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Theoretical Questions

Problem D: Theoretical Questions (from the course text book)

General

1. What is the difference between kernel mode and user mode? Why is the difference important to an operating system?
2. What is multiprogramming?
3. Which of the following instructions should be allowed only in kernel mode? Why?
a. Disable all interrupts.
b. Read the time-of-day clock.
c. Set the time-of-day clock.
d. Change the memory map.
4. Why is the process table needed in a timesharing system? Is it also needed in personal computer systems in which only one process exists, that process taking over the entire machine until it is finished?
Processes

5. Why is multiprogramming central to the operation of a modern operating system?
6. What are the three main states that a process can be in? Describe the meaning of each one briefly.
7. What is the fundamental difference between a process and a thread?
8. In a system with threads, is there normally one stack per thread or one stack per process? Explain.
9. What is a race condition? Give an example of a race condition that could possibly occur when buying airplane tickets for two people to go on a trip together.
10. Give a sketch of how an operating system that can disable interrupts could implement semaphores.
11. Show how counting semaphores (i.e., semaphores that can hold an arbitrarily large value) can be implemented using only binary semaphores and ordinary machine instructions.
12. Round-robin schedulers normally maintain a list of all runnable processes, with each process occurring exactly once in the list. What would happen if a process occurred twice in the list? Can you think of any reason for allowing this?ƒ
Memory management

13. Consider a swapping system in which memory consists of the following hole sizes in memory order: 10 KB, 4 KB, 20 KB, 18 KB, 7 KB, 9 KB, 12 KB, and 15 KB. Which hole is taken for successive segment requests of
a. 12 KB
b. 10 KB
c. 9 KB
for first fit? Now repeat the question for best fit, worse fit, and next fit.
14. A computer has 1 GB of RAM allocated in units of 64KB. How many KB are needed if a bitmap is used to keep track of free memory?
15. Now revisit the previous question using a hole list. How much memory is needed for the list in the best case and in the worst case? Assume the operating system occupies the bottom 512KB of memory.
16. What is the difference between a physical address and a virtual address?
17. If an instruction takes 1 nsec and a page fault takes an additional n nsec, give a formula for the effective instruction time if page faults occur every k instructions.
18. A computer with a 32-bit address uses a two-level page table. Virtual addresses are split into a 9-bit top-level field, an 11-bit second-level page table field, and an offset. How large are the pages and how many are there in the address space?
19. Are there any circumstances in which clock and second chance choose different pages to replace? If so, what are they?
20. Suppose that a computer uses the PFF page replacement algorithm but there is sufficient memory to hold all the processes without page faults. What happens?
I/O

21. Explain what DMA is and why it is used.
22. An alternative to interrupts is polling. Are there any circumstances you can think of in which polling is a better choice?
23. Disk controllers have internal buffers and they are getting larger with each new model. Why?
24. Why do operating system designers attempt to provide device-independent I/O wherever it is possible?
25. Why are output files for the printer normally spooled on disk before being printed?
File systems

26. NTFS uses Unicode for naming files. Unicode supports 16-bit characters. Give an advantage of Unicode file naming over ASCII file naming.
27. Systems that support sequential files always have an operation to rewind files. Do systems that support random access files need this too?

28. Some operating systems provide a system call rename to give a file a new name. Is there any difference at all between using this call to rename a file, and just copying the file to a new file with the new name, followed by deleting the old one?
29. Consider the following directory tree:

If /usr/jim is the working directory, what is the absolute path name for the file whose relative path name is ../ast/x?
30. Consider the following proposal. Instead of having a single root for the file system, give each user a personal root. Does that make the system more flexible? Why or why not?
31. Contiguous allocation of files leads to disk fragmentation. Is this internal fragmentation or external fragmentation? Provide an analogy.
32. Free disk space can be kept track of using a free list or bitmap. Disk addresses require D bits. For a disk with B blocks, F of which are free, state the condition under which the free list uses less space than the bitmap. For D having the value 16 bits, express your answer as a percentage of the disk space that must be free.

III. Submission

For problems A, B and C: –

You will have to hand in a report showing the design, implementation and testing of your solution. In addition, each member of the team must include a breakdown of what their contributions were. For the maximum marks allocated to each team member, it is best that all members work on all aspects of the design and implementation rather than electing one person.
Submit a copy of the Minix virtual machine on CD along with the report.
Demonstrate your code to your lab tutor to the highest specification achieved by your implementation.

For problem D (Theoretical questions):

You will have to answer 15 questions of your own choice (3 per section) as fully as possible including diagrams and examples. Any additional reading can bring extra marks.
All reports have to be submitted in electronic form into the drop box on the NOW portal as well as a physical copy to the student desk.

IV. Assessment Criteria

Each problem A, B, C or D solution will be marked from 0 to 100% according to the following criteria:

Problem A “ Marking Criteria

This problem is the most difficult one of the programming problems “ creating a program according to the specification carries a mark of up to 80% – any implemented features not included in the specification carry extra marks above the 80% mark.

For 40% – the program must be designed properly with the necessary structures in place.

For 50% – in addition the program must compile and some initial correct results obtained and demonstrated.

For 60% – in addition most of the program works and shows results but there are unidentified errors preventing the use of the program by other processes.

For 70% – in addition most of the program works, some minor errors or deficiencies

For 80% – program works as specified in the assignment.

For > 80% – any extra feature not mentioned in the assignment and relevant to the semaphores domain carries extra marks at the discretion of the lab tutor.

Problems B & C“ Marking Criteria

Solving this problem and creating a program according to the specification carries a mark of up to 70% – any implemented features not included in the specification carry extra marks above the 70% mark.

For 40% – the program must be designed properly with the necessary structures in place.

For 50% – in addition the program must compile and some initial correct results obtained and demonstrated.

For 60% – in addition most of the program works and shows results but there are unidentified errors preventing the use of the program by other processes.

For 70% – program works as specified in the assignment.

For > 70% – any extra feature not mentioned in the assignment and relevant to the corresponding B or C problem domain carries extra marks at the discretion of the lab tutor.

Problem D “ Marking Criteria

The solution to this problem is the easiest route to take in this assignment since no programming is required. Hence, the top mark for standard answers to the questions “ i.e. full set of correct answers on the basis of the lecture material (lecture notes or lecture note™s explanations) “ carries a top mark of 60%. Any demonstrable and relevant external reading will carry extra marks.

Each of the 15 questions carry equal marks of 4 “ answer the questions as fully as possible. Extra marks will be awarded for additional information which goes beyond the lecture notes material but this will be at the discretion of the tutor marking the work and only up to a total of 60%.

V. Feedback Opportunities

Formative (Whilst you™re working on the coursework)
You will be given the opportunity to receive informal verbal feedback from your lab tutor regarding your coursework development during the practical sessions.

Summative (After you™ve submitted the coursework)
You will receive specific feedback regarding your coursework submission together with your awarded mark when it is returned to you. Clearly, feedback provided with your coursework is only for developmental purposes so that you can improve for the next assessment or subject-related.

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