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<article>
<articleinfo>
<title>Installing and Using FreeBSD With Other Operating Systems</title>
<authorgroup>
<author>
<firstname>Jay</firstname>
<surname>Richmond</surname>
<affiliation>
<address>
<email>jayrich@sysc.com</email>
</address>
</affiliation>
</author>
</authorgroup>
<pubdate>6 August 1996</pubdate>
<abstract>
<para>This document discusses how to make FreeBSD coexist nicely
with other popular operating systems such as Linux, MS-DOS,
OS/2, and Windows 95. Special thanks to: Annelise Anderson
<email>andrsn@stanford.edu</email>, Randall Hopper
<email>rhh@ct.picker.com</email>, and &a.jkh;.</para>
</abstract>
</articleinfo>
<sect1>
<title>Overview</title>
<para>Most people can not fit these operating systems together
comfortably without having a larger hard disk, so special
information on large EIDE drives is included. Because there are
so many combinations of possible operating systems and hard disk
configurations, the <xref linkend="ch5"> section may be of the
most use to you. It contains descriptions of specific working
computer setups that use multiple operating systems.</para>
<para>This document assumes that you have already made room on
your hard disk for an additional operating system. Any time you
repartition your hard drive, you run the risk of destroying the
data on the original partitions. However, if your hard drive is
completely occupied by DOS, you might find the FIPS utility
(included on the FreeBSD CDROM in the
<filename>\TOOLS</filename> directory or via <ulink
URL="ftp://ftp.FreeBSD.org/pub/FreeBSD/tools/">ftp</ulink>)
useful. It lets you repartition your hard disk without
destroying the data already on it. There is also a commercial
program available called <application>Partition Magic</application>, which lets you size
and delete partitions without consequence.</para>
</sect1>
<sect1 id="ch2">
<title>Overview of Boot Managers</title>
<para>These are just brief descriptions of some of the different
boot managers you may encounter. Depending on your computer
setup, you may find it useful to use more than one of them on
the same system.</para>
<variablelist>
<varlistentry>
<term>Boot Easy</term>
<listitem>
<para>This is the default boot manager used with FreeBSD.
It has the ability to boot most anything, including BSD,
OS/2 (HPFS), Windows 95 (FAT and FAT32), and Linux.
Partitions are selected with the function keys.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OS/2 Boot Manager</term>
<listitem>
<para>This will boot FAT, FAT32, HPFS, FFS (FreeBSD), and EXT2
(Linux). Partitions
are selected using arrow keys. The OS/2 Boot Manager is
the only one to use its own separate partition, unlike the
others which use the master boot record (MBR). Therefore,
it must be installed below the 1024th cylinder to avoid
booting problems. It can boot Linux using LILO when it is
part of the boot sector, not the MBR. Go to <ulink
URL="http://www.linuxresources.com/LDP/HOWTO/HOWTO-INDEX.html">Linux
HOWTOs</ulink> on the World Wide Web for more
information on booting Linux with OS/2's boot
manager.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>OS-BS</term>
<listitem>
<para>This is an alternative to Boot Easy. It gives you more
control over the booting process, with the ability to set
the default partition to boot and the booting timeout.
