Operating Systems Tutorial 3 University of Wales, Cardiff

This tutorial is not assessed, but it is in your interest to attempt it, because SOON there will be one that is.

Cars coming from north and south must pass over a bridge across a river. Unfortunately there is only one lane on the bridge. At any time there can be one or more cars travelling in the same direction on the bridge. Write an outline of procedures (as pseudo-code) which must be called by a process representing a car as it enters and leaves the bridge.

You will require four procedures :

Narrive - North bound car arriving
Ndepart - leaving bridge
Sarrive - South bound car arriving
Sdepart - leaving bridge

Southerly car : 
Sarrive();
  cross
Sdepart()
		
Northerly car :
Narrive();
 cross;
Ndepart

Give an implementation of the procedures using semaphores to provide synchronisation
Hint : use counts for north and south bound cars and semaphores to give mutual exclusion to the counts. Another semaphore is needed to indicate the bridge is free

Discuss the fairness of your solution.

Omer Rana
March 1997.

 
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