JAVA-BASED REAL-TIME PROGRAMMING

2.5. Interrupts, pre-emption, and reentrance
course, the time in software will then be discrete (incremented one step for
each timer interrupt) opposed to the true time which is continuous. Discrete
time is, however, inherent to computer control anyway so it suits our purposes
well.
Note that if we were only simulating our system, the time variable could
be stepwise increased by the scheduler; when all processing is done for one
point of time, the scheduler simply lets time jump to the next time referred
to by the application. But here we aim at software that interacts with the
physical environment which (by the laws of nature) has time as an independent
and continuously increasing variable. The time variable in our software is the
sampled value of the real time.
The timer interrupt actually turns out to be a general and minimum requirement
for both desktop and control computers. Additionally, there are
some low-level and system-specific events that require particularly timely service.
For instance, the Windows-95/98 user may review his/her IRQ settings,
typically finding them allocated by the sound card, the game port, the hard
disk, etc. (The settings are found by selecting IRQ in the panel found by
clicking on Control panel -> System -> Device Manager -> Computer ->
Properties.) Then note that the first interrupt (IRQ 00, having the highest
priority) is used by the “System Timer” which then provides the time base
for the entire operating system. Giving the timer interrupt the highest priority,
allowing it to interrupt other interrupts, is to ensure maintenance of the
time variable. Otherwise, a too excessive number of interrupts could not be
detected.
In embedded computers there are usually also some kind of time-out or
stall-alarm interrupt connected to the very highest (unmaskable) interrupt
level. It is then the obligation of the so called idle loop (the loop being
executed when there are nothing else to do) to reset that timer so it never
expires. But if that should happen, the CPU is overloaded and the controlled
machine must be shut down because without enough time for the feedback
control the system may behave very badly (crazy robot, crashing airplane,
etc.). To prevent this, we will use special programming techniques in later
chapters. In desktop applications, on the other hand, the user simply has to
wait.
2.5.2 Pre-emption
When the scheduler, as a consequence of a timer interrupt, suspends execution
of one activity and lets the program resume execution at another point (where
execution was suspended at some earlier stage), we call it pre-emption. The
term is used also in connection with operating systems; pre-emptive multitasking.
Pre-emption means that execution may be suspended even if there is
nothing in the executing code that explicitly admits the change of execution
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