List of Tables
Table 2.1 |
Execution of a program.................................................................... |
11 |
Table 2.2 |
Duplication of Hardware ................................................................. |
14 |
Table 2.3 |
Semaphore States............................................................................. |
22 |
Table 2.4 |
P() and V() properties ...................................................................... |
24 |
Table 2.5 |
Typical Initial Counter Values ......................................................... |
25 |
TABLE 1. |
Commands available in all menus ................................................... |
97 |
TABLE 2. |
Specific commands .......................................................................... |
97 |
Preface
Every year, millions of microprocessor and microcontroller chips are sold as CPUs for general purpose computers, such as PCs or workstations, but also for devices that are not primarily used as computers, such as printers, TV sets, SCSI controllers, cameras, and even coffee machines. Such devices are commonly called embedded systems. Surprisingly, the number of chips used for embedded systems exceeds by far the number of chips used for general purpose computers.
Both general purpose computers and embedded systems (except for the very simple ones) require an operating system. Most general purpose computers (except mainframes) use either UNIX, Windows, or DOS. For these operating systems, literature abounds. In contrast, literature on operating systems of embedded systems is scarce, although many different operating systems for embedded systems are available. One reason for this great variety of operating systems might be that writing an operating system is quite a challenge for a system designer. But what is more, individually designed systems can be extended in exactly the way required, and the developer does not depend on a commercial microkernel and its flaws.
The microkernel presented in this book may not be any better than others, but at least you will get to know how it works and how you can modify it. Apart from that, this microkernel has been used in practice, so it has reached a certain level of maturity and stability. You will learn about the basic ideas behind this microkernel, and you are provided with the complete source code that you can use for your own extensions.
The work on this microkernel was started in summer 1995 to study the efficiency of an embedded system that was mainly implemented in C++. Sometimes C++ is said to be less efficient than C and thus less suitable for embedded systems. This may be true when using a particular C++ compiler or programming style, but has not been confirmed by the experiences with the microkernel provided in this book. In 1995, there was no hardware platform available to the author on which the microkernel could be tested. So instead, the microkernel was executed on a simulated MC68020 processor. This simulation turned out to be more useful for the development than real hardware, since it provided more information about the execution profile of the code than hardware could have done. By mere coincidence, the author joined a project dealing with automated testing of telecommunication systems. In that project, originally a V25 microcontroller had
2
been used, running a cooperative multitasking operating system. At that time, the system had already reached its limits, and the operating system had shown some serious flaws. It became apparent that at least the operating system called for major redesign, and chances were good that the performance of the microcontroller would be the next bottleneck. These problems had already caused serious project delay, and the most promising solution was to replace the old operating system by the new microkernel, and to design a new hardware based on a MC68020 processor. The new hardware was ready in summer 1996, and the port from the simulation to the real hardware took less than three days. In the two months that followed, the applications were ported from the old operating system to the new microkernel. This port brought along a dramatic simplification of the application as well as a corresponding reduction in source code size. This reduction was possible because serial I/O and interprocess communication were now provided by the microkernel rather than being part of the applications.
Although the microkernel was not designed with any particular application in mind, it perfectly met the requirements of the project. This is neither by accident nor by particular ingenuity of the author. It is mainly due to a good example: the MIRAGE operating system written by William Dowling of Sahara Software Ltd. about twenty years ago. That operating system was entirely written in assembler and famous for its real-time performance. Many concepts of the microkernel presented in this book have been adopted from the MIRAGE operating system.