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Cork Institute of Technology
Higher Certificate in Science in Computing in Information Technology Support – Award
(National Certificate in Science in Computing in Information Technology Support – Award)
(NFQ – Level 8)
Summer 2005
Operating Systems
(Time: 2 Hours)
Instructions Answer four questions All questions are worth equal marks
Examiners: Mr. G. McSweeney Dr. D. Chambers Mr. E. A. Parslow Mr. P. O Connor
1. (a) Explain the following terms with respect to disk access:
(i) Seek time ; (ii) Latency Time ; (iii) Transfer Time. (6 marks)
(b) For the following track request queue, describe the sequence of tracks visited for each of the following disk scheduling algorithms. Assume that the head starts at track 152 and is moving toward track 199. Also assume that the drive has 200 tracks numbered 0-
Request queue: 83, 149, 88, 169, 93, 99, 110, 175, 133 (i) Scan ; (ii) Look; (iii) Circular Look (6 Marks)
(c) In the context of disk space allocation, name the three algorithms which are most used in the contiguous allocation method. (3 marks)
(d) Describe the main disadvantage of contiguous allocation. (3 marks)
(e) State two other methods of disk space allocation. (2 marks) (f) With the aid of a diagram, describe how unix allocates disk space. (5 marks)
2. (a) The command useradd can be used by an administrator to create a new user. How might the potential user be supplied with a password? Describe how the administrator might also provide the user with a home directory to store his/her files. Where might this home directory be placed? ( 7 Marks )
(b) Apart from using (^) useradd , describe two other ways of creating a user account in a standard Linux distribution. ( 3 Marks )
(c) Explain how users are authenticated when they log in to a Linux system. In your answer consider password security. ( 5 Marks )
(d) What are the security risks of a user using a simple password? ( 2 Marks )
(e) What are the advantages of placing users in groups? ( 2 Marks )
(f) What are the different permissions in a Linux group? ( 3 Marks )
(g) Describe how an administrator might use group permissions to limit access to specific files. ( 3 Marks )
3. (a) Describe the difference between a pre-emptive and a non pre-emptive scheduling algorithm. ( 2 Marks )
(b) Give one example of a pre-emptive scheduling algorithm and one example of a non pre- emptive scheduling algorithm. ( 2 Marks )
(c) Describe the main characteristics of the following scheduling algorithms: Shortest Process Next Shortest Remaining Time ( 4 Marks )
(d) Consider the following set of processes with duration given in milliseconds.
Process Name Arrival Time Service Time P1 0 3 P2 2 6 P3 4 2 P4 5 1
Draw a time line to show the execution of each of these processes for the two scheduling algorithms described above. The time slice for the Round Robin algorithm is 2 milliseconds. For each algorithm calculate the Turnaround Time and Normalised Turnaround Time for each process. ( 8 Marks )
(e) Name two other scheduling algorithms. ( 2 Marks )
6.^ (a)^ Write an interactive shell script which displays the following when run. The script should respond to a user’s input appropriately and should keep displaying the menu until the user selects option 5. If a user enters a value outside the range (i.e. from 1 to
- an appropriate message should be displayed and the menu should re-appear.
Select from the menu
**1. Check permissions for your files
- Check who is logged on
- Display Date and Time
- Check what directory you are in
- Exit**
Enter number and press ENTER
( 15 Marks )
(b) Write an interactive shell script which prompts an administrator to enter a potential user’s name and then automatically generates an account for them. The user should be issued with a password and a home drive. ( 10 Marks )
