path: root/en_US.ISO8859-1/books/arch-handbook/boot/chapter.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!--
The FreeBSD Documentation Project

Copyright (c) 2002 Sergey Lyubka <devnull@uptsoft.com>
All rights reserved
Copyright (c) 2014 Sergio Andr?s G?mez del Real <Sergio.G.delReal@gmail.com>
All rights reserved
$FreeBSD$
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<chapter xmlns="http://docbook.org/ns/docbook"
  xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0"
  xml:id="boot">

  <info>
    <title>Bootstrapping and Kernel Initialization</title>

    <authorgroup>
      <author>
	<personname>
	  <firstname>Sergey</firstname>
	  <surname>Lyubka</surname>
	</personname>

	<contrib>Contributed by </contrib>
      </author>
      <!-- devnull@uptsoft.com  12 Jun 2002 -->
    </authorgroup>

    <authorgroup>
      <author>
	<personname>
	  <firstname>Sergio Andr&eacute;s</firstname>
	  <surname> G&oacute;mez del Real</surname>
	</personname>

	<contrib>Updated and enhanced by </contrib>
      </author>
      <!-- Sergio.G.DelReal@gmail.com  Jan 2014 -->
    </authorgroup>
  </info>

  <sect1 xml:id="boot-synopsis">
    <title>Synopsis</title>

    <indexterm><primary>BIOS</primary></indexterm>
    <indexterm><primary>firmware</primary></indexterm>
    <indexterm><primary>POST</primary></indexterm>
    <indexterm><primary>IA-32</primary></indexterm>
    <indexterm><primary>booting</primary></indexterm>
    <indexterm><primary>system initialization</primary></indexterm>
    <para>This chapter is an overview of the boot and system
      initialization processes, starting from the
      <acronym>BIOS</acronym> (firmware) <acronym>POST</acronym>, to
      the first user process creation.  Since the initial
      steps of system startup are very architecture dependent, the
      IA-32 architecture is used as an example.</para>

    <para>The &os; boot process can be surprisingly complex.  After
      control is passed from the <acronym>BIOS</acronym>, a
      considerable amount of low-level configuration must be done
      before the kernel can be loaded and executed.  This setup must
      be done in a simple and flexible manner, allowing the user a
      great deal of customization possibilities.</para>
  </sect1>

  <sect1 xml:id="boot-overview">
    <title>Overview</title>

    <para>The boot process is an extremely machine-dependent
      activity.  Not only must code be written for every computer
      architecture, but there may also be multiple types of booting on
      the same architecture.  For example, a directory listing of
      <filename>/usr/src/sys/boot</filename>
      reveals a great amount of architecture-dependent code.  There is
      a directory for each of the various supported architectures.  In
      the x86-specific <filename>i386</filename>
      directory, there are subdirectories for different boot standards
      like <filename>mbr</filename> (Master Boot Record),
      <filename>gpt</filename> (<acronym>GUID</acronym> Partition
      Table), and <filename>efi</filename> (Extensible Firmware
      Interface).  Each boot standard has its own conventions and data
      structures.  The example that follows shows booting an x86
      computer from an <acronym>MBR</acronym> hard drive with the &os;
      <filename>boot0</filename> multi-boot loader stored in the very
      first sector.  That boot code starts the &os; three-stage boot
      process.</para>

    <para>The key to understanding this process is that it is a series
      of stages of increasing complexity.  These stages are
      <filename>boot1</filename>, <filename>boot2</filename>, and
      <filename>loader</filename> (see &man.boot.8; for more detail).
      The boot system executes each stage in sequence.  The last
      stage, <filename>loader</filename>, is responsible for loading
      the &os; kernel.  Each stage is examined in the following
      sections.</para>

    <para>Here is an example of the output generated by the
      different boot stages.  Actual output
      may differ from machine to machine:</para>

    <informaltable frame="none" pgwide="0">
      <tgroup cols="2">
	<tbody>
	  <row>
	    <entry>&os; Component</entry>
	    <entry>Output (may vary)</entry>
	  </row>

	  <row>
	    <entry><literal>boot0</literal></entry>
	    <entry><screen>F1    FreeBSD
F2    BSD
F5    Disk 2</screen></entry>
	  </row>

	  <row>
	    <entry><literal>boot2</literal>
	      <footnote><para>This prompt will appear if the user
		  presses a key just after selecting an OS to boot at
		  the <literal>boot0</literal>
		  stage.</para></footnote></entry>
	    <entry><screen>&gt;&gt;FreeBSD/i386 BOOT
Default: 1:ad(1,a)/boot/loader
boot:</screen></entry>
	  </row>

	  <row>
	    <entry><filename>loader</filename></entry>
	    <entry><screen>BTX loader 1.00 BTX version is 1.02
Consoles: internal video/keyboard
BIOS drive C: is disk0
BIOS 639kB/2096064kB available memory

FreeBSD/x86 bootstrap loader, Revision 1.1
Console internal video/keyboard
(root@snap.freebsd.org, Thu Jan 16 22:18:05 UTC 2014)
Loading /boot/defaults/loader.conf
/boot/kernel/kernel text=0xed9008 data=0x117d28+0x176650 syms=[0x8+0x137988+0x8+0x1515f8]</screen></entry>
	  </row>

	  <row>
	    <entry>kernel</entry>
	    <entry><screen>Copyright (c) 1992-2013 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
        The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 10.0-RELEASE #0 r260789: Thu Jan 16 22:34:59 UTC 2014
    root@snap.freebsd.org:/usr/obj/usr/src/sys/GENERIC amd64
FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610</screen></entry>
	  </row>
	</tbody>
      </tgroup>
    </informaltable>
  </sect1>

  <sect1 xml:id="boot-bios">
    <title>The <acronym>BIOS</acronym></title>

    <para>When the computer powers on, the processor's registers are
      set to some predefined values.  One of the registers is the
      <emphasis>instruction pointer</emphasis> register, and its value
      after a power on is well defined: it is a 32-bit value of
      <literal>0xfffffff0</literal>.  The instruction pointer register
      (also known as the Program Counter) points to code to be
      executed by the processor.  Another important register is the
      <literal>cr0</literal> 32-bit control register, and its value
      just after a reboot is <literal>0</literal>.  One of
      <literal>cr0</literal>'s bits, the PE (Protection Enabled) bit,
      indicates whether the processor is running in 32-bit protected
      mode or 16-bit real mode.  Since this bit is cleared at boot
      time, the processor boots in 16-bit real mode.  Real mode means,
      among other things, that linear and physical addresses are
      identical.  The reason for the processor not to start
      immediately in 32-bit protected mode is backwards compatibility.
      In particular, the boot process relies on the services provided
      by the <acronym>BIOS</acronym>, and the <acronym>BIOS</acronym>
      itself works in legacy, 16-bit code.</para>

    <para>The value of <literal>0xfffffff0</literal> is slightly less
      than 4&nbsp;GB, so unless the machine has 4&nbsp;GB of physical
      memory, it cannot point to a valid memory address.  The
      computer's hardware translates this address so that it points to
      a <acronym>BIOS</acronym> memory block.</para>

    <para>The <acronym>BIOS</acronym> (Basic Input Output
      System) is a chip on the motherboard that has a relatively small
      amount of read-only memory (<acronym>ROM</acronym>).  This
      memory contains various low-level routines that are specific to
      the hardware supplied with the motherboard.  The processor will
      first jump to the address 0xfffffff0, which really resides in
      the <acronym>BIOS</acronym>'s memory.  Usually this address
      contains a jump instruction to the <acronym>BIOS</acronym>'s
      POST routines.</para>

    <para>The <acronym>POST</acronym> (Power On Self Test)
      is a set of routines including the memory check, system bus
      check, and other low-level initialization so the
      <acronym>CPU</acronym> can set up the computer properly.  The
      important step of this stage is determining the boot device.
      Modern <acronym>BIOS</acronym> implementations permit the
      selection of a boot device, allowing booting from a floppy,
      <acronym>CD-ROM</acronym>, hard disk, or other devices.</para>

    <para>The very last thing in the <acronym>POST</acronym> is the
      <literal>INT 0x19</literal> instruction.  The
      <literal>INT 0x19</literal> handler reads 512 bytes from the
      first sector of boot device into the memory at address
      <literal>0x7c00</literal>.  The term
      <emphasis>first sector</emphasis> originates from hard drive
      architecture, where the magnetic plate is divided into a number
      of cylindrical tracks.  Tracks are numbered, and every track is
      divided into a number (usually 64) of sectors.  Track numbers
      start at 0, but sector numbers start from 1. Track 0 is the
      outermost on the magnetic plate, and sector 1, the first sector,
      has a special purpose.  It is also called the
      <acronym>MBR</acronym>, or Master Boot Record.  The remaining
      sectors on the first track are never used.</para>

    <para>This sector is our boot-sequence starting point.  As we will
      see, this sector contains a copy of our
      <filename>boot0</filename> program.  A jump is made by the
      <acronym>BIOS</acronym> to address <literal>0x7c00</literal> so
      it starts executing.</para>
  </sect1>

  <sect1 xml:id="boot-boot0">
    <title>The Master Boot Record (<literal>boot0</literal>)</title>

    <indexterm><primary>MBR</primary></indexterm>

    <para>After control is received from the <acronym>BIOS</acronym>
      at memory address <literal>0x7c00</literal>,
      <filename>boot0</filename> starts executing.  It is the first
      piece of code under &os; control.  The task of
      <filename>boot0</filename> is quite simple: scan the partition
      table and let the user choose which partition to boot from.  The
      Partition Table is a special, standard data structure embedded
      in the <acronym>MBR</acronym> (hence embedded in
      <filename>boot0</filename>) describing the four standard PC
      <quote>partitions</quote>
      <footnote>
	<para><link
	    xlink:href="http://en.wikipedia.org/wiki/Master_boot_record"></link></para></footnote>.
      <filename>boot0</filename> resides in the filesystem as
      <filename>/boot/boot0</filename>.  It is a small 512-byte file,
      and it is exactly what &os;'s installation procedure wrote to
      the hard disk's <acronym>MBR</acronym> if you chose the
      <quote>bootmanager</quote> option at installation time.  Indeed,
      <filename>boot0</filename> <emphasis>is</emphasis> the
      <acronym>MBR</acronym>.</para>

    <para>As mentioned previously, the <literal>INT 0x19</literal>
      instruction causes the <literal>INT 0x19</literal> handler to
      load an <acronym>MBR</acronym> (<filename>boot0</filename>) into
      memory at address <literal>0x7c00</literal>.  The source file
      for <filename>boot0</filename> can be found in
      <filename>sys/boot/i386/boot0/boot0.S</filename> - which is an
      awesome piece of code written by Robert Nordier.</para>

    <para>A special structure starting from offset
      <literal>0x1be</literal> in the <acronym>MBR</acronym> is called
      the <emphasis>partition table</emphasis>.  It has four records
      of 16 bytes each, called <emphasis>partition records</emphasis>,
      which represent how the hard disk is partitioned, or, in &os;'s
      terminology, sliced.  One byte of those 16 says whether a
      partition (slice) is bootable or not.  Exactly one record must
      have that flag set, otherwise <filename>boot0</filename>'s code
      will refuse to proceed.</para>

    <para>A partition record has the following fields:</para>

    <itemizedlist>
      <listitem>
	<para>the 1-byte filesystem type</para>
      </listitem>

      <listitem>
	<para>the 1-byte bootable flag</para>
      </listitem>

      <listitem>
	<para>the 6 byte descriptor in CHS format</para>
      </listitem>

      <listitem>
	<para>the 8 byte descriptor in LBA format</para>
      </listitem>
    </itemizedlist>

    <para>A partition record descriptor contains information about
      where exactly the partition resides on the drive.  Both
      descriptors, <acronym>LBA</acronym> and <acronym>CHS</acronym>,
      describe the same information, but in different ways:
      <acronym>LBA</acronym> (Logical Block Addressing) has the
      starting sector for the partition and the partition's length,
      while <acronym>CHS</acronym> (Cylinder Head Sector) has
      coordinates for the first and last sectors of the partition.
      The partition table ends with the special signature
      <literal>0xaa55</literal>.</para>

    <para>The <acronym>MBR</acronym> must fit into 512 bytes, a single
      disk sector.  This program uses low-level <quote>tricks</quote>
      like taking advantage of the side effects of certain
      instructions and reusing register values from previous
      operations to make the most out of the fewest possible
      instructions.  Care must also be taken when handling the
      partition table, which is embedded in the <acronym>MBR</acronym>
      itself.  For these reasons, be very careful when modifying
      <filename>boot0.S</filename>.</para>

