COSC 4330                        QUIZ #2          November 15, 1995


CLOSED BOOK.  You can have one sheet of notes.  Each question is worth
20 points.

1.   What is wrong with the following solution to the mutual exclusion
     problem for two processes:
          
          1.   int tie_breaker;
          2.   int want[2]= { 0, 0};
          
          3.   void enter_region(int pid)  // pid must be 0 or 1
          4.   {
          5.        want[pid] = 1;
          6.        tie_breaker = pid;
          7.        while (want[1-pid] && tie_breaker != pid);
          8.   } // enter_region
          
          9.   void leave_region(int pid)  // pid must be 0 or 1
          10.  {
          11.       want[pid] = 0;
          12.  } // leave_region
          
     ___  it does not guarantee mutual exclusion under all
          circumstances

     ___  it may lead to deadlocks

     ___  it may prevent a process to enter its critical section
          even though no other process is accessing the shared
          data

2.   To avoid this problem, line number ____ should be replaced
     by
     
     
3.   What makes an instruction such as the test and set 
     instruction (TSET) discussed in class especially useful 
     for writing user-level solutions to the mutual exclusion
     problem?

4.   What is the major advantage of kernel-supported semaphores?

5.   Rewrite the following fragment of monitor code assuming that all
     signal operations in the monitor are replaced by notify
     operations:
     
     if (nfullslots == 0)     
          notempty.wait;
     take_item_from_buffer(&item);
     nfullslots--;
     notfull.signal; 
--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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