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authorJohn Fieber <jfieber@FreeBSD.org>1997-03-23 16:27:47 +0000
committerJohn Fieber <jfieber@FreeBSD.org>1997-03-23 16:27:47 +0000
commit47a93628dd1bd7e70940ee7f93af97008f4178a4 (patch)
tree77d52c92021d423fed62c80bdf9704fc08169089 /en_US.ISO8859-1
parent33d1a3cbfd1ffa20c377f590c06c9cd7d1af3fc2 (diff)
downloaddoc-47a93628dd1bd7e70940ee7f93af97008f4178a4.tar.gz
doc-47a93628dd1bd7e70940ee7f93af97008f4178a4.zip
Another tutorial converted to Docbook.
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+INDEXLINK= multios.html
-.include <bsd.sgml.mk>
+.include "../../web.mk"
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+<!-- $Id: article.sgml,v 1.1 1997-03-23 16:27:47 jfieber Exp $ -->
+<!DOCTYPE BOOK PUBLIC "-//Davenport//DTD DocBook V3.0//EN">
+<book>
+
+<bookinfo>
+<bookbiblio>
+<title>Installing and Using FreeBSD With Other Operating Systems</title>
+
+<authorgroup>
+<author>
+<firstname>Jay</firstname>
+<surname>Richmond</surname>
+<affiliation>
+<address>
+<email>jayrich@in.net</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 Jordan K. Hubbard
+<email>jkh@time.cdrom.com</email></para></abstract>
+
+</bookbiblio>
+</bookinfo>
+
+<chapter>
+<title>Overview</title>
+
+<para>Most people can't 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 CD-ROM 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 Partition Magic, which lets you size and delete partitions
+without consequence.</para>
+
+</chapter>
+
+<chapter 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, HPFS, FFS (FreeBSD), and EXT2
+(Linux). It will also boot FAT32 partitions. 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.ssc.com/linux/howto.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>\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. 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 is expected to utilitized on computers pre-loaded with Windows
+95 towards the end of 1996. It converts the normal FAT file system
+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>
+
+</chapter>
+
+<chapter 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's how I might do it using these hard disks:
+<itemizedlist>
+
+<listitem>
+<para><filename>/dev/wd0</> (first physical hard disk)</para>
+</listitem>
+
+<listitem>
+<para><filename>/dev/wd1</> (second hard disk)</para>
+</listitem>
+
+</itemizedlist>
+</para>
+
+<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</> utility and make a small 50 meg
+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:</> partition.</para> </step>
+
+<step><para>The next thing I do is install Linux. I'm not sure about
+all the distributions of Linux, but slackware 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 megs for a nice root partition and some swap
+space).</para></step>
+
+<step><para>After I install Linux, and are prompted about installing
+LILO, make SURE 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 megs into the disk with our
+hypothetical 720MB disks). I will use the rest of the hard drive
+(about 270 megs) for the <filename>/usr</> and <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>/usr/src</> slice and swap
+space.</para></step>
+
+<step><para>When viewed with the Windows 95 <command>fdisk</> 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</> in the BIOS. If it is not, I'll have
+the enter my true disk geometry at boot time (to get this, boot
+Windows 95 and consult Microsoft Diagnostics (<filename>MSD.EXE</>),
+or check your BIOS) with the parameter <literal>hd0=1416,16,63</>
+where <replaceable>1416</> is the number of cylinders on my hard
+disk, <replaceable>16</> is the number of heads per track, and
+<replaceable>63</> 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 don't 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>
+
+</chapter>
+
+<chapter 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 partitiin 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</> the MBR, and you will have to reinstall your
+previous boot manager. Boot Easy can be reinstalled by using the
+BOOTINST.EXE utility included in the \TOOLS directory on the CD-ROM,
+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 file systems, but
+currently cannot read HPFS partitions. Linux can read HPFS
+partitions, but can't 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's pretty much the same
+as FAT). Linux can read and write to most file systems. Got that?
