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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 Jordan K. Hubbard
	<email>jkh@time.cdrom.com</email></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 Partition Magic, 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, 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.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>\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 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>
  </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 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:</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 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</filename> and <filename>/</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</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 BOOTINST.EXE utility included in the \TOOLS 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
      file systems, 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 file systems.  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>&gt; 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 manger
      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>
<![ IGNORE [
<!-- dead link -->
    <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 <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">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:/usr/src/sys/i386/boot/biosboot/README.386BSD">file:/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 nown 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) &times; (# heads) &times; (63
	    sectors/track) &times; (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) &times; (16 heads) &times; (63
	    sectors/track) &times; (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&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</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 (&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 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 &times; 16 heads/disk &times; 63
	    sect/(cyl-head) &times; 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&hellip;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>
