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File systems:


FAT (File Allocation Table) is the file system, used by DOS and Windows operating systems. A logical disk formatted in FAT system has the followings partitions:

  • boot sector;
  • file allocation table - FAT (conventionally in two copies);
  • root directory;
  • files.

FAT is allocated immediately after the loading sector and is represented by a chained list intended for search of files on the disk. There are three versions of FAT - FAT12, FAT16 and FAT32. They differ in a number of bits, used for storing of cluster number. FAT12 is mainly used for diskettes, FAT16 – for the drives of small capacity. According to the type (FAT12, FAT16 or FAT32), it has a different structure. But in the most primitive form the idea is the following: each FAT element corresponds to one cluster on disk.The first Fat byte contains the descriptor of the carrier (F8 H for the hard disk). The following several bytes describe the FAT type. They are followed by the table itself. All available space for file storage is divided into clusters. In the root directory there is a reference to the first cluster occupied by the file. The alternatives are the following: the cluster may be free, sacred, defective, the sequence end or the number of the following cluster in a file. If a cluster belongs to a file its cell contains the number of the next cell of this file. If the cell denotes the end of the file, it contains «FFFF» value. The unused clusters are labeled as zero. «Bad» clusters are labeled as a special code. The file is read out until the sequence end code is reached. In DOS (and its direct successor – Windows) two FAT copies are used for safety. Both copies must be identical (they are allocated in succession on the disk).

Created during formatting.

Different FAT extensions were also implemented for different operating systems. For example, there are additional file access attributes in DR-DOS; long filenames support in Windows 95, Linux and Proolix; extended file attributes support in OS/2.

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