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Exam January 2015, questions
Operating Systems (Newcastle University)
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NEWCASTLE UNIVERSITY
SEMESTER 1 2014/2015
OPERATING SYSTEMS
Time allowed - 1½ Hours
Instructions to candidates:
Answer ALL questions
The total marks available for this exam are 100
Marks shown for sub-sections are indicative only
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Question 1
a) What are the two main functions of an operating system?
[4 marks]
b) Explain which of the following instructions should only be
executed in kernel mode and which can be executed in
user mode and why.
i)
Disable all interrupts
ii) Read the time of day clock
iii) Set the time of day clock
iv) Change the value of a local variable
[8 marks]
c) What are the three main states of a process? Describe
each state. With the use of a diagram, explain the
transitions between process states.
[13 marks]
Question 2
a) Consider the following two lines of code:
int ai[10];
int *pai = (int *) calloc(10, sizeof(int));
i)
Explain the differences in memory allocation for ai and
*pai.
[8 marks]
ii) Explain when the memory used by ai and *pai is
available for reuse.
[5 marks]
b) Assuming there are system calls to fork and execute
processes, describe the behaviour of an operating system
shell by explaining the steps that the program repeats. You
may use C language code, pseudocode, or English to
describe a shell.
[12 marks]
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Question 3
a) Consider a system of five jobs A to E with the following
known run times and arrival times.
Job
Running time
Arrival time
A
2 minutes
0 minutes
B
5 minutes
0 minutes
C
3 minutes
3 minutes
D
1 minutes
3 minutes
E
2 minutes
3 minutes
For the following two algorithms, calculate the length of
time each job is in the system and the mean process
turnaround time. Show your working out.
i)
First-come, first-served (assume alphabetic order for
jobs with the same arrival time)
[7 marks]
ii) Shortest remaining time
[8 marks]
b) Consider a system with a high priority process H and a low
priority process L. The processes share the same critical
region. The processes use busy waiting to test whether
they are able to enter the critical region. At one point in
time, L is in the critical region and H becomes ready to run
after completion of an I/O operation.
i)
If we apply the scheduling rule that the highest priority
process must run whenever it is ready, what problem
can occur?
[5 marks]
ii) Why does round robin scheduling avoid the problem
with priority scheduling?
[5 marks]
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Question 4
Consider the following incomplete pseudo-C code for a
bounded buffer, buf, that is shared by producer and consumer
processes.
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#define SIZE 100
int buf[SIZE];
int start = 0; // index of first item
int used = 0; // number of items in buf
// declare semaphores
void producer(void) {
int item;
while(true) {
item = produce_item();
buf[(start + used) % SIZE] = item;
used++;
}
}
void consumer(void) {
int item;
while(true) {
item = buf[start];
start = (start + 1) % SIZE;
used--;
consume_item(item);
}
}
Assume produce_item is a function used by the producer to
produce an item to put in buf and consume_item is a function
used by the consumer to consume an item taken from buf.
a) What are the constraints on access to the buffer that must
be enforced to maintain its integrity?
[5 marks]
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b) Assume that there is a library that defines a semaphore
type and up and down operations on a semaphore. Either
in modifications to the preceding code or in English
language, explain how to use semaphores to maintain the
integrity of the buffer. That is, in a modified program or in
English, answer the following questions.
i)
What semaphores are required and what is their initial
state?
[9 marks]
ii) What are the critical sections of producer and
consumer code?
[3 marks]
iii) How are the semaphores used in the producer and
consumer code?
[8 marks]
END
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