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File Allocation Table facts for kids

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FAT
Developer(s) Microsoft, NCR, SCP, IBM, Compaq, Digital Research, Novell, Caldera
Full name File Allocation Table
Variants 8-bit FAT, FAT12, FAT16, FAT16B, FAT32, ExFAT, FATX, FAT+
Introduced 1977 (1977) with Standalone Disk BASIC-80
Partition IDs MBR/EBR:
  • FAT12: 0x01 e.a. (Extended Attribute)
  • FAT16: 0x040x060x0E e.a.
  • FAT32: 0x0B0x0C e.a.
  • BDP: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
Structures
Directory contents Table
File allocation Linked list
Bad blocks Cluster tagging
Limits
Max volume size
  • FAT12: 32 MB (256 MB for 64 KB clusters)
  • FAT16: 2 GB (4 GB for 64 KB clusters)
  • FAT32: 2 TB (16 TB for 4 KB sectors)
Max file size 4,294,967,295 bytes (4 GB − 1) with FAT16B and FAT32
Max no. of files
  • FAT12: 4,068 for 8 KB clusters
  • FAT16: 65,460 for 32 KB clusters
  • FAT32: 268,173,300 for 32 KB clusters
Max filename length 8.3 filename, or 255 UCS-2 characters when using LFN
Features
Dates recorded
  • Modified date/time, creation date/time (DOS 7.0 and higher only),
  • access date (only available with ACCDATE enabled),
  • deletion date/time (only with DELWATCH 2)
Date range 1980-01-01 to 2099-12-31 (2107-12-31)
Date resolution
  • 2 seconds for last modified time,
  • 10 ms for creation time,
  • 1 day for access date,
  • 2 seconds for deletion time
Forks Not natively
Attributes Read-only, hidden, system, volume, directory, archive
File system
permissions
  • FAT12/FAT16: File, directory and volume access rights for read, write, execute, delete only with DR-DOS, PalmDOS, Novell DOS, OpenDOS, FlexOS, 4680 OS, 4690 OS, Concurrent DOS, Multiuser DOS, System Manager, REAL/32:
    • execute with only FlexOS, 4680 OS, 4690 OS;
    • individual file / directory passwords not with FlexOS, 4680 OS, 4690 OS;
    • world/group/owner permission classes only with multiuser security loaded
  • FAT32: Partial, only with DR-DOS, REAL/32 and 4690 OS
Transparent
compression
  • FAT12/FAT16: Per-volume, SuperStor, Stacker, DoubleSpace, DriveSpace
  • FAT32: No
Transparent
encryption
  • FAT12/FAT16: Per-volume only with DR-DOS
  • FAT32: No

The File Allocation Table (FAT) is a special system that helps computers organize files on storage devices. It was the main way computers like those running MS-DOS and Windows 9x stored information.

FAT was first created in 1977 for floppy disks. Later, it was changed to work with hard disks and other storage. As storage devices got bigger, FAT needed updates. This led to four main versions: FAT12, FAT16, FAT32, and ExFAT.

Even though newer systems like NTFS replaced FAT as the main file system in Windows starting with Windows XP, FAT is still very important. It's used in flash drives, USB flash drives, and many portable devices. This is because it's easy to use and works with many different devices.

How FAT Organizes Files

To understand FAT, imagine your computer's storage as a huge library. When you save a file, the computer needs to know where all its parts are. FAT acts like a special index or map for this library.

The File Allocation Table

The main part of FAT is the File Allocation Table itself. This table is like a list of all the storage spaces on your disk. These storage spaces are called clusters. A cluster is a small, connected area on the disk where data is stored.

When you save a file, it might be too big for just one cluster. So, the computer breaks the file into pieces. Each piece goes into a different cluster. The FAT table then keeps track of which cluster comes next for each file. It's like a linked list of clusters.

For example, if a file starts in Cluster 5, the FAT table will tell the computer that the next part of the file is in Cluster 12. Then, from Cluster 12, it might point to Cluster 20, and so on. This chain continues until a special marker shows the end of the file.

Directory Entries

The root directory is like the main catalog of your disk. It lists the first cluster for each file. Sub-directories are like smaller catalogs inside the main one. They are treated like special files that contain lists of their own files.

Each entry in the FAT table uses a certain number of bits: 12, 16, or 32. The more bits used, the more clusters the system can keep track of. This means it can support larger disks.

