UNIVERSITY OF HOUSTON
                 Department of Computer Science

          COSC 4330: Fundamentals of Operating Systems
                        FINAL EXAMINATION
                        December 15, 1992

CLOSED BOOK: You are allowed one 8.5 by 11" sheet of notes.

1.   What is a TRANSLATION LOOKASIDE BUFFER (10 points)?  What is
     the major difference between a page table entry and a trans-
     lation lookaside buffer entry? (5 points)

A translation lookaside buffer (TLB) is a set of hardware registers
containing the most recently used page table entries.  It is searched
first by the memory management unit before consulting the page table
of the current process.

In addition to the information stored in the page table entry it is
caching, each TLB entry must also contain a PAGE NUMBER.  (This
information is not needed in the page table entry because it is
implicitly indicated by the location of the entry in the page table).

2.   What was the motivation for adding  a  second  hand  to  the
     Clock page replacement algorithm?  (10 points)

 When main memory sizes started to grow in the mid-eighties, it was
found that the hand of the clock was taking too much time to scan the
whole main memory.  Hence, more recent UNIX implementations have added
a second hand that follows the first hand at a fixed angle.  Now the
first hand clears valid bits and the second hand expels the pages that
have not been accessed by the process after their valid bit had been
cleared by the first hand.

3.   Explain why the VMS page replacement algorithm is neither  a
     true local nor a true global algorithm. (10 points) What are
     the major advantages of this approach? (2x5 points)

The VMS page replacement policy is neither a global nor a local
policy because

(a)     VMS allocates to each process a fixed-size partition that
	it manages using a FIFO policy, and

(b)     the pages expelled by the FIFO policy are put at the end
	of a large global queue from which they can be reclaimed.

The two major advantages of the VMS policy are:

(a) 	it provides efficient support for real-time processes
	because VMS can allocate to each real-time process
	enough page frames to keep the whole address space of the
	process in main memory and guarantee that none of its pages
	would ever be expelled.

(b)	it achieves the almost the same overall performance as
	LRU without requiring any special hardware.

4.   What characterizes all processes that run efficiently  on  a
     virtual memory system? (10 points)

All processes that run efficiently on a virtual memory system obey
to the principle of locality, that is they access at any time
a small fraction of their total address space and this fraction does
not change too frequently.

5.   What is the purpose of the dirty bit? (5 points)

The purpose of the dirty bit is to keep track of which pages have been
modified since they were brought into main memory and which pages were
not.  Pages that have been modified since they were brought into main
memory are said to be "dirty" because they need to be copied back to
disk before being expelled from main memory; the other pages are said
to be "clean" because their copy on disk is still valid.

6.   What are  memory mapped files? (10 points)  What  are  their
     main  advantage  (4  points)  and  their  disadvantages (3x2
     points) ?

A mapped file is a file that is mapped into the virtual memory
space of the processes accessing it.  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.

Four major disadvantages are: 

(a)	The OS will never know the exact file size; it knows only
	the number of pages in the file.

(b)	If two or more processes on the same machine access one
	file at the same time, OS must map the file into a shared
	memory segment.

(c)     Shared access by two or more processes on different
	machines is even harder to implement.

(d)	Some files are too large to be mapped into a 32 bit
	virtual address space.

7.   What are the  contents  of  a  UNIX  directory  entry?  (2x5
     points)

A UNIX directory entry only contains a file name and an i-number,
that is the index of the i-node describing the file in the table
of i-nodes of the disk partition.

8.   What information is contained in the access control list  of
     a UNIX file?  (10 points)

The UNIX access control list of a file contains the three 
possible rights (read, write or execute) of three groups of
users (the owner of the file, all users specified in the
"/etc/group" entry associated with the group-ID of the file
and all other users).

--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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