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File Allocation Table (FAT) is a file system that was developed for MS-DOS and used in consumer versions of Microsoft Windows up to and including Windows ME. The FAT file system is considered relatively uncomplicated, and because of that, it is a popular format for floppy disks; moreover, it is supported by virtually all existing operating systems for personal computers, and because of that it is often used to share data between several operating systems booting on the same computer (a multiboot environment). It is also used on solid-state memory sticks and other similar devices.
The FAT filesystem made its debut in August 1980 with the first version of QDOS, the ancestor of Microsoft's PC-DOS and MS-DOS. This initial version of FAT is now referred to as FAT12. As a filesystem for floppy disks it had a number of limitations including a lack of support for directories. It also used cluster addresses that were only 12-bits long, which limited the maximum space of the filesystem to approximately 2MB of space (however this is far more than a typical 360KB floppy could hold at the time).
In 1983 Microsoft introduced a new version of the FAT filesystem with the release of MS-DOS version 2. This new version, now known as FAT16, included support for hard drives and sub-directories. Cluster addresses were increased to 16-bit, allowing for a greater number of clusters (approximately 65,500) and consequently much greater filesystem sizes. Even with 512-byte clusters, this could give up to 32MB of space - enough for the 10MB or 20MB XT hard drives that were typical at the time. As hard drives larger than 32MB were released, large cluster sizes were used. The use of 8,192-byte clusters allowed for filesystem sizes up to 512MB. However this increased the problem of internal fragmentation where small files could result in a great deal of wasted space, for example a 1-byte file stored in a 8,192-byte cluster results in 8,191-bytes of wasted space.
One of the user experience goals for the designers of Windows 95 was the use of long file names in the new operating system. These were implemented using a work-around in the way directory entries are laid out (see below). The new version of the file system became known as VFAT (Virtual FAT), after its Windows 95 VxD device driver. VFAT is supported by Windows 95 and above and Windows NT 4.0 and above.
By 1997, the cluster growth possibility was exhausted. The largest cluster size in Windows FAT was 32KB, giving a maximum volume size 2 gigabytes. Microsoft decided to implement a newer generation of FAT, known as FAT32, with 32-bit cluster numbers, of which 28 bits are currently used. In theory, this should support a total of approximately 268,435,438 clusters, allowing for drive sizes in the multi-terabyte range. However, due to limitations in Microsoft's ScanDisk utility, the FAT is not allowed to grow beyond 4,177,920 clusters, placing the volume limit at 124.55 gigabytes . First supported in Windows 95 OSR 2 and Windows 2000, and incorporating several changes to other filesystem structures, this was a major improvement over previous versions, but is no longer sufficient to cope with today's largest drives.
The alternative IBM PC operating systems, for example OS/2, Linux, FreeBSD, and BeOS, have all supported FAT, and most have gained support for VFAT and FAT32 shortly after the appropriate Windows versions were released. Early Linux distributions also supported a format known as UMSDOS, which was nothing more than FAT with the UNIX file properties (e.g. long file name and access permissions) stored in a separate file called --linux-.---. UMSDOS was mostly dropped after VFAT was released, although it still remains in the Linux kernel sources.
Since Microsoft has announced the discontinuation of the DOS line of succession with Windows ME, it remains unlikely that any new versions of FAT will appear. For most purposes, the NTFS file system that was developed for the Windows NT line is superior to FAT from the points of view of efficiency, performance and reliability. However, FAT is likely to stay for a long time as it is an ideal file system for small drives, like the floppies.
A FAT file system is composed of four different sections.
A partition is divided up into identically sized clusters, small blocks of contiguous space. Cluster sizes vary depending on the type of FAT file system being used and the size of the partition, typically cluster sizes lie somewhere between 2KB and 32KB. Each file may occupy one or more of these clusters depending on its size, a file is represented by a chain of these clusters (referred to as a singly linked list). However these chains are not necessarily stored adjacently on the disk's surface but are often instead fragmented throughout the Data Region.
