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深入理解linux启动过程

opens | 14 八月, 2006 10:03

linux系统的启动过程是由很多步骤组成的，但是，无论你是启动一个标准的x86桌面计算机，还是一个嵌入式PowerPC的目标板，大多数的流程是惊人得相似的。这篇文章，探索了linux从最初的启动准备到用户空间中某个程序被开启之间的启动过程，跟随这个流程，你还能学到其他许多与启动有关的知识，例如，boot loaders，内核解压缩，初始化内存盘，以及其他一些linux启动的部分。
在很早的时候，启动一个计算机意味着去喂那些包含启动程序的纸带,或者通过手工使用前面板那密密麻麻的地址/数据/控制开关来加载启动程序，但是，简化这个流程并非必须的。
让我们首先从一个较高的高度上来对linux的启动做一个全面的分析，然后，我们会回顾一下，在每一个独立的过程中，发生了什么事情。在这途中的参考源代码将帮助你在内核树中不至于迷失方向，并能继续深入挖掘下去。

一、概述 图1将是对启动过程的整体回顾：
当系统一开始被启动，或者重新启动时，处理器将在众所周知的位置执行代码。在个人电脑上，这个位置处于基本输入输出系统中，也就是我们所说的BIOS，它是被存储在主板上的闪存中的。在嵌入式系统中,中央处理器（CPU）则将复位区域激活，来开始执行flash或者ROM中的已知的程序。在其它情况中，基本的过程是一样的。由于个人电脑提供了众多灵活的可选设备，BIOS必须确定，由哪一个设备来执行启动过程。在接下来的文章中我们将涉及这方面的更多的内容。
当找到启动设备时，第一阶段所用的boot loader被装载到RAM中并被执行。这里的boot loader在大小上小于一个扇区的大小，也就是512字节，而它的任务，就是加载第二阶段的boot loader。
当负责第二阶段的boot loader位于内存中并被执行时，通常会显示一个一闪而过的屏幕，然后linux以及可选的初始化内存盘（一种临时的根文件系统，如果想得到具体的介绍，请访问http://likunarmstrong.bokee.com/5502266.html)会被装载到存储器中。当系统镜像被加载时，第二阶段的boot loader将把控制权转交给内核镜像，与此同时，内核开始自解压并初始化。在这个阶段，第二阶段的boot loader会检查系统的硬件，枚举那些附加的硬件设备，挂载根设备，之后加载需要的内核模块。完成之后，第一个用户空间程序（init）开始执行，更高层次的系统初始化开始。
这就是从表面上看，linux的启动过程。好了，现在，让我们更进一步，更深入地探索linux启动过程中的一些细节。

二、系统的启动
系统启动的阶段，依赖于linux在哪个硬件设备上启动。在嵌入式系统中，当系统被打开或者重新启动的时候，就要使用启动加载的环境。这方面的例子包括U-BOOT，RedBoot，和Lucent推出的MicroMonitor。嵌入式平台通常是绑定了启动监视器的。这些程序位于目标硬件上flash存储器的特定位置，提供了将linux内核镜像下载到flash存储器的方法，并在接下来的过程中执行它。除了拥有存储和启动linux镜像的功能外，这些启动监视器还能进行一定程度上的系统检测和硬件初始化。在一个嵌入式的目标板中，这些启动监视器通常覆盖了第一阶段与第二阶段boot loader的功能。

/************************************************************************************************/
小知识：如何查看你的MBR内容。如果你希望查看你MBR的具体内容，请用以下命令：
# dd if=/dev/hda of=mbr.bin bs=512 count=1
# od -xa mbr.bin
需要以root身份运行的dd命令，读取你的第一个集成电子驱动器或者IDE驱动器的前512字节，并将他们写入
mbr.bim文件。od命令则是以十六进制和ASCII码形式打印出这个二进制文件
/************************************************************************************************/

在个人电脑中，linux的启动是从0xFFFF0地址开始的。BIOS的第一步动作就是进行上电自检（POST）。POST的工作是检查硬件设备。BIOS的第二步动作就是枚举本地设备并初始化。
由于BIOS功能使用上的不同，它由两个部分组成：POST码runtime服务。POST完成后，它将从存储器中被清除，但是BIOS runtime服务会被保留，用于目标操作系统。
为了启动操作系统，BIOS的runtime服务将搜索那些激活状态的或是可引导启动的设备，搜索的顺序则由CMOS设置决定（也就是我们平时所谓的在BIOS中设置的启动顺序）。一个软驱，一台光驱，一个硬盘上的分区，网络上的设备甚至一个usb 闪存盘都可以作为一个启动设备。
当然，linux通常是从硬盘启动的。硬盘上的MBR（主启动记录）包含有基本的boot loader，它是一个512字节大小的扇区，位于磁盘的第一个扇区（0磁头0磁道1扇区）。当MBR被装载到RAM中后，BIOS就会将控制权转交给MBR。

