Lecture 7
HOMEWORK PROBLEM 6         INTRODUCTION TO Ada I   586.1

PURPOSE:      To learn about Ada tasking and how to use it.
              Do one of the two homework assignments:
                 A plane chasing a target and firing a missile.
                 An elevator system with three elevators.


END RESULT:   A complete Ada program that is a main procedure
              that WITH's a package you write containing the tasks.
              This is the first of a sequence of three
              assignments that will result in one program.

PROBLEM:      Plane, target and missile problem:
              HW6 is to get the target flying.
              HW7 is to get the chase plane following the target.
              HW8 is to launch a missile at the target.
              Tasking will be used for the target,plane and
              missile . These will be simple tasks like "SHOOT".
              A rendezvous task will be used to access shared
              data. ( REPORT and READ_OUT )
              A special PLOT task is needed that uses only one
              PUT to output the cursor position and character.
              The target is to be started as a task from the main
              procedure. The main procedure must be kept alive
              until the task terminates. 
              The elevator problem is at the end of this handout.

OBSERVE:      All the tasks may be placed in a single package.
              The main program may be used to clean house at the end of
              execution.

READING:      BARNES  14.
              1815A   9.


NOTES: Different compilers will have different algorithms for context
       switching while tasking. VAX Ada does minimum context switching
       as does WADA. TELESOFT seems to stay in a task ( including
       the main procedure as long as computation is possible. The context
       switch occurs when any form of PUT is executed. I/O takes a
       relatively long time, thus if one task has output then all parallel
       tasks that must be kept running must also have output ( or must
       be locked from execution some other way ). The main procedure is
       the sponsor of the SHARED task and thus must be kept running
       until the TARGET, PLANE, and MISSILE task are complete. The global
       time T is used to synchronize all tasks and main program termination.
       Any code can set T to a large number to stop the process. This
       works reliably because the only other action to change T is to
       increment it. It would seem reasonable if GET would suspend waiting
       for input and let parallel task run. It doesn't!

       The entries SHARED.REPORT and SHARED.READ_OUT are to ensure that
       the four values  X,Y,VX,VY are a consistent set independent of
       the context switching algorithm. This is the standard Ada construct
       for shared data.