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1.1.2 Real-Time Systems

One subclass of embedded systems is worthy of an introduction at this point. As commonly defined, a real-time system is a computer system that has timing constraints. In other words, a real-time system is partly specified in terms of its ability to make certain calculations or decisions in a timely manner. These important calculations are said to have deadlines for completion. And, for all practical purposes, a missed deadline is just as bad as a wrong answer.

The issue of what happens if a deadline is missed is a crucial one. For example, if the real-time system is part of an airplane's flight control system, it is possible for the lives of the passengers and crew to be endangered by a single missed deadline. However, if instead the system is involved in satellite communication, the damage could be limited to a single corrupt data packet. The more severe the consequences, the more likely it will be said that the deadline is "hard" and, thus, the system a hard real-time system. Real-time systems at the other end of this continuum are said to have "soft" deadlines.

All of the topics and examples presented in this book are applicable to the designers of real-time systems. However, the designer of a real-time system must be more diligent in his work. He must guarantee reliable operation of the software and hardware under all possible conditions. And, to the degree that human lives depend upon the system's proper execution, this guarantee must be backed by engineering calculations and descriptive paperwork.


1.1.1 History and Future | Programming Embedded Systems in C and C++ | 1.2 Variations on the Theme