02 May 2010
Posted in F
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FAT16 | File Allocation Table(16)
In 1984, IBM released the PC AT, which featured a 20 MB hard disk. Microsoft introduced MS-DOS 3.0 in parallel. (The earlier PC XT was the first PC with a hard drive from IBM, and MS-DOS 2.0 supported that hard drive with FAT12.) Cluster addresses were increased to 16-bit, allowing for up to 65,524 clusters per volume, and consequently much greater file system sizes, at least in theory. However, the maximum possible number of sectors and the maximum (partition, rather than disk) size of 32 MB did not change. Therefore, although technically already "FAT16", this format was not what today is commonly understood as FAT16. With the initial implementation of FAT16 not actually providing for larger partition sizes than FAT12, the early benefit of FAT16 was to enable the use of smaller clusters, making disk usage more efficient, particularly for files several hundred bytes in size, which were far more common at the time. MS-DOS 2.x hard disks larger than 15 MB are incompatible with later versions of MS-DOS.
A 20 MB hard disk formatted under MS-DOS 3.0 was not accessible by the older MS-DOS 2.0 because MS-DOS 2.0 did not support version 3.0's FAT-16. MS-DOS 3.0 could still access MS-DOS 2.0 style 8 kB-cluster partitions under 15 MB. MS-DOS 3.0 also introduced support for high-density 1.2 MB 5.25" diskettes, which notably had 15 sectors per track, hence more space for the FATs.
Partitioning and logical drives
Extended boot record
Apart from improving the structure of the FAT file system itself, a parallel development allowing an increase in the maximum possible FAT size was the introduction of multiple FAT partitions on a hard disk. Extra DOS partitions could not be used as boot partitions. Simply allowing several identical-looking DOS partitions could lead to naming problems: behaviour if more than one partition was marked active was undocumented (although well defined), as was the behaviour if there was more than one hard disk in the computer (which was machine dependent), or if the system was booted from a diskette. The use of third party formatting tools under the DOS complicated the problem even more.
To allow the use of more FAT partitions in a compatible way, a new partition type was introduced (in MS-DOS 3.2, January 1986), the extended partition, which is a container for an additional partition called logical drive and optionally another extended partition containing the next logical drive, and so on. The MBR of a hard disk can either define up to four primary partitions, or an extended partition in addition to up to three primary partitions.
Finally in November 1987, Compaq DOS 3.31 (an OEM version of MS-DOS 3.3 released by Compaq with their machines) introduced what is today called the FAT16 format, with the expansion of the 16-bit disk sector count to 32 bits. The result was initially called the DOS 3.31 Large File System. Although the on-disk changes were minor, the entire DOS disk driver had to be converted to use 32-bit sector numbers, a task complicated by the fact that it was written in 16-bit assembly language. FAT16 partition type hex. 04 indicates less than 65536 sectors (less than 32 MB for sector size 512), type 06 indicates 65536 or more sectors, and hex. 0E indicates LBA instead of CHS addressing — Microsoft's dskprobe tool refers to type 06 as BigFAT.
In 1988 this improvement became more generally available through MS-DOS 4.0 and OS/2 1.1. The limit on partition size was dictated by the 8-bit signed count of sectors per cluster, which had a maximum power-of-two value of 64. With the standard hard disk sector size of 512 bytes, this gives a maximum of 32 KB clusters, thereby fixing the "definitive" limit for the FAT16 partition size at 2 GB for sector size 512. On magneto-optical media, which can have 1 or 2 KB sectors instead of 0.5 KB, this size limit is proportionally larger.
Much later, Windows NT increased the maximum cluster size to 64 KB by considering the sectors-per-cluster count as unsigned. However, the resulting format was not compatible with any other FAT implementation of the time, and it generated greater internal fragmentation. Windows 98 also supported reading and writing this variant, but its disk utilities did not work with it. This contributes to a confusing compatibility situation.
The number of root directory entries available for FAT12 and FAT16 is determined when the volume is formatted, and is stored in a 16-bit field. For a given number RDE and sector size SS the number RDS of root directory sectors is RDS=ceil((RDE×32)/SS), and RDE is normally chosen to fill these sectors, i.e., RDE*32=RDS*SS. FAT12 and FAT16 media typically use 512 root directory entries on non-floppy media. Some third party tools like mkdosfs allow the user to set this parameter.