SUMMARY
The steps for defining dual play fields 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:
o
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 bit-planes
in
each playfield).
o
Building
bit-planes.
Recall
that
playfield 1 is formed from PLANES
1,
3,
and
5 and playfield 2 from PLANES
2,
4,
and
6.
o
Setting
the
modulo
registers.
Write the modulo
to
both BPLIMOD and
BPL2MOD as you will be using both odd- and even-numbered bit-planes.
These steps are added:
o
Defining
priority.
If
you want playfield 2 to have priority, set bit 6 (PF2PRI)
in
BPLCO
N2
to
1.
o
Activating
dual-playfield
mode.
Set bit
10
(DBLPF)
in
BPLCONO to 1.
Bit-planes
and
Display
Windows
of
All
Sizes
You have seen how to form single and dual play fields
in
which the playfield
in
memory
is the same size as the display window. This section shows you how to define and
use
a
play
field
whose big picture
in
memory
is
larger than the display window, how
to
define
display windows
that
are larger or smaller
than
the normal playfield size, and how
to
move the display window
in
the big picture.
WHEN
THE
BIG
PICTURE
IS
LARGER
THAN
THE
DISPLAY
WINDOW
If you design a memory picture larger than the display window, you must choose which
part
of
it
to display. Displaying a portion of a larger playfield differs in the following
ways from displaying the basic playfields described up to now:
Playfield Hardware
65
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