What is Partition Table?

Partition can be considered as a piece of disk space, which is marked thereby runs on some operating system. Partition table is located at the first sector (cylinder 0, head 0 and sector 1, MBR) of each hard disk. It memorizes information about sizes and locations of partitions on hard disk. The partition information is started on offset 1BEH of master boot sector. Each partition entry is 16 bytes long. The total partition table is 64 bytes long. Then partition table is limited to a maximum of 4 entries. That is, there is a maximum of 4 partitions, which is called primary partition and can be created on hard disk.

But there are problems: many people want to create more than 4 partitions. So the extended partition is designed for this demand. Master extended partition is the primary partition. Differing from other partitions, the first sector of extended partition is not a boot sector, but another partition table, which is called logical partition table.

Commonly, there are only two partition entries in logical partition table. One points to a partition, called logical partition, whose boundary must be limited to the extended partition. The other entry, if needed, of the extended partition table points to the next logical partition table. Similarly, its boundary is limited to its parents extended partition. And the next logical partition table may also have two partition entries: one points to a logical partition; the other points to another logical partition table and the rest may be deduced by analogy. Therefore, many partitions could be created in extended partition.

Now let's have a look at the layout of one partition entry.
The 16 bytes of one entry are as follows:

OFFSET BYTE DESCRIPTION
0 1 Boot label. Tell computer toboot from this partition
1 1 Starting head
2 1 Lower 6 bits (bit 0 to bit 5) isstarting sector.
Higher 2 bits (bit 6 to bit 7) is the higher bits of starting cylinder
3 1

The lower 8 bits of starting cylinder

4 1 Partition type
5 1 Ending head
6 1 Lower 6 bits (bit 0 to bit 5) isending sector.
Higher 2 bits (bit 6 to bit 7) is the higher bits of ending cylinder
7 1 The lower 8 bits of ending cylinder
8 4 Leading sectors of this partition
12 4 Number of sectors of this partition

(a) Boot label (offset 0):
Most of the disks have one primary partition. Some people want to have more operating systems on their computers, so they have to create some other primary partitions. To tell the computer from which operating system to boot, one "Active" partition is in need. That's why partition table always keeps an indicator of the currently "Active" partition - the one from which the computer boots. In Partition Table Doctor or Super Fdisk, the active partition is figured out by "Active" with "Yes".

(b) Starting position (offset 1-3):
Describes the partition's starting position, the cylinder, the head and the sector. Also called starting CHS.
starting head = (OFFSET 1)
starting sector = (OFFSET 2) & 0x3f
starting cylinder = (((OFFSET 2) & 0xc0)<<2)|(OFFSET 3)

(c) Partition type (offset 4):
Indicates what file system is in the partition. For example, 06 or 0E indicates a FAT file system. 0B or 0C indicates a FAT32 file system. 07 indicates NTFS or OS/2 HPFS file system.

(d) Ending position (offset 5-7):
Describes the partition's ending position, the cylinder, the head and the sector. Also called ending CHS.

(e) Leading sectors (offset 8-11):
The number of sectors before this partition. If we count all sectors on hard disk in sequence from zero, this field will exactly point to the first sector of this partition.

(f) Number of sectors (offset 12-15):
The total number of sectors on this partition. So the size of this partition will be (Number of sectors)*512/1048576 MB.

Related Articles