Selecting
the
Number
of
Bit-Planes
After deciding how many colors you want and how many bit-planes are required to give
you those colors, you tell the system
how
many bit-planes
to
use.
You select the number of bit-planes by writing the number into the register BPLCONO
(for Bit Plane Control Register 0)
The
relevant bits are bits
14,
13,
and
12,
named
BPU2,
BPUl,
and
BPUO
(for
"Bit
Planes Used"). Table 3-5 shows the values to write
to
these bits and
how
the system assigns bit-plane numbers.
Table
3-5:
Setting the Number of Bit-Planes
Number
of
Name(s)
of
Value
Bit-Planes
Bit-Planes
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 bit-plane is used only in dual-playfield mode and in hold-and-
modify mode (described
in
the section called "Advanced Topics").
NOTE
The
bits
in
the BPLCONO register are not independently set table.
To
set any
one bit, you must reload them all.
Playfield Hardware
37
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