Primary Partition Definition

A primary partition is any of the four possible first-level partitions into which a hard disk drive (HDD) on an IBM-compatible personal computer can be divided.

A partition is a logically independent division on a HDD. An entire operating system can be installed on a single, unpartitioned HDD, and this is often done with the Microsoft Windows operating systems. However, the ability to divide a HDD into multiple partitions offers some important advantages, including the ability to install multiple operating systems on the same HDD, increased efficiency of HDD space usage and enhanced data security.

Although there can only be four primary partitions, it is possible to create numerous additional partitions. This is accomplished by dividing one of the primary partitions into sub-partitions, called logical partitions1. The partition which is divided up is referred to as the extended partition.

Only a primary partition, and only one that is not used as the extended partition, can be designated as the active partition. An active partition is one that contains the operating system that a computer attempts to load into memory by default when it is started or restarted.

The limitation of only four primary partitions exists for historical reasons; that is, it is imposed by the structure of the partition table was developed for IBM-compatible personal computers and still remains in common use. A partition table is a 64-byte segment of data located in the first sector (i.e., the first 512 bytes) of the HDD that provides basic information about the primary partitions (including indicating which is the active partition) for the computer's BIOS (basic input output system) in order to facilitate the boot (i.e., starting up) sequence.

1All partitions are logical divisions, thus the term logical partition may seem redundant. This is another example of the somewhat confusing, and sometimes seemingly contradictory, terminology in the computer field. For an extensive list of such terminology, see Confusing Computer Terminology Explained, The Linux Information Project, March 2006.

Created April 6, 2006.
Copyright © 2006 The Linux Information Project. All Rights Reserved.