File Allocation Table (FAT)

File Allocation Table (FAT) is a file system that was developed 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 considered relatively uncomplicated, and because of that, it is a popular format
for floppy disks; moreover, it is supported by virtually all existing operation systems for personal
computers, and because of that, it is often used to share data between several operation systems
booting on the same computer (a multi-boot environment). It is also used on solid-state memory
cards and other similar devices. It has a serious drawback in that when files are deleted and new
files written to the media, the files can become scattered over the entire media making reading and
writing a slow process. De-fragmentation 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.
FAT is also called 12-bit FAT, the file allocation table (FAT) for a floppy disk. The location of
files on a floppy disk are listed in a one-column table in the FAT. Because the width of each entry
in a floppy disk column is 12 bits, the FAT is called FAT12. As a file system for floppy disks, it
had a number of limitations: no support for hierarchical directories, cluster addresses were “only”
12-bits long (which 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 32MB.
The FAT file system, as is the case with most file systems, does not utilize individual sectors, and
there are several performance reasons for this. By using individual sectors, the process of
managing disks becomes overly cumbersome since files are being broken into 512-byte pieces. If
you were to take a 20 GB disk volume set up with 512 byte sectors and manage them individually,
the disk would have over 40 million individual sectors. Just keeping track of this many pieces of
information is both time, as well as resource, consuming. While some operation systems do
allocate specific sector storage, they also require some advanced intelligence to do so. Bear in
mind how old the FAT file system is, as it was designed many years ago as merely a simple file
system, without the capability to managed individual sectors.
In order for FAT to manage files with some form of efficiency is to group sectors into larger
blocks referred to as clusters, or allocation units. Cluster size, however, is not a predetermined size,
but rather is determined by the size of the disk volume itself, with small volumes (disk sizes)
resulting in smaller clusters, and larger volumes (disk sizes) using larger cluster sizes. For the
most part, a cluster ranges in size from 4 sectors or 2,048 bytes to 64 sectors or 32,768 bytes. You
should be aware that you may, on some occasions, find 128-sector clusters in use at 65,536 bytes
per cluster, as well as some floppy disks with smaller clusters that is usual at just 1 sector per
cluster. In all cases, the sectors in a cluster are continuous, therefore each cluster is a continuous
block of space on the disk.
Cluster sizing, and therefore partition or volume size, as they are directly related, have an
important impact on performance and disk utilization. In all cases, cluster size is determined at the
time a disk volume is partitioned. Certain third-party partitioning utilities such as Partition Magic
by PowerQuest can alter the cluster size of an existing partition within specific parameters.
However, this aside, once the partition size is selected, so are the cluster sizes fixed.
FAT 16 means that file allocation table that uses 16 bits for addressing clusters. It is commonly
used with DOS and Windows 95 systems. A 16-bit DOS and Windows file system (see FAT) that
varies cluster sizes based on hard drive size. Cluster sizes range from 4K (for drives up to 127MB),
to 4K (255MB drives), 8K (511MB drives), 16K (1GB drives). and 32K (for drives up to 2GB).
The ultimate capacity of a FAT16 partition is 2GB.