what it File allocation table?

Answer 1

FAT is the index of data storage in any disc (floppy/hard disk or CD ROM. The index or the details of file name , length, date time are the data visible to us. the clusters used or the location of data's are also written which is machine language, not visible to us. The FAT system was formerly 12 bit, then 16 bit and now in 32 bit. XP (NTFS) is having 32 bit and 64 bit format.

Answer 2

File Allocation Table (FAT) is a partially patented file system developed by Microsoft for MS-DOS and is the primary file system for consumer versions of Microsoft Windows up to and including Windows Me.

The FAT file system is relatively uncomplicated, and is supported by virtually all existing operating systems for personal computers. This ubiquity makes it an ideal format for floppy disks and solid-state memory cards, and a convenient way of sharing data between disparate operating systems installed on the same computer (a dual boot environment).

The most common implementations have a serious drawback in that when files are deleted and new files written to the media, their fragments tend to become scattered over the entire media, making reading and writing a slow process. Defragmentation is one solution to this, but is often a lengthy process in itself and has to be repeated regularly to keep the FAT file system clean

Answer 3

File Allocation Table (FAT) is a partially patented file system developed by Microsoft for MS-DOS and is the primary file system for consumer versions of Microsoft Windows up to and including Windows Me.

The FAT file system is relatively uncomplicated, and is supported by virtually all existing operating systems for personal computers. This ubiquity makes it an ideal format for floppy disks and solid-state memory cards, and a convenient way of sharing data between disparate operating systems installed on the same computer (a dual boot environment).

The most common implementations have a serious drawback in that when files are deleted and new files written to the media, their fragments tend to become scattered over the entire media, making reading and writing a slow process. Defragmentation is one solution to this, but is often a lengthy process in itself and has to be repeated regularly to keep the FAT file system clean.

Answer 4

As the name suggests, FAT is a table that helps to locate the files in computer system's memory. It is similar to an index where the file name and its location is stored by the system. when ever we creat a file, its name is stored in FAT. It is due to FAT only that we are not able to create a new file with same name which we is already given an old file. hope it helps... this will help u to understand, its defination can be taken from any book

Answer 5

FAT for short, it is the logocal portion on a disk drive that maps files with the sectors on a disk, so your OS will know where to find a complete file.

Answer 6

An area on a hard disk or floppy disk where information is stored about the physical location of each piece of every file on the disk and about the location of unusable areas of the disk.

Answer 7

The FAT filesystem was created by Bill Gates and Marc McDonald in 1977 for managing disks in Microsoft Disk BASIC. In August 1980 Tim Paterson incorporated FAT into his 86-DOS operating system for the S-100 8086 CPU boards; the filesystem was the main difference between 86-DOS and CP/M, of which 86-DOS was otherwise mostly a clone.[1]

The name originates from the usage of a table which centralizes the information about which areas belong to files, are free or possibly unusable, and how each file is scattered across the disk. To reduce the management complexity, disk space is allocated to files in contiguous groups of hardware sectors called clusters. The maximum possible number of clusters has dramatically increased over time. Its binary logarithm is used to name the successive major versions of the format, those requiring an operating system upgrade. One major upgrade was not related to increasing this count. The FAT standard has also evolved in several ways where backwards compatibility with existing software has been preserved.


[edit] FAT12
This initial version of FAT is now referred to as FAT12. As a filesystem for disks, it had a number of limitations: no support for hierarchical directories, cluster addresses were "only" 12 bits long (which limited cluster count to 4096 and made the code manipulating the FAT a bit tricky) and the disk size was stored as a 16-bit count of sectors, which limited the size to 32 MB. An entry-level floppy diskette at the time would be 5.25", single-sided, 40 tracks, with 8 sectors per track, resulting in a capacity of slightly less than 160 KB. The above limits exceeded this capacity by one or more orders of magnitude and at the same time allowed all the control structures to fit inside the first track, thus avoiding head movement during read and write operations. The limits were successively lifted in the following years, which improved wait-loss dramatically.

Since the sole root directory had to fit inside the first track as well, the maximum possible number of files was limited to a few dozen.

A limitation which was not addressed until much later was that any bad sector in the control structures area, the track 0, could prevent the diskette from being usable. The DOS formatting tool rejected such diskettes completely. Bad sectors were only allowed in the file area, where they made the entire holding cluster unusable as well.


[edit] Directories
In order to properly support the newer IBM PC XT computer, which featured a 10 MB hard disk, MS-DOS 2.0 was released around the same time, at the beginning of 1983, and introduced hierarchical directories. Apart from allowing for better organization of files, directories allowed it to store many more files on the hard disk, as the maximum number of files was no longer constrained by the (still fixed) root directory size. This number could now be equal to the number of clusters (or even greater, given that zero-sized files do not use any clusters on FAT).

