Remove PHK's existing documentation about the boot process (out of date)
and refer the reader to boot(8) and loader(8). Reviewed by: phk
This commit is contained in:
Nik Clayton
parent
52fb07cf5a
commit
403d96e991
Notes:
svn2git
2020-12-08 03:00:23 +00:00
svn path=/head/; revision=6305
1 changed files with 3 additions and 226 deletions
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@ -1,7 +1,7 @@
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<!--
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The FreeBSD Documentation Project
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$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/internals/chapter.sgml,v 1.13 1999/09/06 06:52:58 peter Exp $
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$FreeBSD: doc/en_US.ISO_8859-1/books/handbook/internals/chapter.sgml,v 1.14 1999/11/07 01:54:47 chris Exp $
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-->
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<chapter id="internals">
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@ -10,231 +10,8 @@
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<sect1 id="booting">
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<title>The FreeBSD Booting Process</title>
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<para><emphasis>Contributed by &a.phk;. v1.1, April
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26th.</emphasis></para>
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<para>Booting FreeBSD is essentially a three step process: load the
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kernel, determine the root filesystem and initialize user-land things.
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This leads to some interesting possibilities shown below.</para>
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<sect2>
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<title>Loading a kernel</title>
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<para>We presently have three basic mechanisms for loading the kernel as
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described below: they all pass some information to the kernel to help
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the kernel decide what to do next.</para>
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<variablelist>
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<varlistentry>
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<term>Biosboot</term>
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<listitem>
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<para>Biosboot is our “bootblocks”. It consists of
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two files which will be installed in the first 8Kbytes of the
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floppy or hard-disk slice to be booted from.</para>
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<para>Biosboot can load a kernel from a FreeBSD filesystem.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Dosboot</term>
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<listitem>
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<para>Dosboot was written by DI. Christian Gusenbauer, and is
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unfortunately at this time one of the few pieces of code that
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will not compile under FreeBSD itself because it is written for
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Microsoft compilers.</para>
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<para>Dosboot will boot the kernel from a MS-DOS file or from a
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FreeBSD filesystem partition on the disk. It attempts to
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negotiate with the various and strange kinds of memory manglers
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that lurk in high memory on MS/DOS systems and usually wins them
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for its case.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Netboot</term>
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<listitem>
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<para>Netboot will try to find a supported Ethernet card, and use
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BOOTP, TFTP and NFS to find a kernel file to boot.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2>
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<title>Determine the root filesystem</title>
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<para>Once the kernel is loaded and the boot-code jumps to it, the
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kernel will initialize itself, trying to determine what hardware is
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present and so on; it then needs to find a root filesystem.</para>
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<para>Presently we support the following types of root
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filesystems:</para>
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<variablelist>
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<varlistentry>
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<term>UFS</term>
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<listitem>
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<para>This is the most normal type of root filesystem. It can
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reside on a floppy or on hard disk.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>MSDOS</term>
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<listitem>
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<para>While this is technically possible, it is not particular
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useful because of the <acronym>FAT</acronym> filesystem's
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inability to deal with links, device nodes and other such
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“UNIXisms”.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>MFS</term>
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<listitem>
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<para>This is actually a UFS filesystem which has been compiled
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into the kernel. That means that the kernel does not really
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need any hard disks, floppies or other hardware to
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function.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>CD9660</term>
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<listitem>
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<para>This is for using a CD-ROM as root filesystem.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>NFS</term>
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<listitem>
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<para>This is for using a fileserver as root filesystem, basically
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making it a diskless machine.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2>
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<title>Initialize user-land things</title>
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<para>To get the user-land going, the kernel, when it has finished
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initialization, will create a process with <literal>pid == 1</literal>
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and execute a program on the root filesystem; this program is normally
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<filename>/sbin/init</filename>.</para>
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<para>You can substitute any program for <command>/sbin/init</command>,
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as long as you keep in mind that:</para>
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<para>there is no stdin/out/err unless you open it yourself. If you
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exit, the machine panics. Signal handling is special for <literal>pid
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== 1</literal>.</para>
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<para>An example of this is the <command>/stand/sysinstall</command>
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program on the installation floppy.</para>
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</sect2>
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<sect2>
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<title>Interesting combinations</title>
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<para>Boot a kernel with a MFS in it with a special
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<filename>/sbin/init</filename> which...</para>
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<variablelist>
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<varlistentry>
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<term>A — Using DOS</term>
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<listitem>
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<itemizedlist>
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<listitem>
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<para>mounts your <filename>C:</filename> as
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<filename>/C:</filename></para>
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</listitem>
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<listitem>
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<para>Attaches <filename>C:/freebsd.fs</filename> on
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<filename>/dev/vn0</filename></para>
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</listitem>
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<listitem>
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<para>mounts <filename>/dev/vn0</filename> as
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<filename>/rootfs</filename></para>
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</listitem>
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<listitem>
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<para>makes symlinks
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<filename>/rootfs/bin</filename> ->
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<filename>/bin</filename>
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<filename>/rootfs/etc</filename> ->
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<filename>/etc</filename>
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<filename>/rootfs/sbin</filename> ->
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<filename>/sbin</filename> (etc...)</para>
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</listitem>
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</itemizedlist>
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<para>Now you are running FreeBSD without repartitioning your hard
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disk...</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>B — Using NFS</term>
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<listitem>
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<para>NFS mounts your <filename>server:~you/FreeBSD</filename> as
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<filename>/nfs</filename>, chroots to <filename>/nfs</filename>
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and executes <filename>/sbin/init</filename> there</para>
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<para>Now you are running FreeBSD diskless, even though you do not
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control the NFS server...</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>C — Start an X-server</term>
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<listitem>
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<para>Now you have an X-terminal, which is better than that dingy
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X-under-windows-so-slow-you-can-see-what-it-does thing that your
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boss insist is better than forking out money on hardware.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>D — Using a tape</term>
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<listitem>
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<para>Takes a copy of <filename>/dev/rwd0</filename> and writes it
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to a remote tape station or fileserver.</para>
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<para>Now you finally get that backup you should have made a year
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ago...</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>E — Acts as a firewall/web-server/what do I
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know...</term>
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<listitem>
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<para>This is particularly interesting since you can boot from a
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write- protected floppy, but still write to your root
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filesystem...</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<para>The FreeBSD booting process is described in the &man.boot.8; and
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&man.loader.8; manual pages.</para>
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</sect1>
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<sect1 id="memoryuse">
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