File Allocation Table - Technical Design - Layout

Layout

A FAT file system is composed of four different sections:

• The Reserved sectors, located at the very beginning.

The first reserved sector (logical sector 0) is the Boot Sector (aka Volume Boot Record (VBR)). It includes an area called the BIOS Parameter Block (with some basic file system information, in particular its type, and pointers to the location of the other sections) and usually contains the operating system's boot loader code.

Important information from the Boot Sector is accessible through an operating system structure called the Drive Parameter Block (DPB) in DOS and OS/2.

The total count of reserved sectors is indicated by a field inside the Boot Sector, and is usually 32 on FAT32 filesystems.

For FAT32 file systems, the reserved sectors include a File System Information Sector at logical sector 1 and a Backup Boot Sector at logical sector 6.

While many other vendors have continued to utilize a single-sector setup (logical sector 0 only) for the bootstrap loader, Microsoft's boot sector code has grown to spawn over logical sectors 0 and 2 since the introduction of FAT32, with logical sector 0 depending on sub-routines in logical sector 2. The Backup Boot Sector area consists of three logical sectors 6, 7, and 8 as well. In some cases, Microsoft also uses sector 12 of the reserved sectors area for an extended boot loader.

• The FAT Region.

This typically contains two copies (may vary) of the File Allocation Table for the sake of redundancy checking, although rarely used, even by disk repair utilities.

These are maps of the Data Region, indicating which clusters are used by files and directories. In FAT12 and FAT16 they immediately follow the reserved sectors.

Typically the extra copies are kept in tight synchronization on writes, and on reads they are only used when errors occur in the first FAT. In FAT32, it is possible to switch from the default behaviour and select a single FAT out of the available ones to be used for diagnosis purposes.

The first two clusters (cluster 0 and 1) in the map contain special values.

• The Root Directory Region.

This is a Directory Table that stores information about the files and directories located in the root directory. It is only used with FAT12 and FAT16, and imposes on the root directory a fixed maximum size which is pre-allocated at creation of this volume. FAT32 stores the root directory in the Data Region, along with files and other directories, allowing it to grow without such a constraint. Thus, for FAT32, the Data Region starts here.

• The Data Region.

This is where the actual file and directory data is stored and takes up most of the partition. Traditionally, the unused parts of the data region are initialized with a filler value of 0xF6 as per the INT 1Eh's Disk Parameter Table (DPT) during format on IBM compatible machines, but also used on the Atari Portfolio. 8-inch CP/M floppies typically came pre-formatted with a value of 0xE5; by way of Digital Research this value was also used on Atari ST formatted floppies. Amstrad used 0xF4 instead. Some modern formatters wipe hard disks with a value of 0x00, whereas a value of 0xFF, the default value of a non-programmed flash block, is used on flash disks to reduce wear. The latter value is typically also used on ROM disks. (Some advanced formating tools allow to configure the format filler byte.)

The size of files and subdirectories can be increased arbitrarily (as long as there are free clusters) by simply adding more links to the file's chain in the FAT. Note however, that files are allocated in units of clusters, so if a 1 KB file resides in a 32 KB cluster, 31 KB are wasted.

FAT32 typically commences the Root Directory Table in cluster number 2: the first cluster of the Data Region.

FAT uses little-endian format for all entries in the header (except for, where explicitly mentioned, for some entries on Atari ST boot sectors) and the FAT(s). It is possible to allocate more FAT sectors than necessary for the number of clusters. The end of the last FAT sector can be unused if there are no corresponding clusters. The total number of sectors (as noted in the boot record) can be larger than the number of sectors used by data (clusters × sectors per cluster), FATs (number of FATs × sectors per FAT), and hidden sectors including the boot sector: this would result in unused sectors at the end of the volume. If a partition contains more sectors than the total number of sectors occupied by the file system it would also result in unused sectors at the end of the volume.