FAT Full Form

Full Form of FAT

FAT stands for File Allocation Table, is a table maintained by the Operating System on a hard disk that provides a map of the clusters in which a file has been stored. The operating system creates a FAT entry for every new file that records each cluster’s location and its sequential order. When we read a file, the operating system (OS) reassembles the file into clusters and then places it as an entirely new file where we want to read it. FAT was designed to help the hard drives and their subdirectories. The earlier FAT12 had a cluster that addresses 12-bit values with 4078 clusters. It allows up to 4084 clusters with UNIX.

Uses of FAT

The File Allocation Table was first introduced in DOS and in Windows 9x, through hard disks. At that time Microsoft introduced their new file system, NTFS. It was only applicable for Windows NT platform in 1993. FAT was used through hard drives. FAT still remained for becoming the standard for all the home users until the invention of NT-based Windows XP in 2001.

Microsoft Windows, later on, added a feature which is a pre-installed tool which converts a FAT file system into NTFS directly. It also does it without rewriting any of the files. But, it can’t be reversed in a direct way.

The naming of the FAT

FAT generally refers to three generations, FAT12, FAT16, and FAT 32. FAT 16 is the actual representation of FAT system file systems with 2 to the power 16 bit cluster entries.

Similarly, with FAT 32, 2 to the power 32 bit cluster entries can be held. Values remain accumulated in the Disk parameter block that can identify the file structures.

VFAT is another form of FAT, where optional extension types for long file names can be kept. It can work on top of any FAT file system. The Volumes which use VFAT file systems can also be read on other operating systems, which do not support any VFAT file system extension.

The general type of file system (FAT12, FAT16, or FAT32) is determined by the width of the cluster entries in the FAT. The previous one was FAT 12, holding 2 to the power 12 entries. In FAT 32, only 2 to the power 28 clusters are used as the other 4 bits are used by the system only.

Types of FATs

8-bit FAT8

Marc Donald designed and implemented the original FAT file system based on a series of discussions between McDonald’s and Bill Gates. It was introduced with 8-bit table elements in 1977or 1978. In 1978, The FAT file system was also used in Microsoft’s Z80 platforms. The Standalone Disk BASIC version supported three FATs, a parameter for MIDAS. Reportedly, MIDAS was also prepared to support 10-bit, 12-bit, and 16-bit FAT variants.

12-bit FAT12

FAT12 is used through 12-bit entries for the cluster address. Some values were reserved for marking the end of a chain of clusters for marking unusable disk areas. So the maximum number of clusters was limited to 4078. To reserve disk space, two 12-bit FAT entries used three consecutive 8-bit bytes on the disk, requiring manipulation to unpack the 12-bit values. This was sufficient for the original floppy disk drives and small hard disks up to 32 megabytes. The FAT 6B version, available with DOS 3.31, supported the 32-bit sector operating system and increased the volume size limit.

16-bit FAT16

Cluster addresses were increased to 16-bit, allowing for up to 65,526 clusters; However, the possible number of sectors and the partition size of 32 MB never changed. Although cluster addresses were 16 bits, this format was not commonly understood as FAT16.

MS-DOS 3.0 to MS-DOS 3.30 could still access FAT12 partitions under 15 MB but required all 16 MB-32 MB partitions to be FAT16, so could not access MS-DOS 2.0 partitions in this size range. MS-DOS 3.31 and higher could access 16 MB-32 MB FAT12 partitions again.

The FAT16B improvement became more generally available through DOS 3.31, PC DOS 4.0, OS/2 1.1, and MS-DOS 4.0. The limit on partition size was dictated by the 8-bit signed count of sectors per cluster, which originally had a maximum power-of-two value of 64. The standard hard disk sector size of 512 bytes gives a maximum of 32 KB cluster size, thereby fixing the “definitive” limit for the FAT16 partition size at 2 GB for sector size 512.

32-bit FAT32

To overcome the size limit of FAT16, Microsoft designed a new version of the file system, FAT32, which supported an increased number of possible clusters. This could reuse most of the existing code to increase the conventional memory by less than 5 KB under DOS. Windows 98 introduced a utility to convert existing hard disks from FAT16 to FAT32 without data loss.

The latter type is also named FAT32X to indicate usage of LBA disk access instead of CHS. On such partitions, CHS-related geometry entries, namely the CHS in the MBR and the number of sectors per track, and the number of heads in the EBPB record may contain no or misleading values and should not be used.

Limitations of FAT

The FAT implementation also has its following limitations:

• It has multiple partitions that are not supported or detected
• Multiple active file handling is allowed only for files opened in reading mode
• The maximum path length is only 260 bytes for FAT. Seeking within files is limited to the area of two gigabytes due to the fseek and ftell limitation set by the ANSI C Standard library
• It does not support UTF 16 and UTF 8

Conclusion

In 2006 the features of Microsoft’s implementation of the FAT system were reversing. Microsoft tried to enforce its patents against Linux. Microsoft gave access to four of TomTom’s patents. TomTom will drop support for the VFAT long filenames from its products. In return, Microsoft will not seek any legal action against TomTom for the five-year duration of the settlement agreement.