File Allocation Table

FAT [fæt] stands for file Allocation Table (on German about: File allocation table) and became of Seattle computer Products as file system for its operating system QDOS, which develops direct predecessor of MS-DOS. It is thoseonly important innovation of QDOS opposite CP/M. In long law cases and a two-year patenting procedure became to 10. January 2006 the patent and associated enormous market power for FAT (FAT12,16,32 etc.) of the manufacturing firm Microsoft awarded. To the family thatFAT Dateisysteme belong:

  • FAT12 (today still for each MS-DOS - or Windows - disk used)
  • FAT16 (today usually on all kinds by mobile data media is used, smaller than 2 gives are.)
  • (VFAT)
  • FAT32 (z becomes. all kinds of mobile memory of more than 2 GB capacity)

the FAT is used a concatenated list in a storage area of a partition, in which information to each cluster (allocation unit) of the partition. This information can say the following:

  • The cluster is not occupied,
    • the cluster is free. (000 16, 0000 16, 00000000 16)
    • the medium is damaged at the position of this cluster. (FF7 16, FFF7 16, FFFFFFF7 16)
  • the cluster is ofa file occupied,
    • the next cluster of the file is the cluster with the number X. (2 < X < FF6 16, 2 < X < FFF6 16, 2 < X < 0FFFFFFF 16)
    • this is thatlast cluster of the file. (FF8 16… FFF 16, FFF8 16… FFFF 16, FFFFFFF8 16… FFFFFFFF 16)

(the numbers in parentheses contain in each case the value for FAT12, FAT16 and FAT32.)

the FAT has with the FAT Dateisystemen in thatRule two copies, in order to still have with overrun functional other FAT. With various programs a data re-establishment is possible in many cases. Some users had bad experiences with FAT repairs by the program ScanDisk.

Table of contents



with the 1980 published first version of QDOS was introduced FAT as file system. At first no sublists were administered. This changed with MS-DOS Version 2.0.

FAT12 becomes only on data media and/or. Partitions up to a size of 16 MIB (16.777.216 byte) assigned; it is until today on all FAT formatted 3.5 " - disks in the use.


  • 12-Bit-Clusternummern, with which 2 12 = 4096 cluster are addressedcan. The numbering the cluster begins on disks with the number 2.
  • Only file names in the pattern 8,3 (eight indications of the file name and three indications of the file name extension) are supported.
  • The cluster size amounts to 512 byte to 4096 byte.
  • ThatGeneral statement (root directory) of the partition is limited to a size of 14 Clustern, thereby a maximum number of 224 entries (listings or files ) results. Normally the first entry is occupied by the designation of the data medium.

Long file names on disks

overlange Dateinamen auf FAT12-Disketten einsetzen zu können, nutztWindows mehrere Verzeichniseinträge vor dem eigentlichen Verzeichniseintrag im FAT12-Format zur Speicherung des langen Dateinamens. Older systems (z. B. ) These listing entries ignore MS-DOS, since them mark by a special combination of attributesare, and. A. as “volume” and “Hidden”.


FAT16 is a file system, which replaced 1983 the old version FAT12. By the increasing size of the assigned non removable disks an extension of the address area became necessary. Now were even with 512-ByteClustern altogether 32 MIB large plates managable.

Since FAT12 is only used for disks, it is guessed/advised nearly in oblivion. Generally today often also the FAT16 is therefore called only FAT. However the server operating system needed Novell Netware up toVersion 4.0 only “DOS” large up to 16 MIB - boat partition, which was thus produced (automatically) with FAT12.

It has the following characteristics:

  • 65,536 entries are possible.
  • Can be addressed 2 16 − 10 = 65,526 cluster (10 cluster neededFAT, therefore not 65,536).
  • 8.3-Dateinamensformat
  • the partition size is limited under MS-DOS and Windows 9x (those on DOS are based) on 2 Gi B. Windows NT can however 4 gives large FAT16-Partitionen to produce and administer. (Cluster size 64 KiB)
  • the cluster are everafter partition size between 512 byte and maximally 32 AI B largely.
  • Files may to 2 give to become large, under Windows NT 4 give.
  • The main (root -) listing must be at a certain position on the data medium.

An advancement took placewith FAT32.


VFAT (virtual FAT) is a file system for the administration of non removable disks under Windows starting from version 95. It is only one extension of the FAT16 - format.

When Windows 95 was published, the non removable disks had ina PC around the 400 MIB capacity. FAT16 administers 2 16 = 65,536 cluster. Even with a cluster size of 8.192 byte 512 MIB is addressable. This was sufficient for conditions at that time (to maximum sizes see FAT16).

Nevertheless the designers hadof Windows 95 the goal of making the use possible from long file names to. This was reached by a trick in the layout of the file system. The file is stored as before as 8.3-Dateiname, with longer names becomes however an alias in the uses, whereby the number is counted up. The long name is then distributed over several listing entries. While so far an entry referred to a file, a file can occupy several entries with ever 32 byte now. The final format permitted toto 255 indications be enough for file names and set consistently on university code as character set with coding UCS-2.

Within so far ranges of the entry with the 8.3-Dateiname, not used by Microsoft systems, now also the date of preparation and the date of the last access were stored.

VFAT is supported in Windows 95 and more highly and in Windows NT 3,5 and more highly. Occasionally in the literature (in particular in www articles) also falsely VFAT is equated with FAT32.


FAT32 is a file system developed by Microsoft,starting from 1997 the previous version FAT16 supplemented. Partitions smaller than 512 MIB are still produced with FAT16, this have however no technical background.

It was called with Windows 95 OSR2 (service release 2), also Windows 95B,introduced. The addressing works with 32 bits, about which 4 bits are reserved, so that 2 28 = 268.435.456 cluster can be addressed.

In addition FAT32 can with Windows 98, Windows 2000, Windows ME and Windows XP as well as - differently than NTFS - problem-free also with FreeDOS and Linux to be used. Since also Macintosh computers can access, it almost offers itself as file system for the change of data.

There up to a partition size of 8 give a cluster only 4 KiB(during the standard formatting), this is large „small and old “plates is relatively better used than with FAT16, where a cluster up to 32 KiB occupies (under Windows NT or Windows 2000 FAT16-Clustergrösse maximally 64 KiB).

FAT32 has the following characteristics:

  • Are used up to 2 28 = 268.435.456 cluster.
  • Since VFAT is supported, long file names (to 255 indications) are possible.
  • The partition size is limited on 2 TiB.
  • The cluster are maximum depending upon partition size between 512 byte and 32KiB largely.
  • Files may up to 4 give − 1 byte (= 4.294.967.295 bytes) to become large (this is probably today the most important restriction).
  • 2 28 files can be put down.
  • The main (root -) listing does not have itself at a certainPosition on the data medium find.
  • Old DOS /Windows versions (until including Windows 95 A and Windows NT before version 4.0) cannot access .
  • Under Windows 2000 and successors may do the user with the inserted program “formatting” maximally 32 give large FAT32-Dateisystemeagain provide (of Microsoft forced upon restriction, see Web on the left of). That access to larger FAT32-Dateisysteme, those with alternative tools (like z. B. with the instruction mkdosfs under Knoppix and/or. Linux were provided), are however always possible.

Web on the left of


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