    <para>Note that the <filename>boot0.S</filename> source file
      is assembled <quote>as is</quote>: instructions are translated
      one by one to binary, with no additional information (no
      <acronym>ELF</acronym> file format, for example).  This kind of
      low-level control is achieved at link time through special
      control flags passed to the linker.  For example, the text
      section of the program is set to be located at address
      <literal>0x600</literal>.  In practice this means that
      <filename>boot0</filename> must be loaded to memory address
      <literal>0x600</literal> in order to function properly.</para>

    <para>It is worth looking at the <filename>Makefile</filename> for
      <filename>boot0</filename>
      (<filename>sys/boot/i386/boot0/Makefile</filename>), as it
      defines some of the run-time behavior of
      <filename>boot0</filename>.  For instance, if a terminal
      connected to the serial port (COM1) is used for I/O, the macro
      <literal>SIO</literal> must be defined
      (<literal>-DSIO</literal>).  <literal>-DPXE</literal> enables
      boot through <acronym>PXE</acronym> by pressing
      <keycap>F6</keycap>.  Additionally, the program defines a set of
      <emphasis>flags</emphasis> that allow further modification of
      its behavior.  All of this is illustrated in the
      <filename>Makefile</filename>.  For example, look at the
      linker directives which command the linker to start the text
      section at address <literal>0x600</literal>, and to build the
      output file <quote>as is</quote> (strip out any file
      formatting):</para>

    <figure xml:id="boot-boot0-makefile-as-is">
      <title><filename>sys/boot/i386/boot0/Makefile</filename></title>

      <programlisting>      BOOT_BOOT0_ORG?=0x600
      LDFLAGS=-e start -Ttext ${BOOT_BOOT0_ORG} \
      -Wl,-N,-S,--oformat,binary</programlisting>
    </figure>

    <para>Let us now start our study of the <acronym>MBR</acronym>, or
      <filename>boot0</filename>, starting where execution
      begins.</para>

    <note>
      <para>Some modifications have been made to some instructions in
	favor of better exposition.  For example, some macros are
	expanded, and some macro tests are omitted when the result of
	the test is known.  This applies to all of the code examples
	shown.</para>
    </note>

    <figure xml:id="boot-boot0-entrypoint">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>start:
      cld			# String ops inc
      xorw %ax,%ax		# Zero
      movw %ax,%es		# Address
      movw %ax,%ds		#  data
      movw %ax,%ss		# Set up
      movw 0x7c00,%sp		#  stack</programlisting>
    </figure>

    <para>This first block of code is the entry point of the program.
      It is where the <acronym>BIOS</acronym> transfers control.
      First, it makes sure that the string operations autoincrement
      its pointer operands (the <literal>cld</literal> instruction)
      <footnote>
	<para>When in doubt, we refer the reader to the official Intel
	  manuals, which describe the exact semantics for each
	  instruction: <link
	    xlink:href="http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html"></link>.</para></footnote>.
      Then, as it makes no assumption about the state of the segment
      registers, it initializes them.  Finally, it sets the stack
      pointer register (<literal>%sp</literal>) to address
      <literal>0x7c00</literal>, so we have a working stack.</para>

    <para>The next block is responsible for the relocation and
      subsequent jump to the relocated code.</para>

    <figure xml:id="boot-boot0-relocation">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>      movw $0x7c00,%si	# Source
      movw $0x600,%di		# Destination
      movw $512,%cx		# Word count
      rep			# Relocate
      movsb			#  code
      movw %di,%bp		# Address variables
      movb $16,%cl		# Words to clear
      rep			# Zero
      stosb			#  them
      incb -0xe(%di)		# Set the S field to 1
      jmp main-0x7c00+0x600	# Jump to relocated code</programlisting>
    </figure>

    <para>As <filename>boot0</filename> is loaded by the
      <acronym>BIOS</acronym> to address <literal>0x7C00</literal>, it
      copies itself to address <literal>0x600</literal> and then
      transfers control there (recall that it was linked to execute at
      address <literal>0x600</literal>).  The source address,
      <literal>0x7c00</literal>, is copied to register
      <literal>%si</literal>.  The destination address,
      <literal>0x600</literal>, to register <literal>%di</literal>.
      The number of bytes to copy, <literal>512</literal> (the
      program's size), is copied to register <literal>%cx</literal>.
      Next, the <literal>rep</literal> instruction repeats the
      instruction that follows, that is, <literal>movsb</literal>, the
      number of times dictated by the <literal>%cx</literal> register.
      The <literal>movsb</literal> instruction copies the byte pointed
      to by <literal>%si</literal> to the address pointed to by
      <literal>%di</literal>.  This is repeated another 511 times.  On
      each repetition, both the source and destination registers,
      <literal>%si</literal> and <literal>%di</literal>, are
      incremented by one.  Thus, upon completion of the 512-byte copy,
      <literal>%di</literal> has the value
      <literal>0x600</literal>+<literal>512</literal>=
      <literal>0x800</literal>, and <literal>%si</literal> has the
      value <literal>0x7c00</literal>+<literal>512</literal>=
      <literal>0x7e00</literal>; we have thus completed the code
      <emphasis>relocation</emphasis>.</para>

    <para>Next, the destination register
      <literal>%di</literal> is copied to <literal>%bp</literal>.
      <literal>%bp</literal> gets the value <literal>0x800</literal>.
      The value <literal>16</literal> is copied to
      <literal>%cl</literal> in preparation for a new string operation
      (like our previous <literal>movsb</literal>).  Now,
      <literal>stosb</literal> is executed 16 times.  This instruction
      copies a <literal>0</literal> value to the address pointed to by
      the destination register (<literal>%di</literal>, which is
      <literal>0x800</literal>), and increments it.  This is repeated
      another 15 times, so <literal>%di</literal> ends up with value
      <literal>0x810</literal>.  Effectively, this clears the address
      range <literal>0x800</literal>-<literal>0x80f</literal>.  This
      range is used as a (fake) partition table for writing the
      <acronym>MBR</acronym> back to disk.  Finally, the sector field
      for the <acronym>CHS</acronym> addressing of this fake partition
      is given the value 1 and a jump is made to the main function
      from the relocated code.  Note that until this jump to the
      relocated code, any reference to an absolute address was
      avoided.</para>

    <para>The following code block tests whether the drive number
      provided by the <acronym>BIOS</acronym> should be used, or
      the one stored in <filename>boot0</filename>.</para>

    <figure xml:id="boot-boot0-drivenumber">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>main:
      testb $SETDRV,-69(%bp)	# Set drive number?
      jnz disable_update	# Yes
      testb %dl,%dl		# Drive number valid?
      js save_curdrive		# Possibly (0x80 set)</programlisting>
    </figure>

    <para>This code tests the <literal>SETDRV</literal> bit
      (<literal>0x20</literal>) in the <emphasis>flags</emphasis>
      variable.  Recall that register <literal>%bp</literal> points to
      address location <literal>0x800</literal>, so the test is done
      to the <emphasis>flags</emphasis> variable at address
      <literal>0x800</literal>-<literal>69</literal>=
      <literal>0x7bb</literal>.  This is an example of the type of
      modifications that can be done to <filename>boot0</filename>.
      The <literal>SETDRV</literal> flag is not set by default, but it
      can be set in the <filename>Makefile</filename>.  When set, the
      drive number stored in the <acronym>MBR</acronym> is used
      instead of the one provided by the <acronym>BIOS</acronym>.  We
      assume the defaults, and that the <acronym>BIOS</acronym>
      provided a valid drive number, so we jump to
      <literal>save_curdrive</literal>.</para>

    <para>The next block saves the drive number provided by the
      <acronym>BIOS</acronym>, and calls <literal>putn</literal> to
      print a new line on the screen.</para>

    <figure xml:id="boot-boot0-savedrivenumber">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>save_curdrive:
      movb %dl, (%bp)		# Save drive number
      pushw %dx			# Also in the stack
#ifdef	TEST	/* test code, print internal bios drive */
      rolb $1, %dl
      movw $drive, %si
      call putkey
#endif
      callw putn		# Print a newline</programlisting>
    </figure>

    <para>Note that we assume <varname>TEST</varname> is not defined,
      so the conditional code in it is not assembled and will not
      appear in our executable <filename>boot0</filename>.</para>

    <para>Our next block implements the actual scanning of the
      partition table.  It prints to the screen the partition type for
      each of the four entries in the partition table.  It compares
      each type with a list of well-known operating system file
      systems.  Examples of recognized partition types are
      <acronym>NTFS</acronym> (&windows;, ID 0x7),
      <literal>ext2fs</literal> (&linux;, ID 0x83), and, of course,
      <literal>ffs</literal>/<literal>ufs2</literal> (&os;, ID 0xa5).
      The implementation is fairly simple.</para>

    <figure xml:id="boot-boot0-partition-scan">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>      movw $(partbl+0x4),%bx	# Partition table (+4)
      xorw %dx,%dx		# Item number

read_entry:
      movb %ch,-0x4(%bx)	# Zero active flag (ch == 0)
      btw %dx,_FLAGS(%bp)	# Entry enabled?
      jnc next_entry		# No
      movb (%bx),%al		# Load type
      test %al, %al		# skip empty partition
      jz next_entry
      movw $bootable_ids,%di	# Lookup tables
      movb $(TLEN+1),%cl	# Number of entries
      repne			# Locate
      scasb			#  type
      addw $(TLEN-1), %di	# Adjust
      movb (%di),%cl		# Partition
      addw %cx,%di		#  description
      callw putx		# Display it

next_entry:
      incw %dx			# Next item
      addb $0x10,%bl		# Next entry
      jnc read_entry		# Till done</programlisting>
    </figure>

    <para>It is important to note that the active flag for each entry
      is cleared, so after the scanning, <emphasis>no</emphasis>
      partition entry is active in our memory copy of
      <filename>boot0</filename>.  Later, the active flag will be set
      for the selected partition.  This ensures that only one active
      partition exists if the user chooses to write the changes back
      to disk.</para>

    <para>The next block tests for other drives.  At startup,
      the <acronym>BIOS</acronym> writes the number of drives present
      in the computer to address <literal>0x475</literal>.  If there
      are any other drives present, <filename>boot0</filename> prints
      the current drive to screen.  The user may command
      <filename>boot0</filename> to scan partitions on another drive
      later.</para>

    <figure xml:id="boot-boot0-test-drives">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>      popw %ax			# Drive number
      subb $0x79,%al		# Does next
      cmpb 0x475,%al		#  drive exist? (from BIOS?)
      jb print_drive		# Yes
      decw %ax			# Already drive 0?
      jz print_prompt		# Yes</programlisting>
    </figure>

    <para>We make the assumption that a single drive is present, so
      the jump to <literal>print_drive</literal> is not performed.  We
      also assume nothing strange happened, so we jump to
      <literal>print_prompt</literal>.</para>

    <para>This next block just prints out a prompt followed by the
      default option:</para>

    <figure xml:id="boot-boot0-prompt">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>print_prompt:
      movw $prompt,%si		# Display
      callw putstr		#  prompt
      movb _OPT(%bp),%dl	# Display
      decw %si			#  default
      callw putkey		#  key
      jmp start_input		# Skip beep</programlisting>
    </figure>

    <para>Finally, a jump is performed to
      <literal>start_input</literal>, where the
      <acronym>BIOS</acronym> services are used to start a timer and
      for reading user input from the keyboard; if the timer expires,
      the default option will be selected:</para>

    <figure xml:id="boot-boot0-start-input">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>start_input:
      xorb %ah,%ah		# BIOS: Get
      int $0x1a			#  system time
      movw %dx,%di		# Ticks when
      addw _TICKS(%bp),%di	#  timeout
read_key:
      movb $0x1,%ah		# BIOS: Check
      int $0x16			#  for keypress
      jnz got_key		# Have input
      xorb %ah,%ah		# BIOS: int 0x1a, 00
      int $0x1a			#  get system time
      cmpw %di,%dx		# Timeout?
      jb read_key		# No</programlisting>
    </figure>

