a bad directory entry. Do not
force the "type" field to zero.
6. Use
the "checksum" field as a value to validate the directory entry. The "first cluster" field is
currently being set to zero, though this might change in future.
Other Notes Relating to FAT
Directories
·
Long File Name directory
entries are identical on all FAT types. See the preceeding sections for
details.
·
DIR_FileSize is a 32-bit field. For FAT32 volumes, your FAT file system driver
must not allow a cluster chain to be created that is longer than 0x100000000
bytes, and the last byte of the last cluster in a chain that long cannot be
allocated to the file. This must be done so that no file has a file size > 0xFFFFFFFF
bytes. This is a fundamental limit of all FAT file systems. The maximum allowed
file size on a FAT volume is 0xFFFFFFFF (4,294,967,295) bytes.
·
Similarly, a FAT file system
driver must not allow a directory (a file that is actually a container for other
files) to be larger than 65,536 * 32 (2,097,152) bytes.
NOTE: This limit does not apply to the number of files in the directory. This limit is on the size of the directory itself and has nothing to do with the content of the directory. There are two reasons for this limit:
NOTE: This limit does not apply to the number of files in the directory. This limit is on the size of the directory itself and has nothing to do with the content of the directory. There are two reasons for this limit:
1. Because FAT directories are not sorted or indexed, it is a bad idea
to create huge directories; otherwise, operations like creating a new entry
(which requires every allocated directory entry to be checked to verify that
the name doesn’t already exist in the directory) become very slow.
2. There are many FAT file system drivers and disk utilities, including
Microsoft’s, that expect to be able to count the entries in a directory using a
16-bit WORD variable. For this reason, directories cannot have more than
16-bits worth of entries.
