File allocation table, known as FAT (see ABBREVIATIONFINDER.ORG). The part of the DOS and OS / 2 file system that keeps track of the location of data stored on a hard disk. When the disk is high-level formatted, the FAT is logged twice and contains a table with one entry for each cluster(cluster) on the hard disk. Each FAT input has a 16-bit or 32-bit extension (it all depends on whether it is a FAT16 or FAT32 input).
The directory listing, which contains the file name, extension, date, etc., points to the entry in the FAT where the file begins. If a file is larger than a cluster, the first entry in the FAT points to the next entry in the file where the second cluster of the file is stored, and so on until the end of the file. If a cluster goes bad, its FAT entry is marked as such and that cluster is not used again.
FAT was created by Bill Gates and Marc McDonald in 1977 in order to handle BASIC disks. It was incorporated for the first time in the operating system QDOS by Tim Paterson in August of 1980, for computers S-100 architecture Intel 8086. This file system was the main difference between QDOS and CP / M.
File Allocation Table.
|Full name||File Allocation Table|
|(12-bit version)||(16-bit version)||(32-bit version)|
|Inserted||1977 (Microsoft Disk BASIC)||July 1988 (MS-DOS 4.0)||August 1996 (Windows 95 OSR2)|
|Partition identifier||0x01 (MBR)||0x04, 0x06, 0x0E (MBR)||0x0B, 0x0C (MBR)
|File location||Linked list|
|Bad blocks||Linked list|
|Maximum file size||32 MB||2 GB||4GB|
|Maximum number of files||4,077||65,517||268,435,437|
|Maximum file name length||8.3 (11) or 255 characters when using LFNs (Long File Names)|
|Maximum volume size||2 MB||2 GB||2 TB|
|Stored dates||Creation, modification, access|
|Supported date range||As January 1 as 1980 – 31 as December as 2107|
|Attributes||Read-only, hidden, system, file, volume name|
|Transparent compression||By volume, Stacker, DoubleSpace, DriveSpace||No|
|Transparent encryption||By volume only with DR-DOS||No|
Floppy file system, so it has several limitations:
- It does not support folder nesting.
- Block addresses only contain 12 bits. This complicates the implementation.
- The disk size is stored as a 16-bit count expressed in sectors, limiting the manageable space to 32 megabytes.
Currently the FAT32 system has completely displaced the old FAT16 file system. This new version allows creating volumes of up to 2 TB, compared to the 2 GB limited by FAT16. The latter system wasted clusters, so if the system had to save files of, for example 2 KB, it wasted by doing so a whole 32KB cluster. With the FAT32 system you would use a 4 KB cluster, thus saving system resources. In addition, a slight increase in the capacity of the hard disk is achieved.
Disadvantages of FAT16.
- The FAT16 file system does not have local file system security or compression features.
- The boot sector is not supported.
- The root folder can only have a maximum of 512 entries, which means that files with long names can greatly decrease the number of available entries.
- FAT16 does not work well with volume sizes that are large.
It first appeared in Windows 95 OSR2. It was necessary to reformat to use the advantages of FAT32. Interestingly, DriveSpace 3 (included with Windows 95 and 98) did not support it. Windows 98 incorporated a tool to convert from FAT16 to FAT32 without data loss. This support was not available on the business line until Windows 2000.
The maximum file size in FAT32 is 4 gigabytes (232-1 bytes), which is cumbersome for video editing and capture applications, as the files generated by these easily exceed this limit.
VFAT and FASTFAT.
Windows 3.11 introduced a new file system access scheme, using 32-bit protected mode (present in Intel 386s and later) bypassing the MS-DOSkernel. To do this, it was directly using the BIOS or the disk drive hardware. This also made it possible to use a cache, speeding up access. All of this was called VFAT or virtual FAT.
Windows NT 3.1 provided the same approach, but called it FASTFAT. However, it was natural for Windows NT drivers to use 32-bit protected mode. It is often confused with LFN (long file names) support as it was enabled by default in Windows 95.
The FAT file system is made up of four sections:
- The region of the root directory. It is the main index of folders and files.
- The boot sector. It is always the first sector of the partition (volume) and includes basic information, pointers to the other sections, and the address of the operating system’s startup routine .
- The FAT region. Contains two copies of the file allocation table (for security purposes). These are partition maps, indicating which clusters are occupied by the files.
- The data region. It is the place where the contents of files and folders are stored. Therefore, it occupies almost the entire partition. The size of any file or folder can be expanded as long as there are enough free clusters. Each cluster is linked to the next one by means of a pointer. If a given cluster is not fully occupied, its remaining space is wasted.