JAVA-BASED REAL-TIME PROGRAMMING

2. Fundamentals
int x = 2;
while (x>1) {
if (x==10) x = 0;
x++;
};
/* What is x now? */
Figure 2.1: Code snippet illustrating sequential execution, trivial to any programmer
but many beginners actually give the wrong answer 0.
Even if computers today are computing quickly enough to do the computations
required by a parallel environment (like our real world).
So, when learning to program, we are first forced into the ‘narrow alley’ of
sequential execution. Compare this with hardware design where each signal
or information flow usually is taken care of by dedicated components ensuring
continuous and timely operation. But for reasons of cost and flexibility, we
cannot implement everything in hardware, so the problem that we are facing
now is that the sequential software execution model does not fit the inherently
parallel problems we want to solve. Even when solving parallel problems,
however, some things are still sequential to their nature (like a sequence of
control actions to be carried out in the correct order). Then, for such parts of
the program, the sequential execution of software will be of great convenience.
2.2 Our physical world is parallel
The object oriented programming (OOP) paradigm, which we want to use
for embedded programming, is based on creating software entities modelling
our environment. Furthermore, since we aim at computer control of physical
devices, some thoughts about the environment to be controlled may be
appropriate 1 .
As we all know, our physical world is inherently parallel. Mathematically,
we need sets of coupled differential equations to describe it. Comparing such
mathematical expressions with the expressions found in programming languages
such as Java, they are not at all expressing the same thing. Basically,
this is related to another beginners problem in programming; the notion of
equality. When an mathematical expression states that x=y+2, that is an
equality which should hold for whatever value x (or y) has. When we in a
Java program write x=y+2, we instruct the computer to take the vale of y,
add 2, and to store that in x. If we in Java write x==y+2, we test if the value
of x equals y+2. The mathematical equality cannot be directly expressed in
Java (or in any other widely used programming language like C, C++, etc.).
Again, computers operate sequential from a programmers point of view.
1 This section (1.2) aims at deeper understanding or wider perspectives, but can be
skipped by the reader only caring about practical programming issues.
18 2012-08-29 16:05