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PC Disk File Systems

The first PC file system, called FAT because it used a "File Allocation Table", was developed in the early 1980's for floppy disks. The File Allocation Table, a map of the location of files on the disk, is located near the beginning of the disk. A disk contains two copies of the FAT, in case one gets damaged.

A disk formatted with the original FAT16 could have only partition. Since FAT16 is limited to 2GB per partition, it could not utilize the entire storage capacity of disk's larger than 2GB. To allow the use of higher capacity hard disks, in January 1986, MS-DOS 3.2 was released with a new partition type called an extended partition. With MS-DOS 3.2 a single disk can have up to four primary partitions (only one of which can be active), or three primary partitions and one extended partition.

In the extended partition, the user can create logical drives. Originally only one logical drive was allowed. in August 1987 MS-DOS 3.3 was released, which allowed up to 24 logical drives in the extended partition. 24 was equal to the maximum number of available letters that could be used for drive names (A and B being reserved for floppy drives).

The FAT file system divides a disk into sections called "clusters". The size of a cluster depends on the size of the disk. Since the largest 16 bit number is 65,535 (64K), the disk can be divided at most into 64K sections. For example, a 512MB disk would have an 8KB cluster size.

512MB / 65,535 = 8KB

When a file is written to the disk, a cluster is the minimum allocation unit. So on a 512 MB disk, a 1KB file wastes 7KB of disk space. Explorer would report the file size as 1KB, but the wasted space would be deducted from the free space on the disk.

A file is written to the first unused block of space on the disk. As files are deleted and rewritten in a different size, unused blocks of space become discontiguous. Parts of individual files become scattered across the disk. That's why it's important to defragment your hard disk occasionally.

A 2GB FAT disk would have a cluster size of 64KB. In this case, a 1KB file wastes 63KB of disk space. No matter how large a file is, unless the file’s size can be evenly divided by the size of a cluster, some space is wasted in the file’s last cluster.

2GB / 65,535 = 64KB

The waste situation was getting out of hand so the size of a FAT partition was limited to 2GB. In the middle of Windows 95 operating system production, Microsoft released OSR2. This revision increased the size of FAT entries to 32 bits. The 32 bit file system was named FAT32, and the 16 bit file system became referred to as FAT16.

Because the largest 32 bit number is more than 2 trillion, the FAT32 cluster size was fixed at 4KB. This helped the disk space waste problem, but the FAT system could not accommodate features like security and damaged file recovery that are required for an advanced operating system. For this reason, Microsoft developed the NTFS (New Technology File System).

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