active partition

DOS could only support one active partition per drive.

The "Status" field in a partition table record is used to indicate an active partition.

The active partition is the partition where the boot flag is set.

It only provides functionality for adding and deleting partitions, but not for setting an active partition.

The slot specifications are still used in current PCs as well as the limitation of having 4 active partitions on a hard disk.

For a harddisk the code in the Master Boot Record (first sector) determines the active partition.

In some cases it is used by Windows XP/2000 to assign the active partition the letter "C:".

Form makes the assumption that the active partition is a DOS FAT partition.

In some cases, the MBR may also attempt to load secondary boot loaders before trying to boot the active partition.

Advanced Active Partition (AAP)

The MBR sector may contain code to locate the active partition and invoke its Volume Boot Record.

In order to clone active partitions without requiring a reboot, Drive Image 7 employed a volume snapshot device driver which was licensed from StorageCraft.

Version 1 could be used to create one FAT12 DOS partition, delete it, change the active partition or display partition data.

PTS-DOS 6.5 and S/DOS 1.0 use this in conjunction with their Advanced Active Partition (AAP) feature.

If an active partition is found, the MBR code loads the boot sector code from that partition, known as Volume Boot Record (VBR), and executes it.

It was also the first version to include a native Windows interface for cloning an active system partition; prior versions required a reboot into a DOS-like environment in order to clone the active partition.

The code in the boot sector of the active partition could then be again a NTLDR boot sector looking for in the root directory of this active partition.

In a more convoluted scenario the active partition can contain a Vista boot sector for the newer Vista boot manager with an entry pointing to another partition with a NTLDR boot sector.

It is the code in the MBR which generally understands disk partitioning, and in turn, is responsible for loading and running the VBR of whichever primary partition is set to boot (the active partition).

In conjunction with LOADER, Multiuser DOS and REAL/32 boot sectors use this to locate the boot sector of the active partition (or another bootstrap loader like IBMBIO.

If there is no active partition, or the active partition's boot sector is invalid, the MBR may load a secondary boot loader which will select a partition (often via user input) and load its boot sector, which usually loads the corresponding operating system kernel.