ABOUT PARTITIONS Partitions are a way
of dividing your physical hard disk up into smaller, logical sections (which
become 'logical' disks to your operating system). For example in
Windows, the first partition is your C: drive, the second is your D: drive,
etc. Windows systems generally arrive
configured such that there is only a single, large partition (C:) covering the
entire physical disk. It is often better to have more than one
partition, for example a partition for Windows and software, and another
partition for data and documents. This makes it possible, for example,
to reinstall Windows on the first partition without disturbing your data.
Another reason for needing more partitions is when a user wants to install
other operating systems (such as Linux) on the same disk and choose which one
to run at boot time. Partition Logic and other tools like it enable you
to shrink your C: partition and create more partitions in the empty space.
Partition information resides in a table, the first part of
which is typically located in the first sector(s) of your disk. The
table is also known as a disk label. There are different schemes for the
format of this table, but on PC-compatible computers the primary table resides
in the first sector, known as the MBR (Master Boot Record) sector. Thus
this scheme is alternately known as MS-DOS partitioning or MBR partitioning.
Other disk label types include Sun/BSD and EFI. As Partition Logic only
supports MS-DOS partitioning at present, the rest of this description relates
only to MS-DOS. THE MS-DOS PARTITIONING
SCHEME Partition tables in the MS-DOS
partitioning scheme contain space for 4 partition entries. The first
sector of an MS-DOS-labeled disk contains some executable code and data, and
the 'primary' partition table. The table entries are each 16 bytes in
length, and describe the starting sector of the partition, the partition size
in sectors, the partition's 'tag' (which is used as an indication of the
partition's filesystem type), a flag indicating whether the partition is
'bootable', and some legacy information about the partition's geometry (its
starting and ending head, cylinder, and sector). Nowadays the geometry
information is not especially relevant, and the fields are not big enough to
describe large partitions on modern disks, but partitioning programs like
Partition Logic attempt to ensure that they are correct nonetheless.
Modern operating systems like Windows and Linux mostly ignore the geometry
information. The MS-DOS partitioning
scheme is fairly old and obsolete, though it is the most common type and used
on all current PC-compatible machines. Newer computers such as Intel
Itanium and Apple's Intel-based Macintosh systems use the more modern EFI GPT
disk label. This scheme may migrate to PC-compatible computers in the
next few years. EXTENDED AND LOGICAL
PARTITIONS A number of years ago it
became apparent that many people would want to use more than the 4 partitions
allowed in the MS-DOS primary partition table. Thus software engineers
dreamed up an extension to this scheme which, while terribly messy and
sometimes inconsistently implemented, allows for a large number of partition
entries. This extension makes use of 'extended' partition tables
containing 'logical' partitions (the ones in the primary table are known as
primary partitions). A user may create
either:
If you have 4 primary partitions, there is no
more room in this scheme for creating additional partitions. This is so
because one primary partition entry is required to describe the first extended
partition, if applicable, which will contain all logical partitions.
When a user creates the first logical partition, the primary
partition entry for the extended partition is created. This primary
extended partition 'wraps around' all logical partitions, and space for a new,
extended partition table is reserved at the beginning of the extended
partition. Then the entry for the logical partition is placed in the
extended table. By convention, extended partition tables use only 2
entries; thus when a second logical partition is created, a second extended
partition is created to wrap around it (inside the first one), and the
remaining free entry in the first extended table describes the second extended
partition. The table entry for the second logical partition resides in a
new table at the start of the second extended partition, and so on. In
other words, for each logical partition, there is an extended partition which
contains it, and extended partitions 'nest' within one another:
Nested extended and logical partitions. 3 primary partitions, followed
by an extended partition, which contains a) a logical partition; and b) a
further extended partition containing a second logical partition.
P=primary, E=extended, L=logical
This scheme is convoluted, and is sometimes implemented
differently depending on the partitioning software used. |