Real-Time Operating Systems

Overview

This chapter covers the essential services that must be provided by a real-time operating system. It focuses on the real-time aspects of operating systems. It is assumed that the reader is already familiar with general operating system concepts. A real-time operating system must provide a predictable service to the application tasks such that the temporal properties of the complete software in a node can be statically analyzed. Many dynamic mechanisms, such as dynamic task creation or virtual memory management, which are standard in workstation operating systems, interfere with this predictability requirement of real-time systems.

The chapter starts with a section on task management. The state transition diagrams of time-triggered and event-triggered tasks, with and without internal blocking, are presented. The application program interface of the different task models is discussed. The simpler the application program interface, the easier it is to write portable application software. Section 10.2 covers the topic of interprocess communication. It is argued that the classic interprocess coordination primitives, such as semaphore operations, are too expensive for many embedded applications and simpler alternatives must be found.

Section 10.3 is devoted to time management. A real-time operating system must provide a clock synchronization service and a number of additional time services that are discussed in this section. Error detection is the topic of Section 10.4. Error detection in the time domain is of particular importance if an architecture is based on the fail-silent assumption.

The final section presents a case study of the real-time operating system ERCOS (Embedded Real-time Control Operating System). ERCOS is an industrial real-time operating system used in embedded automotive applications. It provides many of the services that are discussed in this chapter.