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(itstool) path: entry/screen
Context English State
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.
Overview
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.
Here is an example of the output generated by the different boot stages. Actual output may differ from machine to machine:
FreeBSD Component
Output (may vary)
<literal>boot0</literal>
F1 FreeBSD
F2 BSD
F5 Disk 2
This prompt will appear if the user presses a key just after selecting an OS to boot at the <literal>boot0</literal> stage.
<literal>boot2</literal> <_:footnote-1/>
&gt;&gt;FreeBSD/i386 BOOT
Default: 1:ad(1,a)/boot/loader
boot:
<filename>loader</filename>
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]
kernel
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
The <acronym>BIOS</acronym>
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.
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.
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.
The Master Boot Record (<literal>boot0</literal>)
<primary>MBR</primary>
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 FreeBSD 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-1/>. <filename>boot0</filename> resides in the filesystem as <filename>/boot/boot0</filename>. It is a small 512-byte file, and it is exactly what FreeBSD'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>.
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.
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 FreeBSD'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.
A partition record has the following fields:
the 1-byte filesystem type
the 1-byte bootable flag

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Source information

Source string comment
(itstool) path: entry/screen
Flags
no-wrap, read-only
Source string location
book.translate.xml:390
String age
a year ago
Source string age
a year ago
Translation file
books/arch-handbook.pot, string 45