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Partition / partition table and partition sectors / boot sector

A partition is a set of adjoined sectors on a disk. Any sector of the disk belongs either to no partition or it belongs to a specific partition, but it must not belong to more than one partition at the same time. In other words, partitions must not overlap each other. Typically, different operating systems are installed on separate partitions. An operating system can access all sectors of its partition, while access to sectors of other partitions is not possible sometimes.

There is the old convention that partitions should start and end on cylinder boundaries. No rule is without exception: since the first sector of a hard disk (C/H/S=0/0/1) is always reserved for the MBR and one does not want to leave the complete zeroth cylinder empty, the first partition does not begin on the next cylinder boundary (C/H/S=1/0/1) but directly after the track 0 at C/H/S=0/1/1. Hence only the sectors of the track 0 are left empty (with the exception of the first sector).

There is no compelling technical reason for this convention. All modern operating systems like Linux and Windows do not have any problems when partitions do not start or end on cylinder boundaries. However very old operating systems (e.g. DOS) expect that this convention is obeyed.

With the introduction of SSD drives the alignment on cylinder boundaries became obsolet. On SSDs drives data blocks at certain addresses can be accessed faster than at other addresses. Therefore Microsoft introduced with Windows Vista a new alignment schema. New partitions are now aligned on 1 MiB boundaries to allow the fastest possible access. This alignment is meanwhile the new standard.

Partition table and partition sectors:
Every partition requires an accompanying entry in a partition table. This entry in the partition table describes this partition, i.e. it contains the information about begin and end of the partition and the type of the partition. The partition table itself is stored in one or more separate partition sectors. This applies to MBR as well as GPT partitions.

The partition table for all primary and extended partitions is stored in the MBR (in one sector). On the contrary, to every logical drive belongs a separate partition sector, which describes on one hand the logical partition and on the other hand contains the position of the partition sector for the next logical drive. The partition table for GPT partitions is stored in the 33 sectors following the MBR. For details see format of partition sector.

Boot sector:
The boot sector is the sector at the beginning of a partition. In most cases the boot sector contains data about the file system used for formating the partition. Additionally the boot sector may contain a small program.

When booting by BIOS then loading the boot sector into memory and starting this boot sector code will boot the corresponding operating system from this MBR partition. The contents and the format of the boot sector code depends on the operating system. A certain boot sector can therefore be used only for booting a particular operating system but not for booting any other operating systems.

When booting by UEFI the boot sector code is not used.