FORMING A DUAL-PLAYFIELD DISPLAY
For more flexibility in designing your background display, you can specify two playfields
instead of one. In dual-playfield mode, one playfield is displayed directly in front of the
other. For example, a computer game display might have some action going on in one
playfield in the back-ground, while the other playfield is showing a control panel in the
foreground. You can then change either the foreground or the background without having
to redesign the entire display. You can also move the two playfields independently.
A dual-playfield display is similar to a single-playfield display, differing only in these
aspects:
o Each playfield in a dual display is formed from one, two or three bit planes.
o The colors in each playfield (up to seven plus transparent) are taken from different sets
of color registers.
o You must set a bit to activate dual-playfield mode.
Figure 3-12 shows a dual-playfield display.
In Figure 3-12, one of the colors in each playfield is "transparent" (color 0 in playfield 1
and color 8 in playfield 2). You can use transparency to allow selected features of the
background playfield to show through.
In dual-playfield mode, each playfield is formed from up to three bitplanes. Color registers
0 through 7 are assigned to playfield 1, depending upon how many bit-planes you use.
Color registers 8 through 15 are assigned to playfield 2.
BIT-PLANE ASSIGNMENT IN DUAL-PLAYFIELD MODE
The three odd-numbered bit-planes (1, 3, and 5) are grouped together by the hardware
and may be used in playfield 1. Likewise, the three even-numbered bit-planes (2, 4, and
6) are grouped together and may be used in playfield 2. The bit-planes are assigned
alternately to each playfield, as shown in Figure 3-13.
NOTE
In high-resolution mode, you can have up to two bit-planes in each playfield, bit-planes 1
and 3 in playfield 1 and bit-planes 2 and 4 in playfield 2.
- 62 Playfield Hardware -
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