The beta version of this programs allows you to boot by
selecting the OS with your arrow keys. It is included on
the FreeBSD CD in the <filename class="directory">\TOOLS</filename>
directory, and via <ulink
URL="ftp://ftp.FreeBSD.org/pub/FreeBSD/tools/">ftp</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>LILO, or LInux LOader</term>
<listitem>
<para>This is a limited boot manager. It will boot FreeBSD,
though some customization work is required in the LILO
configuration file.</para>
</listitem>
</varlistentry>
</variablelist>
<note id="fat32">
<title>About FAT32</title>
<para>FAT32 is the replacement to the FAT filesystem included in
Microsoft's OEM SR2 Beta release, which started replacing FAT
on computers pre-loaded with Windows 95 towards the
end of 1996. It converts the normal FAT filesystem and
allows you to use smaller cluster sizes for larger hard
drives. FAT32 also modifies the traditional FAT boot sector
and allocation table, making it incompatible with some boot
managers.</para>
</note>
</sect1>
<sect1 id="ch3">
<title>A Typical Installation</title>
<para>Let's say I have two large EIDE hard drives, and I want to
install FreeBSD, Linux, and Windows 95 on them.</para>
<para>Here is how I might do it using these hard disks:</para>
<itemizedlist>
<listitem>
<para><filename>/dev/wd0</filename> (first physical hard disk)</para>
</listitem>
<listitem>
<para><filename>/dev/wd1</filename> (second hard disk)</para>
</listitem>
</itemizedlist>
<para>Both disks have 1416 cylinders.</para>
<procedure>
<step>
<para>I boot from a MS-DOS or Windows 95 boot disk that
contains the <filename>FDISK.EXE</filename> utility and make a small
50 MB primary partition (35-40 for Windows 95, plus a
little breathing room) on the first disk. Also create a
larger partition on the second hard disk for my Windows
applications and data.</para>
</step>
<step>
<para>I reboot and install Windows 95 (easier said than done)
on the <filename>C:</filename> partition.</para>
</step>
<step>
<para>The next thing I do is install Linux. I am not sure
about all the distributions of Linux, but <ulink URL="http://www.slackware.com">Slackware</ulink> includes
LILO (see <xref linkend="ch2">). When I am partitioning out
my hard disk with Linux <command>fdisk</command>, I would
put all of Linux on the first drive (maybe 300 MB for a
nice root partition and some swap space).</para>
</step>
<step>
<para>After I install Linux, and are prompted about installing
LILO, make <emphasis>sure</emphasis> that I install it on the boot sector of my
root Linux partition, not in the MBR (master boot
record).</para>
</step>
<step>
<para>The remaining hard disk space can go to FreeBSD. I also
make sure that my FreeBSD root slice does not go beyond the
1024th cylinder. (The 1024th cylinder is 528 MB into the
disk with our hypothetical 720 MB disks). I will use the
rest of the hard drive (about 270 MB) for the
<filename class="directory">/usr</filename> and <filename class="directory">/</filename> slices if I wish. The
rest of the second hard disk (size depends on the amount of
my Windows application/data partition that I created in step
1) can go to the <filename class="directory">/usr/src</filename> slice and swap
space.</para>
</step>
<step>
<para>When viewed with the Windows 95 <command>fdisk</command>
utility, my hard drives should now look something like this:
<screen>---------------------------------------------------------------------
Display Partition Information
Current fixed disk drive: 1
Partition Status Type Volume_Label Mbytes System Usage
C: 1 A PRI DOS 50 FAT** 7%
2 A Non-DOS (Linux) 300 43%
Total disk space is 696 Mbytes (1 Mbyte = 1048576 bytes)
Press Esc to continue
---------------------------------------------------------------------
Display Partition Information
Current fixed disk drive: 2
Partition Status Type Volume_Label Mbytes System Usage
D: 1 A PRI DOS 420 FAT** 60%
Total disk space is 696 Mbytes (1 Mbyte = 1048576 bytes)
Press Esc to continue
---------------------------------------------------------------------</screen>
** May say FAT16 or FAT32 if you are using the OEM SR2
update. See <xref linkend="ch2">.</para>
</step>
<step>
<para>Install FreeBSD. I make sure to boot with my first hard
disk set at <quote>NORMAL</quote> in the BIOS. If it is not,
I will have the enter my true disk geometry at boot time (to
get this, boot Windows 95 and consult Microsoft Diagnostics
(<filename>MSD.EXE</filename>), or check your BIOS) with the
parameter <literal>hd0=1416,16,63</literal> where
<replaceable>1416</replaceable> is the number of cylinders on my hard
disk, <replaceable>16</replaceable> is the number of heads per track,
and <replaceable>63</replaceable> is the number of sectors per track on
the drive.</para>
</step>
<step>
<para>When partitioning out the hard disk, I make sure to
install Boot Easy on the first disk. I do not worry about
the second disk, nothing is booting off of it.</para>
</step>
<step>
<para>When I reboot, Boot Easy should recognize my three
bootable partitions as DOS (Windows 95), Linux, and BSD
(FreeBSD).</para>
</step>
</procedure>
</sect1>
<sect1 id="ch4">
<title>Special Considerations</title>
<para>Most operating systems are very picky about where and how
they are placed on the hard disk. Windows 95 and DOS need to be
on the first primary partition on the first hard disk. OS/2 is
the exception. It can be installed on the first or second disk
in a primary or extended partition. If you are not sure, keep
the beginning of the bootable partitions below the 1024th
cylinder.</para>
<para>If you install Windows 95 on an existing BSD system, it will
<quote>destroy</quote> the MBR, and you will have to reinstall your
previous boot manager. Boot Easy can be reinstalled by using
the <filename>BOOTINST.EXE</filename> utility included in the <filename class="directory">\TOOLS</filename> directory on the
CDROM, and via <ulink
URL="ftp://ftp.FreeBSD.org/pub/FreeBSD/tools/">ftp</ulink>.
