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<H2><A NAME="SECTION00571000000000000000">The MBR, boot sectors and partition table</A></H2>
<P>
The information about how a hard disk has been partitioned is
stored in its first sector (that is, the first sector of the
first track on the first disk surface). The first sector is the
<b>master boot record</b> (MBR) of the disk; this is the sector
that the BIOS reads in and starts when the machine is first
booted. The master boot record contains a small program that
reads the partition table, checks which partition is active
(that is, marked bootable), and reads the first sector of that
partition, the partition's <b>boot sector</b> (the MBR is also a
boot sector, but it has a special status and therefore a special
name). This boot sector contains another small program that
reads the first part of the operating system stored on that
partition (assuming it is bootable), and then starts it.
<P>
The partitioning scheme is not built into the hardware, or
even into the BIOS. It is only a convention that many
operating systems follow. Not all operating systems do follow
it, but they are the exceptions. Some operating
systems support partitions, but they occupy one partition on
the hard disk, and use their internal partitioning method
within that partition. The latter type exists peacefully
with other operating systems (including Linux), and does not
require any special measures, but an operating system
that doesn't support partitions cannot co-exist on the same
disk with any other operating system.
<P>
As a safety precaution, it is a good idea to write down the
partition table on a piece of paper, so that if it ever corrupts
you don't have to lose all your files. (A bad partition table
can be fixed with <tt>fdisk</tt><A NAME="1522"> </A>). The relevant information
is given by the <tt>fdisk -l</tt><A NAME="1524"> </A> command:
<BLOCKQUOTE> <TT>
<code>$</code> <i>fdisk -l /dev/hda</i> <BR>
<code></code> <BR>
<code>Disk /dev/hda: 15 heads, 57 sectors, 790 cylinders</code> <BR>
<code>Units = cylinders of 855 * 512 bytes</code> <BR>
<code></code> <BR>
<code> Device Boot Begin Start End Blocks Id System</code> <BR>
<code>/dev/hda1 1 1 24 10231+ 82 Linux swap</code> <BR>
<code>/dev/hda2 25 25 48 10260 83 Linux native</code> <BR>
<code>/dev/hda3 49 49 408 153900 83 Linux native</code> <BR>
<code>/dev/hda4 409 409 790 163305 5 Extended</code> <BR>
<code>/dev/hda5 409 409 744 143611+ 83 Linux native</code> <BR>
<code>/dev/hda6 745 745 790 19636+ 83 Linux native</code> <BR>
<code>$</code>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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