    <para>An interrupt is requested with number
      <literal>0x1a</literal> and argument <literal>0</literal> in
      register <literal>%ah</literal>.  The <acronym>BIOS</acronym>
      has a predefined set of services, requested by applications as
      software-generated interrupts through the <literal>int</literal>
      instruction and receiving arguments in registers (in this case,
      <literal>%ah</literal>).  Here, particularly, we are requesting
      the number of clock ticks since last midnight; this value is
      computed by the <acronym>BIOS</acronym> through the
      <acronym>RTC</acronym> (Real Time Clock).  This clock can be
      programmed to work at frequencies ranging from 2&nbsp;Hz to
      8192&nbsp;Hz.  The <acronym>BIOS</acronym> sets it to
      18.2&nbsp;Hz at startup.  When the request is satisfied, a
      32-bit result is returned by the <acronym>BIOS</acronym> in
      registers <literal>%cx</literal> and <literal>%dx</literal>
      (lower bytes in <literal>%dx</literal>).  This result (the
      <literal>%dx</literal> part) is copied to register
      <literal>%di</literal>, and the value of the
      <varname>TICKS</varname> variable is added to
      <literal>%di</literal>.  This variable resides in
      <filename>boot0</filename> at offset <literal>_TICKS</literal>
      (a negative value) from register <literal>%bp</literal> (which,
      recall, points to <literal>0x800</literal>).  The default value
      of this variable is <literal>0xb6</literal> (182 in decimal).
      Now, the idea is that <filename>boot0</filename> constantly
      requests the time from the <acronym>BIOS</acronym>, and when the
      value returned in register <literal>%dx</literal> is greater
      than the value stored in <literal>%di</literal>, the time is up
      and the default selection will be made.  Since the RTC ticks
      18.2 times per second, this condition will be met after 10
      seconds (this default behavior can be changed in the
      <filename>Makefile</filename>).  Until this time has passed,
      <filename>boot0</filename> continually asks the
      <acronym>BIOS</acronym> for any user input; this is done through
      <literal>int 0x16</literal>, argument <literal>1</literal> in
      <literal>%ah</literal>.</para>

    <para>Whether a key was pressed or the time expired, subsequent
      code validates the selection.  Based on the selection, the
      register <literal>%si</literal> is set to point to the
      appropriate partition entry in the partition table.  This new
      selection overrides the previous default one.  Indeed, it
      becomes the new default.  Finally, the ACTIVE flag of the
      selected partition is set.  If it was enabled at compile time,
      the in-memory version of <filename>boot0</filename> with these
      modified values is written back to the <acronym>MBR</acronym> on
      disk.  We leave the details of this implementation to the
      reader.</para>

    <para>We now end our study with the last code block from the
      <filename>boot0</filename> program:</para>

    <figure xml:id="boot-boot0-check-bootable">
      <title><filename>sys/boot/i386/boot0/boot0.S</filename></title>

      <programlisting>      movw $0x7c00,%bx		# Address for read
      movb $0x2,%ah		# Read sector
      callw intx13		#  from disk
      jc beep			# If error
      cmpw $0xaa55,0x1fe(%bx)	# Bootable?
      jne beep			# No
      pushw %si			# Save ptr to selected part.
      callw putn		# Leave some space
      popw %si			# Restore, next stage uses it
      jmp *%bx			# Invoke bootstrap</programlisting>
    </figure>

    <para>Recall that <literal>%si</literal> points to the selected
      partition entry.  This entry tells us where the partition begins
      on disk.  We assume, of course, that the partition selected is
      actually a &os; slice.</para>

    <note>
      <para>From now on, we will favor the use of the technically
	more accurate term <quote>slice</quote> rather than
	<quote>partition</quote>.</para>
    </note>

    <para>The transfer buffer is set to <literal>0x7c00</literal>
      (register <literal>%bx</literal>), and a read for the first
      sector of the &os; slice is requested by calling
      <literal>intx13</literal>.  We assume that everything went okay,
      so a jump to <literal>beep</literal> is not performed.  In
      particular, the new sector read must end with the magic sequence
      <literal>0xaa55</literal>.  Finally, the value at
      <literal>%si</literal> (the pointer to the selected partition
      table) is preserved for use by the next stage, and a jump is
      performed to address <literal>0x7c00</literal>, where execution
      of our next stage (the just-read block) is started.</para>
  </sect1>

  <sect1 xml:id="boot-boot1">
    <title><literal>boot1</literal> Stage</title>

    <para>So far we have gone through the following sequence:</para>

    <itemizedlist>
      <listitem>
	<para>The <acronym>BIOS</acronym> did some early hardware
	  initialization, including the <acronym>POST</acronym>.  The
	  <acronym>MBR</acronym> (<filename>boot0</filename>) was
	  loaded from absolute disk sector one to address
	  <literal>0x7c00</literal>.  Execution control was passed to
	  that location.</para>
      </listitem>

      <listitem>
	<para><filename>boot0</filename> relocated itself to the
	  location it was linked to execute
	  (<literal>0x600</literal>), followed by a jump to continue
	  execution at the appropriate place.  Finally,
	  <filename>boot0</filename> loaded the first disk sector from
	  the &os; slice to address <literal>0x7c00</literal>.
	  Execution control was passed to that location.</para>
      </listitem>
    </itemizedlist>

    <para><filename>boot1</filename> is the next step in the
      boot-loading sequence.  It is the first of three boot stages.
      Note that we have been dealing exclusively
      with disk sectors.  Indeed, the <acronym>BIOS</acronym> loads
      the absolute first sector, while <filename>boot0</filename>
      loads the first sector of the &os; slice.  Both loads are to
      address <literal>0x7c00</literal>.  We can conceptually think of
      these disk sectors as containing the files
      <filename>boot0</filename> and <filename>boot1</filename>,
      respectively, but in reality this is not entirely true for
      <filename>boot1</filename>.  Strictly speaking, unlike
      <filename>boot0</filename>, <filename>boot1</filename> is not
      part of the boot blocks
      <footnote>
	<para>There is a file <filename>/boot/boot1</filename>, but it
	  is not the written to the beginning of the &os; slice.
	  Instead, it is concatenated with <filename>boot2</filename>
	  to form <filename>boot</filename>, which
	  <emphasis>is</emphasis> written to the beginning of the &os;
	  slice and read at boot time.</para></footnote>.
      Instead, a single, full-blown file, <filename>boot</filename>
      (<filename>/boot/boot</filename>), is what ultimately is
      written to disk.  This file is a combination of
      <filename>boot1</filename>, <filename>boot2</filename> and the
      <literal>Boot Extender</literal> (or <acronym>BTX</acronym>).
      This single file is greater in size than a single sector
      (greater than 512 bytes).  Fortunately,
      <filename>boot1</filename> occupies <emphasis>exactly</emphasis>
      the first 512 bytes of this single file, so when
      <filename>boot0</filename> loads the first sector of the &os;
      slice (512 bytes), it is actually loading
      <filename>boot1</filename> and transferring control to
      it.</para>

    <para>The main task of <filename>boot1</filename> is to load the
      next boot stage.  This next stage is somewhat more complex.  It
      is composed of a server called the <quote>Boot Extender</quote>,
      or <acronym>BTX</acronym>, and a client, called
      <filename>boot2</filename>.  As we will see, the last boot
      stage, <filename>loader</filename>, is also a client of the
      <acronym>BTX</acronym> server.</para>

    <para>Let us now look in detail at what exactly is done by
      <filename>boot1</filename>, starting like we did for
      <filename>boot0</filename>, at its entry point:</para>

    <figure xml:id="boot-boot1-entry">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>start:
	jmp main</programlisting>
    </figure>

    <para>The entry point at <literal>start</literal> simply jumps
      past a special data area to the label <literal>main</literal>,
      which in turn looks like this:</para>

    <figure xml:id="boot-boot1-main">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>main:
      cld			# String ops inc
      xor %cx,%cx		# Zero
      mov %cx,%es		# Address
      mov %cx,%ds		#  data
      mov %cx,%ss		# Set up
      mov $start,%sp		#  stack
      mov %sp,%si		# Source
      mov $0x700,%di		# Destination
      incb %ch			# Word count
      rep			# Copy
      movsw			#  code</programlisting>
    </figure>

    <para>Just like <filename>boot0</filename>, this
      code relocates <filename>boot1</filename>,
      this time to memory address <literal>0x700</literal>.  However,
      unlike <filename>boot0</filename>, it does not jump there.
      <filename>boot1</filename> is linked to execute at
      address <literal>0x7c00</literal>, effectively where it was
      loaded in the first place.  The reason for this relocation will
      be discussed shortly.</para>

    <para>Next comes a loop that looks for the &os; slice.  Although
      <filename>boot0</filename> loaded <filename>boot1</filename>
      from the &os; slice, no information was passed to it about this
      <footnote>
	<para>Actually we did pass a pointer to the slice entry in
	  register <literal>%si</literal>.  However,
	  <filename>boot1</filename> does not assume that it was
	  loaded by <filename>boot0</filename> (perhaps some other
	  <acronym>MBR</acronym> loaded it, and did not pass this
	  information), so it assumes nothing.</para></footnote>,
      so <filename>boot1</filename> must rescan the
      partition table to find where the &os; slice starts.  Therefore
      it rereads the <acronym>MBR</acronym>:</para>

    <figure xml:id="boot-boot1-find-freebsd">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>      mov $part4,%si		# Partition
      cmpb $0x80,%dl		# Hard drive?
      jb main.4			# No
      movb $0x1,%dh		# Block count
      callw nread		# Read MBR</programlisting>
    </figure>

    <para>In the code above, register <literal>%dl</literal>
      maintains information about the boot device.  This is passed on
      by the <acronym>BIOS</acronym> and preserved by the
      <acronym>MBR</acronym>.  Numbers <literal>0x80</literal> and
      greater tells us that we are dealing with a hard drive, so a
      call is made to <literal>nread</literal>, where the
      <acronym>MBR</acronym> is read.  Arguments to
      <literal>nread</literal> are passed through
      <literal>%si</literal> and <literal>%dh</literal>.  The memory
      address at label <literal>part4</literal> is copied to
      <literal>%si</literal>.  This memory address holds a
      <quote>fake partition</quote> to be used by
      <literal>nread</literal>.  The following is the data in the fake
      partition:</para>

    <figure xml:id="boot-boot2-make-fake-partition">
      <title><filename>sys/boot/i386/boot2/Makefile</filename></title>

      <programlisting>      part4:
	.byte 0x80, 0x00, 0x01, 0x00
	.byte 0xa5, 0xfe, 0xff, 0xff
	.byte 0x00, 0x00, 0x00, 0x00
	.byte 0x50, 0xc3, 0x00, 0x00</programlisting>
    </figure>

    <para>In particular, the <acronym>LBA</acronym> for this fake
      partition is hardcoded to zero.  This is used as an argument to
      the <acronym>BIOS</acronym> for reading absolute sector one from
      the hard drive.  Alternatively, CHS addressing could be used.
      In this case, the fake partition holds cylinder 0, head 0 and
      sector 1, which is equivalent to absolute sector one.</para>

    <para>Let us now proceed to take a look at
      <literal>nread</literal>:</para>

    <figure xml:id="boot-boot1-nread">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>nread:
      mov $0x8c00,%bx		# Transfer buffer
      mov 0x8(%si),%ax		# Get
      mov 0xa(%si),%cx		#  LBA
      push %cs			# Read from
      callw xread.1		#  disk
      jnc return		# If success, return</programlisting>
    </figure>

    <para>Recall that <literal>%si</literal> points to the fake
      partition.  The word
      <footnote>
	<para>In the context of 16-bit real mode, a word is 2
	  bytes.</para></footnote>
      at offset <literal>0x8</literal> is copied to register
      <literal>%ax</literal> and word at offset <literal>0xa</literal>
      to <literal>%cx</literal>.  They are interpreted by the
      <acronym>BIOS</acronym> as the lower 4-byte value denoting the
      LBA to be read (the upper four bytes are assumed to be zero).
      Register <literal>%bx</literal> holds the memory address where
      the <acronym>MBR</acronym> will be loaded.  The instruction
      pushing <literal>%cs</literal> onto the stack is very
      interesting.  In this context, it accomplishes nothing.
      However, as we will see shortly, <filename>boot2</filename>, in
      conjunction with the <acronym>BTX</acronym> server, also uses
      <literal>xread.1</literal>.  This mechanism will be discussed in
      the next section.</para>

    <para>The code at <literal>xread.1</literal> further calls
      the <literal>read</literal> function, which actually calls the
      <acronym>BIOS</acronym> asking for the disk sector:</para>

