Page 99 - DCAP608_REAL TIME SYSTEMS

Real Time Systems

Notes the calculated bounds were too high to guarantee the correct timing behaviour of an
application already at design time.

 The self-checking coverage of the MARS components has to be high because the
fault tolerance mechanisms of MARS are based on it. Whereas the old kernel was not
specifically designed in order to meet this requirement, MARS-2 uses both hardware
and software mechanisms to increase the self-checking coverage to a sufficiently
high degree.MARS-2 is based on a microkernel operating system architecture in
contrast to the monolithic kernel of MARS-1.
Questions:
1. What are the functions of time driven real-time systems?

2. Explain in what ways, the MARS system was different from other systems.
3. Describe the self-checking coverage concept.

Source: dl.acm.org/citation.cfm?id=506416

9.5 Summary

 Clock-driven schedulers schedule periodic tasks according to a cyclic schedule.
 Aperiodic and sporadic jobs can be scheduled, if they do not influence other scheduled
jobs.
 Applicable to static systems, with a small number of aperiodic jobs.
 The scheduling decision times partition the time line into intervals called frames.

 A straight forward way to implement the scheduler is to store the precomputed schedule
as a table.
 A constraint enforced by this structure is that scheduling decisions are made periodically,
rather than at arbitrary times.
 The scheduling decision times partition the timeline into intervals called frames.
 Each frame has length f which is the frame size.

 Scheduling decisions are made only at the end of every frame so there is no preemption in
a frame.
 Phase is always a multiple of frame size.

9.6 Keywords

Clock-driven Schedulers: The clock-driven schedulers are those in which the scheduling points
are determined by the interrupts received from a clock.
Cyclic Executive: It refers to a scheduler that deterministically interleaves and sequentializes the
execution of periodic-tasks on a CPU according to a given cyclic schedule.
Event-driven Schedulers: In the event-driven ones, the scheduling points are defined by certain
events which precludes clock interrupts.

Frame Size: Each frame has length f which is the frame size.
Frames: The scheduling decision times partition the timeline into intervals called frames.

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