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finding that commercial real-time operating systems are expensive to purchase
and support. Future RTOS designs will be developed in-house and leverage the
vast amount of open-source code available for real-time systems, which will de-
mand the use of Design for Six Sigma (DFSS) to optimize their design. In ad-
dition to the features found in standard operating systems such as memory man-
agement, task scheduling, and peripheral communication, the operating system
must provide a method for ensuring time deadlines are met. This is not to say
that all real-time systems will always meet their deadlines because other factors
need to be considered, factors that are out of the control of the operating sys-
tem. The real-time operating system has additional features such as timers and
preemption.
A real-time operating system must have a deterministic kernel, which means
that system calls that are handled by the operating system must complete within
a predetermined and known time (Kalinsky, 2003). If a task makes a system call,
the time to perform the system call should be consistent, but the worst-case time to
perform the system call must be known. This is essential for programmers to ensure
that the task will always meet their deadlines. If a system uses an operating system
that is nondeterministic, there is no time guarantee that a call will finish in time to
allow the task to complete by its deadline.
3.2
RTOS HARD VERSUS SOFT REAL-TIME SYSTEMS
There are three types of real-time systems: soft, hard, and firm. Hard systems are
defined as ones that experience catastrophic failure if deadlines are not meant. Failure
is deemed catastrophic if the system cannot recover from such an event. A hard real-
time system would not be able to recover if deadlines were missed and the effects
could be disastrous. Examples of this are vehicle and flight controllers; if a deadline
were missed in these systems, the vehicle or plan may crash causing devastating
damage and people may lose their lives.
Soft systems are those that can sustain some missed deadlines and the system
will not cause devastating results. For example, a machine that records television
programs is a real-time system because it must start and stop at a certain time in order
to record the appropriate program. But, if the system does not start/stop the recording
at the correct time, it may be annoying but will not cause catastrophic damage. An
operating system must be designed so that it can meet the requirements of the type
of system in which it is used.
A firm system falls somewhere in between soft and hard, where occasional failures
may be tolerated. But if the issue persists, the system may experience failures because
deadlines that are repeatedly missed may not be recoverable. This may indicate a
system that is overused. If system utilization is occurring, meaning that the central
processing unit (CPU) is overused and unable to support the task deadlines, before new
hardware is purchased there may be optimization techniques that can be performed
on the system and improve efficiencies (Furr, 2002).
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