CSCI-UA.0202-002: Operating Systems Final Exam 2020

Important Notes- READ BEFORE SOLVING THE EXAM

•    If you perceive any ambiguity in any of the questions, state your assumptions clearly and solve  the problem based on your  assumptions. We will grade both your solutions and your assumptions.

•    This exam is take-home.

•    The exam is posted, on NYU classes, at 10am EST of Wed Dec 16th.

•    You have up to 23 hours and 55 minutes to submit on NYU classes.

•    You are allowed only one submission, unlike assignments and labs.

•    Your answers must be very focused. You may be penalized for wrong answers and for putting irrelevant information in your answers.

•    This exam contains five problems, with a total of 100 points.

•    Your answer sheet must be organized as follows:

-      The very first page of your answer must contain:

. You Last Name

. Your First Name

. Your NetID

. Copy and paste the honor code showed in the rectangle at the bottom of this page.

-     In your  answer sheet, answer one problem per page (even if your answer takes one line).

Honor code (copy and paste to the first page of your exam)

Problem 1

Suppose we have a system that implements virtual memory as a two-level page table. The processor is single core (i.e. single CPU). When a process wants to access the memory,

the MMU takes the address from the CPU and divides the bits of that address into several fields, as we saw in class.

a)   [2 points] Is the address generated by the CPU a physical address or virtual address?

b)   [8 points] How many fields does the MMU need to divide the address into? What are these fields?

c)   [4 points] Which field(s) is/are used to access the TLB?

d)   [4 points] If we have eight cores in the system instead of one, which of your answers in a, b, and c above will change, if any, and why?

e)   [2 points] If we have a system with eight cores, how many MMUs dowe have?

Problem 2

Answer the following questions about virtual memory.

a)   [5 points] Can a page fault in one process result in a page fault in another process in the future? Explain.

b)   [5 points] What is/are the piece(s) of hardware that must  exist for a system to support  virtual  memory  using  pages?  For  each  piece  you  cite,  explain  what happens if it this piece did not exist. If your answer is that no hardware support is needed and the OS can take care of the whole virtual memory scheme, state, in a bulleted  list,  the  steps  that  will  be  done  by  the  OS  from  the  time  the  CPU generates the virtual address till the physical address is ready.

c)   [10 points] A  2D integer array, A[64][64],   is  stored  in memory in row-major order (i,e, A[0][0] is followed by A[0][1], then A[0][2],  … etc). Which of the following two pieces of code results in more page faults? Justify your choice. Assume the whole physical memory can hold only four pages and that the size of each page is 512 bytes.

Problem 3

[20 points]

Suppose you wrote a multithreaded program where you spawn x threads. During execution time we may see less than x threads, from that process, executing in parallel. That is, as a programmer, you want, for example, five threads to be executing in parallel. But, during execution, you see that there are less than five threads really executing. State five scenarios that may cause this to happen.

Problem 4

[20 points]

Suppose we have p processes in the system. Each one of the p processes requires m instances of resource R. What is the minimum number of instances of resource R (e.g. if R is a printer, then how many printers) that we need to have to ensure no deadlock? Give an answer in terms of m and p and explain your logic how you reached this conclusion.