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\markright{\footnotesize CS 481 Operating Systems Principles, Fall 2000}

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\section*{Homework 4 --- due on Thursday 5 October 2000}

Total number of points available on this homework is 100. Full credit
is equivalent to 100 points.

\begin{enumerate}


\item (10 pts) \textbf{Synchronization}.  Basic definitions.
\begin{enumerate}
\item What is mutual exclusion?
\item What is an atomic operation?
\item What is a semaphore?
\item What is busy waiting?
\end{enumerate}



\item (10 pts) \textbf{Synchronization.} What advantages does
  the test\&set instruction have over enabling and disabling
  interrupts?  In which circumstances may we still prefer enabling and
  disabling interupts?



\item (10 pts) \textbf{Synchronization.}  Suppose you have a
process with two threads of execution in it.  In general, what code
should you place in a critical section?  How would you ensure that only one
thread at a time was in the critical section?



\item (20 pts) \textbf{Synchronization.} Making Water.
  You need two hydrogen atoms (H), and one oxygen (O) to make
  water.  Write algorithms that generate water as soon as the
  atoms are available.  You do not have to submit actual code (although
you may).  Pseudocode is sufficient.

Assume that there are two threads that are making atoms, one for hydrogen
and one for oxygen.  Each thread has its own queue in which it puts its
newly created atoms.  A third thread reads atoms off the queues and makes
water out of them.

\begin{enumerate}
  \item Solve the problem using semaphores.
  \item Solve the problem using monitors.
\end{enumerate}

\item (25 pts) \textbf{Semaphores.}  Suppose a building has a
limit on the number of people that may be in it at one time,
because of fire code.  Suppose this is a very popular place to visit, so
the number of people inside must be monitored closely.  Further,
suppose that this building has more than one entrance and exit.
Construct an algorithm that can be used to control a set of turnstiles 
so that the room can 
be filled but is never allowed to exceed its legal capacity.

Assume that each person is a thread that can either request to enter or
to leave.  The algorithm is a common piece of code and data shared by
all person-threads.  As before,
pseudocode is sufficient.

\item (25 pts) \textbf{Monitors.} Consider a family with three children
(Bobby, Suzy, and Pat).  Each of the three children received one
critical piece of ski equipment from their grandmother (Bobby has
boots, Suzy has skis, and Pat has poles).  However, to go skiing, a
child needs a full set of equipment.  Each
morning the parents put
out two distinct pieces of equipment (either skis and boots, skis and
poles, or boots and poles), and the child with the remaining piece of
equipment can then go skiing.  

Write monitor code for Bobby, Suzy, and Pat that will ensure that one
child skis everyday (i.e., there are no deadlocks).  You may assume a
random number generator that determines which of the two items the
parents puts out every morning.  Make the Lock\_Acquire() and
Lock\_Release() explicit in your code.

Assume separate threads for Bobby, Suzy, Pat, and another thread
for the parents.
Bobby, Suzy, and Pat will use the same code.  All four threads will use
the same data structures.
As before, pseudocode is sufficient.

\end{enumerate}

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