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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Logical Structures and File Systems | Partitioning, Partition Sizes and Drive Lettering ]

FAT Sizes: FAT12, FAT16 and FAT32

Throughout my discussion of file systems, I have referred to the FAT family of file systems. This includes several different FAT-related file systems, as described here. 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 the maximize size of the table. 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 very small volumes, and is used on floppy disks and hard disk partitions smaller than about 16 MB (the latter being rare today.)
  • FAT16: The FAT used for most older systems, and for small partitions on modern systems, 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, even with FAT32 now more popular than FAT16. 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. VFAT is a variant of FAT16.
  • FAT32: The newest FAT type, FAT32 is supported by newer versions of Windows, including Windows 95's OEM SR2 release, as well as Windows 98, Windows ME and Windows 2000. 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 very small hard disk volumes

Small to moderate- sized hard disk volumes

Medium-sized 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

Tip: If you are not sure of whether a given disk volume on your PC is formatted using FAT16 or FAT32, you can use this procedure to find out.

Next: FAT Partition Efficiency: Slack


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