The format of the FAT itself did not change. The 10 MB hard disk on the PC XT had 4 KB clusters. If a 20 MB hard disk was later installed, and formatted with MS-DOS 2.0, the resultant cluster size would be 8 KB, the boundary at 15.9 MB.


[edit] Initial FAT16
In 1984 IBM released the PC AT, which featured a 20 MB hard disk. Microsoft introduced MS-DOS 3.0 in parallel. Cluster addresses were increased to 16-bit, allowing for a greater number of clusters (up to 65,517) and consequently much greater file-system sizes. However, the maximum possible number of sectors and the maximum (partition, rather than disk) size of 32 MB did not change. Therefore, although technically already "FAT16", this format was not yet what today is commonly understood under this name. A 20 MB hard disk formatted under MS-DOS 3.0 was not accessible by the older MS-DOS 2.0. Of course, MS-DOS 3.0 could still access MS-DOS 2.0 style 8 KB cluster partitions.

MS-DOS 3.0 also introduced support for high-density 1.2 MB 5.25" diskettes, which notably had 15 sectors per track, hence more space for FAT. This probably prompted a dubious optimization of the cluster size, which went down from 2 sectors to just 1. The net effect was that high density diskettes were significantly slower than older double density ones.


[edit] Extended partition and logical drives
Apart from improving the structure of the FAT filesystem itself, a parallel development allowing an increase in the maximum possible FAT storage space was the introduction of multiple FAT partitions. Originally partitions were supposed to be used only for sharing the disk between operating systems, typically DOS and Xenix at the time, so DOS was only prepared to handle one FAT partition. It was not possible to create multiple DOS partitions using DOS tools, and third party tools would warn that such a scheme would not be compatible with DOS. Simply allowing several identical-looking DOS partitions could lead to naming problems: should C: be the first FAT partition on disk, for simplicity, or rather the partition marked as active in the partition table, so that several DOS versions can co-exist? Which partition should be C: if the system was booted from a diskette?

To allow the use of more FAT partitions in a compatible way a new partition type was introduced (in MS-DOS 3.2, January 1986), the extended partition, which was actually just a container for additional partitions called logical drives. Originally only 1 logical drive was possible, allowing the use of hard-disks up to 64 MB. In MS-DOS 3.3 (August 1987) this limit was increased to 24 drives; it probably came from the compulsory letter-based disk naming (A and B being reserved for the two floppy drives). The logical drives were described by on-disk structures which closely resemble the master boot record (MBR) of the disk (which describes the primary partitions), probably to simplify coding, and they were chained/nested in a way analogous to Russian matryoshka dolls. Only one extended partition was allowed. Logical drives were not bootable, and the extended partition could only be created after the primary FAT partition (except with third party formatting tools), which removed all ambiguity, but also the possibility of booting several DOS versions from the same hard disk.

A useful side-effect of the extended partition scheme was to significantly increase the maximum number of partitions possible on a PC hard disk, beyond the 4 which could be described by the MBR alone.

Prior to the introduction of extended partitions, some hard disk controllers (which at that time were separate option boards, since the IDE standard did not yet exist) could make large hard disks appear as two separate disks. Alternatively, special software drivers, like Ontrack's Disk Manager could be installed for the same purpose


[edit] Final FAT16
Finally in November 1987, Compaq DOS 3.31 introduced what is today called the FAT16 format, with the expansion of the 16-bit disk sector index to 32 bits. The result was initially called the DOS 3.31 Large File System. Although the on-disk changes were apparently minor, the entire DOS disk code had to be converted to use 32-bit sector numbers, a task complicated by the fact that it was written in 16-bit assembly language.

In 1988 the improvement became more generally available through MS-DOS 4.0. The limit on partition size was now dictated by the 8-bit signed count of sectors-per-cluster, which had a maximum power-of-two value of 64. With the usual hard disk sector size of 512 bytes, this gives 32 KB clusters, thereby fixing the "definitive" limit for the FAT16 partition size at 2 gigabytes. On magneto-optical media, which can have 1 or 2 KB sectors, the limit is proportionally greater.

Much later, Windows NT increased the maximum cluster size to 64 KB by considering the sectors-per-cluster count as unsigned. However, the resulting format was not compatible with any other FAT implementation of the time, and it generated massive internal fragmentation. Windows 98 also supported reading and writing this variant, but its disk utilities didn't work with it.

The FAT16 format limits the number of entries in the root directory to 512 (entries being file and/or folder names in the old 8.3 format).[2] Use of long file names reduces this further. Even today, this limitation still applies to certain MP3 players that require the use of the FAT16 file system format.


