UNIVERSITY OF HOUSTON
                 Department of Computer Science
                                
          COSC 4330--Fundamentals of Operating Systems
                        FINAL EXAMINATION
                          May 16, 1994

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

1.   A computer has 32 bit addresses and a page size of 4
     Kbytes.(2x5 points)
     
     (a)    How many bits are used to represent the page number ?  

     12 bits because 2**12 = 4096
     
     (b)    If each page table entry occupies four bytes, what is the
            maximum size of a process page table?  

     A process address space has 2**32 bytes that is 2**32/2**12=2**20
     pages.

     Since each page table entry occupies 4 bytes, the maximum size of
     a page table is 2**20*4=2*22= 4 Megabytes.

2.   Which of the following statements apply to these five page
     replacement policies:
     
     FIFO      LRU       Clock       VMS      Denning's Working Set
     
     rarely expels the right page:  FIFO
     
     is a good approximation of LRU: Clock  (and maybe VMS)
     
     cannot be implemented efficiently: LRU & Denning's WS
    
     is neither truly local nor truly global: VMS
    
     (20 points)
 
3.   Why is it difficult to prevent the circular wait condition
     in message passing systems? (10 points)
     
Preventing the condition would require all messages to move in one
single direction.

4.   A binary semaphore is a semaphore than can only have two
     values, namely zero and one. Write the two P(...) and V(..)
     procedures necessary to implement a regular semaphore using
     only binary semaphores assuming that the initial value of
     the regular semaphore is always zero. (20 points)
     
     Your global declarations (with their initial values):
     

     Your two procedures:
     
5.   UNIX semaphores are identified by a 32 bit key that uniquely
     identifies the semaphore.  What is the  major disadvantage
     of this solution? (5 points)  Do you have a better solution?
     (5 points)

There is one single key space shared by all users, which makes it
difficult to know if a given key value is not already used by
some application.  A better solution would be to associate file
names with semaphores:  two different users could use the same
semaphore names in different directories.

6.   What are the factors to consider when selecting the page
     size of a virtual memory system? (10 points)

(1)  Large page sizes reduce the overall number of page faults
     and the number of page table entries in each page table;

(2)  Small page sizes minimize internal fragmentation and require
     less main memory space.

(3)  There no point however in increasing page sizes over 4
     or 8 kilobytes since it would then be likely that most of the
     information brought into main memory would  be useless.
     
7.   What are memory mapped files? (5 points)  What is their
     major advantage? (5 points)

Memory mapped files are files that are mapped into the virtual memory
space of the processes accessing them.  Once a file is mapped, it
becomes part of the address space of the process.

The major advantage of memory mapped files is than their contents
can be accessed through standard programming constructs: Hence
standard assignments can replace replace reads and writes, which
reduces the number system calls and context switches.

8.   What are the advantages and disadvantages of contiguous
     allocation of file blocks in term of  (a) access times and
     (b) efficient utilization of the disk space? (2x5 points)
     
(a)  Contiguous allocation provides the fastest access of all disk
     allocation policies;

(b)  it will cause EXTERNAL FRAGMENTATION and does not use disk
     space efficiently.
    
--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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