MS-DOS Partitioning Summary

• Microsoft MS-DOS operating system versions 2.x, 3.x, 4.x, 5.x, 6.0, 6.2, 6.21, 6.22
• Microsoft Windows 95

SUMMARY

This article contains a summary of MS-DOS partitioning information. For information on how MS-DOS assigns drive letters, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q51978
   TITLE     : Order in Which MS-DOS Assigns Drive Letters

MORE INFORMATION

A hard disk's master boot record (MBR) is located in the first sector of the disk (cylinder 0, side 0, sector 1). The partition table is located at offset 01BE, containing up to four 16-byte entries. The fourth byte of each partition table entry is used to mark the partition type.

MS-DOS began supporting hard disks in version 2.0.

MS-DOS Versions 2.x

MS-DOS 2.x supports one type 01 partition of up to 15 megabytes (MB) in size, which uses a 12-bit file allocation table (FAT). Fdisk creates only one MS-DOS partition per drive.

MS-DOS 3.0

MS-DOS 3.0 supports partitions larger than 15 MB using a 16-bit FAT, which allows a smaller cluster size and more efficient disk usage. As a result, MS-DOS 2.x hard disks larger than 15 MB are incompatible with later versions of MS-DOS. Fdisk creates only one MS-DOS partition per drive.

MS-DOS 3.3

MS-DOS 3.3 introduces support for more than one logical drive per hard disk. Logical drives are treated as completely separate disks under MS-DOS, even though they may occupy the same physical hard disk.

This is supported by using nonbootable MS-DOS partitions known as extended MS-DOS partitions. Fdisk reports these as EXT DOS; other MS-DOS partitions are reported as PRI DOS (for primary MS-DOS). Each primary MS-DOS partition is a logical drive, and extended MS-DOS partitions contain from 1 to 23 logical drives (MS-DOS supports drive letters up to Z). Logical drives in extended MS-DOS partitions have the same FAT type as a primary MS-DOS partition of the same size.

Only one PRI DOS partition and one EXT DOS partition is allowed per drive. On computers with two physical hard disks, a PRI DOS partition is not required on the second physical disk. A PRI DOS partition is required on the first physical disk. (MS-DOS does not support more than two physical disks.)

MS-DOS 4.0

MS-DOS versions 4.0 and later support logical drives larger than 32 MB. Full usage of these logical drives requires the MS-DOS program Share.exe to be loaded in MS-DOS 4.0.

The following table explains the versions 4.x and later MS-DOS partition types:

   Partition  Fdisk                                    Starting in
   Type       Reports      Size            FAT Type    MS-DOS version
   ------------------------------------------------------------------
   01         PRI DOS      0-15 MB         12-Bit      2.0 (a)
   04         PRI DOS      16-32 MB        16-Bit      3.0
   05         EXT DOS      0-2 GB (b)      n/a         3.3
   06         PRI DOS      32 MB-2 GB (b)  16-bit      4.0

   (a) 15-MB size limitation extended in version 3.0.

   (b) 2 GB (gigabytes) includes a limit of 1024 cylinders per drive
       imposed by the standard AT ROM BIOS interrupt 13 protocol.

MS-DOS 5.0

MS-DOS versions 5.0 and later support up to eight physical hard disks. Share.exe is not required for full large-drive support; this support is included in the MS-DOS kernel.

MS-DOS versions 5.0 and later support the same partitioning strategy as version 4.x, including Fdisk's inability to create more than one primary MS- DOS partition on a physical disk. However, because some original equipment manufacturer (OEM) partitioning software allows you to create more than one primary MS-DOS partition, MS-DOS versions 5.0 and later have kernel support for up to four primary MS-DOS partitions. This makes it easier to upgrade from previous versions of OEM-modified MS-DOS. Fdisk still creates only one PRI DOS partition on a physical disk.

NOTE: Many OEMs have changed their versions of MS-DOS to support more than one primary MS-DOS partition, larger type 04 partitions, and new partition types.

Windows 95

Windows 95 supports two new partition types (0E and 0F) to support the logical block addressing (LBA) INT13h extensions as specified in the Windows 95 Driver Development Kit (DDK).

   Partition  Fdisk                                    Starting in
   Type       Reports      Size            FAT Type    version
   ------------------------------------------------------------------
   01         PRI DOS      0-15 MB         12-Bit      MS-DOS 2.0
   04         PRI DOS      16-32 MB        16-Bit      MS-DOS 3.0
   05         EXT DOS      0-2 GB          n/a         MS-DOS 3.3
   06         PRI DOS      32 MB-2 GB      16-bit      MS-DOS 4.0
   0E         PRI DOS      32 MB-2 GB      16-bit      Windows 95
   OF         EXT DOS      0-2 GB          n/a         Windows 95