Hardware Reference
In-Depth Information
13
SOLUTIONS TO THE EXERCISES
SOLUTIONS FOR CHAPTER 1
1.1
Fast computing tends to minimize the average response time of computation
activities, whereas real-time computing is required to guarantee the timing con-
straints of each task.
1.2
The main limitations of the current real-time kernels are mainly due to the fact
that they are developed to minimize runtime overhead (hence functionality)
rather than offering support for a predictable execution. For example, short in-
terrupt latency is good for servicing I/O devices, but introduces unpredictable
delays in task execution for the high priority given to the interrupt handlers.
Scheduling is mostly based on fixed priority, and explicit timing constraints
cannot be specified on tasks. No specific support is usually provided for peri-
odic tasks and no aperiodic service mechanism is available for handling event-
driven activities. Access to shared resources is often realized through classical
semaphores, which are efficient, but prone to priority inversion if no protocol
is implemented for entering critical sections. Finally, no temporal protection
or resource reservation mechanism is usually available in current real-time ker-
nels for coping with transient overload conditions, so a task executing too much
may introduce unbounded delays on the other tasks.
1.3
A real-time kernel should allow the user to specify explicit timing constraints
on application tasks and support a predictable execution of real-time activi-
ties with specific real-time mechanisms, including scheduling, resource man-
agement, synchronization, communication, and interrupt handling. In critical
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