Editing File Allocation Table (section)

=== Logical sectored FAT ===
{{See also|Extended boot record}}
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MS-DOS and PC&nbsp;DOS implementations of FAT12 and FAT16 could not access disk partitions larger than 32 megabytes. Several manufacturers developed their own FAT variants within their OEM versions of MS-DOS.<ref name="Novell_1993_FYI.M.1101"/>

Some vendors ([[AST Research|AST]] and [[NEC]]<ref name="Novell_1993_FYI.M.1101"/>) supported [[AST MBR<!-- NEC MBR -->|eight]], instead of the standard [[Partition table (master boot record)|four]], primary partition entries in their custom extended ''[[Master Boot Record]]'' (''MBR''), and they adapted MS-DOS to use more than a single primary partition.

Other vendors worked around the volume size limits imposed by the 16-bit sector entries by increasing the apparent ''size'' of the sectors the file system operated on. These ''logical sectors'' were larger (up to 8192&nbsp;bytes) than the ''physical sector'' size (still 512&nbsp;bytes) on the disk. The DOS-BIOS or System BIOS would then combine multiple physical sectors into logical sectors for the file system to work with.

These changes were transparent to the file system implementation in the DOS kernel. The underlying DOS-BIOS translated these logical sectors into physical sectors according to partitioning information and the drive's physical geometry.

The drawback of this approach was increased memory used for sector buffering and deblocking. Since older DOS versions could not use large logical sectors, the OEMs introduced new partition IDs for their FAT variants in order to hide them from off-the-shelf issues of MS-DOS and PC DOS. Known partition IDs for logical sectored FATs include: <code>[[Partition type#PID_08h|0x08]]</code> ([[Commodore International|Commodore]] MS-DOS 3.x), <code>[[Partition type#PID_11h|0x11]]</code> ([[Leading Edge Hardware Products|Leading Edge]] MS-DOS 3.x), <code>[[Partition type#PID_14h|0x14]]</code> (AST MS-DOS 3.x), <code>[[Partition type#PID_24h|0x24]]</code> (NEC MS-DOS 3.30<ref name="Novell_1993_FYI.M.1101"/>), <code>[[Partition type#PID_56h|0x56]]</code> ([[AT&T Corporation|AT&T]] MS-DOS 3.x), <code>[[Partition type#PID_E5h|0xE5]]</code> ([[Tandy Computers|Tandy]] MS-DOS), <code>[[Partition type#PID_F2h|0xF2]]</code> ([[Sperry Corporation|Sperry IT]] MS-DOS 3.x, [[Unisys]] MS-DOS 3.3 – also used by [[Digital Research]] [[DOS Plus]] 2.1).<ref name="Brouwer_2002_Partition-IDs" /> OEM versions like Toshiba&nbsp;MS-DOS, Wyse&nbsp;MS-DOS&nbsp;3.2<!-- with 1024 bytes/sector --> and 3.3,<ref name="Microsoft_2000_Wyse-DOS" /> as well as Zenith&nbsp;MS-DOS are also known to have utilized logical sectoring.<ref name="Microsoft_2000_Logical-sectoring" /><!-- but partition IDs and other specifics of these formats are not known -->

While non-standard and sub-optimal, these FAT variants are perfectly valid according to the specifications of the file system itself.{{citation needed|date=June 2022}} Therefore, even if default issues of MS-DOS and PC&nbsp;DOS were not able to cope with them, most of these vendor-specific FAT12 and FAT16 variants can be mounted by more flexible file system implementations in operating systems such as DR-DOS, simply by changing the partition ID to one of the recognized types.<ref group="nb" name="NB_DRDOS_Logical-Sectoring">DR-DOS is able to boot off FAT12/FAT16 logical sectored media with [[BPB logical sector size|logical sector size]]s up to 1024 bytes.</ref> Also, if they no longer need to be recognized by their original operating systems, existing partitions can be "converted" into FAT12 and FAT16 volumes more compliant with versions of MS-DOS/PC&nbsp;DOS&nbsp;4.0–6.3, which do not support sector sizes different from 512&nbsp;bytes,<ref name="Brouwer_2002_Logical_FAT" /> by switching to a [[DOS 3.31 BPB|BPB with 32-bit entry]] for the number of sectors, as introduced since DOS&nbsp;3.31 (see [[#FAT16B|FAT16B]] below), keeping the cluster size and reducing the [[BPB logical sector size|logical sector size in the BPB]] down to 512&nbsp;bytes, while at the same time increasing the counts of logical sectors per cluster, reserved logical sectors, total logical sectors, and logical sectors per FAT by the same factor.

A parallel development in MS-DOS / PC&nbsp;DOS which allowed an increase in the maximum possible FAT size was the introduction of multiple FAT partitions on a hard disk. To allow the use of more FAT partitions in a compatible way, a new partition type was introduced in PC&nbsp;DOS&nbsp;3.2 (1986), the ''[[Extended boot record|extended partition]]'' (EBR),<ref name="Duncan_1988_MS-DOS_Encyclopedia" /> which is a container for an additional partition called ''logical drive''. Since PC&nbsp;DOS&nbsp;3.3 (April 1987), there is another, optional extended partition containing the next ''logical drive'', and so on. The [[Master Boot Record|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.