KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q49326: Determining Number of Partitions Per Hard Disk

Article: Q49326
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:3.x,4.x,5.0
Operating System(s): 
Keyword(s): 
Last Modified: 17-DEC-2000

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft MS-DOS operating system versions 3.1, 3.2, 3.21, 3.3, 3.3a, 4.0, 4.01, 5.0 
-------------------------------------------------------------------------------

SUMMARY
=======

This article covers how MS-DOS decides how many partitions there are for each
hard disk.

MORE INFORMATION
================

MS-DOS looks through all of the hard drives supported by the ROM BIOS and the
resident disk device driver to find all of the drives. It understands extended
partitions and how to find logical drives in them and reserves a letter for each
drive defined.

In the main partition table, only two MS-DOS partitions are allowed: the primary
partition and the extended partition. With MS-DOS Version 3.30, the values for
the primary partition structure are 01 and 04. With MS-DOS Versions 4.00 and
4.01, the numbers are 01, 04, and 06 (greater than 32 MB). The extended
partition is marked with a 05 system byte.

The first sector of an extended partition is the first sector of the first
logical drive in that partition (usually Drive D). This sector contains the
three jump bytes, followed by a BPB, followed by code. The code is similar to
the boot code in the main boot sector, but since extended partition volumes
cannot be booted, this code generally displays a message stating that an
extended volume cannot be booted.

Following the code at the same offset as the master partition table is the
extended logical drive table. This table has the same structure as the master
partition table, but can only have two nonzero entries. The first kind of entry
(not necessarily the first entry, most likely it is the fourth) is either a 01,
04, or 06 type drive, and defines the current drive.

If you only had Drives C and D, the logical drive table would have an entry for
Drive D, and then contain three entries of 0's (zeros).

If more than one logical drive is defined within the extended partition, a second
entry must be made in the logical drive table that points to the next entry.

If you had a Drive C with an extended partition that included Drives D and E, the
first sector in Drive D would have a logical drive table that included a 01, 04,
or 06 drive type for D with its starting and ending head/cylinder/sector
numbers, and it would have a 05 entry for Drive E and its starting and ending
head/cylinder/sector numbers. Drive E would have an entry in its logical drive
table for itself, and three entries filled with 0's (zeros).

The first sectors in extended drives are analogous to boot sectors in the main
partitions. The only difference is that they have drive tables and cannot be
booted. They have BPBs and the ending 55AAH bytes that signify that the sector
is valid.

Additional query words: 3.30 4.00 4.01 5.00

======================================================================
Keywords          :  
Technology        : kbMSDOSSearch kbMSDOS321 kbMSDOS400 kbMSDOS320 kbMSDOS330a kbMSDOS310 kbMSDOS500 kbMSDOS330 kbMSDOS401
Version           : MS-DOS:3.x,4.x,5.0

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.