The steps for defining dual playfields are almost the same as those for
defining the basic playfield. Only in the following steps does the
dual-playfield creation process differ from that used for the basic
playfield:

   *   loading colors into the registers .
      -----------------------------------
      Keep in mind that color registers 0-7 are used by playfield 1
      and registers 8 through 15 are used by playfield 2 (if there are
      three bitplanes in each playfield).

   *   building bitplanes .
      --------------------
      Recall that playfield 1 is formed from PLANES 1, 3, and 5 and
      playfield 2 from PLANES 2, 4, and 6.

   *   setting the modulo registers .
      ------------------------------
      Write the modulo to both  bpl1mod and bpl2mod  as you will be
      using both odd- and even-numbered bitplanes.

These steps are added:

   *   defining priority .
      -------------------
      If you want playfield 2 to have priority, set bit 6 (PF2PRI)
      in  bplcon2  to 1.

   *   activating dual-playfield mode .
      --------------------------------
      Set bit 10 ( dblpf ) in  bplcon0  to 1.