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DUAL-PLAYFIELD PRIORITY AND CONTROL
Either playfield 1 or 2 may have priority; that is, either one may be displayed in front of
the other, although playfield 1 normally has priority. The bit known as PF2PRI (bit 6)
in register BPLCON2 is used to control priority. When PF2PRI
=
1, playfield 2 has
priority over playfield
1.
When PF2PRI
=
0, play field 1 has priority.
You can also control the relative priority of play fields and sprites. Chapter 7, "System
Control Hardware," shows you how to control the priority of these objects.
You can control the two playfields separately as follows:
o
They can have different-sized representations in memory, and different portions
of each one can be selected for display.
o
They can be scrolled separately.
NOTE:
You must take special care when scrolling one play field and holding the other
stationary. When you are scrolling low-resolution play fields, you must fetch one
word more than the width of the play field you are trying to scroll (two words
more in high-resolution mode) in order to provide some data to display when the
actual scrolling takes place. Only one data-fetch start register and one data-
fetch stop register are available, and these are shared by both playfields. If you
want to scroll one playfield and hold the other, you must adjust the data-fetch
start and data-fetch stop to handle the playfield being scrolled. Then, you must
adjust the modulo and the bit-plane pointers of the playfield that is
not
being
scrolled to maintain its position on the display. In low-resolution mode, you
adjust the pointers by -2 and the modulo by -2. In high-resolution mode, you
adjust the pointers by -4 and the modulo by -4.
ACTIVATING DUAL-PLAYFIELD MODE
Writing a 1 to bit 10 (called DBLPF) of the bit-plane control register BPLCONO selects
dual-playfield mode. Selecting dual-playfield mode changes both the way the hardware
groups the bit-planes for color interpretation-all odd-numbered bit-planes are grouped
together and all even-numbered bit-planes are grouped together-and the way hardware
can move the bit-planes on the screen.
64 Play field Hardware
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