It has been mentioned several times already in this book that real-time programming represents a major application area for Ada, and particularly for Ada tasking. The Real-Time Systems Annex specifies additional characteristics of the language that facilitate the programming of embedded and real-time systems. If an implementation supports the Real-Time Systems Annex then it must also support the Systems Programming Annex (see previous chapter). All issues discussed in the Real-Time Systems Annex affect the tasking facilities of the language. They can be grouped together into the following topics.

• Time and clocks – introduced in Chapter 1.

• Scheduling – how to allocate system resources, in particular the processor.

• Resource control – how to monitor and manage the use of the processor by individual tasks or groups of tasks.

• Optimisations and restrictions – specifically the Ravenscar profile.

All of these topics are discussed in this and the next two chapters; starting with the important issue of scheduling.

Scheduling

The functional correctness of a concurrent program should not depend on the exact order in which tasks are executed. It may be necessary to prove that the non-determinism of such programs cannot lead to deadlock or livelock (that is, progress is always taking place), but it is not necessary to program the order in which all actions must occur explicitly.