UNIVERSITY OF HOUSTON
                 Department of Computer Science
                                
           COSC 4330-Fundamentals of Operating Systems
                        FINAL EXAMINATION
                           May 6, 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 correct or incorrect (2 points), and why
     (3 points).

     a)   There are some problems that can be solved using semaphores
          and cannot be solved using monitors.

          FALSE: SINCE YOU CAN EMULATE SEMAPHORES WITH MONITORS THEY
          ARE EQUALLY POWERFUL

     b)   It  is impossible to prevent deadlocks in client-server
          programs.

          TRUE: NONE OF THE FOUR DEADLOCK PREVENTION METHODS WOULD
          WORK
         
     c)   VMS stores process page tables in main memory.
          
          FALSE: PROCESS PAGE TABLE ARE STORED IN THE SYSTEM'S
          VIRTUAL MEMORY          
   
     d)   UNIX stores the name of a file in its i-node.

          FALSE: FILE NAMES ARE STORED IN DIRECTORIES (BECAUSE
          OTHERWISE FILES COULD NOT HAVE MULTIPLE NAMES)



2.   A  process has to send a message through either one  of  two
     channels, one that is cheap but very busy and the other that
     is faster but also more expensive.  Assuming that
     
     a) several processes will be competing for these channels and

     b) each process using a channel must have exclusive access to
        that channel,

     implement  a  policy  where the cheap  channel  is  selected
     whenever  its queue length is lesser than MAXQ and the  fast
     channel is selected otherwise :

     Global Declarations:

     integer cheap_queue_length = 0;
     semaphore cheap_channel = 1;
     semaphore fast_channel  = 1;
     semaphore access        = 1;

     Process Code: (5×4 points)
     ___________________________________
     if(cheap_queue_length < MAXQ) {
        /* pick cheap channel */
        __________________________________
        cheap_queue_length++;
        __________________________________
        P(&cheap_channel);
        __________________________________
        cheap_queue_length--;
        __________________________________
        cheap_send(..);
        V(&cheap_channel);
        __________________________________
     } else {
        /* pick fast channel */
        __________________________________
        P(&fast_channel);
        __________________________________
        fast_send (..);
        V(&fast_channel);
     } /* if */

3.   Fragmentation:
    
     a)   What is the difference between internal and  external
          fragmentation? (5 points)

          (SEE NOTES)


     b)   Which of them should be avoided the most and why?  (5
          points)

	  EXTERNAL FRAGMENTATION BECAUSE INTERNAL FRAGMENTATION
	  ONLY WAISTES AN AVERAGE OF HALF A PAGE PER PROCESS

4.   Describe in some detail the VMS page replacement policy.(10
     points: minus 3 points if there is no drawing)

     (SEE NOTES:  TEH VMS POLICY IS THAT WITH FIXED PARTITIONS
      AND A SINGLE GLOBAL QUEUE)

5.   A  virtual  memory system has a virtual address space  of  4
     Gigabytes  and a page size of 4 kilobytes.  Each page  table
     entry occupies 4 bytes. (2x5 points)

     a)   How many bits are used for the page number?  20 bits
       
     b)   What is the maximum size of a page table?  2^22 pages
       
6.   What  are  memory mapped files? (5 points)   What  is  their
     major advantage? (5 points)

     (SEE NOTES:  WITH MEMORY MAPPED FILES, THE FILE CONTENTS ARE
     DIRECTLY ACCESSIBLE TO THE PROGRAM)

7.   What is the major advantage of access control lists over
     tickets? (5 points)
     
     IT IS MUCH EASIER TO REVOKE ACCESS FOR SOME BUT NOT ALL USERS
     (BUT TICKETS ALLOW FOR MUCH FASTER ACCESS CONTROL)

8.   Are the following statements true or false for the three
     following file allocation methods? (5 points per correct line;
     no partial credits)
     
                                                         
     This file allocation      Sequential   Linked    Indexed
     method                        
                                                         
     handles well direct         T          T         T    
     access
                                                         
     is used by UNIX                  F          F    T     
                                                         
     avoids external                  F     T         T    
     fragmentation
                      
--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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