You can also re-start the installation process and go to the
partition editor. From there, mark the FreeBSD partition as
bootable, select Boot Manager, and then type W to (W)rite out
the information to the MBR. You can now reboot, and Boot Easy
should then recognize Windows 95 as DOS.</para>
<para>Please keep in mind that OS/2 can read FAT and HPFS
partitions, but not FFS (FreeBSD) or EXT2 (Linux) partitions.
Likewise, Windows 95 can only read and write to FAT and FAT32
(see <xref linkend="ch2">) partitions. FreeBSD can read most
filesystems, but currently cannot read HPFS partitions. Linux
can read HPFS partitions, but can not write to them. Recent
versions of the Linux kernel (2.x) can read and write to Windows
95 VFAT partitions (VFAT is what gives Windows 95 long file
names - it is pretty much the same as FAT). Linux can read and
write to most filesystems. Got that? I hope so.</para>
</sect1>
<sect1 id="ch5">
<title>Examples</title>
<para><emphasis>(section needs work, please send your example to
<email>jayrich@sysc.com</email>)</emphasis>.</para>
<para>FreeBSD+Win95: If you installed FreeBSD after Windows 95,
you should see <literal>DOS</literal> on the Boot Easy menu. This is
Windows 95. If you installed Windows 95 after FreeBSD, read
<xref linkend="ch4"> above. As long as your hard disk does not
have 1024 cylinders you should not have a problem booting. If
one of your partitions goes beyond the 1024th cylinder however,
and you get messages like <errorname>invalid system disk</errorname>
under DOS (Windows 95) and FreeBSD will not boot, try looking
for a setting in your BIOS called <quote>> 1024 cylinder
support</quote> or <quote>NORMAL/LBA</quote> mode. DOS may need LBA
(Logical Block Addressing) in order to boot correctly. If the
idea of switching BIOS settings every time you boot up does not
appeal to you, you can boot FreeBSD through DOS via the
<filename>FBSDBOOT.EXE</filename> utility on the CD (It should find your
FreeBSD partition and boot it.)</para>
<para>FreeBSD+OS/2+Win95: Nothing new here. OS/2's boot manager
can boot all of these operating systems, so that should not be a
problem.</para>
<para>FreeBSD+Linux: You can also use Boot Easy to boot both
operating systems.</para>
<para>FreeBSD+Linux+Win95: (see <xref linkend="ch3">)</para>
</sect1>
<sect1 id="sources">
<title>Other Sources of Help</title>
<para>There are many <ulink
URL="http://www.linuxresources.com/LDP/HOWTO/HOWTO-INDEX.html">Linux
HOW-TOs</ulink> that deal with multiple operating systems on
the same hard disk.</para>
<para>The <ulink
URL="http://www.linuxresources.com/LDP/HOWTO/mini/Linux+DOS+Win95+OS2.html">Linux+DOS+Win95+OS2
mini-HOWTO</ulink> offers help on configuring the OS/2 boot
manager, and the <ulink
URL="http://www.linuxresources.com/LDP/HOWTO/mini/Linux+FreeBSD.html">Linux+FreeBSD
mini-HOWTO</ulink> might be interesting as well. The <ulink
URL="http://www.in.net/~jkatz/win95/Linux-HOWTO.html">Linux-HOWTO</ulink>
is also helpful.</para>
<para>The <ulink
URL="http://www.tburke.net/info/ntldr/ntldr_hacking_guide.htm">NT
Loader Hacking Guide</ulink> provides good information on
multibooting Windows NT, '95, and DOS with other operating
systems.</para>
<para>And Hale Landis's <quote>How It Works</quote> document pack contains some
good info on all sorts of disk geometry and booting related
topics. You can find it at
<ulink url="ftp://fission.dt.wdc.com/pub/otherdocs/pc_systems/how_it_works/allhiw.zip"></ulink>.</para>
<para>Finally, do not overlook FreeBSD's kernel documentation on
the booting procedure, available in the kernel source
distribution (it unpacks to <ulink
URL="file://localhost/usr/src/sys/i386/boot/biosboot/README.386BSD">/usr/src/sys/i386/boot/biosboot/README.386BSD</ulink>.</para>
</sect1>
<sect1>
<title>Technical Details</title>
<para><emphasis>(Contributed by Randall Hopper,
<email>rhh@ct.picker.com</email>)</emphasis></para>
<para>This section attempts to give you enough basic information
about your hard disks and the disk booting process so that you
can troubleshoot most problems you might encounter when getting
set up to boot several operating systems. It starts in pretty
basic terms, so you may want to skim down in this section until
it begins to look unfamiliar and then start reading.</para>
<sect2>
<title>Disk Primer</title>
<para>Three fundamental terms are used to describe the location
of data on your hard disk: Cylinders, Heads, and Sectors.