    <figure xml:id="boot-boot1-xread1">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>xread.1:
	pushl $0x0		#  absolute
	push %cx		#  block
	push %ax		#  number
	push %es		# Address of
	push %bx		#  transfer buffer
	xor %ax,%ax		# Number of
	movb %dh,%al		#  blocks to
	push %ax		#  transfer
	push $0x10		# Size of packet
	mov %sp,%bp		# Packet pointer
	callw read		# Read from disk
	lea 0x10(%bp),%sp	# Clear stack
	lret			# To far caller</programlisting>
    </figure>

    <para>Note the long return instruction at the end of this block.
      This instruction pops out the <literal>%cs</literal> register
      pushed by <literal>nread</literal>, and returns.  Finally,
      <literal>nread</literal> also returns.</para>

    <para>With the <acronym>MBR</acronym> loaded to memory, the actual
      loop for searching the &os; slice begins:</para>

    <figure xml:id="boot-boot1-find-part">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>	mov $0x1,%cx		 # Two passes
main.1:
	mov $0x8dbe,%si # Partition table
	movb $0x1,%dh		 # Partition
main.2:
	cmpb $0xa5,0x4(%si)	 # Our partition type?
	jne main.3		 # No
	jcxz main.5		 # If second pass
	testb $0x80,(%si)	 # Active?
	jnz main.5		 # Yes
main.3:
	add $0x10,%si		 # Next entry
	incb %dh		 # Partition
	cmpb $0x5,%dh		 # In table?
	jb main.2		 # Yes
	dec %cx			 # Do two
	jcxz main.1		 #  passes</programlisting>
    </figure>

    <para>If a &os; slice is identified, execution continues at
      <literal>main.5</literal>.  Note that when a &os; slice is found
      <literal>%si</literal> points to the appropriate entry in the
      partition table, and <literal>%dh</literal> holds the partition
      number.  We assume that a &os; slice is found, so we continue
      execution at <literal>main.5</literal>:</para>

    <figure xml:id="boot-boot1-main5">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>main.5:
	mov %dx,0x900			   # Save args
	movb $0x10,%dh			   # Sector count
	callw nread			   # Read disk
	mov $0x9000,%bx			   # BTX
	mov 0xa(%bx),%si		   # Get BTX length and set
	add %bx,%si			   #  %si to start of boot2.bin
	mov $0xc000,%di			   # Client page 2
	mov $0xa200,%cx			   # Byte
	sub %si,%cx			   #  count
	rep				   # Relocate
	movsb				   #  client</programlisting>
    </figure>

    <para>Recall that at this point, register <literal>%si</literal>
      points to the &os; slice entry in the <acronym>MBR</acronym>
      partition table, so a call to <literal>nread</literal> will
      effectively read sectors at the beginning of this partition.
      The argument passed on register <literal>%dh</literal> tells
      <literal>nread</literal> to read 16 disk sectors.  Recall that
      the first 512 bytes, or the first sector of the &os; slice,
      coincides with the <filename>boot1</filename> program.  Also
      recall that the file written to the beginning of the &os;
      slice is not <filename>/boot/boot1</filename>, but
      <filename>/boot/boot</filename>.  Let us look at the size of
      these files in the filesystem:</para>

    <screen xml:id="boot-boot1-filesize">-r--r--r--  1 root  wheel   512B Jan  8 00:15 /boot/boot0
-r--r--r--  1 root  wheel   512B Jan  8 00:15 /boot/boot1
-r--r--r--  1 root  wheel   7.5K Jan  8 00:15 /boot/boot2
-r--r--r--  1 root  wheel   8.0K Jan  8 00:15 /boot/boot</screen>

    <para>Both <filename>boot0</filename> and
      <filename>boot1</filename> are 512 bytes each, so they fit
      <emphasis>exactly</emphasis> in one disk sector.
      <filename>boot2</filename> is much bigger, holding both
      the <acronym>BTX</acronym> server and the
      <filename>boot2</filename> client.  Finally, a file called
      simply <filename>boot</filename> is 512 bytes larger than
      <filename>boot2</filename>.  This file is a
      concatenation of <filename>boot1</filename> and
      <filename>boot2</filename>.  As already noted,
      <filename>boot0</filename> is the file written to the absolute
      first disk sector (the <acronym>MBR</acronym>), and
      <filename>boot</filename> is the file written to the first
      sector of the &os; slice; <filename>boot1</filename> and
      <filename>boot2</filename> are <emphasis>not</emphasis> written
      to disk.  The command used to concatenate
      <filename>boot1</filename> and <filename>boot2</filename> into a
      single <filename>boot</filename> is merely
      <command>cat boot1 boot2 &gt; boot</command>.</para>

    <para>So <filename>boot1</filename> occupies exactly the first 512
      bytes of <filename>boot</filename> and, because
      <filename>boot</filename> is written to the first sector of the
      &os; slice, <filename>boot1</filename> fits exactly in this
      first sector.  When <literal>nread</literal> reads the first
      16 sectors of the &os; slice, it effectively reads the entire
      <filename>boot</filename> file
      <footnote>
	<para>512*16=8192 bytes, exactly the size of
	  <filename>boot</filename></para></footnote>.
      We will see more details about how <filename>boot</filename> is
      formed from <filename>boot1</filename> and
      <filename>boot2</filename> in the next section.</para>

    <para>Recall that <literal>nread</literal> uses memory address
      <literal>0x8c00</literal> as the transfer buffer to hold the
      sectors read.  This address is conveniently chosen.  Indeed,
      because <filename>boot1</filename> belongs to the first 512
      bytes, it ends up in the address range
      <literal>0x8c00</literal>-<literal>0x8dff</literal>.  The 512
      bytes that follows (range
      <literal>0x8e00</literal>-<literal>0x8fff</literal>) is used to
      store the <emphasis>bsdlabel</emphasis>
      <footnote>
	<para>Historically known as <quote>disklabel</quote>.  If you
	  ever wondered where &os; stored this information, it is in
	  this region.  See &man.bsdlabel.8;</para></footnote>.</para>

    <para>Starting at address <literal>0x9000</literal> is the
      beginning of the <acronym>BTX</acronym> server, and immediately
      following is the <filename>boot2</filename> client.  The
      <acronym>BTX</acronym> server acts as a kernel, and executes in
      protected mode in the most privileged level.  In contrast, the
      <acronym>BTX</acronym> clients (<filename>boot2</filename>, for
      example), execute in user mode.  We will see how this is
      accomplished in the next section.  The code after the call to
      <literal>nread</literal> locates the beginning of
      <filename>boot2</filename> in the memory buffer, and copies it
      to memory address <literal>0xc000</literal>.  This is because
      the <acronym>BTX</acronym> server arranges
      <filename>boot2</filename> to execute in a segment starting at
      <literal>0xa000</literal>.  We explore this in detail in the
      following section.</para>

    <para>The last code block of <filename>boot1</filename> enables
      access to memory above 1MB
      <footnote>
	<para>This is necessary for legacy reasons.  Interested
	  readers should see <link
	    xlink:href="http://en.wikipedia.org/wiki/A20_line"/>.</para></footnote>
	and concludes with a jump to the starting point of the
      <acronym>BTX</acronym> server:</para>

    <figure xml:id="boot-boot1-seta20">
      <title><filename>sys/boot/i386/boot2/boot1.S</filename></title>

      <programlisting>seta20:
	cli			# Disable interrupts
seta20.1:
	dec %cx			# Timeout?
	jz seta20.3		# Yes

	inb $0x64,%al		# Get status
	testb $0x2,%al		# Busy?
	jnz seta20.1		# Yes
	movb $0xd1,%al		# Command: Write
	outb %al,$0x64		#  output port
seta20.2:
	inb $0x64,%al		# Get status
	testb $0x2,%al		# Busy?
	jnz seta20.2		# Yes
	movb $0xdf,%al		# Enable
	outb %al,$0x60		#  A20
seta20.3:
	sti			# Enable interrupts
	jmp 0x9010		# Start BTX</programlisting>
    </figure>

    <para>Note that right before the jump, interrupts are
      enabled.</para>
  </sect1>

  <sect1 xml:id="btx-server">
    <title>The <acronym>BTX</acronym> Server</title>

    <para>Next in our boot sequence is the
      <acronym>BTX</acronym> Server.  Let us quickly remember how we
      got here:</para>

    <itemizedlist>
      <listitem>
	<para>The <acronym>BIOS</acronym> loads the absolute sector
	  one (the <acronym>MBR</acronym>, or
	  <filename>boot0</filename>), to address
	  <literal>0x7c00</literal> and jumps there.</para>
      </listitem>

      <listitem>
	<para><filename>boot0</filename> relocates itself to
	  <literal>0x600</literal>, the address it was linked to
	  execute, and jumps over there.  It then reads the first
	  sector of the &os; slice (which consists of
	  <filename>boot1</filename>) into address
	  <literal>0x7c00</literal> and jumps over there.</para>
      </listitem>

      <listitem>
	<para><filename>boot1</filename> loads the first 16 sectors
	  of the &os; slice into address <literal>0x8c00</literal>.
	  This 16 sectors, or 8192 bytes, is the whole file
	  <filename>boot</filename>.  The file is a
	  concatenation of <filename>boot1</filename> and
	  <filename>boot2</filename>.  <filename>boot2</filename>, in
	  turn, contains the <acronym>BTX</acronym> server and the
	  <filename>boot2</filename> client.  Finally, a jump is made
	  to address <literal>0x9010</literal>, the entry point of the
	  <acronym>BTX</acronym> server.</para>
      </listitem>
    </itemizedlist>

    <para>Before studying the <acronym>BTX</acronym> Server in detail,
      let us further review how the single, all-in-one
      <filename>boot</filename> file is created.  The way
      <filename>boot</filename> is built is defined in its
      <filename>Makefile</filename>
      (<filename>/usr/src/sys/boot/i386/boot2/Makefile</filename>).
      Let us look at the rule that creates the
      <filename>boot</filename> file:</para>

    <figure xml:id="boot-boot1-make-boot">
      <title><filename>sys/boot/i386/boot2/Makefile</filename></title>

      <programlisting>      boot: boot1 boot2
	cat boot1 boot2 > boot</programlisting>
    </figure>

    <para>This tells us that <filename>boot1</filename> and
      <filename>boot2</filename> are needed, and the rule simply
      concatenates them to produce a single file called
      <filename>boot</filename>.  The rules for creating
      <filename>boot1</filename> are also quite simple:</para>

    <figure xml:id="boot-boot1-make-boot1">
      <title><filename>sys/boot/i386/boot2/Makefile</filename></title>

      <programlisting>      boot1: boot1.out
	objcopy -S -O binary boot1.out boot1

      boot1.out: boot1.o
	ld -e start -Ttext 0x7c00 -o boot1.out boot1.o</programlisting>
    </figure>

    <para>To apply the rule for creating
      <filename>boot1</filename>, <filename>boot1.out</filename> must
      be resolved.  This, in turn, depends on the existence of
      <filename>boot1.o</filename>.  This last file is simply the
      result of assembling our familiar <filename>boot1.S</filename>,
      without linking.  Now, the rule for creating
      <filename>boot1.out</filename> is applied.  This tells us that
      <filename>boot1.o</filename> should be linked with
      <literal>start</literal> as its entry point, and starting at
      address <literal>0x7c00</literal>.  Finally,
      <filename>boot1</filename> is created from
      <filename>boot1.out</filename> applying the appropriate rule.
      This rule is the <filename>objcopy</filename> command applied to
      <filename>boot1.out</filename>.  Note the flags passed to
      <filename>objcopy</filename>: <literal>-S</literal> tells it to
      strip all relocation and symbolic information;
      <literal>-O binary</literal> indicates the output format, that
      is, a simple, unformatted binary file.</para>

    <para>Having <filename>boot1</filename>, let us take a look at how
      <filename>boot2</filename> is constructed:</para>

    <figure xml:id="boot-boot1-make-boot2">
      <title><filename>sys/boot/i386/boot2/Makefile</filename></title>

