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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