[edit] Long File Names (VFAT, LFNs)
One of the "user experience" goals for the designers of Windows 95 was the ability to use long filenames (LFNs), in addition to classic 8.3 names. LFNs were implemented using a work-around in the way directory entries are laid out (see below). The version of the file system with this extension is usually known as VFAT after the Windows 95 VxD device driver.

Interestingly, the VFAT driver actually appeared before Windows 95, in Windows for Workgroups 3.11, but was only used for implementing 32-bit File Access, a higher performance protected mode file access method, bypassing DOS and directly using either the BIOS, or, better, the Windows-native protected mode disk drivers. It was a backport; Microsoft's ads for WfW 3.11 said 32-bit File Access was based on "the 32-bit file system from our Chicago project."[citation needed]

In Windows NT, support for long filenames on FAT started from version 3.5.


[edit] FAT32
In order to overcome the volume size limit of FAT16, while still allowing DOS real-mode code to handle the format without unnecessarily reducing the available conventional memory, Microsoft decided to implement a newer generation of FAT, known as FAT32, with cluster counts held in a 32-bit field, of which 28 bits are currently used.

In theory, this should support a total of approximately 268,435,438 (< 228) clusters, allowing for drive sizes in the range of 2 terabytes. However, due to limitations in Microsoft's scandisk utility, the FAT is not allowed to grow beyond 4,177,920 (< 222) clusters, placing the volume limit at 124.55 gigabytes, unless “scandisk” is not needed.[3]

FAT32 was introduced with Windows 95 OSR2, although reformatting was needed to use it, and DriveSpace 3 (the version that came with Windows 95 OSR2 and Windows 98) never supported it. Windows 98 introduced a utility to convert existing hard disks from FAT16 to FAT32 without loss of data. In the NT line, native support for FAT32 arrived in Windows 2000. Windows NT 4 supported FAT32 with free driver by Winternals company, later acquired by Microsoft.

Windows 2000 and Windows XP can read and write to FAT32 filesystems of any size, but the format program on these platforms can only create FAT32 filesystems up to 32 GB. Thompson and Thompson (2003) write[4] that “Bizarrely, Microsoft states that this behavior is by design.” Microsoft's knowledge base article 184006[3] indeed confirms the limitation and the by design statement, but gives no rationale or explanation. Peter Norton's opinion[5] is that “Microsoft has intentionally crippled the FAT32 file system.”

The maximum possible size for a file on a FAT32 volume is 4 GiB minus 2 B (232−2 bytes). For most users, this has become the most nagging limit of FAT32 as of 2005, since video capture and editing applications and some other software can easily exceed this limit. Most new windows machines now ship with NTFS and thus avoid these problems but those who run dual boot systems or who move external data drives between computers with different operating systems have little choice but to stick with FAT32 (although between Windows and Linux it is possible to use ext2 through the use of external drivers such as ext2 IFS; two good drivers can be found at [1] and [2]).


[edit] Third party support
The alternative IBM PC operating systems — such as Linux, FreeBSD, and BeOS — have all supported FAT, and most added support for VFAT and FAT32 shortly after the corresponding Windows versions were released. Early Linux distributions also supported a format known as UMSDOS, which was FAT with Unix file attributes (such as long file name and access permissions) stored in a separate file called --linux-.---. UMSDOS fell into disuse after VFAT was released and is not enabled by default in Linux kernels from version 2.5.7 onwards [6]. The Mac OS X operating system also supports the FAT filesystems on volumes other than the boot disk.


[edit] FAT and Alternate Data Streams
The FAT filesystem itself is not designed for supporting ADS, but some operating systems that heavily depend on them have devised various methods for handling them in FAT drives. Such methods either store the additional information in extra files and directories (Mac OS), or give new semantics to previously unused fields of the FAT on-disk data structures (OS/2 and Windows NT). The second design, while presumably more efficient, prevents any copying or backing-up of those volumes using non-aware tools; manipulating such volumes using non-aware disk utilities (e.g. defragmenters or CHKDSK) will probably lose the information.

Mac OS using PC Exchange stores its various dates, file attributes and long filenames in a hidden file called FINDER.DAT, and Resource Forks (a common Mac OS ADS) in a subdirectory called RESOURCE.FRK, in every directory where they are used. From PC Exchange 2.1 onwards, they store the Mac OS long filenames as standard FAT long filenames and convert FAT filenames longer than 31 characters to unique 31-character filenames, which can then be made visible to Macintosh applications.

Mac OS X stores metadata (Resource Forks, file attributes, other ADS) in a hidden file with a name constructed from the owner filename prefixed with "._", and Finder stores some folder and file metadata in a hidden file called ".DS_Store".

