JAVA-BASED REAL-TIME PROGRAMMING

Chapter 2
Fundamentals
Goal: Review of basic properties of software execution as a preparation
for dealing with concurrency and real-time issues.
Already having some programming experience means that you, dear reader,
have a lot of knowledge about how a computer interprets or executes a computer
program. When you know it, it is all natural, and most programmers
are not aware of the insights they have gained. We are now about to write
programs that behave concurrently even if we only have one processor. Before
doing so, it could be a good idea to take a step back and review the basic
properties of program execution. This is the first thing that will be done in
this chapter. Then, different notions of concurrency in hardware and software
will be treated. Finally, we state the duties of the software systems we are
about to develop.
The eager and experienced programmer may skip this chapter, but beware,
you may be a very skilled programmer without having the profound
understanding you think you have.
2.1 Software execution is performed sequentially
To recall the sequential nature of program execution, assume you write an
ordinary program to be run on an ordinary computer having one processor
(CPU). A piece of the code may look as shown in Figure 2.1, so what value
will x have after leaving the loop?
You know that the resulting value of x will be one. The novice answering
zero believes that the loop-condition is continuously evaluated in parallel with
evaluation of the expressions in the loop. Thus, the fact that the computer
treats our instructions sequentially one after another cannot be considered
natural to all of us; some of us have just learned that this is the way it works.
Actually, it is a severe restriction that only one thing at a time can be done.
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