The maximum number of colors that a viewport can display is determined by
the depth of the bitmap that the viewport displays. the depth is specified
when the BitMap is initialized. See the section below called
"preparing the bitmap structure."

Depth determines the number of bitplanes used to define the colors of the
rectangular image you are trying to build (the raster image) and the
number of different colors that can be displayed at the same time within a
viewport. for any single pixel, the system can display any one of 4,096
possible colors.

The following table shows depth values and the corresponding number of
possible colors for each value.


    Table 27-1: Depth Values and Number of Colors in the ViewPort


          Colors    Depth Value
          ------    -----------
              2          1
              4          2
              8          3         (Note 1)
             16          4         (Notes 1,2)
             32          5         (Notes 1,2,3)
             16          6         (Notes 1,4)
             64          6         (Notes 1,2,3,5)
          4,096          6         (Notes 1,2,3,6)

        Notes:
        1. Not available for superhires.
        2. Single-playfield mode only - dualpf not one of the
           viewport's attributes.
        3. Low-resolution mode only - neither hires nor
           superhires one of the viewport attributes.
        4. Dual Playfield mode - dualpf is an attribute of this
           ViewPort.  Up to eight colors (in three planes) for
           each playfield.
        5. Extra-Half-Brite mode - extra_halfbrite is an
           attribute of this ViewPort.
        6. Hold-And-Modify mode only - ham is an attribute of
           this ViewPort.


The color palette used by a viewport is specified in a colormap. see the
section called "preparing the colormap" for more information.

Depending on whether single- or dual-playfield mode is used, the system
will use different color register groupings for interpreting the on-screen
colors. The table below details how the depth and the different viewport
modes affect the registers the system uses.


      Table 27-2: Color Registers Used in Single-playfield Mode


                        Color
           Depth   Registers Used
           -----   --------------
             1          0,1
             2          0-3
             3          0-7
             4          0-15
             5          0-31
             6          0-31       (if extra_halfbrite is an
                                    attribute of this viewport.)

             6          0-15       (if ham is an attribute of
                                    this viewport.)


The following table shows the five possible combinations when dualpf is an
attribute of the viewport.


        Table 27-3: Color Register Used in Dual-playfield Mode


           Depth       Color       Depth       Color
           (PF-1)    Registers     (PF-2)    Registers
           ------    ---------     ------    ---------
             1          0,1          1          8,9
             2          0-3          1          8,9
             2          0-3          2          8-11
             3          0-7          2          8-11
             3          0-7          3          8-15