.\"	$NetBSD: prep,v 1.13 2002/06/29 22:45:30 lukem Exp $
.
First and foremost, before beginning the installation process,
.Em 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.
.Pp
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.
.Pp
The
.Tn BIOS
has a limit of 1024 cylinders and 63 sectors per track for
doing
.Tn BIOS
I/O.
This is because of the old programming interface
to the
.Tn 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
.Tn 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
.Tn BIOS
can internally perform a translation.
This can be activated in most modern BIOSes by using
.Em Large
or
.Em LBA
mode for the disk.
.Pp
.Nx
does not have the mentioned limitations with regard to the geometry.
However, since the
.Tn BIOS
has to be used during startup,
it is important to know about the geometry the
.Tn BIOS
uses.
The
.Nx
kernel should be on a part of the disk where it can be loaded using the
.Tn BIOS ,
within the limitations of the
.Tn 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.
.Pp
If you have not yet installed any other systems on the hard disk
that you plan to install
.Nx
on, or if you plan to use the
disk entirely for
.Nx ,
you may wish to check your
.Tn BIOS
settings for the
.Sq Large
or
.Sq LBA
modes, and activate
them for the hard disk in question.
While they are not needed by
.Nx
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
.Em not
change these settings if you already have
data on the disk that you want to preserve!
.Pp
In any case, it is wise to check your the
.Tn 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.
.Pp
The geometry that the
.Tn BIOS
uses will be referred to as the
.Tn Em BIOS
.Em geometry ,
the geometry that
.Nx
uses is the
.Em real geometry .
.Pp
.Ic sysinst ,
the
.Nx
installation program, will try to discover both the real geometry and
.Tn BIOS
geometry.
.Pp
It is
.Em important
that
.Ic sysinst
know the proper
.Tn BIOS
geometry to be able
to get
.Nx
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
.Nx .
If you intend to have several OSes on your disk, this becomes
a much larger issue.