It is not particularly important to know what these terms
relate to except to know that, together, they identify where
data is physically on your disk.</para>
<para>Your disk has a particular number of cylinders, number of
heads, and number of sectors per cylinder-head (a
cylinder-head also known now as a track). Collectively this
information defines the <quote>physical disk geometry</quote> for your hard
disk. There are typically 512 bytes per sector, and 63
sectors per track, with the number of cylinders and heads
varying widely from disk to disk. Thus you can figure the
number of bytes of data that will fit on your own disk by
calculating:</para>
<informalexample>
<para>(# of cylinders) × (# heads) × (63
sectors/track) × (512 bytes/sect)</para>
</informalexample>
<para>For example, on my 1.6 Gig Western Digital AC31600 EIDE hard
disk, that is:</para>
<informalexample>
<para>(3148 cyl) × (16 heads) × (63
sectors/track) × (512 bytes/sect)</para>
</informalexample>
<para>which is 1,624,670,208 bytes, or around 1.6 Gig.</para>
<para>You can find out the physical disk geometry (number of
cylinders, heads, and sectors/track counts) for your hard
disks using ATAID or other programs off the net. Your hard
disk probably came with this information as well. Be careful
though: if you are using BIOS LBA (see <xref
linkend="limits">), you can not use just any program to get
the physical geometry. This is because many programs (e.g.
<filename>MSD.EXE</filename> or FreeBSD fdisk) do not identify the
physical disk geometry; they instead report the
<firstterm>translated geometry</firstterm> (virtual numbers from using
LBA). Stay tuned for what that means.</para>
<para>One other useful thing about these terms. Given 3
numbers—a cylinder number, a head number, and a
sector-within-track number—you identify a specific
absolute sector (a 512 byte block of data) on your disk.
Cylinders and Heads are numbered up from 0, and Sectors are
numbered up from 1.</para>
<para>For those that are interested in more technical details,
information on disk geometry, boot sectors, BIOSes, etc. can
be found all over the net. Query Lycos, Yahoo, etc. for
<literal>boot sector</literal> or <literal>master boot record</literal>.
Among the useful info you will find are Hale Landis's
<citetitle>How It Works</citetitle> document pack. See the <xref
linkend="sources"> section for a few pointers to this
pack.</para>
<para>Ok, enough terminology. We are talking about booting
here.</para>
</sect2>
<sect2 id="booting">
<title>The Booting Process</title>
<para>On the first sector of your disk (Cyl 0, Head 0, Sector 1)
lives the Master Boot Record (MBR). It contains a map of your
disk. It identifies up to 4 <firstterm>partitions</firstterm>, each of
which is a contiguous chunk of that disk. FreeBSD calls
partitions <firstterm>slices</firstterm> to avoid confusion with its
own partitions, but we will not do that here. Each partition can
contain its own operating system.</para>
<para>Each partition entry in the MBR has a <firstterm>Partition
ID</firstterm>, a <firstterm>Start Cylinder/Head/Sector</firstterm>, and an
<firstterm>End Cylinder/Head/Sector</firstterm>. The Partition ID
tells what type of partition it is (what OS) and the Start/End
tells where it is. <xref linkend="tbl-pid"> lists a
smattering of some common Partition IDs.</para>
<table id="tbl-pid">
<title>Partition IDs</title>
<tgroup cols="2">
<thead>
<row>
<entry>ID (hex)</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>01</entry>
<entry>Primary DOS12 (12-bit FAT)</entry>
</row>
<row>
<entry>04</entry>
<entry>Primary DOS16 (16-bit FAT)</entry>
</row>
<row>
<entry>05</entry>
<entry>Extended DOS</entry>
</row>
<row>
<entry>06</entry>
<entry>Primary big DOS (> 32MB)</entry>
</row>
<row>
<entry>0A</entry>
<entry>OS/2</entry>
</row>
<row>
<entry>83</entry>
<entry>Linux (EXT2FS)</entry>
</row>
<row>
<entry>A5</entry>
<entry>FreeBSD, NetBSD, 386BSD (UFS)</entry>
</row>
</tbody>
</tgroup>
</table>
<para>Note that not all partitions are bootable (e.g. Extended
