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Cork Institute of Technology
Bachelor of Science in Computing –Stage 2
(NFQ - Level 7)
Summer 2007
Operating Systems
(Time: 3 Hours)
Answer ANY FOUR questions. All questions carry equal marks.
Examiners: Ms. M. Meagher Ms. A. Brown Mr. P. Rothwell.
- (a) List and briefly discuss^ the three goals of an operating system. [3 marks] Briefly describe (^) the role of the command interpreter. [3 marks] Briefly outline the notion of an interrupt and comment on its central role. [4 marks]
(b) At any time a process may be ‘blocked’, ‘suspended’ or ‘ready’. Define each of these states and give a simple example of when a process may enter each state. [3 marks] Summarise the typical contents of a process control block (PCB). [3 marks] Briefly explain what the PCB is used for. [2 marks]
(c) Outline the steps involved in a process switch. [4 marks] Briefly distinguish between a process switch and a context switch. [3 marks] [Total: 25 marks]
- (a) Given the following skeleton pseudocode complete the ‘wait’ and ‘signal’ methods. class semaphore { int value; ListofProcesses L = new ListofProcesses( );
void wait( ) { //complete this }//end wait method
void signal ( ) { //complete this } //end signal method } //end class semaphore [4 marks]
Based on the code given, briefly explain the semaphore object. [2 marks]
(b) Several proposed software and hardware solutions to the mutual exclusion problem involve ‘busy waiting.’ Explain ‘busy waiting’ in this context. Fully explain how the use of a semaphore avoids the ‘busy waiting’ problem. [4 marks] Using pseudocode, explain how a semaphore can be used to solve the general mutual exclusion problem. [4 marks] Clearly indicate the semaphore’s initial value and justify your choice. [2 marks]
(c) Fully describe the producer/consumer problem. In the context of your description clearly explain the mutual exclusion and synchronisation problems. [9 marks] [Total: 25 marks]
- (a) With reference to the paged scheme of memory management, use diagrams, examples, and definitions to give full descriptions of : (i) the efficiency of use of physical memory; [3 marks] (ii) address mapping; [6 marks]
(b) Assuming memory is managed using a paged virtual memory management scheme, outline the notion of a page fault and list and briefly describe the steps involved in the page fault handling routine. [8 marks]
(c) Briefly explain and comment upon each of the following characteristics that may be taken into account in the choice of a victim page when replacing a page in virtual memory management: recency, age, frequency, modify/dirty bit. [8 marks] [Total: 25 marks]
- (a) Explain the terms ‘pre-emptive’ and ‘non-pre-emptive’ in the context of CPU scheduling algorithms. [2 marks] Indicate broadly the importance of both of these notions for CPU scheduling. [2 marks] (b) Given the following set of process information, for ONE of the following scheduling algorithms briefly characterise the algorithm and complete the table below: (i) Round Robin (time slice = 3); (ii) Shortest Process Next; (iii) Highest Response Ratio Next [5 marks] Fully explain how you got your answer with the help of a table on graph paper. [5 marks] Proc arrival time service time wait time Turnaround NTT Ratio 0 0 6 1 2 6 2 3 2 (c) Briefly define and give examples of the following categories of criteria for evaluating scheduling algorithms: User oriented, system oriented, performance related, non-performance related. [8 marks] Briefly explain what is meant by ‘processing burst time’ of a process. [3 marks] [Total: 25 marks]
- (a) Why is the performance of disk access seen as highly important? [2 marks]
(b) For the following disk track request queue, diagram and calculate the head movement required under TWO of the following disk scheduling algorithms. Request queue: 27 129 110 186 147 41 10 64 120 Where necessary assume that the disk head is initially at track 100, is moving toward track 0, and maximum track number is 200. [6 marks] (i) First-in First-out; (ii) C-Scan; (iii) Look. For both algorithms chosen, briefly discuss their fairness and efficiency. [4 marks]
(c) Fully describe the three methods of disk space allocation: contiguous, linked (or chained), and indexed. [6 marks] With the aid of a diagram, fully describe (^) the method of disk space allocation used in UNIX. Include a brief discussion^ of its advantages and disadvantages in your description. [7 marks] [Total: 25 marks]
