Source Code Secrets

Before you get too excited about a new book about the Linux kernel, let me clear the air—The Basic Kernel is about 386BSD. If you are a savvy OS person, you've already figured that out from the names of the authors; Bill and Lynne Jolitz are the creators of 386BSD. So, why review it? Because it is a good book that covers the internals of operating systems design—and 386BSD, the system it covers, is available in source code.

The Basic Kernel is the first volume in a series entitled Operating System Source Code Secrets. The second book in the series is titled Virtual Memory. The first thirty pages of the book explain why such a book is necessary, and define both what a kernel is and the notation used in the book.

The book then talks about what code needs to be done for the kernel, presenting code fragments with explanations. About 60 pages cover the assembly code necessary to interface to the 386 processor. Not being a 386 assembly language programmer (but having extensive systems programming experience), I had some problems here. The explanations were clearly for someone with an assembly language background. I would have liked to see more in the way of “how it works” explanations. However, if you are interested in 386 code, it is probably fine.

I felt more comfortable with the next chapter, covering C code. The beginning of that chapter is typical of what follows, so I will use it as an example. The trap.c routine is introduced, along with a listing and brief explanation of its functions, followed by a detailed explanation of what the functions are, how to call them and how they are implemented. The implementation details consist of code fragments with text explanations.

The book continues in this style, covering i386/trap.c, i386/cpu.c, kern/config.c, kern/malloc.c, kern/fork.c, kern/exit.c, kern/sig.c, kern/cred.c, kern/priv.c, kern/synch.c, kern/lock.c, kern/execve.c and kern/descrip.c. If you have looked at or worked with a POSIX kernel, you can probably guess what functions are found in these files.

Three appendices cover kernel source organization, network-level security and dynamic make files. The book ends with answers to the exercises included in the chapters, and an index with entries for all the functions described.

The book is well-written, and, I expect, accurate. If you want to know how the 386BSD kernel works, this book will tell you. And, if you want to know how to implement low-level functionality that requires dealing with 386 hardware, this book has a wealth of information on the subject.

What this book won't tell you is how Linux is implemented. While the same sort of tasks have to be done in Linux, different design decisions were made for 386BSD than for Linux. As I was reading the text, many times I was thinking, “Gee, I wonder if this is done the same way in Linux?” If that comparison is of interest to you, buying both The Basic Kernel and Linux Kernel Internals (reviewed in Linux Journal, December, 1996) would probably fill your needs.