COSC 4330                    FINAL                             8/11/98

Closed Book.   You can have one sheet of notes.

1.   For each of the  statements below, indicate  in  one  sentence
     whether or not the statement is true or false, and why (5 points
     each):
     
     a)   There is no easy way to implement the at most once semantics in
          remote procedure calls.

	  FALSE.  You just need to add a unique ID to each client request.
     
     b)   If your computer has a test and set instruction, you do not need
          semaphores to avoid busy waits.

	  FALSE. All user level solutions require busy waits.

     c)   One page fault every five hundred references is an acceptable
          page fault rate for a virtual memory system.

          FALSE.  An acceptable page rate is one fault each 50,000 to
          200,000 references.

     d)   The LRU policy always expels the page that has been the least
          frequently referenced since it was brought into main memory.

	  FALSE.  Itexpels the page that has been the least recently 
          referenced since it was brought into main memory.

     e)   There is no efficient way to pass huge arrays in a remote
          procedure call.

	  TRUE.  You must send a copy of each array.

     f)   Dynamic address translation is normally done by the software.

          FALSE.  It is done by the Memory Management Unit.

  
2.   A computer has 32 bit addresses and a page size of one
     kilobyte.(2×5 points)
     
     a)   How many bits are used to represent the page number ? _22_ bits
          
     b)   What is the maximum number of entries in a process page table?
          __2^22= 4 M = around 4 million___ entries

3.   What  is  the  difference  between  internal  fragmentation   and
     external fragmentation? (10 points)  Which of them wastes more memory
     than the other and what can be done about it?  (2×5 points)

     Internal fragmetnation happens in paged virtual memory systems and
     wastes an average of  one half a page frame per process.

     External fragmentation happens in ssytems using variable partitions
     to manage their memory.  It results from the fact that free blocks
     tend to become very small because each process occupying the memory
     space freed by another process cannot normally occupy all the freed
     space. 

     The only real solution to memory fragmentation is compaction. It
     involves merging all free blocks into a single large free block by
     moving around some of the processes.

     Its most serious disadvantage is the cost of moving processes around,
     which is a function of  their sizes and, ultimately, of the size of
     the main memory.  Hence, memory compaction is a much lesser attractive
     solution today than it was when main memories were much smaller.


4.   Compare and contrast semaphores and monitor conditions? (2×5 points)

     Unlike semaphores monitor conditions have NO VALUE.

     Hence, a signal operation on a monitor condition will have NO EFFECT
     unless there is a process waiting on that condition.  This is NOT TRUE
     for V() operations on semaphores.

5.   What are the major advantage and disadvantage of the FIFO  page
     replacement policy? (2×5 points)  Name a page replacement policy that
     uses FIFO and explain what it does to correct the deficiencies of that
     policy. (2×5 points)

     FIFO is very easy to implement (and cheap to run) but often expels the
     wrong page.

     The Mach page replacement uses global FIFO but puts expelled pages at the
     tail end of a global queue from which they can be reclaimed by processes 
     trying to access them.

     OR

     The VMS page replacement assigns a fixed partition to each process and
     manages each of them using a FIFO policy. Expelled pages are put at the tail
     end of a global queue from which they can be reclaimed by processes trying
     to access them.