+I hope so.</para>
+
+</chapter>
+
+<chapter id="ch5">
+<title>Examples</title>
+
+<para><emphasis>(section needs work, please send your example to
+<email>jayrich@in.net</email>)</emphasis>.</para>
+
+<para>FreeBSD+Win95: If you installed FreeBSD after Windows 95, you
+should see <literal>DOS</> 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</> under DOS (Windows 95)
+and FreeBSD will not boot, try looking for a setting in your BIOS
+called <quote>&gt; 1024 cylinder support</> or <quote>NORMAL/LBA</>
+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 doesn't appeal to you, you can boot FreeBSD through DOS via
+the <filename>FBSDBOOT.EXE</> 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 manger
+can boot all of these operating systems, so that shouldn't 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>
+
+</chapter>
+
+<chapter id="sources">
+<title>Other Sources of Help</title>
+
+<para>There are many <ulink
+URL="http://www.ssc.com/linux/howto.html">Linux HOW-TOs</ulink> that
+deal with multiple operating systems on the same hard disk.</para>
+
+<para>The <ulink
+URL="http://sunsite.unc.edu/mdw/HOWTO/mini/Linux+OS2+DOS">Linux+OS/2+DOS
+Mini-HOWTO</ulink> offers help on configuring the OS/2 boot
+manager. 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.dorsai.org/~dcl/publications/NTLDR_Hacking">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 "How It Works" document pack contains some good info
+on all sorts of disk geometry and booting related topics. Here are a few
+links that might help you find it: <ulink URL="ftp://fission.dt.wdc.com/pub/otherdocs/pc_systems/how_it_works/allhiw.zip">ftp://fission.dt.wdc.com/pub/otherdocs/pc_systems/how_it_works/allhiw.zip</ulink>,
+<ulink URL="http://www.cs.yorku.ca/People/frank/docs/">http://www.cs.yorku.ca/People/frank/docs/</ulink>.</para>
+
+<para>Finally, don't overlook FreeBSD's kernel documentation on the booting
+procedure, available in the kernel source distribution (it unpacks to
+<ulink URL="file:/usr/src/sys/i386/boot/biosboot/README.386BSD">file:/usr/src/sys/i386/boot/biosboot/README.386BSD</ulink>.</para>
+
+</chapter>
+
+<chapter>
+<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>
+
+
+<sect1>
+<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's 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 nown as a track). Collectively this information defines the
+"physical disk geometry" 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'll fit on your own disk by
+calculating: <informalexample><para>(# of cylinders) &times; (#
+heads) &times; (63 sectors/track) &times; (512
+bytes/sect)</></informalexample> For example, on my 1.6 Gig Western
+Digital AC31600 EIDE hard disk,that's: <informalexample><para>(3148
+cyl) &times; (16 heads) &times; (63 sectors/track) &times (512
+bytes/sect)</para></informalexample></para>
+
+<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're using
+BIOS LBA (see <xref linkend="limits">), you can't use just any
+program to get the physical geometry. This is because many programs
+(e.g. <filename>MSD.EXE</> or FreeBSD fdisk) don't identify the
+physical disk geometry; they instead report the
+<firstterm>translated geometry</> (virtual numbers from using LBA).
+Stay tuned for what that means.</para>
+
+<para>One other useful thing about these terms. Given 3
+numbers&mdash;a cylinder number, a head number, and a
+sector-within-track number&mdash;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</> or <literal>master boot record</>. Among the useful info
+you'll find are Hale Landis's <citetitle>How It Works</> document
+pack. See the <xref linkend="sources"> section for a few pointers to
+this pack.</para>
+
+<para>Ok, enough terminology. We're talking about booting
+here.</para>
+
+</sect1>
+
+<sect1 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</>, each of which is a
+contiguous chunk of that disk. FreeBSD calls partitions
+<firstterm>slices</> to avoid confusion with it's own partitions, but
+we won't do that here. Each partition can contain its own operating
+system.</para>
+
+<para>Each partition entry in the MBR has a <firstterm>Partition
+ID</>, a <firstterm>Start Cylinder/Head/Sector</>, and an
+<firstterm>End Cylinder/Head/Sector</>. 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</>
+<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 (&gt; 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&mdash;some aren't. What makes a partition bootable is the
+configuration of the <firstterm>Partition Boot Sector</> 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's important to know. All of your
+hard disks have MBRs. However, the one that's important is the one on the
+disk that's 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've just described.</para>
+
+</sect1>
+
+<sect1 id="limits">
+<title>Booting Limitations and Warnings</title>
+
+<para>Now the interesting stuff that you need to watch out for.</para>
+
+<sect2>
+<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's 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:
+<itemizedlist>
+<listitem><para>1024 cylinders, max</para></listitem>
+<listitem><para>256 heads , max</para></listitem>
+<listitem><para>64 cylinders, max (actually 63, <literal>0</> isn't
+available)</para></listitem>
+</itemizedlist>
+</para>
+
+<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't 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:
+<informalexample>
+<para>1024 cyl/disk &times; 16 heads/disk &times; 63 sect/(cyl-head)
+&times; 512 bytes/sector</para>
+</informalexample>
+</para>
+
+<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, it's
+physical geometry is:
+<informalexample>
+<para>(3148 cyl, 16 heads, 63 sectors/track, 512 bytes/sector)</para>
+</informalexample>
+</para>
+
+<para>However, my BIOS LBA remaps this to:
+<informalexample>
+<para>( 787 cyl, 64 heads, 63 sectors/track, 512 bytes/sector)</para>
+</informalexample>
+</para>
+
+<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>
+
+</sect2>
+
+<sect2>
+<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's 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's typically free&hellip;typically.</para>
+
+<para>That's 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's where it'll 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's one that occupies a few extra
+sectors after the MBR, it'll 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's 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's 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:
+<itemizedlist>
+
+<listitem>
+<para>OS-BS 1.35</para>
+</listitem>
+
+<listitem>
+<para>Boot Easy</para>
+</listitem>
+
+<listitem>
+<para>LILO</para>
+</listitem>
+
+</itemizedlist>
+</para>
+
+<para>These boot managers use a few additional sectors after the
+Master Boot Sector:
+<itemizedlist>
+
+<listitem>
+<para>OS-BS 2.0 Beta 8 (sectors 2-5)</para>
+</listitem>
+
+<listitem>
+<para>OS/2's boot manager</para>
+</listitem>
+
+</itemizedlist>
+</para>
+
+</sect2>
+
+<sect2>
+<title>What if your machine won't boot?</title>
+
+<para>At some point when installing boot managers, you might leave the
+MBR in a state such that your machine won't 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:
+<informalexample>
+<screen>A:\> <userinput>FDISK /MBR</></screen>
+</informalexample>
+</para>
+
+<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>
+
+</sect2>
+</sect1>
+</chapter>
+</book>