File Allocation Table (FAT) is an area on the hard disk or floppy disk containing information of every file stored and unallocated spaces on the disk and first developed for MS-DOS. Most of the PCs operating system support FAT file system hence, external devices like memory cards, SD, pen drives etc., supports FAT file system. FAT12 is the oldest type of FAT file system. FAT12 indicates that the size of each entry in the file allocation table of a disk is 12 bits. FAT12 is therefore suitable for small volumes (floppy disk) having size of 16MB or less. It has a cluster size ranging from 0.5KB to 4KB.

FAT12 Cluster:

The size of the FAT determines the number of clusters the disk volume can contain. FAT12 can store 4086 clusters (2 12- few clusters,   approx). Floppy disks have FAT12 file system. The hierarchy from top level to bottom is as follows:
Disk → Partition (one or more) → Clusters (one or more) → Sectors (one or more, depending on size)

The entry provides the number of the next cluster. FAT is something like table of contents in a book. If the file allocation table is lost or damaged, then reading the disk becomes a problem.

Regions of FAT12 disk or disk organization

    • Reserved or Boot Sector
    • FAT
    • Root directory
    • Data area
  • Reserved area or boot sector contains information like size of sector, number of sectors in a cluster, number of copies of FAT table, BIOS Parameter Block (BPB). On disk usage, the information from the boot sector is read and required information is extracted.
  • FAT (File Allocation Table) area contains information regarding the used and free clusters. Each entry has the address of the next cluster file. If the value of the entry is 0, then it is a free cluster. FAT has values in its entries like last cluster of the file, next data cluster of current file, unused data cluster etc.   
  • Root directory has finite size and it is the primary directory of the disk. Approximately 224 entries can be present in FAT12.
  • Data area contains files and directories. Cluster 2 is always the first cluster. Size of file / directory names is 8 bytes.

Disadvantages of FAT12

  • Cluster size is less
  • Does not support hierarchical directories
  • Over writing to the media results in scattering of data over entire media thereby decreasing reading and writing speed
  • As the number of files on disk increases, it slows down

Related Info

In order to overcome the limitations of the previous FAT file systems, FAT 32 was created. With FAT 32 file system you can format external drives or your computer's hard disk as well as partitions. Due to accidental formatting data from logical drives can be lost. To recover FAT32 partition or external disk drive, use the best recovery utility.

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