Disk Partitioning
Partitioning Primer
Principles of partitioning
The first block of a hard disk used in a PC is called the Master Boot Record (or MBR), and part of this comprises a Partition Table. This table has space for exactly four entries, which define the primary partitions on the disk. Each entry simply contains numbers which define the starting disk block of the partition, and the number of blocks it occupies. If there are fewer than four partitions the unused slots contain all-zero values.
One of the partitions can be marked Active, which means that it is the partition used to boot the computer. The computer can be made to boot from a different partition by changing which one is marked Active.
Partitions can also be marked Hidden, which is simply an instruction to the operating system to ignore them. They are still visible to partition-management software.
To allow for more than four partitions in total, one primary partition can be defined as an Extended partition. Such a partition is then divided into one or more Logical Volumes. An extended partition can contain a virtually unlimited number of logical volumes, but only occupies one slot in the master partition table. Each volume gets a drive letter just like a primary partition, and in nearly all respects there is no practical difference from the operating system's point of view.
When using DOS it was a rule that at most only one primary partition and one extended partition could be visible at one time. This rule no longer applies to any current version of Windows, which can happily handle up to the maximum of four primary or three primary and one extended partition.
Terminology
Partitions behave somewhat differently in different versions of Windows, and in most cases it's the difference between Windows product families that's important rather than the specific version. To keep things succinct, the following terms are used throughout this article:
- Win9x
- Refers generically to Windows 95 / 98 / ME
- Win2k
- Refers generically to Windows NT / 2000 / XP
Words such as “Windows 2000” refer to a specific version of Windows rather than a generic family.
The FAT file-system also comes in two variants. The term FAT as used here applies generically to both, the specific variants are referred to as FAT16 and FAT32 respectively.
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