      <programlisting>      boot2: boot2.ld
	@set -- `ls -l boot2.ld`; x=$$((7680-$$5)); \
	    echo "$$x bytes available"; test $$x -ge 0
	dd if=boot2.ld of=boot2 obs=7680 conv=osync

      boot2.ld: boot2.ldr boot2.bin ../btx/btx/btx
	btxld -v -E 0x2000 -f bin -b ../btx/btx/btx -l boot2.ldr \
	    -o boot2.ld -P 1 boot2.bin

      boot2.ldr:
	dd if=/dev/zero of=boot2.ldr bs=512 count=1

      boot2.bin: boot2.out
	objcopy -S -O binary boot2.out boot2.bin

      boot2.out: ../btx/lib/crt0.o boot2.o sio.o
	ld -Ttext 0x2000 -o boot2.out

      boot2.o: boot2.s
	${CC} ${ACFLAGS} -c boot2.s

      boot2.s: boot2.c boot2.h ${.CURDIR}/../../common/ufsread.c
	${CC} ${CFLAGS} -S -o boot2.s.tmp ${.CURDIR}/boot2.c
	sed -e '/align/d' -e '/nop/d' "MISSING" boot2.s.tmp > boot2.s
	rm -f boot2.s.tmp

      boot2.h: boot1.out
	${NM} -t d ${.ALLSRC} | awk '/([0-9])+ T xread/ \
	    { x = $$1 - ORG1; \
	    printf("#define XREADORG %#x\n", REL1 + x) }' \
	    ORG1=`printf "%d" ${ORG1}` \
	    REL1=`printf "%d" ${REL1}` > ${.TARGET}</programlisting>
    </figure>

    <para>The mechanism for building <filename>boot2</filename> is
      far more elaborate.  Let us point out the most relevant facts.
      The dependency list is as follows:</para>

    <figure xml:id="boot-boot1-make-boot2-more">
      <title><filename>sys/boot/i386/boot2/Makefile</filename></title>

      <programlisting>      boot2: boot2.ld
      boot2.ld: boot2.ldr boot2.bin ${BTXDIR}/btx/btx
      boot2.bin: boot2.out
      boot2.out: ${BTXDIR}/lib/crt0.o boot2.o sio.o
      boot2.o: boot2.s
      boot2.s: boot2.c boot2.h ${.CURDIR}/../../common/ufsread.c
      boot2.h: boot1.out</programlisting>
    </figure>

    <para>Note that initially there is no header file
      <filename>boot2.h</filename>, but its creation depends on
      <filename>boot1.out</filename>, which we already have.  The rule
      for its creation is a bit terse, but the important thing is that
      the output, <filename>boot2.h</filename>, is something like
      this:</para>

    <figure xml:id="boot-boot1-make-boot2h">
      <title><filename>sys/boot/i386/boot2/boot2.h</filename></title>

      <programlisting>#define XREADORG 0x725</programlisting>
    </figure>

    <para>Recall that <filename>boot1</filename> was relocated (i.e.,
      copied from <literal>0x7c00</literal> to
      <literal>0x700</literal>).  This relocation will now make sense,
      because as we will see, the <acronym>BTX</acronym> server
      reclaims some memory, including the space where
      <filename>boot1</filename> was originally loaded.  However, the
      <acronym>BTX</acronym> server needs access to
      <filename>boot1</filename>'s <literal>xread</literal> function;
      this function, according to the output of
      <filename>boot2.h</filename>, is at location
      <literal>0x725</literal>.  Indeed, the
      <acronym>BTX</acronym> server uses the
      <literal>xread</literal> function from
      <filename>boot1</filename>'s relocated code.  This function is
      now accessible from within the <filename>boot2</filename>
      client.</para>

    <para>We next build <filename>boot2.s</filename> from files
      <filename>boot2.h</filename>, <filename>boot2.c</filename> and
      <filename>/usr/src/sys/boot/common/ufsread.c</filename>.  The
      rule for this is to compile the code in
      <filename>boot2.c</filename> (which includes
      <filename>boot2.h</filename> and <filename>ufsread.c</filename>)
      into assembly code.  Having <filename>boot2.s</filename>, the
      next rule assembles <filename>boot2.s</filename>, creating the
      object file <filename>boot2.o</filename>.  The
      next rule directs the linker to link various files
      (<filename>crt0.o</filename>,
      <filename>boot2.o</filename> and <filename>sio.o</filename>).
      Note that the output file, <filename>boot2.out</filename>, is
      linked to execute at address <literal>0x2000</literal>.  Recall
      that <filename>boot2</filename> will be executed in user mode,
      within a special user segment set up by the
      <acronym>BTX</acronym> server.  This segment starts at
      <literal>0xa000</literal>.  Also, remember that the
      <filename>boot2</filename> portion of <filename>boot</filename>
      was copied to address <literal>0xc000</literal>, that is, offset
      <literal>0x2000</literal> from the start of the user segment, so
      <filename>boot2</filename> will work properly when we transfer
      control to it.  Next, <filename>boot2.bin</filename> is created
      from <filename>boot2.out</filename> by stripping its symbols and
      format information; boot2.bin is a <emphasis>raw</emphasis>
      binary.  Now, note that a file <filename>boot2.ldr</filename> is
      created as a 512-byte file full of zeros.  This space is
      reserved for the bsdlabel.</para>

    <para>Now that we have files <filename>boot1</filename>,
      <filename>boot2.bin</filename> and
      <filename>boot2.ldr</filename>, only the
      <acronym>BTX</acronym> server is missing before creating the
      all-in-one <filename>boot</filename> file.  The
      <acronym>BTX</acronym> server is located in
      <filename>/usr/src/sys/boot/i386/btx/btx</filename>; it has its
      own <filename>Makefile</filename> with its own set of rules for
      building.  The important thing to notice is that it is also
      compiled as a <emphasis>raw</emphasis> binary, and that it is
      linked to execute at address <literal>0x9000</literal>.  The
      details can be found in
      <filename>/usr/src/sys/boot/i386/btx/btx/Makefile</filename>.</para>

    <para>Having the files that comprise the <filename>boot</filename>
      program, the final step is to <emphasis>merge</emphasis> them.
      This is done by a special program called
      <filename>btxld</filename> (source located in
      <filename>/usr/src/usr.sbin/btxld</filename>).  Some arguments
      to this program include the name of the output file
      (<filename>boot</filename>), its entry point
      (<literal>0x2000</literal>) and its file format
      (raw binary).  The various files are
      finally merged by this utility into the file
      <filename>boot</filename>, which consists of
      <filename>boot1</filename>, <filename>boot2</filename>, the
      <literal>bsdlabel</literal> and the
      <acronym>BTX</acronym> server.  This file, which takes
      exactly 16 sectors, or 8192 bytes, is what is
      actually written to the beginning of the &os; slice
      during installation.  Let us now proceed to study the
      <acronym>BTX</acronym> server program.</para>

    <para>The <acronym>BTX</acronym> server prepares a simple
      environment and switches from 16-bit real mode to 32-bit
      protected mode, right before passing control to the client.
      This includes initializing and updating the following data
      structures:</para>

    <indexterm><primary>virtual v86 mode</primary></indexterm>
    <itemizedlist>
      <listitem>
	<para>Modifies the
	  <literal>Interrupt Vector Table (IVT)</literal>.  The
	  <acronym>IVT</acronym> provides exception and interrupt
	  handlers for Real-Mode code.</para>
      </listitem>

      <listitem>
	<para>The <literal>Interrupt Descriptor Table (IDT)</literal>
	  is created.  Entries are provided for processor exceptions,
	  hardware interrupts, two system calls and V86 interface.
	  The IDT provides exception and interrupt handlers for
	  Protected-Mode code.</para>
      </listitem>

      <listitem>
	<para>A <literal>Task-State Segment (TSS)</literal> is
	  created.  This is necessary because the processor works in
	  the <emphasis>least</emphasis> privileged level when
	  executing the client (<filename>boot2</filename>), but in
	  the <emphasis>most</emphasis> privileged level when
	  executing the <acronym>BTX</acronym> server.</para>
      </listitem>

      <listitem>
	<para>The <acronym>GDT</acronym> (Global Descriptor Table) is
	  set up.  Entries (descriptors) are provided for
	  supervisor code and data, user code and data, and real-mode
	  code and data.
	  <footnote>
	    <para>Real-mode code and data are necessary when switching
	      back to real mode from protected mode, as suggested by
	      the Intel manuals.</para></footnote></para>
      </listitem>
    </itemizedlist>

    <para>Let us now start studying the actual implementation.  Recall
      that <filename>boot1</filename> made a jump to address
      <literal>0x9010</literal>, the <acronym>BTX</acronym> server's
      entry point.  Before studying program execution there,
      note that the <acronym>BTX</acronym> server has a special header
      at address range <literal>0x9000-0x900f</literal>, right before
      its entry point.  This header is defined as follows:</para>

    <figure xml:id="btx-header">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>start:						# Start of code
/*
 * BTX header.
 */
btx_hdr:	.byte 0xeb			# Machine ID
		.byte 0xe			# Header size
		.ascii "BTX"			# Magic
		.byte 0x1			# Major version
		.byte 0x2			# Minor version
		.byte BTX_FLAGS			# Flags
		.word PAG_CNT-MEM_ORG>>0xc	# Paging control
		.word break-start		# Text size
		.long 0x0			# Entry address</programlisting>
    </figure>

    <para>Note the first two bytes are <literal>0xeb</literal> and
      <literal>0xe</literal>.  In the IA-32 architecture, these two
      bytes are interpreted as a relative jump past the header into
      the entry point, so in theory, <filename>boot1</filename> could
      jump here (address <literal>0x9000</literal>) instead of address
      <literal>0x9010</literal>.  Note that the last field in the
      <acronym>BTX</acronym> header is a pointer to the client's
      (<filename>boot2</filename>) entry point.  This field is patched
      at link time.</para>

    <para>Immediately following the header is the
      <acronym>BTX</acronym> server's entry point:</para>

    <figure xml:id="btx-init">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>/*
 * Initialization routine.
 */
init:		cli				# Disable interrupts
		xor %ax,%ax			# Zero/segment
		mov %ax,%ss			# Set up
		mov $0x1800,%sp		#  stack
		mov %ax,%es			# Address
		mov %ax,%ds			#  data
		pushl $0x2			# Clear
		popfl				#  flags</programlisting>
    </figure>

    <para>This code disables interrupts, sets up a working stack
      (starting at address <literal>0x1800</literal>) and clears the
      flags in the EFLAGS register.  Note that the
      <literal>popfl</literal> instruction pops out a doubleword (4
      bytes) from the stack and places it in the EFLAGS register.
      As the value actually popped is <literal>2</literal>, the
      EFLAGS register is effectively cleared (IA-32 requires that bit
      2 of the EFLAGS register always be 1).</para>

    <para>Our next code block clears (sets to <literal>0</literal>)
      the memory range <literal>0x5e00-0x8fff</literal>.  This range
      is where the various data structures will be created:</para>

    <figure xml:id="btx-clear-mem">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>/*
 * Initialize memory.
 */
		mov $0x5e00,%di		# Memory to initialize
		mov $(0x9000-0x5e00)/2,%cx	# Words to zero
		rep				# Zero-fill
		stosw				#  memory</programlisting>
    </figure>

    <para>Recall that <filename>boot1</filename> was originally loaded
      to address <literal>0x7c00</literal>, so, with this memory
      initialization, that copy effectively disappeared.  However,
      also recall that <filename>boot1</filename> was relocated to
      <literal>0x700</literal>, so <emphasis>that</emphasis> copy is
      still in memory, and the <acronym>BTX</acronym> server will make
      use of it.</para>

    <para>Next, the real-mode <acronym>IVT</acronym> (Interrupt Vector
      Table is updated.  The <acronym>IVT</acronym> is an array of
      segment/offset pairs for exception and interrupt handlers.  The
      <acronym>BIOS</acronym> normally maps hardware interrupts to
      interrupt vectors <literal>0x8</literal> to
      <literal>0xf</literal> and <literal>0x70</literal> to
      <literal>0x77</literal> but, as will be seen, the 8259A
      Programmable Interrupt Controller, the chip controlling the
      actual mapping of hardware interrupts to interrupt vectors, is
      programmed to remap these interrupt vectors from
      <literal>0x8-0xf</literal> to <literal>0x20-0x27</literal> and
      from <literal>0x70-0x77</literal> to
      <literal>0x28-0x2f</literal>.  Thus, interrupt handlers are
      provided for interrupt vectors <literal>0x20-0x2f</literal>.
      The reason the <acronym>BIOS</acronym>-provided handlers are not
      used directly is because they work in 16-bit real mode, but not
      32-bit protected mode.  Processor mode will be switched to
      32-bit protected mode shortly.  However, the
      <acronym>BTX</acronym> server sets up a mechanism to effectively
      use the handlers provided by the <acronym>BIOS</acronym>:</para>

    <figure xml:id="btx-ivt">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>/*
 * Update real mode IDT for reflecting hardware interrupts.
 */
		mov $intr20,%bx			# Address first handler
		mov $0x10,%cx			# Number of handlers
		mov $0x20*4,%di			# First real mode IDT entry
init.0:		mov %bx,(%di)			# Store IP
		inc %di				# Address next
		inc %di				#  entry
		stosw				# Store CS
		add $4,%bx			# Next handler
		loop init.0			# Next IRQ</programlisting>
    </figure>