DOS). Some are—some are not. What makes a partition
bootable is the configuration of the <firstterm>Partition Boot
Sector</firstterm> that exists at the beginning of each
partition.</para>
<para>When you configure your favorite boot manager, it looks up
the entries in the MBR partition tables of all your hard disks
and lets you name the entries in that list. Then when you
boot, the boot manager is invoked by special code in the
Master Boot Sector of the first probed hard disk on your
system. It looks at the MBR partition table entry
corresponding to the partition choice you made, uses the Start
Cylinder/Head/Sector information for that partition, loads up
the Partition Boot Sector for that partition, and gives it
control. That Boot Sector for the partition itself contains
enough information to start loading the operating system on
that partition.</para>
<para>One thing we just brushed past that is important to know.
All of your hard disks have MBRs. However, the one that is
important is the one on the disk that is first probed by the
BIOS. If you have only IDE hard disks, its the first IDE disk
(e.g. primary disk on first controller). Similarly for SCSI
only systems. If you have both IDE and SCSI hard disks
though, the IDE disk is typically probed first by the BIOS, so
the first IDE disk is the first probed disk. The boot manager
you will install will be hooked into the MBR on this first
probed hard disk that we have just described.</para>
</sect2>
<sect2 id="limits">
<title>Booting Limitations and Warnings</title>
<para>Now the interesting stuff that you need to watch out
for.</para>
<sect3>
<title>The dreaded 1024 cylinder limit and how BIOS LBA helps</title>
<para>The first part of the booting process is all done
through the BIOS, (if that is a new term to you, the BIOS is
a software chip on your system motherboard which provides
startup code for your computer). As such, this first part
of the process is subject to the limitations of the BIOS
interface.</para>
<para>The BIOS interface used to read the hard disk during
this period (INT 13H, Subfunction 2) allocates 10 bits to
the Cylinder Number, 8 bits to the Head Number, and 6 bits
to the Sector Number. This restricts users of this
interface (i.e. boot managers hooked into your disk's MBR as
well as OS loaders hooked into the Boot Sectors) to the
following limits:</para>
<itemizedlist>
<listitem>
<para>1024 cylinders, max</para>
</listitem>
<listitem>
<para>256 heads, max</para>
</listitem>
<listitem>
<para>64 sectors/track, max (actually 63, <literal>0</literal>
is not available)</para>
</listitem>
</itemizedlist>
<para>Now big hard disks have lots of cylinders but not a lot
of heads, so invariably with big hard disks the number of
cylinders is greater than 1024. Given this and the BIOS
interface as is, you can not boot off just anywhere on your
hard disk. The boot code (the boot manager and the OS
loader hooked into all bootable partitions' Boot Sectors)
has to reside below cylinder 1024. In fact, if your hard
disk is typical and has 16 heads, this equates to:</para>
<informalexample>
<para>1024 cyl/disk × 16 heads/disk × 63
sect/(cyl-head) × 512 bytes/sector</para>
</informalexample>
<para>which is around the often-mentioned 528MB limit.</para>
<para>This is where BIOS LBA (Logical Block Addressing) comes
in. BIOS LBA gives the user of the BIOS API calls access to
physical cylinders above 1024 though the BIOS interfaces by
redefining a cylinder. That is, it remaps your cylinders
and heads, making it appear through the BIOS as though the
disk has fewer cylinders and more heads than it actually
does. In other words, it takes advantage of the fact that
hard disks have relatively few heads and lots of cylinders
by shifting the balance between number of cylinders and
number of heads so that both numbers lie below the
above-mentioned limits (1024 cylinders, 256 heads).</para>
<para>With BIOS LBA, the hard disk size limitation is
virtually removed (well, pushed up to 8 Gigabytes anyway).
