Operating Systems--Spring 2011

    Home | | Schedule | | Assignments | | Lecture Notes

    Homework 1

    Due: Thursday, February 17, in class

    Answer the following questions. Your answers should be typed.

    Question 1. Briefly define each of these terms (one or two sentences each):

      a. Operating System
      b. interrupt
      c. system call
      d. kernel
      e. cache
      f. argv
      g. argc
      h. multiporgramming.
      i. DMA
      j. Virtual Machine

    Question 2. When using a memory cache, one can define a "hit ratio", H, as the fraction of memory accesses that are found in the cache versus in the main memory. Using the hit ratio and the speeds of memory access for the two different memory types, one can calculate the average time to access a byte. If the speed of the faster memory (cache) is given by T1 and the speed of the slower memory is given by T2, then the average access time is given by:

      T = H * (T1) + (1 - H)(T1 + T2)

    a) Suppose a computer's cache has an access time of 10 ns, and the access time for main memory is 100 ns. Compute the average access time for the following hit ratios: 0.2, 0.5 and 0.95.

    b) What do you notice about the access time as the hit ratio increases?

    c) One way to increase the hit ratio is to increase the size of the cache. Why is it not a good idea to have a very large cache?

    Question 3. What is the purpose of system calls? How do system calls relate to the OS and to the concept of dual mode (monitor mode and user mode) operation?

    Question 4. Which of the following instructions should be priveleged? Briefly justify your answer.

      a. Set the value of the timer
      b. Read the clock
      c. Clear memory
      d. Turn off interrupts
      e. Switch from user to monitor mode

    Question 5. Some early computers protected the operating system by placing it in a memory partition that could not be modified by either the user job or the operating system itself. Describe what problem(s) you think could arise from such a scheme.

    Question 6. What is the purpose of the command interpreter? Why is it usually separate from the kernel?

    Question 7. What is a PCB? List five items that are (usually) maintained in a PCB and briefly state the purpose of each item.

    Question 8. Describe the difference among short-term, medium-term and long-term scheduling. Which scheduler will execute the most often? Which scheduler will execute the least often?

    Completed homework:

    • Turn in your typed answers to the homework questions by Thursday, February 17 in class.

    Home | | Schedule | | Assignments | | Lecture Notes

    Constance Royden--croyden@mathcs.holycross.edu
    Computer Science 346--Operating Systems
    Date Created: January 9, 2004
    Last Modified: January 7, 2011
    Page Expires: January 8, 2012