Translation

(itstool) path: sect1/para
English
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.
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Context English Chinese (Simplified) (zh_CN) State
&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
the 8 byte descriptor in LBA format 8字节 LBA格式描述符
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>. 一个分区记录描述符包含某一分区在硬盘上的确切位置信息。 LBA和CHS两种描述符指示相同的信息，但是指示方式有所不同：LBA (逻辑块寻址，Logical Block Addressing)指示分区的起始扇区和分区长度， 而CHS(柱面 磁头 扇区)指示首扇区和末扇区。
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>.
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.
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):
<filename>sys/boot/i386/boot0/Makefile</filename> <filename>sys/boot/i386/boot0/Makefile</filename>
BOOT_BOOT0_ORG?=0x600
LDFLAGS=-e start -Ttext ${BOOT_BOOT0_ORG} \
-Wl,-N,-S,--oformat,binary 		BOOT_BOOT0_ORG?=0x600
LDFLAGS=-e start -Ttext ${BOOT_BOOT0_ORG} \
-Wl,-N,-S,--oformat,binary
Let us now start our study of the <acronym>MBR</acronym>, or <filename>boot0</filename>, starting where execution begins.
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.
<filename>sys/boot/i386/boot0/boot0.S</filename> <filename>sys/boot/i386/boot0/boot0.S</filename>

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Source string comment
(itstool) path: sect1/para
Source string location
book.translate.xml:498
String age
a year ago
Source string age
a year ago
Translation file
books/zh_CN/arch-handbook.po, string 56