What Is a Partition Table?

A partition table is a piece of code used to indicate different partitions or sections of a computer Hard Disk Drive (HDD). This is typically part of a Master Boot Record (MBR) on an HDD, which is frequently accessed on startup of a computer to indicate the part of the drive that should be used to start it. Different partitions can be indicated on a single table, allowing the HDD to be split up into sections that are recognized and written to individually. A partition table is typically written in hexadecimal notation, which uses numbers and letters to express a wide range of numeric values.

The basic structure of a partition table is fairly complex when first looking at it, though the information provided in it is actually rather simple. It is just one part of the MBR on a computer's HDD, which is used when the computer is first started up, a process referred to as bootstrapping or "booting." When the computer is turned on, a microchip on the motherboard containing the Basic Input/Output System (BIOS) for that computer begins the startup. During booting, however, the BIOS hands over control of the system and the rest of startup to the HDD in the computer.

When this happens, the MBR is used by the computer to determine the way in which bootstrapping should continue. Part of the MBR is a partition table, which provides information about the various partitions on the HDD, though there might only be one in many basic systems. In this partition table, one section of the HDD is established as the boot partition, which is used for startup, and other sections might simply be used for storage. Multiple partitions can be used on a computer to contain a different Operating System (OS) in each one, allowing the machine to start in one OS, but providing the user with different systems that can be used on the same computer.

The information in a partition table is written in hexadecimal notation, which uses the numbers "0" through "9" and the letters "A" through "F," which indicate "10" through "15." This is a base 16 system, which means that the number indicated is then multiplied by 16 raised to a certain power. For example, the hexadecimal number "A3" would be 10x16^1 added to 3x16^0, or 163. A longer hexadecimal entry such as "34B" would be 3x16^2 added to 4x16^1 and 11x16^0, or 843. Different information can be conveyed through hexadecimal notation in a partition table, including the identity of the boot partition and the sizes of different sectors within one or more partitions on the HDD.