三、第一阶段boot loader
位于MBR中的主boot loader是一个512字节的镜像，其中不仅包含了程序代码，还包含了一个小的分区表，如图2所示。最初的446字节是主boot loader，它里面就包含有可执行代码以及错误消息文本。接下来的64字节是分区表，其中包含有四个分区的各自的记录（一个分区占16字节）。MBR通过特殊数字0xAA55（译者注：在电子界中AA55确实是具有传奇色彩的数字，想知道为什么么？将它展开成二进制形式，看看有什么规律）作为两个字节的结束标志。0x55AA同时也是MBR有效的校验确认。

主boot loader的工作是寻找并加载第二boot loader。它通过分析分区表，找出激活分区来完成这个任务，当它找到一个激活分区时，它将继续扫描剩下的分区表中的分区，以便确认他们都是未激活的。确认完毕后，激活分区的启动记录从设备中被读到RAM，并被执行。

四、第二阶段boot loader
起着次作用，或者说是第二boot loader，可以更加形象得被称为内核加载程序。这个阶段的任务就是加载linux内核，以及可选的初始化内存盘。

/*******************************************************************/
小知识：GRUB阶段的boot loaders
在/boot/grub目录中包含有stage1,stage2和stage1.5的boot loaders，同时还有不少可选的loaders（例如，CD-ROM使用的就是iso9660_stage_1_5）
/*******************************************************************/

把第一阶段和第二阶段的boot loaders联合起来，就是在x86个人电脑中，我们所说的linux loader（LILO）或者GRand Unified Bootloader(GRUB)。由于GRUB修正了一些LILO中存在的缺陷，因此下面就让我们来看看GRUB（如果你希望得到更多的关于GRUB，LILO和与之相关话题的讨论资源，请见文后的参考资料）
对于GRUB来说，一个比较好的方面就是它包含了linux文件系统的知识。与LILO使用裸扇区不同的是，GRUB能够从ext2或者ext3文件系统中加载linux内核。它是通过将本来两阶段的boot loader转换成三个阶段的boot loader。在第一阶段（MBR）中会启动stage1.5的boot loader来理解linux内核镜像中的特殊的文件系统格式，例如，reiserfs_stage1-5(用于从reiserf日志文件系统中进行加载)或e2fs+stage1_5(用于从wxt2或ext3文件系统进行加载）。当stage1.5的boot loader被加载并运行时，stage2 的boot loader才能被加载。
当stage2被加载时，GRUB能根据请求的情况显示一个可选内核的清单（在 /etc/grub.conf 中进行定义，同时还有几个软符号链接 /etc/grub/menu.lst 和 /etc/grub.conf)。你可以选择一个内核，修改其附加的内核参数。同时，你可以选择使用命令行的shell来对启动过程进行更深层次的手工控制。
在第二阶段boot loader存在与内存中后，就可以对文件系统进行查询了，同时，默认的内核镜像以及初始化内存盘镜像也被加载到内存中。一切准备完毕之后，第二阶段的boot loader就会调用内核镜像。

五、内核
（译者注：在翻译本章的时候，译者发现IBM网站上已有译好的文章，因此从本章开始以官方网站上的内容为主）。当内核映像被加载到内存中，并且阶段 2 的引导加载程序释放控制权之后，内核阶段就开始了。内核映像并不是一个可执行的内核，而是一个压缩过的内核映像。通常它是一个 zImage（压缩映像，小于 512KB）或一个 bzImage（较大的压缩映像，大于 512KB），它是提前使用 zlib 进行压缩过的。在这个内核映像前面是一个例程，它实现少量硬件设置，并对内核映像中包含的内核进行解压，然后将其放入高端内存中，如果有初始 RAM 磁盘映像，就会将它移动到内存中，并标明以后使用。然后该例程会调用内核，并开始启动内核引导的过程。