    <para>The next block creates the <acronym>IDT</acronym> (Interrupt
      Descriptor Table).  The <acronym>IDT</acronym> is analogous, in
      protected mode, to the <acronym>IVT</acronym> in real mode.
      That is, the <acronym>IDT</acronym> describes the various
      exception and interrupt handlers used when the processor is
      executing in protected mode.  In essence, it also consists of an
      array of segment/offset pairs, although the structure is
      somewhat more complex, because segments in protected mode are
      different than in real mode, and various protection mechanisms
      apply:</para>

    <figure xml:id="btx-idt">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>/*
 * Create IDT.
 */
		mov $0x5e00,%di			# IDT's address
		mov $idtctl,%si			# Control string
init.1:		lodsb				# Get entry
		cbw				#  count
		xchg %ax,%cx			#  as word
		jcxz init.4			# If done
		lodsb				# Get segment
		xchg %ax,%dx			#  P:DPL:type
		lodsw				# Get control
		xchg %ax,%bx			#  set
		lodsw				# Get handler offset
		mov $SEL_SCODE,%dh		# Segment selector
init.2:		shr %bx				# Handle this int?
		jnc init.3			# No
		mov %ax,(%di)			# Set handler offset
		mov %dh,0x2(%di)		#  and selector
		mov %dl,0x5(%di)		# Set P:DPL:type
		add $0x4,%ax			# Next handler
init.3:		lea 0x8(%di),%di		# Next entry
		loop init.2			# Till set done
		jmp init.1			# Continue</programlisting>
    </figure>

    <para>Each entry in the <literal>IDT</literal> is 8 bytes long.
      Besides the segment/offset information, they also describe the
      segment type, privilege level, and whether the segment is
      present in memory or not.  The construction is such that
      interrupt vectors from <literal>0</literal> to
      <literal>0xf</literal> (exceptions) are handled by function
      <literal>intx00</literal>; vector <literal>0x10</literal> (also
      an exception) is handled by <literal>intx10</literal>; hardware
      interrupts, which are later configured to start at interrupt
      vector <literal>0x20</literal> all the way to interrupt vector
      <literal>0x2f</literal>, are handled by function
      <literal>intx20</literal>.  Lastly, interrupt vector
      <literal>0x30</literal>, which is used for system calls, is
      handled by <literal>intx30</literal>, and vectors
      <literal>0x31</literal> and <literal>0x32</literal> are handled
      by <literal>intx31</literal>.  It must be noted that only
      descriptors for interrupt vectors <literal>0x30</literal>,
      <literal>0x31</literal> and <literal>0x32</literal> are given
      privilege level 3, the same privilege level as the
      <filename>boot2</filename> client, which means the client can
      execute a software-generated interrupt to this vectors through
      the <literal>int</literal> instruction without failing (this is
      the way <filename>boot2</filename> use the services provided by
      the <acronym>BTX</acronym> server).  Also, note that
      <emphasis>only</emphasis> software-generated interrupts are
      protected from code executing in lesser privilege levels.
      Hardware-generated interrupts and processor-generated exceptions
      are <emphasis>always</emphasis> handled adequately, regardless
      of the actual privileges involved.</para>

    <para>The next step is to initialize the <acronym>TSS</acronym>
      (Task-State Segment).  The <acronym>TSS</acronym> is a hardware
      feature that helps the operating system or executive software
      implement multitasking functionality through process
      abstraction.  The IA-32 architecture demands the creation and
      use of <emphasis>at least</emphasis> one <acronym>TSS</acronym>
      if multitasking facilities are used or different privilege
      levels are defined.  Since the <filename>boot2</filename>
      client is executed in privilege level 3, but the
      <acronym>BTX</acronym> server runs in privilege level 0, a
      <acronym>TSS</acronym> must be defined:</para>

    <figure xml:id="btx-tss">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>/*
 * Initialize TSS.
 */
init.4:		movb $_ESP0H,TSS_ESP0+1(%di)	# Set ESP0
		movb $SEL_SDATA,TSS_SS0(%di)	# Set SS0
		movb $_TSSIO,TSS_MAP(%di)	# Set I/O bit map base</programlisting>
    </figure>

    <para>Note that a value is given for the Privilege Level 0 stack
      pointer and stack segment in the <acronym>TSS</acronym>.  This
      is needed because, if an interrupt or exception is received
      while executing <filename>boot2</filename> in Privilege Level 3,
      a change to Privilege Level 0 is automatically performed by the
      processor, so a new working stack is needed.  Finally, the I/O
      Map Base Address field of the <acronym>TSS</acronym> is given a
      value, which is a 16-bit offset from the beginning of the
      <acronym>TSS</acronym> to the I/O Permission Bitmap and the
      Interrupt Redirection Bitmap.</para>

    <para>After the <acronym>IDT</acronym> and <acronym>TSS</acronym>
      are created, the processor is ready to switch to protected mode.
      This is done in the next block:</para>

    <figure xml:id="btx-prot">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>/*
 * Bring up the system.
 */
		mov $0x2820,%bx			# Set protected mode
		callw setpic			#  IRQ offsets
		lidt idtdesc			# Set IDT
		lgdt gdtdesc			# Set GDT
		mov %cr0,%eax			# Switch to protected
		inc %ax				#  mode
		mov %eax,%cr0			#
		ljmp $SEL_SCODE,$init.8		# To 32-bit code
		.code32
init.8:		xorl %ecx,%ecx			# Zero
		movb $SEL_SDATA,%cl		# To 32-bit
		movw %cx,%ss			#  stack</programlisting>
    </figure>

    <para>First, a call is made to <literal>setpic</literal> to
      program the 8259A <acronym>PIC</acronym> (Programmable Interrupt
      Controller).  This chip is connected to multiple hardware
      interrupt sources.  Upon receiving an interrupt from a device,
      it signals the processor with the appropriate interrupt vector.
      This can be customized so that specific interrupts are
      associated with specific interrupt vectors, as explained before.
      Next, the <acronym>IDTR</acronym> (Interrupt Descriptor Table
      Register) and <acronym>GDTR</acronym> (Global Descriptor Table
      Register) are loaded with the instructions
      <literal>lidt</literal> and <literal>lgdt</literal>,
      respectively.  These registers are loaded with the base address
      and limit address for the <acronym>IDT</acronym> and
      <acronym>GDT</acronym>.  The following three instructions set
      the Protection Enable (PE) bit of the <literal>%cr0</literal>
      register.  This effectively switches the processor to 32-bit
      protected mode.  Next, a long jump is made to
      <literal>init.8</literal> using segment selector SEL_SCODE,
      which selects the Supervisor Code Segment.  The processor is
      effectively executing in CPL 0, the most privileged level, after
      this jump.  Finally, the Supervisor Data Segment is selected for
      the stack by assigning the segment selector SEL_SDATA to the
      <literal>%ss</literal> register.  This data segment also has a
      privilege level of <literal>0</literal>.</para>

    <para>Our last code block is responsible for loading the
      <acronym>TR</acronym> (Task Register) with the segment selector
      for the <acronym>TSS</acronym> we created earlier, and setting
      the User Mode environment before passing execution control to
      the <filename>boot2</filename> client.</para>

    <figure xml:id="btx-end">
      <title><filename>sys/boot/i386/btx/btx/btx.S</filename></title>

      <programlisting>/*
 * Launch user task.
 */
		movb $SEL_TSS,%cl		# Set task
		ltr %cx				#  register
		movl $0xa000,%edx		# User base address
		movzwl %ss:BDA_MEM,%eax		# Get free memory
		shll $0xa,%eax			# To bytes
		subl $ARGSPACE,%eax		# Less arg space
		subl %edx,%eax			# Less base
		movb $SEL_UDATA,%cl		# User data selector
		pushl %ecx			# Set SS
		pushl %eax			# Set ESP
		push $0x202			# Set flags (IF set)
		push $SEL_UCODE			# Set CS
		pushl btx_hdr+0xc		# Set EIP
		pushl %ecx			# Set GS
		pushl %ecx			# Set FS
		pushl %ecx			# Set DS
		pushl %ecx			# Set ES
		pushl %edx			# Set EAX
		movb $0x7,%cl			# Set remaining
init.9:		push $0x0			#  general
		loop init.9			#  registers
		popa				#  and initialize
		popl %es			# Initialize
		popl %ds			#  user
		popl %fs			#  segment
		popl %gs			#  registers
		iret				# To user mode</programlisting>
    </figure>

    <para>Note that the client's environment include a stack segment
      selector and stack pointer (registers <literal>%ss</literal> and
      <literal>%esp</literal>).  Indeed, once the
      <acronym>TR</acronym> is loaded with the appropriate stack
      segment selector (instruction <literal>ltr</literal>), the stack
      pointer is calculated and pushed onto the stack along with the
      stack's segment selector.  Next, the value
      <literal>0x202</literal> is pushed onto the stack; it is the
      value that the EFLAGS will get when control is passed to the
      client.  Also, the User Mode code segment selector and the
      client's entry point are pushed.  Recall that this entry
      point is patched in the <acronym>BTX</acronym> header at link
      time.  Finally, segment selectors (stored in register
      <literal>%ecx</literal>) for the segment registers
      <literal>%gs, %fs, %ds and %es</literal> are pushed onto the
      stack, along with the value at <literal>%edx</literal>
      (<literal>0xa000</literal>).  Keep in mind the various values
      that have been pushed onto the stack (they will be popped out
      shortly).  Next, values for the remaining general purpose
      registers are also pushed onto the stack (note the
      <literal>loop</literal> that pushes the value
      <literal>0</literal> seven times).  Now, values will be started
      to be popped out of the stack.  First, the
      <literal>popa</literal> instruction pops out of the stack the
      latest seven values pushed.  They are stored in the general
      purpose registers in order
      <literal>%edi, %esi, %ebp, %ebx, %edx, %ecx, %eax</literal>.
      Then, the various segment selectors pushed are popped into the
      various segment registers.  Five values still remain on the
      stack.  They are popped when the <literal>iret</literal>
      instruction is executed.  This instruction first pops
      the value that was pushed from the <acronym>BTX</acronym>
      header.  This value is a pointer to <filename>boot2</filename>'s
      entry point.  It is placed in the register
      <literal>%eip</literal>, the instruction pointer register.
      Next, the segment selector for the User Code Segment is popped
      and copied to register <literal>%cs</literal>.  Remember that
      this segment's privilege level is 3, the least privileged
      level.  This means that we must provide values for the stack of
      this privilege level.  This is why the processor, besides
      further popping the value for the EFLAGS register, does two more
      pops out of the stack.  These values go to the stack
      pointer (<literal>%esp</literal>) and the stack segment
      (<literal>%ss</literal>).  Now, execution continues at
      <literal>boot0</literal>'s entry point.</para>

    <para>It is important to note how the User Code Segment is
      defined.  This segment's <emphasis>base address</emphasis> is
      set to <literal>0xa000</literal>.  This means that code memory
      addresses are <emphasis>relative</emphasis> to address 0xa000;
      if code being executed is fetched from address
      <literal>0x2000</literal>, the <emphasis>actual</emphasis>
      memory addressed is
      <literal>0xa000+0x2000=0xc000</literal>.</para>
  </sect1>

  <sect1 xml:id="boot2">
    <title><application>boot2</application> Stage</title>

    <para><literal>boot2</literal> defines an important structure,
      <literal>struct bootinfo</literal>.  This structure is
      initialized by <literal>boot2</literal> and passed to the
      loader, and then further to the kernel.  Some nodes of this
      structures are set by <literal>boot2</literal>, the rest by the
      loader.  This structure, among other information, contains the
      kernel filename, <acronym>BIOS</acronym> harddisk geometry,
      <acronym>BIOS</acronym> drive number for boot device, physical
      memory available, <literal>envp</literal> pointer etc.  The
      definition for it is:</para>