OS/2 heavily depends on extended attributes (EAs) and stores them in a hidden file called "EA DATA. SF" in the root directory of the FAT12 or FAT16 volume. This file is indexed by 2 previously reserved bytes in the file's (or directory's) directory entry. In the FAT32 format, these bytes hold the upper 16 bits of the starting cluster number of the file or directory, hence making it difficult to store EAs on FAT32. Extended attributes are accessible via the Workplace Shell desktop, through REXX scripts, and many system GUI and command-line utilities (such as 4OS2).[7]

Windows NT supports the handling of extended attributes in HPFS, NTFS, and FAT. It stores EAs on FAT using exactly the same scheme as OS/2, but does not support any other kind of ADS as held on NTFS volumes. Trying to copy a file with any ADS other than EAs from an NTFS volume to a FAT volume gives a warning message with the names of the ADSs that will be lost.

Windows 2000 onward acts exactly as Windows NT, except that it ignores EAs when copying to FAT32 without any warning (but shows the warning for other ADSs, like "Macintosh Finder Info" and "Macintosh Resource Fork").


[edit] Future
Microsoft has recently secured patents for VFAT and FAT32 (but not the original FAT), which is causing concern that the company might later seek royalties from Linux distributions and from media vendors that pre-format their products (see FAT Licensing below). Despite two earlier rulings against them, Microsoft prevailed and was awarded the patents.

Since Microsoft has announced the discontinuation of its MS-DOS-based consumer operating systems 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; its main drawbacks are the size overhead for small volumes and the very limited support by anything other than the NT-based versions of Windows, since the exact specification is a trade secret of Microsoft, which in turn makes it difficult to use a DOS floppy for recovery purposes. Microsoft provided a recovery console to work around this issue, but for security reasons it severely limited what could be done through the Recovery Console by default. The movement of recovery utilities to boot CDs based on BartPE or Linux is finally eroding this drawback.

FAT is still the normal filesystem for removable media (with the exception of CDs and DVDs), with FAT12 used on floppies, and FAT16 on most other removable media (such as flash memory cards for digital cameras and USB flash drives). Most removable media are not yet large enough to benefit from FAT32, although some larger flash drives do make use of it. FAT16 is used on these drives for reasons of compatibility and size overhead.

The FAT32 formatting support in Windows 2000 and XP is limited to volumes of 32 GB, which effectively forces users of modern hard drives either to use NTFS, to partition the drive into smaller volumes (below 32 GB), or to format the drive using third party tools such as a port of mkdosfs, fat32format or using another operating system such as os x or linux.


[edit] exFAT
exFAT is an incompatible replacement for FAT filesystems that will be introduced with Windows CE 6.0. It is intended to be used on flash drives, where FAT is used today. Windows XP and Vista file system drivers will be offered by Microsoft shortly after the release of Windows CE 6.0. exFAT introduces a free space bitmap allowing faster space allocation and faster deletes, support for files up to 2^64 bytes, larger cluster sizes (up to 32 MB in the first implementation), an extensible directory structure and name hashes for filenames for faster comparizons. It does not have short 8.3 DOS names anymore. It does not appear to have security access control lists or file system journaling like NTFS.


[edit] Design

[edit] Main disk structures
The following is an overview of the order of structures in a FAT partition or disk:

Boot
sector More reserved
sectors
(optional) File
Allocation
Table #1 File
Allocation
Table #2 Root
Directory
(FAT12/16 only) Data Region (for files and directories) ...
(To end of partition or disk)

A FAT file system is composed of four different sections.

The Reserved sectors, located at the very beginning. The first reserved sector is the Boot Sector (aka Partition Boot Record). It includes an area called the BIOS Parameter Block (with some basic file system information, in particular its type, and pointers to the location of the other sections) and usually contains the operating system's boot loader code. The total count of reserved sectors is indicated by a field inside the Boot Sector. Important information from the Boot Sector is accessible through an operating system structure called the Drive Parameter Block in DOS and OS/2.
The FAT Region. This contains two copies of the File Allocation Table for the sake of redundancy, although the extra copy is rarely used, even by disk repair utilities. These are maps of the Data Region, indicating which clusters are used by files and directories.
The Root Directory Region. This is a Directory Table that stores information about the files and directories located in the root directory. It is only used with FAT12 and FAT16 and means that the root directory has a fixed maximum size which is pre-allocated at creation of this volume. FAT32 stores the root directory in the Data Region along with files and other directories instead, allowing it to grow without such a restraint.
The Data Region. This is where the actual file and directory data is stored and takes up most of the partition. The size of files and subdirectories can be increased arbitrarily (as long as there are free clusters) by simply adding more links to the file's chain in the FAT. Note however, that each cluster can be taken only by one file, and so if a 1 KB file resides in a 32 KB cluster, 31 KB are wasted.