The Real-Time Environment

Overview

The purpose of this introductory chapter is to describe the environment of real-time computer systems from a number of different perspectives. A solid understanding of the technical and economic factors which characterize a real-time application helps to interpret the demands that the system designer must cope with. The chapter starts with the definition of a real-time system and with a discussion of its functional and metafunctional requirements. Particular emphasis is placed on the temporal requirements that are derived from the well-understood properties of control applications. The objective of a control algorithm is to drive a process so that a performance criterion is satisfied. Random disturbances occurring in the environment degrade system performance and must be taken into account by the control algorithm. Any additional uncertainty that is introduced into the control loop by the control system itself, e.g., a non-predictable jitter of the control loop, results in a degradation of the quality of control.

In the Sections 1.2 to 1.5 real-time applications are classified from a number of viewpoints. Special emphasis is placed on the fundamental differences between hard and soft real-time systems. Because soft real-time systems do not have catastrophic failure modes, a less rigorous approach to their design is often followed. Sometimes resource-inadequate solutions that will not handle the rarely occurring peak-load scenarios are accepted on economic arguments. In a hard real-time application, such an approach is unacceptable because the safety of a design in all specified situations, even if they occur only very rarely, must be demonstrated vis-a-vis a certification agency. In Section 1.6, a brief analysis of the real-time system market is carried out with emphasis on the field of embedded real-time systems. An embedded real-time system is a part of a self-contained product, e.g., a television set or an automobile. In the future, embedded real-time systems will form the most important market segment for real-time technology.