Hardware Reference
In-Depth Information
Video playing;
audio/video encoding and decoding;
on-line image processing;
sensory data transmission in distributed systems.
Soft tasks are typically related to system-user interactions. Thus, they include:
The command interpreter of the user interface;
handling input data from the keyboard;
displaying messages on the screen;
representation of system state variables;
graphical activities; and
saving report data.
1.2.2
LIMITS OF CURRENT REAL-TIME SYSTEMS
Most of the real-time computing systems used to support control applications are
based on kernels [AL86, Rea86, HHPD87, SBG86], which are modified versions of
timesharing operating systems. As a consequence, they have the same basic features
found in timesharing systems, which are not suited to support real-time activities. The
main characteristics of such real-time systems include the following:
Multitasking . A support for concurrent programming is provided through a set
of system calls for process management (such as create , activate , terminate , de-
lay , suspend , and resume ). Many of these primitives do not take time into account
and, even worse, introduce unbounded delays on tasks' execution time that may
cause hard tasks to miss their deadlines in an unpredictable way.
Priority-based scheduling . This scheduling mechanism is quite flexible, since
it allows the implementation of several strategies for process management just by
changing the rule for assigning priorities to tasks. Nevertheless, when application
tasks have explicit time requirements, mapping timing constraints into a set of
priorities may not be simple, especially in dynamic environments. The major
problem comes from the fact that these kernels have a limited number of priority
levels (typically 128 or 256), whereas task deadlines can vary in a much wider
range. Moreover, in dynamic environments, the arrival of a new task may require
the remapping of the entire set of priorities.
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