UNIVERSITY OF HOUSTON
                 Department of Computer Science
          COSC 4330-- Fundamentals of Operating Systems
                          FIRST MIDTERM
                        February 28, 1994

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

1.   Why is it necessary to have a supervisor  mode  and  a  user
     mode to protect user files against unauthorized access?  (10
     points)  Explain how this mechanism can be defeated  if  the
     computer lacks memory protection.  (10 points)

2.   Complete the following program fragment in such a  way  that
     all   stderr  output will be redirected to the file logfile.
     (10 points)

             int fildes;
             fildes = open("logfile", O_CREAT | O_WRONLY);

3.   What is the purpose of the ready list ? (10 points)  Why  is
     it  often organized as a priority queue instead of a regular
     FIFO queue? (10points)

4.   Describe the states visited  by a process from the moment it
     issues  an  I/O  call  to  the moment it regains the CPU and
     explain in a few words each state transition. (15 points)

5.   What is the major disadvantage of busy waits ?  (10  points)
     Describe one case where they are nevertheless the best solu-
     tion. (5 points)

6.   Consider the following two program fragments,  which  access
     concurrently  the  same  shared  variable  count and add the
     necessary semaphore  calls  to  guarantee  mutual  exclusion
     within the critical sections of both programs (20 points)

     Global Variables:
                             int count = 0;
                             semaphore ____________ = _____;
                             semaphore ____________ = _____;

     Program Fragments:

     increment (int delta){                  int collect (){
                                                     int tmp;
             ___________________                     ___________________
             count = count + delta;                  tmp = count;
             ___________________                     ___________________
             printf("0one!
             ___________________                     ___________________
                                                     return(tmp);
                                                     ___________________
     } /* increment */                               } /* collect */


--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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