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After deciding how many colors you want and how many bitplanes are
required to give you those colors, you tell the system how many bitplanes
to use.

You select the number of bitplanes by writing the number into the register
 BPLCON0  (for Bitplane Control Register 0) The relevant bits are bits 14,
13, and 12, named BPU2, BPU1, and BPU0 (for "Bitplanes Used"). Table 3-5
shows the values to write to these bits and how the system assigns
bitplane numbers.

          Table 3-5: Setting the Number of Bitplanes

                    Number of     Name(s) of
          Value     Bitplanes     Bitplanes
          -----     ---------     ----------
           000       None *
           001         1           PLANE 1
           010         2           PLANES 1 and 2
           011         3           PLANES 1 - 3
           100         4           PLANES 1 - 4
           101         5           PLANES 1 - 5
           110         6           PLANES 1 - 6 **
           111                     Value not used.

     *  Shows only a background color; no playfield is visible.

     ** Sixth bitplane is used only in  dual-playfield mode  and in
         hold-and-modify mode  (described in the section called
        Advanced Topics.

   About the  BPLCON0  register.
   The bits in the  BPLCON0  register cannot be set independently.  To set
   any one bit, you must reload them all.

The following example shows how to tell the system to use two low
resolution bitplanes.

        MOVE.W  #$2200,BPLCON0+CUSTOM   ; Write to it

Because register  BPLCON0  is used for setting other characteristics of the
display and the bits are not independently settable, the example above
also sets other parameters (all of these parameters are described later
in the chapter).

   *   Hold-and-modify mode is turned off.
   *   Single-playfield mode is set.
   *   Composite video color is enabled.  (Not applicable in all models.)
   *   Genlock audio is disabled.
   *   Light pen is disabled.
   *   Interlaced mode is disabled.
   *   External resynchronization is disabled. (genlock)

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