Introduction

This is the 16-bit version of the FAT file system. The 16-bit part describes the way units are allocated on the drive. The FAT16 file system uses a 16-bit number to identify each allocation unit (called cluster), and this gives it a total of 65.536 clusters. The size of each cluster is defined in the boot sector of the volume (volume = partition). The File System ID number usually associated with FAT16 volumes are 04h and 06h. The first is used on volumes with less than 65536 sectors (typical this is on drives less than 32 Mb in size), and the latter one is used on volumes with more than 65536 sectors. There is also another variant which is used with the LBA address mode, that variant has a File System ID of 0Eh. I do not know if the LBA variant is different from the CHS type. So far I don't se why anything should be changed to support LBA addresses.

Basic Structure

The FAT16 file system structure contains the following regions:


The first sector (boot sector) contain information which is used to calculate the sizes and locations of the other regions. The boot sector also contain code to boot the operating system installed on the volume. The data region is split up into logical blocks called clusters. Each of these clusters has an accompanying entry in the FAT region. The cluster specific entry can either contain a value of the next cluster which contain data from the file, or a so called End-of-file value which means that there are no more clusters which contain data from the file. The root directory and its sub-directories contain filename, dates, attribute flags and starting cluster information about the filesystem objects.

Boot Sector

The first sector in the reserved region is the boot sector. Though this sector is typical 512 bytes in can be longer depending on the media. The boot sector typical start with a 3 byte jump instruction to where the bootstrap code is stored, followed by an 8 byte long string set by the creating operating system. This is followed by the BIOS Parameter Block, and then by an Extended BIOS Parameter Block. Finally the boot sector contain boot code and a signature.

BIOS Parameter Block

The BIOS Parameter Block contains basic information about the overall structure of the FAT file system. That is information such as sector and cluster size, location information of FAT copies, the root directory size etc..

Extended BIOS Parameter Block

The Extended BIOS Parameter Block contains information that is only used in the FAT16 file system.

Bootstrap Code

The bootstrap code is different between operating system and versions which are intended to be loaded of this FAT16 volume. The responsability of the bootstrap code is to continue the boot sequence. If ex. MS-DOS is installed the bootstrap code will locate the file IO.SYS in the file system, load part of it into memory and then jump to a specific entrypoint in IO.SYS. What the bootstrap code does is vary between operating system.

Boot Sector Signature

The word at offset 1FEh should contain the signature AA55h. This will indicate to the BIOS that the sector is executable. The signature is also used by other applications when validating that the correct sector has been loaded. No matter what the Bytes Per Sector value is this signature should always be at offset 1FEh.

File Allocation Table

The FAT structure contain linked lists of files in the file system. Any file or directory entry in a (sub)directory list contain a cluster number for the first chunk of the file/directory. This cluster number also has an associated entry in the FAT. At this entry in the FAT a single word value either points to the next cluster/chunk or it contain an End-of-file value. These are the valid values:


Each FAT copy start with a value of FFxxh for the first cluster, where xx is equal to the Media Descriptor field in the BIOS Parameter Block. The FAT entry for the second cluster is set to the End-of-file value (FFFFh). The two highest bits of this value may be used for dirty volume flags in the following way.


It should be noted that the last entry in the FAT should be calculated by taking the number of clusters in the data area, and not relying on the Sectors Per FAT field in the BIOS Parameter Block. This is because the number of entries in the FAT doesn't have to be an even multiple of the bytes per sector value.

The maximum size of each FAT copy is 128 Kb (2 * 65536).

Directory Entry Structure

Each of the entries in a directory list is 32 byte long. The only directory which is in a fixed location is the root directory. This is also the only directory which may contain an entry with the Volume Label attribute set. The size of the root directory is defined in the BIOS Parameter Block.

Sub-directories are created by allocating a cluster which then are cleared so it doesn't contain any directory entries. Two default entries are then created: The '.' entry point to the directory itself, and the '..' entry points to the parent directory. If the contents of a sub-directory grows beyond what can be in the cluster a new cluster is allocated in the same way as for files.

This is the format of the directory entries:

Filename & Extension

The filename is 8 byte long. Shorter names must be trailing padded with space bytes (ASCII: 20h). The extension is 3 byte long and shorter names also have to be trailing padded. The characters allowed in the filename and extension are basicly the uppercase letters of the english alphabet, plus the digits 0 to 9.

The first byte in the filename is treated special. The following rules apply:

1. A value of 00h is interpertated as 'stop the search, there is no more entries in this directory'.
2. A value of 05h is should be replaced with the ASCII character E5h. The character is used in Japan.
3. Must not contain the value 20h.
4. A value of E5h is interpertated as 'the entry is free'.
5. Any of the values mentioned below are allowed.

The following characters are not allowed in the filename or its extension:

1. Any character below 20h, except the 05h character.
2. Any character in the following list: 22h ("), 2Ah (*), 2Bh (+), 2Ch (,), 2Eh (.), 2Fh (/), 3Ah (:), 3Bh (;), 3Ch (<), 3Dh (=), 3Eh (>), 3Fh (?), 5Bh ([), 5Ch (\), 5Dh (]), 7Ch (|).

For compatibility reasons it is recommended to limit the characters to:

1. Any uppercase characters from A to Z.
2. Any digit from 0 to 1.
3. Any of the following characters: #, $, %, &, ', (, ), -, @

Attribute Byte

The attribute byte defines a set of flags which can be set for directories, volume name, hidden files, system files, etc. These are the flags:

Read Only

This flag is used to prevent programs from not automatically overwriting or deleting this entry.

Hidden

This flag indicates to the system that the file should be hidden when doing normal directory listings. But in a lot of programs this can be overwritten by the user.

System

This flag indicates that the file/directory is important for the system, and shouldn't be manipulated without any concern.

Volume Name

When this flag is set, the directory entry is not pointing to a file, but to nothing. Thus the the Starting cluster must point the cluster 0. The only information used from this entry is the filename (8 bytes) plus the filename extension (3 bytes). These bytes form an 11 byte long volume label (without any .) There may be only one valid entry in the entire volume with this flag set. This entry must be in the root directory and preferably among the first entries, if not, then MS-DOS can have trouble displaying the right volume label if there are long file names present. This volume name should be the same as the one in the boot sector.

Calculation Algorithms

How to calculate the starting location of each region. The VolumeStart variable contain a LBA address of the first sector in the volume. On drives which are not partitioned the VolumeStart value is 0. On drives which are partitioned the VolumeStart variable contain the sector number at which the partition start.

Special Notes

When creating a FAT16 volume special care should be taken to ensure best compatibility. Following these rules ensure the best compatibility:

• A FAT16 partition may not have less than 4085 clusters or more than 65524 clusters.

Conclusion

The FAT family of file systems are relative simple file systems. The complexity can be enhanced by adding support for long file names, using the VFAT Long File Names. Also have a look at the 32 bit version of the FAT file system.