2013년 12월 13일 금요일

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


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 Microsoft Corporation in the United States and/or other countries. Other product and company names mentioned herein may be the trademarks of their respective owners.
© 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:



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.
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.

Here are two more important notes about the FAT region of a FAT volume:
  1. 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.
  2. 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

In FAT directories all names are unique. Look at the first three examples earlier. Those different names all refer to the same file, and there can only be one file with DIR_Name set to “FOO     BAR” in any directory.

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

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

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
//                          11 bytes long.
        //      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);
        }

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.
}
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
}
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
       }
}
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.
}
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.   */

}

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.   */
}

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:

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.

 



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