IF01093
Partition
Partitions as used in MS-DOS, Microsoft
Windows and Linux; for partitions as used in other operating systems, see Slice
(disk) and BSD disklabel.
Technically, a hard disk should contain
either as many as four primary partitions, or one to three primaries along with
a single extended partition. Each of these partitions are described by a 16-byte
entry in the Partition Table which is located in the Master Boot Record.
The "type" of a partition is identified by
a 1-byte code found in its partition table entry. Some of these codes (such as
0x05 and 0x0F) may be used to indicate the presence of an extended partition,
but most are used by operating systems that examine partition tables to decide
if a partition contains a file system they can mount/access for reading or
writing data.
Once a specific partition's type has been
identified, additional information about its purpose and probable contents may
be found (see: Andries Brouwer. List of partition identifiers for PCs. as one
such resource). For example, some type codes are used to hide a partition's
contents from various operating systems. However, if an OS or some partitioning
tool has been programmed to also examine the boot sectors of any partition, then
its file system may no longer remain hidden. (Note: There are no officially
assigned partition types; thus, more than one kind of file system may lay claim
to the same code value.)
Primary (or Logical)
A primary (or logical) partition contains
one file system. In MS-DOS and earlier versions of Microsoft Windows systems,
the first partition (C:) must be a "primary partition". Other operating systems
may not share this limitation; however, this can depend on other factors, such
as a PC's BIOS.
The "partition type" code for a primary or
logical partition can either correspond to a file system contained within (e.g.
0x07 means either an NTFS or an OS/2 HPFS file system) or indicate the partition
has a special use (e.g. code 0xBC may mean an Acronis Secure Zone and code 0x82
usually indicates a Linux swap file partition). The FAT16 and FAT32 file systems
have made use of quite a number of partition type codes over time due to the
limits of various DOS and Windows OS versions. Though a Linux operating system
may recognize a number of different file systems (ext2, ext3, reiserfs, etc.),
they have all consistently used the same partition type code: 0x83 (Linux
native).
Extended
An extended partition is secondary to the
primary partition(s). A hard disk may contain only one extended partition; which
can then be sub-divided into logical drives, each of which is (under DOS and
Windows) assigned additional drive letters.
For example, under either DOS or Windows, a
hard disk with one primary partition and one extended partition, the latter
containing two logical drives, would typically be assigned the three drive
letters: C:, D: and E: (in that order).
Extended partitions are useful if you want
more than four partitions on a single physical drive. Technically, the number of
logical drives is no longer limited in later operating systems, but under
Windows there is an effective limit if they are to be assigned drive letters
(only 24 letters, C: through Z:, are generally available. Later operating
systems may allow more drives to be mounted/accessed by using A-A:, A-B:, A-C:,
etc.).
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