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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.

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