Linux bootloader 编写方法-红联Linux系统门户

对于移植 linux 到其它开发板的人来说，编写 boot loader 是一个不可避免的过程。对于学习linux的人来讲，编写 bootloader 也是一个很有挑战性的工作。本文通过对 linux引导协议进行分析，详细阐述了如何编写一个可以在 i386 机器上引导 2.4.20内核的基本的bootloader。

1.概述

linux运行在保护模式下，但是当机器启动复位的时候却处于实模式下。所以写bootloader做的工作也是在实模式之下的。

linux 的内核有多种格式，老式的zImage和新型的bzImage。它们之间最大的差别是对于内核体积大小的限制。由于zImage内核需要放在实模式1MB 的内存之内，所以其体积受到了限制。目前采用的内核格式大多为bzImage，这种格式没有1MB内存限制。本文以下部分主要以bzImage为例进行分析.

2.bzImage格式内核的结构

bzImage 内核从前向后分为3个部分，前512字节被称为bootsect，这就是软盘引导linux时用到的bootloader，如果不从软盘引导，这部分就没有用，其中存储了一些编译时生成的内核启动选项的默认值。从512个字节开始的512*n个字节称为setup部分，这是linux内核的实模式部分，这部分在实模式下运行，主要功能是为保护模式的linux内核启动准备环境。这个部分最后会切换进入保护模式，跳转到保护模式的内核执行。最后的部分就是保护模式的内核，也就是真正意义上的linux内核。其中n的大小可以从bootsect后半部得到，详细地址可以参阅linux boot protocol。

3.引导过程概述

第一步，打开冰箱门；第二步把大象放到冰箱里……不要笑，过程就是这么简单。首先需要把linux内核的setup部分拷贝到9020H:0开始的地址，然后把保护模式内核拷贝到1MB开始的地址，然后根据Linux Boot Protocol 2.03的内容设定参数区的内容，基地址就是9000H：0，最后使用一条ljmp $0x9020,$0跳转到setup段，剩下的事情就是linux自己的了^_^,果然简单吧！

4.THE LINUX/I386 BOOT PROTOCOL

这个就是我们引导linux所使用的协议，它的位置在：Documetation/i386/boot.txt中。里面详细的写了引导linux所需要知道的一切知识，对于其它体系结构的CPU，也一定存在着类似的东东，仿照本文的方法就可以了。

5.细节一：基本引导参数

当然我们不指定任何参数linux内核也可以启动，但是这样有可能启动进入一个我们不支持的framebuffer模式，导致没有任何屏幕显示；也可能 mount了错误的根分区失败，导致No Init Found的kernel panic。所以我们必须要指定一些东西。

如果你像我一样是一个懒人，那么可以直接把bootsect拷到9000H：0的位置，使用软盘引导时它会把自己复制到这个地方的，这里面有些默认的设置，详情请见boot.txt。

首先是root的位置，这里bootsect_pos指向的是9000H:0的地址。

bootsect_pos[0x1fc] = root_minor;bootsect_pos[0x1fd] = root_major;

其中root_minor和root_major分别是root的主设备号和次设备号。

当前显示模式：

bootsect_pos[0x1fa] = 0xff;bootsect_pos[0x1fb] = 0xff;

这两个数值相当于引导参数vga=0xHHH的值，两个0xff代表文本模式。

bootsect_pos[0x210] = 0xff;

这是在设定你的 bootloader的类型，其实只要不是0就行，因为0代表的loader太旧无法引导新的内核，setup发现这个后就会停下来。按照规范你应该写成 0xff，这表示未知的boot loader，如果你的bootloader已经得到了一个官方分配的type id，那就写上自己的数值。

6.细节二：如何加载内核

如果你现在的环境是一无所有，那么必须使用bios中断或者ATA指令去读硬盘了，不过如果你手中如果有基本的DOS系统，那么就可以使用DOS的程序了。为了能够操作整个4GB的地址空间，我使用了WATCOM C写了个小程序读内核，不过你可以仿照bootsect里面的做法，在实模式中读一部分，然后进入到保护模式拷贝到1MB以上，然后再从实模式读一部分……需要注意1:9000H:0也是DOS占用的地址空间，所以读完内核后就不要返回DOS了，否则会有问题；

注意 2:一定保证是纯DOS，不要加载HIMEM或者EMM386这样的东西，它们会使上面的引导过程失败。loadlin倒是可以来者通吃几乎所有的 DOS，不过它的作者也是这方面的大牛，对DOS下的内存管理非常的熟悉。我们现在研究这些古老的东西很难找资料了，况且我们是在写 bootloader，不是DOS killer。

7.引导时的高级功能

1)initrd

initrd是启动时的一个小虚拟盘，一般用它来实现模块化的内核。引导initrd的方法主要有两个要点：
第一，把initrd读入内存，我们可以仿照大多数boot loader的方法把它放在内存的最高端；
第二，设定initrd的起始位置和长度

bootsect_pos[0x218]开始的4个字节放的是起始物理地址，bootsect_pos[0x21c]开始的4个字节放的是initrd的长度。

2)command_line支持

用command_line 你可以给内核传一些参数，自己定制内核的行为。我是这样做的，首先把command_line放在9900H:0的地址里，然后把9900H:0的物理地址存放在bootsect_pos[0x228]开始的4个字节里面。注意一定是物理地址，所以你应该放99000H这个数，然后内核就会识别你的 command_line了。

8.结束语

写本文的目的主要是为了用最少的语言和最短的时间说明bootloader的原理，真正的权威资料还是要看linux内核源码和boot.txt文件。我曾经写过一个例子loaderx，使用WATCOM C和TASM，WATCOM C是一个可以在DOS下生成能访问4GB物理地址程序的C编译器，里面也有详细的注释和文档说明。

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