What is the significance of FAT? What are the limitations of FAT 16? 4m jun2006

By | January 30, 2015

What is the significance of FAT? What are the limitations of FAT 16?  4m jun2006


The FAT maps the usage of data space of the disk. It contains information about the space used by each individual file, the unused disk space and the space that is unusable due to defects in the disk. Since FAT contains vital information, two copies of FAT are stored on the disk, so that in case one gets destroyed, the other can be used.

A FAT entry can contain any of the following:

  • unused cluster
  • reserved cluster
  • bad cluster
  • last cluster in file
  • next cluster number in the file.

The DOS file system maintains a table of pointers called FAT (File allocation table) which consists of an array of 16-bit values. There is one entry in the FAT for each cluster in the file area, i.e., each entry of the FAT (except the two) corresponds to one cluster of disk space. If the value in the FAT entry doesn’t mark an unused, reserved or defective cluster, then the cluster corresponding to the FAT entry is part of a file and the value in the FAT entry would indicate the next cluster in the file.

The first two entries (0 & 1) in FAT are reserved for use by the operating system.

Therefore, the cluster number 2 corresponds to the first cluster in the data space of the disk. Prior to any data being written on to the disk, the FAT entries are all set to zero indicating a ‘free’ cluster .The FAT chain for a file ends with the hexadecimal value, i.e., FFFF. The FAT structure can be shown as in Figure 2 below.


 Limitation of FAT16: The DOS designers decided to use clusters with at least four sectors in them (thus a cluster size of at least 2KB) for all FAT16 hard disks. That size suffices for any hard disk with less than a 128MB total capacity. The largest logical disk drives that DOS can handle comfortably have capacities up to 2GB. For such a large volume, the cluster size is 32KB. This means that even if a file contains only a single byte of data, writing it to the disk uses one entire 32KB region of the disk, making that area unavailable for any other file’s data storage.

The most recent solution to these large-disk problems was introduced by Microsoft in its OSR2 release of Windows 95 and it was named FAT32. The cluster entry for FAT32 uses 32-bit numbers. The minimum size for a FAT32 volume is 512MB.

Microsoft has reserved the top four bits of every cluster number in a FAT32 file allocation table. That means there are only 28-bits for the cluster number, so the maximum cluster number possible is 268,435,456.