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File Allocation Table (FAT)

Definition - What does File Allocation Table (FAT) mean?

A file allocation table (FAT) is a file system developed for hard drives that originally used 12 or 16 bits for each cluster entry into the file allocation table. It is used by the operating system (OS) to manage files on hard drives and other computer systems. It is often also found on in flash memory, digital cameras and portable devices. It is used to store file information and extend the life of a hard drive.

Most hard drives require a process known as seeking; this is the actual physical searching and positioning of the read/write head of the drive. The FAT file system was designed to reduce the amount of seeking and thus minimize the wear and tear on the hard disc.

FAT was designed to support hard drives and subdirectories. The earlier FAT12 had a cluster addresses to 12-bit values with up to 4078 clusters; it allowed up to 4084 clusters with UNIX. The more efficient FAT16 increased to 16-bit cluster address allowing up to 65,517 clusters per volume, 512-byte clusters with 32MB of space, and had a larger file system; with the four sectors it was 2,048 bytes.

Techopedia explains File Allocation Table (FAT)

FAT16 was introduced in 1983 by IBM with the simultaneous releases of IBM's personal computer AT (PC AT) and Microsoft’s MS-DOS (disk operating system) 3.0 software. In 1987 Compaq DOS 3.31 released an expansion of the original FAT16 and increased the disc sector count to 32 bits. Because the disc was designed for a 16-bit assembly language, the whole disc had to be altered to use 32-bit sector numbers.

In 1997 Microsoft introduced FAT32. This FAT file system increased size limits and allowed DOS real mode code to handle the format. FAT32 has a 32-bit cluster address with 28 bits used to hold the cluster number for up to approximately 268 million clusters. The highest level division of a file system is a partition. The partition is divided into volumes or logical drives. Each logical drive is assigned a letter such as C, D or E.

A FAT file system has four different sections, each as a structure in the FAT partition. The four sections are:

  • Boot Sector: This is also known as the reserved sector; it is located on the first part of the disc. It contains: the OS's necessary boot loader code to start a PC system, the partition table known as the master boot record (MRB) that describes how the drive is organized, and the BIOS parameter block (BPB) which describes the physical outline of the data storage volume.
  • FAT Region: This region generally encompasses two copies of the File Allocation Table which is for redundancy checking and specifies how the clusters are assigned.
  • Data Region: This is where the directory data and existing files are stored. It uses up the majority of the partition.
  • Root Directory Region: This region is a directory table that contains the information about the directories and files. It is used with FAT16 and FAT12 but not with other FAT file systems. It has a fixed maximum size that is configured when created. FAT32 usually stores the root directory in the data region so it can be expanded if needed.

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