An active partition is a partition that contains the operating system that a computer attempts to load into memory by default when it is started or restarted. A partition is a logically independent section of a hard disk drive (HDD).
When booting (i.e., starting) a computer, the master boot record (MBR) looks at the partition table and selects the primary partition that is marked active in that table. The MBR is a small program that is executed when a computer starts up in order to find the operating system and load it into memory. It also contains the partition table and other information needed by the BIOS (basic input output system) to access the hard disk drive (HDD).
A partition table is a 64-byte data structure that describes to the operating system each primary partition on a personal computer's HDD, including where it starts, where it ends, the filesystem type on it, and whether it is the active partition. A data structure is an efficient way of organizing data.
Primary partitions are the first four partitions on a HDD. Only a primary partition can be an active partition. However, in the case of Linux, it is possible to boot into partitions other than the active partition, including into logical partitions, because control of the booting process is handed over to LILO (Linux loader) or GRUB (grand unified boot loader). Logical partitions are partitions that are created by carving up the fourth primary partition, which is then referred to as an extended partition.
Only one partition can be the active partition at any one time. However, the active partition can be changed by using fdisk, which is a widely available command line (i.e., all-text mode), but menu-driven, program for creating and manipulating partitions. To use fdisk to view or change the active partition, it is necessary to first become the root (i.e., administrative) user, which can be accomplished by using the su (i.e., substitute user) command. fdisk can then be launched to examine the first HDD (designated by /dev/hda) by entering the following and pressing the ENTER key:
Then pressing the p (for print) key followed by the ENTER key causes fdisk to display the partition data for the system. The active partition is shown by an asterisk (i.e., a star character) in the column labeled Boot. The short sequence of steps to change the active partition is started by pressing the a key followed by the ENTER key; however, this can cause a system to become unbootable if done incorrectly, and thus it is only recommended for advanced users or for those willing to take risks.
Created March 22, 2006.