Level of Description
Linux source code for all supported architectures is contained in more than 14,000 C and assembly language files stored in about 1000 subdirectories; it consists of roughly 6 million lines of code, which occupy over 230 megabytes of disk space. Of course, this book can cover only a very small portion of that code. Just to figure out how big the Linux source is, consider that the whole source code of the book you are reading occupies less than 3 megabytes. Therefore, we would need more than 75 books like this to list all code, without even commenting on it! So we had to make some choices about the parts to describe. This is a rough assessment of our decisions:
• We describe process and memory management fairly thoroughly.
• We cover the Virtual Filesystem and the Ext2 and Ext3 filesystems, although many functions are just mentioned without detailing the code; we do not discuss other filesystems supported by Linux.
• We describe device drivers, which account for roughly 50% of the kernel, as far as the kernel interface is concerned, but do not attempt analysis of each specific driver. The book describes the official 2.6.11 version of the Linux kernel, which can be downloaded from the web site http://www.kernel.org.
关于本书讨论内容的层次
支持全部体系架构的Linux源码包含14000多行C和汇编文件并分布在1000多个子目录中；源码包共包含越六百万行代码，占据超过230M硬盘空间。当然，本书仅覆盖很小一部分的源码。
Linux源代码究竟有多大呢？您正在阅读的这本书（900多页）中所占据的空间不超过3M空间。因此需要超过75本这样的书才能容得下全部的代码——这还不包括注释哦！这样一来我们不得不对探讨的部分做出一些取舍。我们对内容的分配大致是这样的：
- 对进程和内存管理我们进行了完整的分析；
- 对于文件系统，尽管我们只是提到了许多功能而并未做详尽的描述，但是我们的分析依然覆盖了虚拟文件系统、Ext2和Ext3文件系统，对于其他Linux支持的文件系统没有纳入讨论范围；
- 对于内核接口而言，我们剖析了约占内核50%的设备驱动，但是并未试图分析每一个特定驱动的实现。
本书描述的内容基于官方发布的2.6.11版本内核源码，你可以在http://www.kernel.org中下载该源码。