The
bit-planes for a full-screen-sized, interlaced display are 400, rather
than
200, lines
long. Assuming
that
the playfield
in
memory is the normal 320 pixels wide,
data
for the
interlaced picture begins
at
the following locations (these are all byte addresses):
Line 1 START
Line 2 START+40
Line 3 START+80
Line 4 START+120
and so on. Therefore, you
use
a modulo of 40 to skip the lines in the other
field.
For
odd fields, the bit-plane pointers begin
at
START. For even fields, the bit-plane
pointers begin
at
START+40.
You can use the Copper
to
handle resetting of the bit-plane pointers for interlaced
displays.
DISPLAYING
AND
REDISPLAYING
THE
PLAYFIELD
You
start
playfield display by making certain
that
the bit-plane pointers are set and
bit-plane
DMA
is
turned on. You turn on bit-plane
DMA
by writing a 1 to bit BPLEN
in the DMACON (for
DMA
control) register. See chapter
7,
"System Control
Hardware," for instructions
on
setting this register.
Each time the playfield
is
redisplayed, you have to reset the bit-plane pointers. Reset-
ting
is
necessary because the pointers have been incremented
to
point to each successive
word in memory and must be repointed
to
the first word for the next display. You write
Copper instructions to handle the redisplay or perform this operation as
part
of a verti-
cal blanking task.
ENABLING
THE
COLOR
DISPLAY
To
enable color rather than black and white display, you need
to
set bit 9
in
BPLCONO.
Doing so enables the color burst signal on composite video;
it
does not affect RGB video.
52 Playfield Hardware
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