If a file is very small, it still takes up a whole cluster. This can waste space if you have many tiny files and large clusters.

Where FAT Is Used

FAT has been used in many places over the years. It's still important for certain types of storage.

Past Uses of FAT

FAT was the standard file system for DOS and Windows 9x computers. This means most home computers used FAT until Windows XP came out in 2001. Windows Me was the last Windows version to use FAT as its main file system.

For floppy disks, FAT12 and FAT16 were standardized. This helped different computers use floppy disks in the same way.

Modern Uses of FAT

Today, FAT is still used in many places.

  • Booting Computers: Modern computers use FAT for a special part called the EFI system partition. This helps the computer start up.
  • Sharing Files: FAT is great for USB flash drives and memory cards. This is because it works with many different operating systems like Windows, Linux, and older DOS systems.
  • Portable Devices: Devices like digital cameras, camcorders, and media players often use FAT. This is why you can easily plug your camera into a computer and see your photos.
  • Digital Cameras: Almost all digital cameras use a system called DCF. This system requires using FAT12, FAT16, FAT32, or exFAT to store photos.

Different Versions of FAT

Over time, FAT has been updated to handle bigger storage devices. Each major update is named after the number of bits it uses for its entries.

Original 8-bit FAT

The very first FAT system used 8-bit entries. It was created in 1977 or 1978 by Marc McDonald at Microsoft. This early version was used on 8-inch floppy disks. It was a simple system without sub-directories.

FAT12

In 1980, Tim Paterson at Seattle Computer Products (SCP) improved FAT. He made the entries 12 bits long. This version, called FAT12, was used in QDOS and later in MS-DOS and IBM PC DOS.

FAT12 could handle up to 4,078 clusters. This was good for floppy disks and small hard disks up to 32 megabytes. To save space, two 12-bit entries were packed into three 8-bit bytes.

FAT16

In 1984, with the release of PC DOS 3.0 and MS-DOS 3.0, the cluster addresses were increased to 16 bits. This allowed for up to 65,526 clusters. This version was called FAT16.

The first FAT16 could handle partitions up to 32 megabytes. Later, in 1987, Compaq introduced an improved FAT16, often called FAT16B. This version used 32-bit numbers for sectors, allowing for much larger partitions, up to 2 GB.

Windows NT later increased the maximum cluster size for FAT16 to 64 KB, allowing even larger volumes, but this was not compatible with older systems.

FAT32

To handle even larger disks, Microsoft created FAT32 in 1996 with Windows 95 OSR2. This version uses 32-bit numbers for clusters, but only 28 bits are actually used for the cluster number.

FAT32 can support very large volumes, up to 2 terabytes with standard sector sizes, and even up to 16 TB with larger sectors. However, a single file on a FAT32 volume cannot be larger than 4 GB minus 1 byte. This is a limit of how file sizes are stored in the directory.

Windows 98 included a tool to convert FAT16 disks to FAT32 without losing data. Windows 2000 and later versions of Windows also support FAT32.

FAT Extensions

Over the years, different features have been added to FAT to make it more useful.

Long File Names (VFAT)

Older FAT systems only allowed short file names, like "MYFILE.TXT" (8 characters for the name, 3 for the extension). With Windows 95, users wanted to use longer, more descriptive names.

This led to the VFAT (Virtual FAT) extension. VFAT allows file names up to 255 characters long. It does this by storing the long name in a special way that doesn't change the basic FAT structure. This means older systems that don't understand VFAT can still see and use the files, but only with their short names.

Extended Attributes

Some operating systems, like OS/2 and Windows NT, use "extended attributes" (EAs). These are extra pieces of information about a file, like its creator or special settings. OS/2 stores these in a hidden file in the root directory. Windows NT also supports this for FAT12 and FAT16.

FATX

FATX is a special version of FAT used for Microsoft's Xbox video game console. It's similar to FAT16 and FAT32 but has some key differences, making it incompatible with regular FAT systems. For example, file names can be up to 42 characters long, and the dates files were created or changed start from the year 2000 instead of 1980.

exFAT

exFAT is a newer file system introduced by Microsoft in 2006. It's designed for flash drives and memory cards, especially larger ones like SDXC cards. The main advantage of exFAT is that it can handle files much larger than 4 GB, which is a limit for FAT32. It stores file sizes using 8 bytes instead of 4, allowing for extremely large files.

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