Disks formatted using FAT are organized as shown in Figure 20-1. They have a boot sector that stores information about the disk type, starting and ending sectors, the active partition, and a bootstrap program that executes at startup and boots the operating system. This is followed by a reserve area that can be one or more sectors in length.

Figure 20-1. An overview of FAT16 volume structure

The reserve area is followed by the primary file allocation table, which provides a reference table for the clusters on the volume. Each reference in the table relates to a specific cluster and defines the cluster's status as follows:

If a cluster is in use, the cluster entry identifies the number of the next cluster in the file or that it is the last cluster of a file, in which case the end of the file has been reached.

FAT volumes also have the following features:

When an application attempts to read a file, the operating system looks up the starting cluster of the file in the root directory table and then uses the file allocation table to find and read all the clusters in the file.

Although FAT supports basic file and folder operations, its features are rather limited. By using FAT, you have the following capabilities:

In addition, although FAT16 supports small cluster sizes, FAT32 does not. Table 20-2 provides a summary of FAT16 and FAT32.

Inside Out: FAT on very small media

It is important to note that FAT volumes are structured differently depending on volume size. When you format a volume that is less than 32,680 sectors (16 megabytes [MB]), the format program uses 12 bits for FAT12. This means less space is reserved for each entry in the table and more space is made available for data. This technique is meant to be used with very small media, such as floppy disks.

In FAT, disk sectors are 512 bytes. By default Windows Server 2003 sets the size of clusters and the number of sectors per cluster based on the size of the volume. Disk geometry also is a factor in determining cluster size because the number of clusters on the volume must fit into the number of bits used by the file system. The actual amount of data you can store on a single FAT volume is a factor of the maximum cluster size and the maximum number of clusters you can use per volume. This can be written out as a formula:

ClusterSize × MaximumNumberOfClusters = MaximumVolumeSize

FAT16 supports a maximum of 65,526 clusters and a maximum cluster size of 64 KB. This is where the limitation of 4 gigabytes (GB) for volume size comes from. With disks less that 32 MB but more than 16 MB in size, the cluster size is 512 bytes and there is one sector per cluster. This changes as the volume size increases up to the largest cluster size of 64 KB with 128 sectors per cluster on 2-GB to 4-GB volumes.

FAT32 volumes using 512-byte sectors can be up to 2 terabytes (TB) in size and can use clusters of up to 64 KB. To control the maximum number of clusters allowed, the Windows operating system reserves the upper 4 bits, however, limiting FAT32 to a maximum 28 bits worth of clusters. With a maximum recommended cluster size of 32 KB (instead of the maximum allowable 64 KB), this means a FAT32 volume on the Windows operating system can be up to 32 GB in size. Because the smallest cluster size allowed for FAT32 volumes is 512 bytes, the smallest FAT32 volume you can create is 33 MB.

Inside Out: Getting volume format and feature information

A quick way to check the file system type and available features of a volume is to type fsutil fsinfo volumeinfo DriveDesignator at the command prompt, where DriveDesignator is the drive letter of the volume followed by a colon, such as C:. For a FAT or FAT32 volume, you'll see output similar to the following:

Volume Name : LogData
Volume Serial Number : 0x70692a2e
Max Component Length : 255
File System Name : FAT32
Preserves Case of filenames
Supports Unicode in filenames