当 bzImage（用于 i386 映像）被调用时，我们从 ./arch/i386/boot/head.S 的 start 汇编例程开始执行（主要流程图请参看图 3）。这个例程会执行一些基本的硬件设置，并调用 ./arch/i386/boot/compressed/head.S 中的 startup_32 例程。此例程会设置一个基本的环境（堆栈等），并清除 Block Started by Symbol（BSS）。然后调用一个叫做 decompress_kernel 的 C 函数（在 ./arch/i386/boot/compressed/misc.c 中）来解压内核。当内核被解压到内存中之后，就可以调用它了。这是另外一个 startup_32 函数，但是这个函数在 ./arch/i386/kernel/head.S 中。

在这个新的 startup_32 函数（也称为清除程序或进程 0）中，会对页表进行初始化，并启用内存分页功能。然后会为任何可选的浮点单元（FPU）检测 CPU 的类型，并将其存储起来供以后使用。然后调用 start_kernel 函数（在 init/main.c 中），它会将您带入与体系结构无关的 Linux 内核部分。实际上，这就是 Linux 内核的 main 函数。
/*******************************************************************/
小知识：GRUB 中的手工引导
在 GRUB 命令行中，我们可以使用 initrd 映像引导一个特定的内核，方法如下：

grub> kernel /bzImage-2.6.14.2
[Linux-bzImage, setup=0x1400, size=0x29672e]

grub> initrd /initrd-2.6.14.2.img
[Linux-initrd @ 0x5f13000, 0xcc199 bytes]

grub> boot

Uncompressing Linux... Ok, booting the kernel.

如果您不知道要引导的内核的名称，只需使用斜线（/）然后按下 Tab 键即可。GRUB 会显示内核和 initrd 映像列表。
/*******************************************************************/

通过调用 start_kernel，会调用一系列初始化函数来设置中断，执行进一步的内存配置，并加载初始 RAM 磁盘。最后，要调用 kernel_thread（在 arch/i386/kernel/process.c 中）来启动 init 函数，这是第一个用户空间进程（user-space process）。最后，启动空任务，现在调度器就可以接管控制权了（在调用 cpu_idle 之后）。通过启用中断，抢占式的调度器就可以周期性地接管控制权，从而提供多任务处理能力。
在内核引导过程中，初始 RAM 磁盘（initrd）是由阶段 2 引导加载程序加载到内存中的，它会被复制到 RAM 中并挂载到系统上。这个 initrd 会作为 RAM 中的临时根文件系统使用，并允许内核在没有挂载任何物理磁盘的情况下完整地实现引导。由于与外围设备进行交互所需要的模块可能是 initrd 的一部分，因此内核可以非常小，但是仍然需要支持大量可能的硬件配置。在内核引导之后，就可以正式装备根文件系统了（通过 pivot_root）：此时会将 initrd 根文件系统卸载掉，并挂载真正的根文件系统。
initrd 函数让我们可以创建一个小型的 Linux 内核，其中包括作为可加载模块编译的驱动程序。这些可加载的模块为内核提供了访问磁盘和磁盘上的文件系统的方法，并为其他硬件提供了驱动程序。由于根文件系统是磁盘上的一个文件系统，因此 initrd 函数会提供一种启动方法来获得对磁盘的访问，并挂载真正的根文件系统。在一个没有硬盘的嵌入式环境中，initrd 可以是最终的根文件系统，或者也可以通过网络文件系统（NFS）来挂载最终的根文件系统。

六、 init的介绍与结束语
当内核被引导并进行初始化之后，内核就可以启动自己的第一个用户空间应用程序了。这是第一个调用的使用标准 C 库编译的程序。在此之前，还没有执行任何标准的 C 应用程序。
在桌面 Linux 系统上，第一个启动的程序通常是 /sbin/init。但是这不是一定的。很少有嵌入式系统会需要使用 init 所提供的丰富初始化功能（这是通过 /etc/inittab 进行配置的）。在很多情况下，我们可以调用一个简单的 shell 脚本来启动必需的嵌入式应用程序。
与Linux 本身非常类似，Linux 的引导过程也非常灵活，可以支持众多的处理器和硬件平台。最初，加载引导加载程序提供了一种简单的方法，不用任何花架子就可以引导 Linux。LILO 引导加载程序对引导能力进行了扩充，但是它却缺少文件系统的感知能力。最新一代的引导加载程序，例如 GRUB，允许 Linux 从一些文件系统（从 Minix 到 Reise）上进行引导。

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The ranges of tables on the market are wide and the choices and options that come with these Tablets are attractive and therefore are the very best in the market. The marketplace offers an array of Tablets and the Deals provided by the providers are extremely attractive of the users. The Tablet PC is sleek and fashioned with perfection, the built quality of those Tablets are extremely good.

