发信人: wenbobo()
整理人: wenbobo(2002-12-31 10:49:44), 站内信件
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版主注：
我现在打算翻译Write Your Own Operating System [FAQ]，希望有兴趣的朋友帮助翻译，我觉得翻译这样的文章是个很好的锻炼，至少比盲目写程序要值得。希望大家支持。有意承担翻译任务者请给我发email。
本文发表于http://www.mega-tokyo.com/os/os-faq.html
作者为 Stuart 'Dark Fiber' George <[email protected]>
本文的译者为 wenbobo <[email protected]>
转载请勿删除以上版权宣示信息
Boot Loaders / Boot Menus
引导装载器/引导菜单
Boot Loaders and Boot Menus... two similar but different types of programs.
Boot Loader和Boot Menu……两个相似但不同类型的程序。
The distinction between the two is simple, a boot menu allows you to select the operating system you want to boot but does not load anypart of that os into memory.
两者之间的区别很简单，Boot Menu允许你选择你打算引导的操作系统，但是并不把系统的任何部分装载入内存。
A boot loader will allow you to select the os you want to boot AND load parts of it into memory.
Boot Loader则允许你选择打算引导的操作系统并把此系统的一部分装载入内存。
For example: System Commander is a boot menu. GRUB is a bootloader.
例如：System Commander是boot menu。GRUB是boot loader。
By using a boot loader over a boot menu, you alleviate a lot of the early angst hobbyist OS programmers usually get when trying to load their kernel. The boot loader loads your kernel for you, regardless of filesystem. Good boot loaders also recognise some executable fileformats.
通过在boot menu上使用boot loader，你可以减轻许多早期的操作系统编程狂在装载内核时通常尝到的痛苦。Boot loader可以文件系统无关地为你装载内核。好的Boot loader也可以识别某些可执行文件格式。
If you ignore using a boot loader, you have to write your own loader to read in your kernel file from the file system.
如果你忽视使用boot loader，那你就不得不自己写一个loader来从文件系统读入你的内核文件。
Boot Sectors
引导扇区
The computer follows a set sequence, one of the first things it does is perform its POST, Power On Self Test.
计算机遵循一个固定的顺序运行，其中它做的第一件事是完成POST，开机自检。
The POST is designed to detect faults in the system (no display, no keyboard, etc) and look for a bootable device (floppy, scsi, hard disk, cdrom)
POST被设计成检测系统里的错误（例如没有显示设备，没有键盘等）并查找一个可启动的设备（软盘，scsi，硬盘，cdrom等）
In modern machines you can set the order the BIOS determins what to boot from, "A, C, CDROM / C Only / CDROM, C, A" etc.
在现代的机器里你可以设置BIOS的引导检测顺序，如“A, C, CDROM / C Only / CDROM, C, A”等等。
Back in the "old" days, the BIOS would first look for a floppy disk in drive A: and then it would look in drive C:.
回到“旧”时代，BIOS首先查找的是A:驱动器里的软盘，接下来查找硬盘C:。
Once the BIOS finds a boot sector with a bootable ID (0x55, 0xAA), it loads it into memory at a specifically set location, 0x0000:0x7C00, segment 0x0000, address 0x7C00.
一旦BIOS找到一个带有可启动ID号（0x55, 0xAA）的引导扇区，就把它装载进内存的一个特定位置，0x0000:0x7C00，段基址0x0000，偏移量0x7C00。
Once the sector is loaded, execution starts at this address.
一旦此扇区装载完成，计算机就从这个地址开始运行。
BIOS, all being made by different manufacturers, set its registers up differently according to who its maker is. *NEVER* assume what the contents of registers might be, for example, NEVER assume that DS/SS/ES etc will be initialised to 0 because not all bios do this. (and this has caught many people unawares!)
所有不同制造商出品的BIOS，寄存器的设置都依照它的制造者而互不相同。*永远也别*假设寄存器的内容也许是0，例如，不要假设DS/SS/ES等会被初始化成0因为并非所有的BIOS都这么做（这是很多人所不了解的！）
Floppy disks and hard disks boot differently. There are slight differences in the way they go about things.
软盘和硬盘的引导是不同的。它们的运作有些微小的差别。
Floppies are pretty simple, the BIOS loads the bootsector into 0x0000:0x7C00 and proceeds to execute it. Its a one step process.
软盘引导非常简单，BIOS把引导区装载到0x0000:0x7C00并执行它。这是一步完成的过程。
A hard disk is different. A hard disk has an MBR (Master Boot Record), this is because a hard disk can have multiple partitions and any one of them can be bootable.
硬盘则不同。硬盘有一个MBR(主引导记录)，这是因为一个硬盘可以有多个分区，并且其中的任何一个分区都可以是可启动的。
The BIOS loads the MBR (sector 0 of the hard disk) into memory at 0x0000:0x7C00 and executes it, now typically (ie: there is no standard) that sector moves its code down to 0x0000:0x0600 area and continues to execute itself from there.
BIOS把MBR（硬盘的0扇区）装载到内存的0x0000:0x7C00并执行它，接下来典型的做法是（注意：这里并没有标准）MBR程序把自身的代码放入0x0000:0x0600区域，然后继续从那里执行自身。
Once it has moved itself, it will proceed to scan the partition table for a bootable partition. When it finds a bootable partition it loads the first sector of that partition into 0x0000:0x7C00 and executes it. (Now you see why it moves its code down!)
一旦它移动好自身，接下来就扫描分区表以寻找一个可启动分区。找到一个可启动分区后，它把此分区的第一个扇区装载到0x0000:0x7C00并执行它。（现在你该知道为什么MBR要把自身向下移了吧！）
The entire process is quite simple. But things get much harder when you decide you want your bootsector to load a file or an operating system kernel.
整个过程非常简单。但是当你打算让自己的引导扇区装载一个文件或者系统内核的时候，事情就变得非常麻烦了。
(see the examples bootr01.zip and smbmbr03.zip)
（看看这两个例子bootr01.zip 和 smbmbr03.zip）
Master Boot Record
主引导记录
Every hard disk has an MBR (Master Boot Record) as the very first sector of the disk.
每个硬盘都有一个MBR(主引导记录)，就是磁盘的第一个扇区。
The MBR contains a table of four entries that are used for what is known as Primary Partitions.
MBR包含一张表，表内有四个项，用以记录各个分区信息。
This table is filled in by the disk paritioning programs such as FDisk and Disk Druid, etc.
该表被硬盘分区程序填充，例如 FDisk 和 Disk Druid，等等。
The layout of this table corresponds to:
表项的内容如下：
Offset Size (bytes) Description
偏移量 大小（字节） 说明
0x00 1 Boot Indicator (0x80=bootable, 0x00=not bootable)
启动标志（0x80=可启动的，0x00=不可启动的）
0x01 1 Starting Head Number
起始头号
0x02 2 Starting Cylinder Number (10 bits) and Sector (6 bits)
起始柱面号和扇区号
0x04 1 Descriptor (Type of partition/filesystem)
分区描述信息
0x05 1 Ending Head Number
结束头号
0x06 2 Ending Cylinder and Sector numbers
结束柱面号和扇区号
0x08 4 Starting Sector (relative to begining of disk)
起始扇区（相对于硬盘的开始）
0x0C 4 Number of Sectors in partition
分区的扇区数目
(see the examples bootr01.zip and smbmbr03.zip)
（看看这两个例子bootr01.zip 和 smbmbr03.zip）
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