    <programlisting><filename>/usr/include/machine/bootinfo.h:</filename>
struct bootinfo {
	u_int32_t	bi_version;
	u_int32_t	bi_kernelname;		/* represents a char * */
	u_int32_t	bi_nfs_diskless;	/* struct nfs_diskless * */
				/* End of fields that are always present. */
#define	bi_endcommon	bi_n_bios_used
	u_int32_t	bi_n_bios_used;
	u_int32_t	bi_bios_geom[N_BIOS_GEOM];
	u_int32_t	bi_size;
	u_int8_t	bi_memsizes_valid;
	u_int8_t	bi_bios_dev;		/* bootdev BIOS unit number */
	u_int8_t	bi_pad[2];
	u_int32_t	bi_basemem;
	u_int32_t	bi_extmem;
	u_int32_t	bi_symtab;		/* struct symtab * */
	u_int32_t	bi_esymtab;		/* struct symtab * */
				/* Items below only from advanced bootloader */
	u_int32_t	bi_kernend;		/* end of kernel space */
	u_int32_t	bi_envp;		/* environment */
	u_int32_t	bi_modulep;		/* preloaded modules */
};</programlisting>

    <para><literal>boot2</literal> enters into an infinite loop
      waiting for user input, then calls <function>load()</function>.
      If the user does not press anything, the loop breaks by a
      timeout, so <function>load()</function> will load the default
      file (<filename>/boot/loader</filename>).  Functions
      <function>ino_t lookup(char *filename)</function> and
      <function>int xfsread(ino_t inode, void *buf, size_t
      nbyte)</function> are used to read the content of a file into
      memory.  <filename>/boot/loader</filename> is an
      <acronym>ELF</acronym> binary, but where the
      <acronym>ELF</acronym> header is prepended with
      <filename>a.out</filename>'s <literal>struct
      exec</literal> structure.  <function>load()</function> scans the
      loader's ELF header, loading the content of
      <filename>/boot/loader</filename> into memory, and passing the
      execution to the loader's entry:</para>

    <programlisting><filename>sys/boot/i386/boot2/boot2.c:</filename>
    __exec((caddr_t)addr, RB_BOOTINFO | (opts &amp; RBX_MASK),
	   MAKEBOOTDEV(dev_maj[dsk.type], 0, dsk.slice, dsk.unit, dsk.part),
	   0, 0, 0, VTOP(&amp;bootinfo));</programlisting>
  </sect1>

  <sect1 xml:id="boot-loader">
    <title><application>loader</application> Stage</title>

    <para><application>loader</application> is a
      <acronym>BTX</acronym> client as well.  I will not describe it
      here in detail, there is a comprehensive man page written by
      Mike Smith, &man.loader.8;.  The underlying mechanisms and
      <acronym>BTX</acronym> were discussed above.</para>

    <para>The main task for the loader is to boot the kernel.  When
      the kernel is loaded into memory, it is being called by the
      loader:</para>

    <programlisting><filename>sys/boot/common/boot.c:</filename>
    /* Call the exec handler from the loader matching the kernel */
    module_formats[km-&gt;m_loader]-&gt;l_exec(km);</programlisting>
  </sect1>

  <sect1 xml:id="boot-kernel">
    <title>Kernel Initialization</title>

    <para>Let us take a look at the command that links the kernel.
      This will help identify the exact location where the loader
      passes execution to the kernel.  This location is the kernel's
      actual entry point.</para>

    <programlisting><filename>sys/conf/Makefile.i386:</filename>
ld -elf -Bdynamic -T /usr/src/sys/conf/ldscript.i386  -export-dynamic \
-dynamic-linker /red/herring -o kernel -X locore.o \
&lt;lots of kernel .o files&gt;</programlisting>

    <indexterm><primary>ELF</primary></indexterm>
    <para>A few interesting things can be seen here.  First, the
      kernel is an ELF dynamically linked binary, but the dynamic
      linker for kernel is <filename>/red/herring</filename>, which is
      definitely a bogus file.  Second, taking a look at the file
      <filename>sys/conf/ldscript.i386</filename> gives an idea about
      what <application>ld</application> options are used when
      compiling a kernel.  Reading through the first few lines, the
      string</para>

    <programlisting><filename>sys/conf/ldscript.i386:</filename>
ENTRY(btext)</programlisting>

    <para>says that a kernel's entry point is the symbol `btext'.
      This symbol is defined in <filename>locore.s</filename>:</para>

    <programlisting><filename>sys/i386/i386/locore.s:</filename>
	.text
/**********************************************************************
 *
 * This is where the bootblocks start us, set the ball rolling...
 *
 */
NON_GPROF_ENTRY(btext)</programlisting>

    <para>First, the register EFLAGS is set to a predefined value of
      0x00000002.  Then all the segment registers are
      initialized:</para>

    <programlisting><filename>sys/i386/i386/locore.s:</filename>
/* Don't trust what the BIOS gives for eflags. */
	pushl	$PSL_KERNEL
	popfl

/*
 * Don't trust what the BIOS gives for %fs and %gs.  Trust the bootstrap
 * to set %cs, %ds, %es and %ss.
 */
	mov	%ds, %ax
	mov	%ax, %fs
	mov	%ax, %gs</programlisting>

    <para>btext calls the routines
      <function>recover_bootinfo()</function>,
      <function>identify_cpu()</function>,
      <function>create_pagetables()</function>, which are also defined
      in <filename>locore.s</filename>.  Here is a description of what
      they do:</para>

    <informaltable frame="none" pgwide="1">
      <tgroup cols="2" align="left">
	<tbody>
	  <row>
	    <entry><function>recover_bootinfo</function></entry>
	    <entry>This routine parses the parameters to the kernel
	      passed from the bootstrap.  The kernel may have been
	      booted in 3 ways: by the loader, described above, by the
	      old disk boot blocks, or by the old diskless boot
	      procedure.  This function determines the booting method,
	      and stores the <literal>struct bootinfo</literal>
	      structure into the kernel memory.</entry>
	  </row>

	  <row>
	    <entry><function>identify_cpu</function></entry>
	    <entry>This functions tries to find out what CPU it is
	      running on, storing the value found in a variable
	      <varname>_cpu</varname>.</entry>
	  </row>

	  <row>
	    <entry><function>create_pagetables</function></entry>
	    <entry>This function allocates and fills out a Page Table
	      Directory at the top of the kernel memory area.</entry>
	  </row>
	</tbody>
      </tgroup>
    </informaltable>

    <para>The next steps are enabling VME, if the CPU supports
      it:</para>

    <programlisting>	testl	$CPUID_VME, R(_cpu_feature)
	jz	1f
	movl	%cr4, %eax
	orl	$CR4_VME, %eax
	movl	%eax, %cr4</programlisting>

    <para>Then, enabling paging:</para>

    <programlisting>/* Now enable paging */
	movl	R(_IdlePTD), %eax
	movl	%eax,%cr3			/* load ptd addr into mmu */
	movl	%cr0,%eax			/* get control word */
	orl	$CR0_PE|CR0_PG,%eax		/* enable paging */
	movl	%eax,%cr0			/* and let's page NOW! */</programlisting>

    <para>The next three lines of code are because the paging was set,
      so the jump is needed to continue the execution in virtualized
      address space:</para>

    <programlisting>	pushl	$begin				/* jump to high virtualized address */
	ret

/* now running relocated at KERNBASE where the system is linked to run */
begin:</programlisting>

    <para>The function <function>init386()</function> is called with
      a pointer to the first free physical page, after that
      <function>mi_startup()</function>.  <function>init386</function>
      is an architecture dependent initialization function, and
      <function>mi_startup()</function> is an architecture independent
      one (the 'mi_' prefix stands for Machine Independent).  The
      kernel never returns from <function>mi_startup()</function>, and
      by calling it, the kernel finishes booting:</para>

    <programlisting><filename>sys/i386/i386/locore.s:</filename>
	movl	physfree, %esi
	pushl	%esi				/* value of first for init386(first) */
	call	_init386			/* wire 386 chip for unix operation */
	call	_mi_startup			/* autoconfiguration, mountroot etc */
	hlt		/* never returns to here */</programlisting>

    <sect2>
      <title><function>init386()</function></title>

      <para><function>init386()</function> is defined in
	<filename>sys/i386/i386/machdep.c</filename> and performs
	low-level initialization specific to the i386 chip.  The
	switch to protected mode was performed by the loader.  The
	loader has created the very first task, in which the kernel
	continues to operate.  Before looking at the code, consider
	the tasks the processor must complete to initialize protected
	mode execution:</para>

      <itemizedlist>
	<listitem>
	  <para>Initialize the kernel tunable parameters, passed from
	    the bootstrapping program.</para>
	</listitem>

	<listitem>
	  <para>Prepare the GDT.</para>
	</listitem>

	<listitem>
	  <para>Prepare the IDT.</para>
	</listitem>

	<listitem>
	  <para>Initialize the system console.</para>
	</listitem>

	<listitem>
	  <para>Initialize the DDB, if it is compiled into
	    kernel.</para>
	</listitem>

	<listitem>
	  <para>Initialize the TSS.</para>
	</listitem>

	<listitem>
	  <para>Prepare the LDT.</para>
	</listitem>

	<listitem>
	  <para>Set up proc0's pcb.</para>
	</listitem>
      </itemizedlist>

      <indexterm><primary>parameters</primary></indexterm>
      <para><function>init386()</function> initializes the tunable
	parameters passed from bootstrap by setting the environment
	pointer (envp) and calling <function>init_param1()</function>.
	The envp pointer has been passed from loader in the
	<literal>bootinfo</literal> structure:</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
		kern_envp = (caddr_t)bootinfo.bi_envp + KERNBASE;

	/* Init basic tunables, hz etc */
	init_param1();</programlisting>

      <para><function>init_param1()</function> is defined in
	<filename>sys/kern/subr_param.c</filename>.  That file has a
	number of sysctls, and two functions,
	<function>init_param1()</function> and
	<function>init_param2()</function>, that are called from
	<function>init386()</function>:</para>

      <programlisting><filename>sys/kern/subr_param.c:</filename>
	hz = HZ;
	TUNABLE_INT_FETCH("kern.hz", &amp;hz);</programlisting>

      <para>TUNABLE_&lt;typename&gt;_FETCH is used to fetch the value
	from the environment:</para>

      <programlisting><filename>/usr/src/sys/sys/kernel.h:</filename>
#define	TUNABLE_INT_FETCH(path, var)	getenv_int((path), (var))</programlisting>

      <para>Sysctl <literal>kern.hz</literal> is the system clock
	tick.  Additionally, these sysctls are set by
	<function>init_param1()</function>: <literal>kern.maxswzone,
	kern.maxbcache, kern.maxtsiz, kern.dfldsiz, kern.maxdsiz,
	  kern.dflssiz, kern.maxssiz, kern.sgrowsiz</literal>.</para>

      <indexterm>
	<primary>Global Descriptors Table (GDT)</primary>
      </indexterm>

      <para>Then <function>init386()</function> prepares the Global
	Descriptors Table (GDT).  Every task on an x86 is running in
	its own virtual address space, and this space is addressed by
	a segment:offset pair.  Say, for instance, the current
	instruction to be executed by the processor lies at CS:EIP,
	then the linear virtual address for that instruction would be
	<quote>the virtual address of code segment CS</quote> + EIP.
	For convenience, segments begin at virtual address 0 and end
	at a 4Gb boundary.  Therefore, the instruction's linear
	virtual address for this example would just be the value of
	EIP. Segment registers such as CS, DS etc are the selectors,
	i.e., indexes, into GDT (to be more precise, an index is not a
	selector itself, but the INDEX field of a selector).
	FreeBSD's GDT holds descriptors for 15 selectors per
	CPU:</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
union descriptor gdt[NGDT * MAXCPU];	/* global descriptor table */

<filename>sys/i386/include/segments.h:</filename>
/*
 * Entries in the Global Descriptor Table (GDT)
 */
#define	GNULL_SEL	0	/* Null Descriptor */
#define	GCODE_SEL	1	/* Kernel Code Descriptor */
#define	GDATA_SEL	2	/* Kernel Data Descriptor */
#define	GPRIV_SEL	3	/* SMP Per-Processor Private Data */
#define	GPROC0_SEL	4	/* Task state process slot zero and up */
#define	GLDT_SEL	5	/* LDT - eventually one per process */
#define	GUSERLDT_SEL	6	/* User LDT */
#define	GTGATE_SEL	7	/* Process task switch gate */
#define	GBIOSLOWMEM_SEL	8	/* BIOS low memory access (must be entry 8) */
#define	GPANIC_SEL	9	/* Task state to consider panic from */
#define GBIOSCODE32_SEL	10	/* BIOS interface (32bit Code) */
#define GBIOSCODE16_SEL	11	/* BIOS interface (16bit Code) */
#define GBIOSDATA_SEL	12	/* BIOS interface (Data) */
#define GBIOSUTIL_SEL	13	/* BIOS interface (Utility) */
#define GBIOSARGS_SEL	14	/* BIOS interface (Arguments) */</programlisting>