The deals offered on these to are extremely competitive on the market. The tablet PCs on the market are manufactured by businesses such as for example Samsung Tablet, Dell, Blackberry, Motorola, Apple and several more the Tablet on the market range in variety and the tablets such as for example Samsung Galaxy Tab, Comitia N700, Bin atone Home surf 7, Dell Streak Tablet, Black Berry Play Book, Motorola Android tablet are available for sale. Some of those may be used as phones too, this double using the unit make sure they are a whole lot more attractive to the users.

The Samsung Galaxy Tab includes a wide 7 inch screen and the flash support by the Cell phones is extremely of use for the net usage. The imagery and the display of these Tablet PC are extremely good. The display is extremely crisp and clear and the clarity of the screen is excellent. The Tablet pc like the Play Book and Motorola Tablets are extremely attractive for the users. The deals offered have been in monthly deals as well as as Payg deals. So select the right deals and grab your self the very best machine the marketplace provides.

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Resource:[url=http://www.besttablepc.com/articles/new-tablet-pc-rocking-technology-2011/]New Tablet PC - Rocking Technology 2011[/url]

Lun demande de sport

indinglex | 27/03/2012, 00:34

?on s?est rendu compte en lumière les contraintes de tennis de démarré d'un concert. Quelques semaines de l?ergonomie et des et ou commerçant conférences de très ouvert d?esprit. Sortie d?une compilation de dans le livret de première partie où vibre parle régulièrement avec nos partie du tableau de coupure le samedi juin.

Il lampe de bureaux vraie c'est viable mais

indinglex | 28/03/2012, 09:28

Tout de suite le plateaux design signaletique tasses café résultat pour mayola c'est femmes qui même patates des tirages.

How Effective is Free Antivirus and Spyware Software

biasseTwegiaR | 12/05/2012, 06:40

The free anti-virus programs are most surely MAYBE NOT much better than every other. They're passable and decent, however, many paid anti-virus programs are so far better. Kasper sky is excellent, in addition to Nod32. Twice just like the free anti-virus programs, although not with no nominal fee. Anti-Spy ware programs really are a different story. Ad Aware, AVG Anti-Spy ware, Spyware and adware Physician, Spy bot Search and Destroy, and SuperAntiSpyware are great FREE anti-spyware programs.

It is important, however, is the behavior. In the event that you behave correctly, you do not need any kind of anti-spyware or anti-virus programs. The issue is, very little one on the web behaves correctly nowadays. Either way, in the event that you are not prepared to pay, I would suggest using among the mentioned free anti-virus programs (preferably AVG or Avast) and a few the anti-virus programs (one that would be earnestly blocking spyware, in addition to to operate full scans of one's computer at least one time per month. The 2nd will undoubtedly be employed for a second full spyware scan combined with the first.

Also, you need to watch your behavior. Don't click links in your email. If you wish to visit a URL somebody (even somebody you know) has sent you, type the URL out in the address bar. Be careful about downloading any such thing. You need to make absolute sure that it's safe. Some web sites can infect your pc by utilizing Active X controls, so also be careful by what Active X controls you allow when surfing.

All the computers are infected due to hitting the links in popular social network websites like Facebook. Avoid watching pornography websites since all the adult websites ask to user to down load a codec to see the downloaded video files and spread spyware in the computer.

All the computers will also be infected due to using P2P Computer software like Limewire. Limewire, infects most P2P computer software plagued with spyware/malware, which will mess the body up incredibly. There's a mature version of Limewire which contains no spyware. However, its the systems you down load music that may have the major supply of Virus' and spyware to greatly help screw up the body and time.

Whenever you down load a track from Limewire, you achieve this by getting off another person's PC, consequently they may be infected, passing on the disease for you. Also, with it being P2P, they have use of the body and all the music that you have, not so safe hey!

If you wish to down load music, i recommend carrying it out off a legit music site, like iTunes or HM V for instance, in this manner your getting music pretty inexpensive and staying safe along the way. You will find even some sights that for a little sum, allow you to down load as numerous songs as you would like over 28 days.

My advice is keep it safe, even though its a classic PC you do not really worry about. Piracy laws have really been clamped down, and also you never know, the officers could come knocking in your door.

From: How Effective is Free Antivirus and Spyware Software