UNIVERSITY OF HOUSTON
                 Department of Computer Science
                                
           COSC 4330—Fundamentals of Operating Systems
                       MIDTERM EXAMINATION
                          March 4, 1996

This exam is closed book.  You can have one sheet of notes.

1.   For  each  of the statements below, indicate in one sentence
     whether the statement is true or false (2 points), and why (3
     points).
     a)   Memory protection prevents user processes from directly
          accessing the file system.

     b)   Most scheduling policies decrease the priority of processes
          that have exhausted their slice of CPU time.
          
     c)   Most  servers process all incoming requests in a purely
          sequential fashion.
          
     d)   Streams are a special kind of virtual circuits.
2.
     Which system calls should you use to write:

     a)   one program executing as two processes? (5 points)
     
     b)   two programs executing as a single process? (5 points)   
     
3.   What  are  the  major  advantage (5 points)  and  the  major
     disadvantage (5 points) of kernel-supported threads?

4.   What is the main difference between the at most once and the
     all or nothing semantics in remote procedure calls?  (5 points)
     Which one is the easiest to implement and why?  (5 points)

5.
     What are the major disadvantages of:

     a)   non-preemptive scheduling policies?  (5 points)               
               
     b)   the round-robin policy?  (5 points)
                
               
6.   Consider the following main program fragment:

                    int alpha;
                    alpha = 1;
                    modify(&alpha,&alpha);
                    printf("result = %d", alpha);

     and the following procedure:

                    modify(int *pone, *ptwo)
                    {
                                   (*pone) = (*pone)*3;
                                   (*ptwo) = (*ptwo)*2;
                    } // modify

     What would the program print out:

     a)   if modify was called through a regular procedure call? (5
          points)
          
     b)   if modify was called through a remote procedure call? (5
          points)
          
           
7.   What are the major differences between blocking and non-blocking
     send primitives? (10 points)

8.   When  is  it  a  good idea to disable interrupts  to  ensure
     mutual exclusion?  (2×5 points)

9.   Fill  the  missing  parts  of this solution  to  the  mutual
     exclusion problem:  (10 points)

     #define FALSE 0
     #define TRUE  1
     
     int can_go;    // process that can go
     int beg[2];
     beg[0] = beg[1] = _____________;
     
     enter_region(int me) {
       int other; // pid of other process
       other = 1 - me; // 0 if 1 and 1 if 0
       beg[me] = TRUE;
       can_go = other;
       while (beg[other]&&________________________________);
     } // enter_region
     
     leave_region(int me)
     {
       ____________________________________________________;
          } // leave_region
                    
       ____________________________________________________;
     } // leave_region

--IMPORTANT----IMPORTANT----IMPORTANT----IMPORTANT----IMPORTANT--
Please do not try to answer these questions by yourself BEFORE
having a clear idea of what's on the test for which you are
studying.

Materials covered in each test DO VARY from semester to semester
because the pace of the course can change, some materials are be
added and other deleted almost every semester.
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