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<personname> <firstname>Sergey</firstname> <surname>Lyubka</surname> </personname> <contrib>Contributed by </contrib>
<primary>system initialization</primary>
The FreeBSD 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.
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 FreeBSD <filename>boot0</filename> multi-boot loader stored in the very first sector. That boot code starts the FreeBSD three-stage boot process.
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 <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>8</manvolnum></citerefentry> for more detail). The boot system executes each stage in sequence. The last stage, <filename>loader</filename>, is responsible for loading the FreeBSD kernel. Each stage is examined in the following sections.
F1 FreeBSD
F5 Disk 2
<literal>boot2</literal> <_:footnote-1/>
&gt;&gt;FreeBSD/i386 BOOT
Default: 1:ad(1,a)/boot/loader
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
(, 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]
Copyright (c) 1992-2013 The FreeBSD Project.
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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 amd64
FreeBSD clang version 3.3 (tags/RELEASE_33/final 183502) 20130610
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.
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.
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.
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.