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Answer Posted / vinod singh kushwah
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.
| Is This Answer Correct ? | 2 Yes | 0 No |
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