Test 2
Operating Systems and Networking
CS330
Spring 2007
Answer only five from the following
Allotted time: 1 hr and 50 minutes
Date: March 19, 2007
Policy: Open notes, text and reference books.
1. CPU periodically receives data from devices like keyboard, mouse, network interface card, etc. To
figure out if there’s any data from a device for CPU, it must employ some method like periodic
polling from CPU or interrupt from the device to CPU. Briefly discuss when one is more efficient
than the other.
2. For a system of interest, assume the context switching time in a RR system is negligible. Each
process when it arrives at CPU gets
units of time-slice. Will I/O utilization increase or
decrease if
is increased? State your rationale.
3. Consider the same system as outlined in the previous question. If
is reduced, will the reduction
cause some programs to take longer times to finish? Why, or why not?
4. In the lecture, it was pointed out that a busy-waiting by a process to get into its critical section
should generally be avoided, if possible. Indicate why. Could there be any situation, however,
where busy-waiting might not be all that bad? Indicate how.
5. The two processes shown below access a shared printer to do their print tasks via a semaphore S
which is initialized to 0. It is conjectured that the following output strings are either individually
valid or invalid when the two processes are run concurrently. Mark the output strings either as
valid or invalid strings.
Process P: Process Q:
{ …. { ….
while (TRUE) while (TRUE)
{ {
wait(S); signal(S);
print(P); print(Q);
} }
} // end_P } // end_Q
Possible output claims are:
A) QPQPQPQP B) QQQPPQPP
C) PQPQPQPQ D) QPPPQPQQ
6. Consider the following situation. Peter, Paul and Mary are all distinguished members of Caribbean
Pirates Unconventional. Three of them share a treasure chest which contains valuable trinkets and
gold coins. Periodically, though, Peter and Paul steals from the chest (naturally without anybody’s
knowledge) to purchase rum and cigars when they run out of money without alerting anyone in the
group. Mary has an inkling of what’s going on, but she has decided not to open her mouth and
create even a bigger problem. Every time she has some valuable to store into the chest she does so
without letting any of her partners know.