If you have an LBA BIOS, you can put FreeBSD or any OS
anywhere you want and not hit the 1024 cylinder
limit.</para>
<para>To use my 1.6 Gig Western Digital as an example again,
its physical geometry is:</para>
<informalexample>
<para>(3148 cyl, 16 heads, 63 sectors/track, 512
bytes/sector)</para>
</informalexample>
<para>However, my BIOS LBA remaps this to:</para>
<informalexample>
<para>(787 cyl, 64 heads, 63 sectors/track, 512
bytes/sector)</para>
</informalexample>
<para>giving the same effective size disk, but with cylinder
and head counts within the BIOS API's range (Incidentally, I
have both Linux and FreeBSD existing on one of my hard disks
above the 1024th physical cylinder, and both operating
systems boot fine, thanks to BIOS LBA).</para>
</sect3>
<sect3>
<title>Boot Managers and Disk Allocation</title>
<para>Another gotcha to watch out when installing boot
managers is allocating space for your boot manager. It is
best to be aware of this issue up front to save yourself
from having to reinstall one or more of your OSs.</para>
<para>If you followed the discussion in <xref
linkend="booting"> about the Master Boot Sector (where the
MBR is), Partition Boot Sectors, and the booting process,
you may have been wondering just exactly where on your hard
disk that nifty boot manager is going to live. Well, some
boot managers are small enough to fit entirely within the
Master Boot Sector (Cylinder 0, Head 0, Sector 0) along with
the partition table. Others need a bit more room and
actually extend a few sectors past the Master Boot Sector in
the Cylinder 0 Head 0 track, since that is typically
free…typically.</para>
<para>That is the catch. Some operating systems (FreeBSD
included) let you start their partitions right after the
Master Boot Sector at Cylinder 0, Head 0, Sector 2 if you
want. In fact, if you give FreeBSD's sysinstall a disk with
an empty chunk up front or the whole disk empty, that is
where it will start the FreeBSD partition by default (at least
it did when I fell into this trap). Then when you go to
install your boot manager, if it is one that occupies a few
extra sectors after the MBR, it will overwrite the front of
the first partition's data. In the case of FreeBSD, this
overwrites the disk label, and renders your FreeBSD
partition unbootable.</para>
<para>The easy way to avoid this problem (and leave yourself
the flexibility to try different boot managers later) is
just to always leave the first full track on your disk
unallocated when you partition your disk. That is, leave
the space from Cylinder 0, Head 0, Sector 2 through Cylinder
0, Head 0, Sector 63 unallocated, and start your first
partition at Cylinder 0, Head 1, Sector 1. For what it is
worth, when you create a DOS partition at the front of your
disk, DOS leaves this space open by default (this is why
some boot managers assume it is free). So creating a DOS
partition up at the front of your disk avoids this problem
altogether. I like to do this myself, creating 1 Meg DOS
partition up front, because it also avoids my primary DOS
drive letters shifting later when I repartition.</para>
<para>For reference, the following boot managers use the
Master Boot Sector to store their code and data:</para>
<itemizedlist>
<listitem>
<para>OS-BS 1.35</para>
</listitem>
<listitem>
<para>Boot Easy</para>
</listitem>
<listitem>
<para>LILO</para>
</listitem>
</itemizedlist>
<para>These boot managers use a few additional sectors after
the Master Boot Sector:</para>
<itemizedlist>
<listitem>
<para>OS-BS 2.0 Beta 8 (sectors 2-5)</para>
</listitem>
<listitem>
<para>OS/2's boot manager</para>
</listitem>
</itemizedlist>
</sect3>
<sect3>
<title>What if your machine will not boot?</title>
<para>At some point when installing boot managers, you might
leave the MBR in a state such that your machine will not boot.
This is unlikely, but possible when re-FDISKing underneath
an already-installed boot manager.</para>
<para>If you have a bootable DOS partition on your disk, you
can boot off a DOS floppy, and run:</para>
<informalexample>
<screen>A:\> <userinput>FDISK /MBR</userinput></screen>
</informalexample>
<para>to put the original, simple DOS boot code back into the
system. You can then boot DOS (and DOS only) off the hard
drive. Alternatively, just re-run your boot manager
installation program off a bootable floppy.</para>
</sect3>
</sect2>
</sect1>
</article>