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What Are Overlays?


   Overlays are portions or segments of a program that do not reside in
   RAM until the execution of the segment is required.  The main program
   code, or root portion of the program, is always resident in memory.
   Overlays are stored on disk until the root calls them into memory.
   The portion of memory overlays share is called the overlay area.  An
   overlay is loaded whenever a routine in the overlay is called.  When
   the root calls another overlay that is not currently in memory, the
   new overlay overwrites the first.  Note that the largest overlay
   determines the amount of memory allocated for all of the overlays.