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SMIE-141: Introduction to Embedded Systems
Exercise 4: Resource sharing
Kai Huang
November 18, 2014
1 Exercise 4.1
? Explain the difference between private, shared and exclusive resources and give an
example of problems encountered in exclusive resources.
? Solution: Private resources are used exclusively by one process/thread, whereas
shared resources can be used by at least two possibly competing processes/threads.
Simultaneous use of a resource by multiple processes/threads without further coun-
termeasures, for example, can raise inconsistencies or illegible spending on printer
or monitor. Such resources are also called exclusive resources, there really should
always just one process/thread exclusively take on it.
? Propose three ways such that the problem occurring in Resource Sharing Problem
of mutual exclusion (mutual exclusion) can be solved.
? Solution: Non-preemptive tasks, interrupts disabled, static scheduling, Use of
semaphores.
? Explain the application of a possible solution of the previous part.
? Solution: A (binary) semaphore S is a data structure that can be accessed only
by the atomic commands, wait (s), bzw. P(s) and signal (s), bzw. V(S). Assume
a process A want to enter a critical section, it executes a wait(s) on the semaphore
and claims the right to entry into the critical section. If the process hears that the
semaphore is free, he enters the critical section and releases the semaphore with
signal(s) after he left its critical section. If the call of the semaphore wait(s) is
blocked by another process B, A waits until B releases the semaphore.
2 Exercise 4.2 - Priority Inversion
? In the lecture you learned about the Priority Inheritance Protocol (PIP). Does the
PIP solve the problem of deadlocks? Give a brief explanation for your response.
1
? Solution:No, the PIP does not prevent deadlocks.
? Given are the four tasks J1, J2, J3 and J4. The table below contains information
on their arrival times, deadlines, their execution time and priorit
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