UNIVERSITY OF HOUSTON
                 Department of Computer Science

                   COSC 6360-Operating Systems
                          FINAL PART I
                        December 10, 1990


Be concise but complete. The examination is closed text  but  you
can have four pages of notes.

1.   What conditions must an encryption function  fulfill  to  be
     usable  for  digital signatures and public-key cryptosystems?
     (10 points)

     THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE
     READING LIST.  IT WILL BE NOT BE ASKED AGAIN.

2.   A time-sharing system has 20 terminals.  The  average  think
     time  is  8 seconds.  The average disk access time is 25 ms.
     Each user job requires on the average 750 ms of CPU time and
     24 disk accesses.

     The CPU utilization  are  respectively  75%  and  60%.   (10
     points)

     (a)  What is the disk utilization?        
     (b)  What is the system's throughput?   obs/s
     (c)  What is the system's response time?  

     (d)  What is the absolute maximum for  the  system's 
          throughput? obs/s

     (e)  What is the absolute minimum for the system's response 
          time?
     
     THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE
     READING LIST.  IT WILL BE NOT BE ASKED AGAIN.

3.   Sketch the solution proposed by Tanenbaum et  al.  to  build
     object  capabilities  that  can be easily downgraded without
     going back to the server.  (10 points)

4.   What is the difference between  optimistic  and  pessimistic
     replication strategies?  (10 points)

5.   What  are  the  advantages  and  disadvantages  of  Sprite's
     delayed-write policy?

6.   What are the advantages and disadvantages of Andrew's  call-
     back policy?

                    Department of Computer Science

                     COSC 6360-Operating Systems
                            FINAL PART II
                   OS QUALIFYING EXAMINATION PART A
                          December 10, 1990

Be concise but complete. The examination is closed text  but  you
can have four pages of notes.

1.   Are local disks back to stay?

     While the workstations of the previous generations were gen-
     erally  diskless,  today's  workstations often come packaged
     with a local hard disk.

     (a)  What technical reasons can we give for that change? ( 8
          points)

     (b)  What technical advances are needed before  the  balance
          swings  again  in  favor  of  diskless workstations? (6
          points)

     (c)  What  is  the  likelyhood  of  having  these   advances
          achieved  and  implemented  in  the  near  future?   (6
          points)

2.   File System Security:

     Sketch a protection mechanism for a network file system that pro-
     vides the same amount of protection as the current UNIX file sys-
     tem but would operate  correctly  in  environments  where  client
     sites  cannot  be trusted and messages can be easily intercepted.
     (You don't have the right to assume the existence of any  special
     hardware, including Amoeba's F-boxes.) (20 points)