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<?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
$FreeBSD$
-->
<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 Andrés</firstname>
<surname> Gómez 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>>>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 GB, so unless the machine has 4 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 Hz to
8192 Hz. The <acronym>BIOS</acronym> sets it to
18.2 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 > 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 & RBX_MASK),
MAKEBOOTDEV(dev_maj[dsk.type], 0, dsk.slice, dsk.unit, dsk.part),
0, 0, 0, VTOP(&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->m_loader]->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 \
<lots of kernel .o files></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", &hz);</programlisting>
<para>TUNABLE_<typename>_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 = &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, &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 & 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 >= 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)->func))((*sipp)->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 = &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 =
&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(&proc0, RFFDG | RFPROC, &initproc);
if (error)
panic("cannot fork init: %d\n", error);
initproc->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>
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