Overview of the Book

To make life easier, Chapter 1 presents a general picture of what is inside a Unix kernel and how Linux competes against other well-known Unix systems.

The heart of any Unix kernel is memory management. Chapter 2 explains how 80 x 86 processors include special circuits to address data in memory and how Linux exploits them.

Processes are a fundamental abstraction offered by Linux and are introduced in Chapter 3. Here we also explain how each process runs either in an unprivileged User Mode or in a privileged Kernel Mode. Transitions between User Mode and Kernel Mode happen only through well-established hardware mechanisms called interrupts and exceptions. These are introduced in Chapter 4.

In many occasions, the kernel has to deal with bursts of interrupts coming from different devices. Synchronization mechanisms are needed so that all these requests can be serviced in an interleaved way by the kernel: they are discussed in Chapter 5 for both uniprocessor and multiprocessor systems.

One type of interrupt is crucial for allowing Linux to take care of elapsed time; further details can be found in Chapter 6.

Next we focus again on memory: Chapter 7 describes the sophisticated techniques required to handle the most precious resource in the system (besides the processors, of course), available memory. This resource must be granted both to the Linux kernel and to the user applications. Chapter 8 shows how the kernel copes with the requests for memory issued by greedy application programs.

Chapter 9 explains how a process running in User Mode makes requests to the kernel, while Chapter 10 describes how a process may send synchronization signals to other processes. Chapter 11 explains how Linux executes, in turn, every active process in the system so that all of them can progress toward their completions. Now we are ready to move on to another essential topic, how Linux implements the filesystem. A series of chapters cover this topic. Chapter 12 introduces a general layer that supports many different filesystems. Some Linux files are special because they provide trapdoors to reach hardware devices; Chapter 13 offers insights on these special files and on the corresponding hardware device drivers.

Another issue to consider is disk access time; Chapter 14 shows how a clever use of RAM reduces disk accesses, therefore improving system performance significantly. Building on the material covered in these last chapters, we can now explain in Chapter 15 how user applications access normal files. Chapter 16 completes our discussion of Linux memory management and explains the techniques used by Linux to ensure that enough memory is always available. The last chapter dealing with files is Chapter 17 which illustrates the most frequently used Linux filesystem, namely Ext2 and its recent evolution, Ext3.

Chapter 18 deals with the lower layers of networking.

The last two chapters end our detailed tour of the Linux kernel: Chapter 19 introduces communication mechanisms other than signals available to User Mode processes; Chapter

Last, but not least, are the appendixes: Appendix A sketches out how Linux is booted, while Appendix B describes how to dynamically reconfigure the running kernel, adding and removing functionalities as needed. Appendix C is just a list of the directories that contain the Linux source code.

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