      <para>Note that those #defines are not selectors themselves, but
	just a field INDEX of a selector, so they are exactly the
	indices of the GDT.  for example, an actual selector for the
	kernel code (GCODE_SEL) has the value 0x08.</para>

      <indexterm><primary>Interrupt Descriptor Table
	  (IDT)</primary></indexterm>
      <para>The next step is to initialize the Interrupt Descriptor
	Table (IDT).  This table is referenced by the processor when a
	software or hardware interrupt occurs.  For example, to make a
	system call, user application issues the
	<literal>INT 0x80</literal> instruction.  This is a software
	interrupt, so the processor's hardware looks up a record with
	index 0x80 in the IDT.  This record points to the routine that
	handles this interrupt, in this particular case, this will be
	the kernel's syscall gate.  The IDT may have a maximum of 256
	(0x100) records.  The kernel allocates NIDT records for the
	IDT, where NIDT is the maximum (256):</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
static struct gate_descriptor idt0[NIDT];
struct gate_descriptor *idt = &amp;idt0[0];	/* interrupt descriptor table */</programlisting>

      <para>For each interrupt, an appropriate handler is set.  The
	syscall gate for <literal>INT 0x80</literal> is set as
	well:</para>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
	setidt(0x80, &amp;IDTVEC(int0x80_syscall),
			SDT_SYS386TGT, SEL_UPL, GSEL(GCODE_SEL, SEL_KPL));</programlisting>

      <para>So when a userland application issues the
	<literal>INT 0x80</literal> instruction, control will transfer
	to the function <function>_Xint0x80_syscall</function>, which
	is in the kernel code segment and will be executed with
	supervisor privileges.</para>

      <para>Console and DDB are then initialized:</para>
      <indexterm><primary>DDB</primary></indexterm>

      <programlisting><filename>sys/i386/i386/machdep.c:</filename>
	cninit();
/* skipped */
#ifdef DDB
	kdb_init();
	if (boothowto &amp; RB_KDB)
		Debugger("Boot flags requested debugger");
#endif</programlisting>

      <para>The Task State Segment is another x86 protected mode
	structure, the TSS is used by the hardware to store task
	information when a task switch occurs.</para>

      <para>The Local Descriptors Table is used to reference userland
	code and data.  Several selectors are defined to point to the
	LDT, they are the system call gates and the user code and data
	selectors:</para>

      <programlisting><filename>/usr/include/machine/segments.h:</filename>
#define	LSYS5CALLS_SEL	0	/* forced by intel BCS */
#define	LSYS5SIGR_SEL	1
#define	L43BSDCALLS_SEL	2	/* notyet */
#define	LUCODE_SEL	3
#define	LSOL26CALLS_SEL	4	/* Solaris &gt;= 2.6 system call gate */
#define	LUDATA_SEL	5
/* separate stack, es,fs,gs sels ? */
/* #define	LPOSIXCALLS_SEL	5*/	/* notyet */
#define LBSDICALLS_SEL	16	/* BSDI system call gate */
#define NLDT		(LBSDICALLS_SEL + 1)</programlisting>

      <para>Next, proc0's Process Control Block
	(<literal>struct pcb</literal>) structure is initialized.
	proc0 is a <literal>struct proc</literal> structure that
	describes a kernel process.  It is always present while the
	kernel is running, therefore it is declared as global:</para>

      <programlisting><filename>sys/kern/kern_init.c:</filename>
    struct	proc proc0;</programlisting>

      <para>The structure <literal>struct pcb</literal> is a part of a
	proc structure.  It is defined in
	<filename>/usr/include/machine/pcb.h</filename> and has a
	process's information specific to the i386 architecture, such
	as registers values.</para>
    </sect2>

    <sect2>
      <title><function>mi_startup()</function></title>

      <para>This function performs a bubble sort of all the system
	initialization objects and then calls the entry of each object
	one by one:</para>

      <programlisting><filename>sys/kern/init_main.c:</filename>
	for (sipp = sysinit; *sipp; sipp++) {

		/* ... skipped ... */

		/* Call function */
		(*((*sipp)-&gt;func))((*sipp)-&gt;udata);
		/* ... skipped ... */
	}</programlisting>

      <para>Although the sysinit framework is described in the <link
	  xlink:href="&url.doc.langbase;/books/developers-handbook">Developers'
	  Handbook</link>, I will discuss the internals of it.</para>

      <indexterm><primary>sysinit objects</primary></indexterm>
      <para>Every system initialization object (sysinit object) is
	created by calling a SYSINIT() macro.  Let us take as example
	an <literal>announce</literal> sysinit object.  This object
	prints the copyright message:</para>

      <programlisting><filename>sys/kern/init_main.c:</filename>
static void
print_caddr_t(void *data __unused)
{
	printf("%s", (char *)data);
}
SYSINIT(announce, SI_SUB_COPYRIGHT, SI_ORDER_FIRST, print_caddr_t, copyright)</programlisting>

      <para>The subsystem ID for this object is SI_SUB_COPYRIGHT
	(0x0800001), which comes right after the SI_SUB_CONSOLE
	(0x0800000).  So, the copyright message will be printed out
	first, just after the console initialization.</para>

      <para>Let us take a look at what exactly the macro
	<literal>SYSINIT()</literal> does.  It expands to a
	<literal>C_SYSINIT()</literal> macro.  The
	<literal>C_SYSINIT()</literal> macro then expands to a static
	<literal>struct sysinit</literal> structure declaration with
	another <literal>DATA_SET</literal> macro call:</para>

      <programlisting><filename>/usr/include/sys/kernel.h:</filename>
      #define C_SYSINIT(uniquifier, subsystem, order, func, ident) \
      static struct sysinit uniquifier ## _sys_init = { \ subsystem, \
      order, \ func, \ ident \ }; \ DATA_SET(sysinit_set,uniquifier ##
      _sys_init);

#define	SYSINIT(uniquifier, subsystem, order, func, ident)	\
	C_SYSINIT(uniquifier, subsystem, order,			\
	(sysinit_cfunc_t)(sysinit_nfunc_t)func, (void *)ident)</programlisting>

      <para>The <literal>DATA_SET()</literal> macro expands to a
	<literal>MAKE_SET()</literal>, and that macro is the point
	where all the sysinit magic is hidden:</para>

      <programlisting><filename>/usr/include/linker_set.h:</filename>
#define MAKE_SET(set, sym)						\
	static void const * const __set_##set##_sym_##sym = &amp;sym;	\
	__asm(".section .set." #set ",\"aw\"");				\
	__asm(".long " #sym);						\
	__asm(".previous")
#endif
#define TEXT_SET(set, sym) MAKE_SET(set, sym)
#define DATA_SET(set, sym) MAKE_SET(set, sym)</programlisting>

      <para>In our case, the following declaration will occur:</para>

      <programlisting>static struct sysinit announce_sys_init = {
	SI_SUB_COPYRIGHT,
	SI_ORDER_FIRST,
	(sysinit_cfunc_t)(sysinit_nfunc_t)  print_caddr_t,
	(void *) copyright
};

static void const *const __set_sysinit_set_sym_announce_sys_init =
    &amp;announce_sys_init;
__asm(".section .set.sysinit_set" ",\"aw\"");
__asm(".long " "announce_sys_init");
__asm(".previous");</programlisting>

      <para>The first <literal>__asm</literal> instruction will create
	an ELF section within the kernel's executable.  This will
	happen at kernel link time.  The section will have the name
	<literal>.set.sysinit_set</literal>.  The content of this
	section is one 32-bit value, the address of announce_sys_init
	structure, and that is what the second
	<literal>__asm</literal> is.  The third
	<literal>__asm</literal> instruction marks the end of a
	section.  If a directive with the same section name occurred
	before, the content, i.e., the 32-bit value, will be appended
	to the existing section, so forming an array of 32-bit
	pointers.</para>

      <para>Running <application>objdump</application> on a kernel
	binary, you may notice the presence of such small
	sections:</para>

      <screen>&prompt.user; <userinput>objdump -h /kernel</userinput>
  7 .set.cons_set 00000014  c03164c0  c03164c0  002154c0  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  8 .set.kbddriver_set 00000010  c03164d4  c03164d4  002154d4  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  9 .set.scrndr_set 00000024  c03164e4  c03164e4  002154e4  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 10 .set.scterm_set 0000000c  c0316508  c0316508  00215508  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 11 .set.sysctl_set 0000097c  c0316514  c0316514  00215514  2**2
                  CONTENTS, ALLOC, LOAD, DATA
 12 .set.sysinit_set 00000664  c0316e90  c0316e90  00215e90  2**2
                  CONTENTS, ALLOC, LOAD, DATA</screen>

      <para>This screen dump shows that the size of .set.sysinit_set
	section is 0x664 bytes, so <literal>0x664/sizeof(void
	*)</literal> sysinit objects are compiled into the kernel.
	The other sections such as <literal>.set.sysctl_set</literal>
	represent other linker sets.</para>

      <para>By defining a variable of type <literal>struct
	  linker_set</literal> the content of
	<literal>.set.sysinit_set</literal> section will be
	<quote>collected</quote> into that variable:</para>

      <programlisting><filename>sys/kern/init_main.c:</filename>
      extern struct linker_set sysinit_set; /* XXX */</programlisting>

      <para>The <literal>struct linker_set</literal> is defined as
	follows:</para>

      <programlisting><filename>/usr/include/linker_set.h:</filename>
  struct linker_set {
	int	ls_length;
	void	*ls_items[1];		/* really ls_length of them, trailing NULL */
};</programlisting>

      <para>The first node will be equal to the number of a sysinit
	objects, and the second node will be a NULL-terminated array
	of pointers to them.</para>

      <para>Returning to the <function>mi_startup()</function>
	discussion, it is must be clear now, how the sysinit objects
	are being organized.  The <function>mi_startup()</function>
	function sorts them and calls each.  The very last object is
	the system scheduler:</para>

      <programlisting><filename>/usr/include/sys/kernel.h:</filename>
enum sysinit_sub_id {
	SI_SUB_DUMMY		= 0x0000000,	/* not executed; for linker*/
	SI_SUB_DONE		= 0x0000001,	/* processed*/
	SI_SUB_CONSOLE		= 0x0800000,	/* console*/
	SI_SUB_COPYRIGHT	= 0x0800001,	/* first use of console*/
...
	SI_SUB_RUN_SCHEDULER	= 0xfffffff	/* scheduler: no return*/
};</programlisting>

      <para>The system scheduler sysinit object is defined in the file
	<filename>sys/vm/vm_glue.c</filename>, and the entry point for
	that object is <function>scheduler()</function>.  That
	function is actually an infinite loop, and it represents a
	process with PID 0, the swapper process.  The proc0 structure,
	mentioned before, is used to describe it.</para>

      <para>The first user process, called <emphasis>init</emphasis>,
	is created by the sysinit object
	<literal>init</literal>:</para>

      <programlisting><filename>sys/kern/init_main.c:</filename>
static void
create_init(const void *udata __unused)
{
	int error;
	int s;

	s = splhigh();
	error = fork1(&amp;proc0, RFFDG | RFPROC, &amp;initproc);
	if (error)
		panic("cannot fork init: %d\n", error);
	initproc-&gt;p_flag |= P_INMEM | P_SYSTEM;
	cpu_set_fork_handler(initproc, start_init, NULL);
	remrunqueue(initproc);
	splx(s);
}
SYSINIT(init,SI_SUB_CREATE_INIT, SI_ORDER_FIRST, create_init, NULL)</programlisting>

      <para>The <function>create_init()</function> allocates a new
	process by calling <function>fork1()</function>, but does not
	mark it runnable.  When this new process is scheduled for
	execution by the scheduler, the
	<function>start_init()</function> will be called.  That
	function is defined in <filename>init_main.c</filename>.  It
	tries to load and exec the <filename>init</filename> binary,
	probing <filename>/sbin/init</filename> first, then
	<filename>/sbin/oinit</filename>,
	<filename>/sbin/init.bak</filename>, and finally
	<filename>/stand/sysinstall</filename>:</para>

      <programlisting><filename>sys/kern/init_main.c:</filename>
static char init_path[MAXPATHLEN] =
#ifdef	INIT_PATH
    __XSTRING(INIT_PATH);
#else
    "/sbin/init:/sbin/oinit:/sbin/init.bak:/stand/sysinstall";
#endif</programlisting>
    </sect2>
  </sect1>
</chapter>