The file allocation table or FAT stores information about
the clusters on the disk in a table. There are three different varieties of this file
allocation table, which vary based on their size. The system utility that you use to
partition the disk will normally choose the correct type of FAT for the volume you are
using, but sometimes you will be given a choice of which you want to use.
Since each cluster has one entry in the FAT, and these entries are used to hold the
cluster number of the next cluster used by the file, the size of the FAT is the limiting
factor on how many clusters any disk volume can contain. The following are the three
different FAT versions now in use:
- FAT12: The oldest type of FAT uses a 12-bit binary number to hold the cluster
number. A volume formatted using FAT12 can hold a maximum of 4,086 clusters, which is 2^12
minus a few values (to allow for reserved values to be used in the FAT). FAT12 is
therefore most suitable for smaller volumes, and is used on floppy disks and hard disk
partitions smaller than about 16 MB.
- FAT16: The FAT used for most hard disk partitions uses a 16-bit binary number to
hold cluster numbers. When you see someone refer to a "FAT" volume generically,
they are usually referring to FAT16, because it is the de facto standard for hard disks. A
volume using FAT16 can hold a maximum of 65,526 clusters, which is 2^16 less a few values
(again for reserved values in the FAT). FAT16 is used for hard disk volumes ranging in
size from 16 MB to 2,048 MB.
- FAT32: The newest FAT type, FAT32 is supported by Windows 95's OEM SR2 release,
as well as Windows 98. FAT32 uses a 28-bit binary cluster number--not 32, because 4
of the 32 bits are "reserved". 28 bits is still enough to permit ridiculously
huge volumes--FAT32 can theoretically handle volumes with over 268 million clusters, and
will support (theoretically) drives up to 2 TB in size. However to do this the size of the FAT grows very large; see here for details on FAT32's limitations.
Here's a summary table showing how the three types of FAT compare:
Attribute |
FAT12 |
FAT16 |
FAT32 |
Used For |
Floppies and small hard disk volumes |
Small to large hard disk volumes |
Medium to very large hard disk volumes |
Size of Each FAT
Entry |
12 bits |
16 bits |
28 bits |
Maximum Number of
Clusters |
4,086 |
65,526 |
~268,435,456 |
Cluster Size Used |
0.5 KB to 4 KB |
2 KB to 32 KB |
4 KB to 32 KB |
Maximum Volume Size |
16,736,256 |
2,147,123,200 |
about 2^41 |
Next: FAT Partition Efficiency: Slack
