xref: /src/distrib/notes/i386/prep

	$NetBSD: prep,v 1.6 1998/01/09 18:46:32 perry Exp $	

First and foremost, before beginning the installation process, MAKE
SURE YOU HAVE A RELIABLE BACKUP of any data on your hard disk that you
wish to keep. Mistakes in partitioning your hard disk may lead
to data loss.

Before you begin, you should be aware of the geometry issues that may
arise in relation to your hard disk.  First of all, you should know
about sector size. You can count on this to be 512 bytes; other sizes
are rare (and currently not supported). Of particular interest are
the number of sectors per track, the number of tracks per cylinder
(also known as the number of heads), and the number of cylinders.
Together they describe the disk geometry.

The BIOS has a limit of 1024 cylinders and 63 sectors per track for
doing BIOS I/O. This is because of the old programming interface
to the BIOS that restricts these values. Most of the big disks
currently being used have more than 1024 real cylinders. Some have
more than 63 sectors per track. Therefore, the BIOS can be instructed
to use a fake geometry that accesses most of the disk and the fake
geometry has less than or equal to 1024 cylinders and less than or
equal to 63 sectors. This is possible because the disks can be
addressed in a way that is not restricted to these values, and the
BIOS can internally perform a translation. This can be activated
in most modern BIOSes by using 'Large' or 'LBA' mode for the disk.

NetBSD does not have the mentioned limitations with regard to the
geometry. However, since the BIOS has to be used during startup,
it is important to know about the geometry the BIOS uses. The
NetBSD kernel should be on a part of the disk where it can be
loaded using the BIOS, within the limitations of the BIOS
geometry. The install program will check this for you, and
will give you a chance to correct this if this is not the case.

If you have not yet installed any other systems on the hard disk
that you plan to install NetBSD on, or if you plan to use the
disk entirely for NetBSD, you may wish to check your BIOS
settings for the 'Large' or 'LBA' modes, and activate
them for the hard disk in question. While they are not needed
by NetBSD as such, doing so will remove the limitations mentioned
above, and will avoid hassle should you wish to share the disk with
other systems. Do NOT change these settings if you already have
data on the disk that you want to preserve!

In any case, it is wise to check your the BIOS settings for the
hard disk geometry before beginning the installation, and write
them down. While this should usually not be needed, it enables
you to verify that the install program determines these values
correctly.

The geometry that the BIOS uses will be referred to as the "BIOS
geometry", the geometry that NetBSD uses is the "real geometry".

Sysinst will try to discover both the real geometry and BIOS
geometry. If a Master Boot Record (MBR) has been written to
your disk by DOS, Windows 95, NT, or another system, sysinst should
have no trouble find the BIOS geometry. If the disk has not had a MBR
written to it, it may be much more difficult to get the BIOS geometry.

It is *important* that sysinst know the proper BIOS geometry to be able
to get NetBSD to boot, regardless of where on your disk you put it.
It is less of a concern if the disk is going to be used entirely for
NetBSD.  If you intend to have several OSes on your disk, this becomes
a much larger issue.

Another issue with geometry is with SCSI disks.  The geometry reported
by the disk often does not address all addressable sectors on the disk.
sysinst will assist you if you want to choose a fake geometry for
NetBSD to use so it can get access to more sectors.  This is not
the same as the BIOS fake geometry.