Partition Table Definition

A partition table is a 64-byte data structure that provides basic information for a computer's operating system about the division of the hard disk drive (HDD) into primary partitions.

A data structure is an efficient way of organizing data. A partition is a division of a HDD into logically independent sections. Primary partitions are the first four partitions on a HDD.

The partition table is part of the master boot record (MBR), which is a small program that is executed when a computer boots (i.e., starts up) in order to find the operating system and load it into memory. The MBR also contains other information needed by the BIOS (basic input output system) in order to access the HDD (on which the operating system is usually located).

The MBR, and thus the partition table, is stored in the boot sector, which is the first physical sector of a HDD. A sector is a segment of a track on a magnetic disk (i.e., a floppy disk or a platter in a HDD).

A track is any of the concentric circles on the magnetic media on a disk or platter over which one magnetic head (i.e., a device used for reading and writing data on the disk) passes while the head is stationary but the disk is spinning. A platter is a thin, high-precision aluminum or glass disk that is coated on both sides with a high-sensitivity magnetic material and which is used by a HDD to store data.

The MBR reads the partition table to determine which partition is the active partition. The active partition is the partition that contains the operating system that a computer attempts to load into memory by default when it is booted or rebooted.

The partition table begins at the hexadecimal (i.e., base 16) position 0x1BE in the boot sector. It contains four entries, each of which is 16 bytes in length, one for each partition.

The partition table entry for each partition consists of six items: the active flag, with 0x00 for off and 0x80 for on (one byte); the starting head, cylinder and sector (three bytes); the filesystem descriptor (one byte); the ending head, cylinder and sector (three bytes); the starting sector relative to the disk beginning (four bytes); and the number of sectors in the partition (four bytes).

A cylinder is any set of all of tracks of equal diameter in a HDD. It can be visualized as a single, imaginary, circle that cuts through all of the platters (and both sides of each platter) in the drive. Modern HDDs contain multiple platters, all of which are mounted on a single shaft, in order to maximize the data storage surface in a given volume of space.

Created March 24, 2006.
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