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The <acronym>BIOS</acronym>
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Overview 总览
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. 启动过程与计算机架构息息相关。不仅必须为每个计算机体系结构编写代码，而且在同一体系结构上也可能有多个不同类型的引导。例如,<filename>/usr/src/sys/boot</filename>的目录列表显示了大量依赖于体系结构的代码。每个受支持的体系结构都有一个目录。在特定于 x86 的 <filename>i386</filename> 目录中，有不同引导标准的子目录，如<filename>mbr</filename>(主引导记录)、<filename>gpt</filename>(<acronym>GUID</acronym>分区表)和<filename>efi</filename>(可扩展固件接口)。每个引导标准都有自己的约定和数据结构。以下示例显示,使用存储在第一扇区的 FreeBSD <filename>boot0</filename> 多引导加载程序，从<acronym>MBR</acronym> 硬盘驱动器启动 x86 计算机。该引导代码启动 FreeBSD 三阶段引导过程。
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. 理解此过程的关键在于这些阶段的复杂度逐级增加。这些阶段是<filename>boot1</filename>，<filename>boot2</filename> 和 <filename>loader</filename>（更多详情请参阅 <citerefentry><refentrytitle>boot</refentrytitle><manvolnum>8</manvolnum></citerefentry>）。这些阶段在系统启动时按顺序执行。最后一步，<filename>loader</filename>，负责加载 FreeBSD 内核。以下章节将介绍各个阶段。
Here is an example of the output generated by the different boot stages. Actual output may differ from machine to machine: 下面是由不同引导阶段生成的输出的示例。实际输出可能因机器而异:
FreeBSD Component FreeBSD 组件
Output (may vary) 输出（可能有所不同）
<literal>boot0</literal> <literal>boot0</literal>
F1 FreeBSD
F2 BSD
F5 Disk 2 		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. 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/> <literal>boot2</literal> <_:footnote-1/>
&gt;&gt;FreeBSD/i386 BOOT
Default: 1:ad(1,a)/boot/loader
boot: 		&gt;&gt;FreeBSD/i386 BOOT
Default: 1:ad(1,a)/boot/loader
boot:
<filename>loader</filename> <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] 		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 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 		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> 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. 当PC加电后，处理器的寄存器被设为某些特定值。在这些寄存器中， <emphasis>指令指针</emphasis>寄存器被设为32位值0xfffffff0。 指令指针寄存器指向处理器将要执行的指令代码。<literal>cr1</literal>， 一个32位控制寄存器，在刚启动时值被设为0。cr1的PE(Protected Enabled， 保护模式使能)位用来指示处理器是处于保护模式还是实地址模式。 由于启动时该位被清位，处理器在实地址模式中引导。在实地址模式中， 线性地址与物理地址是等同的。
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. 值 <literal>0xfffffff0</literal> 略小于4G，因此计算机没有 4G 字节物理内存，这就不会是一个有效的内存地址。计算机硬件将这个地址转指向 <acronym>BIOS</acronym> 存储块。
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. BIOS表示<emphasis>Basic Input Output System</emphasis> (基本输入输出系统)。在主板上，它被固化在一个相对容量较小的 只读存储器(Read-Only Memory, ROM)。BIOS包含各种各样为主板硬件 定制的底层例程。就这样，处理器首先指向常驻BIOS存储器的地址 0xfffffff0。通常这个位置包含一条跳转指令，指向BIOS的POST例程。
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. POST表示<emphasis>Power On Self Test</emphasis>(加电自检)。 这套程序包括内存检查，系统总线检查和其它底层工具， 从而使得CPU能够初始化整台计算机。这一阶段中有一个重要步骤， 就是确定引导设备。现在所有的BIOS都允许手工选择引导设备。 你可以从软盘、光盘驱动器、硬盘等设备引导。
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. <acronym>POST</acronym>的最后一步是执行<literal>INT 0x19</literal>指令。 这个指令从引导设备第一个扇区读取512字节，装入地址<literal>0x7c00</literal>。 <emphasis>第一个扇区</emphasis>的说法最早起源于硬盘的结构，硬盘面被分为若干圆柱形轨道。给轨道编号，同时又将轨道分为 一定数目(通常是64)的扇形。0号轨道是硬盘的最外圈，1号扇区， 第一个扇区(轨道、柱面都从0开始编号，而扇区从1开始编号) 有着特殊的作用，它又被称为主引导记录(Master Boot Record, <acronym>MBR</acronym>)。 第一轨剩余的扇区常常不使用。
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>) 主引导记录 (<literal>boot0</literal>)
<primary>MBR</primary> <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. 如前所述， <literal>INT 0x19</literal> 指令装载 MBR， 也就是 <filename>boot0</filename> 的内容至内存地址 0x7c00。 再看文件 <filename>sys/boot/i386/boot0/boot0.S</filename>， 可以猜想这里面发生了什么 - 这是引导管理器， 一段由 Robert Nordier书写的令人起敬的程序片段。
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. MBR里，也就是<filename>boot0</filename>里， 从偏移量0x1be开始有一个特殊的结构，称为 <emphasis>分区表</emphasis>。其中有4条记录 (称为<emphasis>分区记录</emphasis>)，每条记录16字节。 分区记录表示硬盘如何被划分，在FreeBSD的术语中， 这被称为slice(d)。16字节中有一个标志字节决定这个分区是否可引导。 有仅只能有一个分区可设定这一标志。否则， <filename>boot0</filename>的代码将拒绝继续执行。
A partition record has the following fields: 一个分区记录有如下域：
the 1-byte filesystem type 1字节 文件系统类型
the 1-byte bootable flag 1字节 可引导标志
the 6 byte descriptor in CHS format 6字节 CHS格式描述符

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The <acronym>BIOS</acronym>
<primary>BIOS(基本输入输出系统, Basic Input Output System)</primaryThe <acronym>BIOS</acronym>
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(itstool) path: sect1/title
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books/zh_CN/arch-handbook.po, string 46