The steps for defining a scrolled playfield are the same as those for
defining the basic playfield, except for the following steps:

   *   defining the data fetch .
      -------------------------
      Fetch one extra word per horizontal line and start it 16 pixels
      before the normal (unscrolled) data-fetch start.

   *   defining the modulo .
      ---------------------
      The modulo is two counts less than when there is no scrolling.

These steps are added:

   *  For  vertical scrolling , reset the bitplane pointers
      -----------------------------------------------------
      for the amount of the scrolling increment.
      -----------------------------------------
      Reset  bplxpth and bplxptl  during the  vertical blanking  interval.

   *  For  horizontal scrolling , specify the delay.
      ---------------------------------------------
      Set bits 7-0 in  bplcon1  for 0 to 15 bits of delay.