On the PC, partitions traditionally use a structure called Partition Table, located at the end of the master boot record (MBR, Master Boot Record).
This table can not contain more than 4 partition records (also called partition descriptors), specific to its beginning, end and size in different addressing modes, as well as a single number, called partition type, and a flag indicating whether the partition is active or not (there can only be one active partition at a time).
The marker is used during startup, after the BIOS loads the master boot record into memory and run it, check the DOS MBR partition table to a close and locates the active partition.
Then load the boot sector of that partition into memory and executes it. Unlike the master boot record (which is usually independent of the operating system), the boot sector is installed along with the operating system.
After loading the second stage, the boot sector can load any disk partitions (allowing the user to select the partition). Or if the manager knows how to locate the kernel (core) of the operating system on one partition (can allow the user to specify additional kernel options for strategic purposes of recovery).
Extended and logical partitions
Any version of DOS can only read FAT primary partition on the hard drive. This coupled with the deterioration of the FAT with the use and increased size of the disks moved Microsoft to create a relatively simple improved scheme.
One of the entries in the table of main partition was renamed the extended partition and received a number associated with special partition (0×05).
The start field of the location of the partition has first descriptor of the extended partition, which in turn has a similar field with the location of the next, and a linked list of descriptors.
Other fields of an extended partition are undefined, they have allocated space and can not be used to store data. The initial partition of the elements of the linked list are called logical drives that are assigned spaces and can store data.
Older operating systems ignored the extended partitions with type number 0×05. This scheme replaces the old as all partitions on a hard disk can be placed within a single extended partition.
For some reason, Microsoft did not update its DOS operating system to boot from an extended partition, because the need for primary partitions is preserved.
Above this still would have allowed a primary FAT partition per drive, meaning all other primary FAT partitions must have their type numbers prior to changing DOS boot partition so that it is able to proceed.
This technique, used by several popular boot managers is called concealment partition. However, we must take account fifth partition can be compressed but not highly recommended.