The File Allocation Table (FAT) is a list of entries that map to each cluster on the partition. Each entry records one of five things:
Each version of the FAT file system uses a different size for FAT entries. The size is indicated by the name, for example the FAT16 file system uses 16 bits for each entry while the FAT32 file system uses 32 bits. This difference means that the File Allocation Table of a FAT32 system can map a greater number of clusters than FAT16, allowing for larger partition sizes with FAT32. This also allows for more efficient use of space than FAT16, because on the same hard drive a FAT32 table can address smaller clusters which means less wasted space.
A Directory Table is a special type of file that represents a directory. Each file or directory stored within it is represented by a 32 byte entry in the table. Each entry records the name, extension, attributes (archive, directory, hidden, read-only, system and volume), the date and time of creation, the address of the first cluster of the file/directory's data and finally the size of the file/directory.
Aside from the Root Directory Table in FAT12 and FAT16 file systems which occupies the special Root Directory Region location, all Directory Tables are stored in the Data Region.
Long File Names (LFN) are stored on a FAT32 file system using a trick - adding phoney entries into the Directory Tables. The entries are marked with a Volume Label attribute which is impossible for a regular file and because of that they're ignored by most old MS-DOS programs. Each phoney entry can contain up to 13 UCS-2 characters (26 bytes), gaining about 15 bytes in addition to the old 8 + 3 by using fields in the record which contained file size or time stamps (but for security versus disk checking tools the starting cluster field is left unused with a 0 value). See 8.3 for additional explanations.
Although technical details of the FAT file system derive from CP/M and have been widely known and widely disseminated in the PC community for many years, and although the file system itself is widely considered to be obsolete, it should be noted that in 2003 Microsoft made a point of asserting intellectual property claims to the system. Citing patents, Microsoft claimed that licensing fees are required for its use in such applications as removable solid state media, and consumer devices using such media.
This claim by Microsoft is controversial, for a number of reasons. Microsoft itself has admitted that it developed its first FAT file system in 1976, so no patents could apply to an implementation of the original version. Copyright law could not prevent a re-implementation of FAT either.
Microsoft has cited 4 patents, dating from 1995 on, for FAT. All of these patents relate to storing both long and short filenames in a single file system. A Slashdot discussion on December 4, 2003, analyzed these patents and participants (particularly Svartalf) reported the following:
In addition, there also seems to be prior art for at least the first, third, and fourth patents in the Rock Ridge Interchange Protocol standard for UNIX, which was an IEEE draft specification on at least July 13th, 1993. This protocol defined a method to support long and short names on the same media (as well as additional information) to support Unix systems.
And of course, there is the simple question if this is really an innovative idea at all. Patents are supposed to be granted for new ideas; the notion of not removing short names, but adding long name information as well, is an option that is immediately obvious to any practitioner in the field. Indeed, the standard technique for adding new fields (where backward compatibility is important) is to add new fields with the information, placing them wherever they fit, while not removing the old ones. And there's nothing clever about how the new data is stored, according to most practioners who have examined it.
In short, many have concluded that these patents only cover implementations that include support for long filenames, so it's likely that removable solid state media and consumer devices only using short names would be unaffected. Also, there is good reason to believe the patents would be found invalid or unnecessary if challenged. In particular, many or all such patents are believed by many to be invalid because of previous public release, prior art, or because the technique would be obvious to a practitioner.
In addition, there is some reason to believe that none of this licensing could impact operating system or firmware implementations. In the document "Microsoft Extensible Firmware Initiative FAT 32 File System Specification, FAT: General Overview of On-Disk Format" by Microsoft (version 1.03, December 6, 2000), Microsoft specifically grants a number of rights, and many readers of that document have interpreted it as permitting operating system vendors to implement FAT.
Many see the Microsoft patents as wrongly issued and unsound, and believe that Microsoft has applied for many patents only as a move in its fight against open source. As a result, many petitioned the US Patent and Trade Office (PTO) to re-examine the patent. In particular, the Public Patent Foundation submitted a large set of prior art, including information from Xerox and IBM. The US PTO acknowledged that the evidence raised "substantial new question[s] of patentability", and opened an investigation into the validity of Microsoft's FAT patents. Patents are rejected in about 70% of all re-examinations, so it is quite possible that these patents will be eventually rejected.
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