If you design a memory picture larger than the display window, you must
choose which part of it to display. Displaying a portion of a larger
playfield differs in the following ways from displaying the basic
playfields described up to now:

   *  If the big picture in memory is larger than the display window,
      you must respecify the modulos. The modulo must be some value
      other than 0.

   *  You must allocate more memory for the larger memory picture.

 specifying the modulo 
 specifying the data fetch 
 memory allocation 
 selecting the display window starting position 
 selecting the stopping position