Operating Systems Exam for Computer Science Degree, Exams of Operating Systems

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Cork Institute of Technology

Bachelor of Science (Honours) in Software Development – Stage 2

Bachelor of Science (Honours) in Software Development and Computer

Networking – Stage 2

(NFQ Level 8)

Summer 2006

Operating Systems

(Time: 3 Hours)

Instructions Answer FIVE questions. All questions carry equal marks.

Examiners: Mr. G. Mac Sweeney Dr. M. O’Cinneide Mr. M. Donnelly Dr. J. Buckley Dr. A. Kinsella

Q1. Consider the following entry from a typical /etc/passwd file on a Linux system: jane:x:1004:100:Jane:/home/jane:/bin/bash (a) Explain each item in the entry. (7 Marks) (b) Explain how a user is authenticated when he / she logs onto a Linux system. (3 Marks) (c) Explain how Linux maintains the secrecy of users’ passwords. (3 Marks) (d) Explain the purpose of the environmental variable PATH. (2 Marks) (e) In a Linux system, how would you enable one group of users to be able to run a particular program and prevent another group from running it? (2 Marks) (f) State some of the advantages of partitioning a disk. (3 Marks)

Q2. (a) What are the main criteria for File Organisation? (5 Marks) (b) Serial (Pile) is a type of file organisation. Name two others. (2 Marks) (c) With reference to Serial (Pile), give an advantage, a disadvantage and an example of where this type of file organisation is appropriate. (3 Marks) (d) In a client-server environment, name three services a server might offer its clients. (3 Marks) (e) In client-server applications, why is host-based processing not regarded as true in a client server relationship? (2 Marks) (f) What is the usual minimum application required for a client in a client-server relationship? (2 Marks) (g) Give three reasons why process migration may be desirable? (3 Marks)

Q3. (a) What is deadlock? (3 Marks) (b) What are the three conditions for deadlock? (3 Marks) (c) What is meant by circular wait? (2 Marks) (d) Explain the terms deadlock avoidance and deadlock prevention. (2 Marks) (e) What is a reusable resource? Give an example. (2 Marks) A system with four processes and three resource types is characterised by the Total Resource Vector and Current Resource Allocation as follows: Total Resource Vector R1 R2 R 7 7 7 Current Resource Allocation R1 R2 R P1 2 0 3 P2 1 3 2 P3 2 1 2 P4 1 1 0 (f) Calculate the available resource vector. (2 Marks)

(g) The following matrix describes the Total Resource Claims for each of the current processes. Is the system currently in a safe state? Explain how you derived your answer. (6 Marks)

R1 R2 R P1 5 2 4 P2 5 3 6 P3 3 3 2 P4 7 7 7

Q4. (a) Using an appropriate diagram, describe how UNIX allocates disk space. (7 Marks) (b) State three advantages of the UNIX disk space allocation method. (3 Marks) (c) If the capacity of a UNIX block is 1K , how many indexes can it hold? (2 Marks) (d) 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 125 and is moving towards track 0. Also assume the drive has two hundred tracks numbered 0 – 199.

Request queue: 45, 138, 60, 51, 178, 37, 86, 78, 102, 134, 145 (i) Scan ; (ii) Circular Scan ; (iii) Circular Look (6 Marks)

(e) Explain what is meant by internal fragmentation. (2 Marks)

Q5. (a) Draw the Seven State Model transition diagram. In your diagram, carefully label each state and each transition. (14 Marks) (b) Describe the steps in a process switch. (6 Marks)

Q6. The following is part of the output of a process status command.

PID PPID TTY TIME CMD 4097 4088 pts/2 0:00 ps (a) Explain the meaning of PID, PPID, TTY, TIME and CMD. (5 Marks) A further process status command has the following output; PID PPID TTY TIME CMD 4114 4113 pts/3 0:00 bash 4115 4114 pts/3 0:00 ps –af (b) Explain the significance of the value 4114 in the PID and PPID columns. (2 Marks) (c) In UNIX , how would you kill a process (and prevent it from finishing)? (2 Marks)

Arrival Time

Service Time P1 0 3 P2 2 6 P3 3 2 P4 5 4 (d) Draw a time line to show the execution of each of the following processes: Round Robin (time slice = 2) Shortest Remaining Time

For each algorithm, calculate the Turnaround Time and the Normalised Turnaround Time. (8 Marks)

(e) What do the Round Robin and Shortest Remaining Time algorithms have in common? (1 Mark)

(f) Name two other scheduling algorithms. (2 Marks)

Q7. (a) In memory management what is meant by paging? (2 Marks) (b) What is another method of address mapping? (2 Marks) (c) Explain the terms page and frame. (2 Marks) (d) What is the offset of the logical address 1011011011101001 if the page number uses four bits? (2 Marks) (e) If a page number uses four bits, how many pages are there and how many locations per page? (2 Marks)

If a logical address is 0101010110011011 and the page table is as follows: 0 0111 1 1001 2 0001 3 1011 4 0010 5 1110 6 1101 (f) Calculate the absolute address (page number uses 4 bits). (6 Marks) (g) With reference to virtual memory, what is meant by the following?

• real memory
• thrashing
• resident set
• principle of locality. (4 Marks)