---
Data
on
the
Screen
Odd
field
- line 1
Even
field
- line 1
Odd
field
- line 2
Even
field
- line 2
Odd
field
- last line
Even
field
- last line
Data
in
Memory
Line 1
Line 2
Line 3
Line 4
Line
399
Line 400
Figure
2-1:
Interlaced Bit-Plane in RAM - 400 Lines Long
The
system retrieves
data
for bit-plane displays by using pointers
to
the starting address
of the
data
in memory. As you can see, the starting address for the even-numbered
fields is one line greater
than
the starting address for the odd-numbered fields. There-
fore, the bit-plane pointer must contain a different value for alternate fields of the inter-
laced display. This means
that
two separate Copper instruction lists are required.
To
get the Copper
to
execute the correct list, you set an interrupt
to
the 68000
just
after the first line of the display. When the interrupt is executed, you change the con-
tents of the
COPILC
location register
to
point
to
the second list. Then, during the
vertical blanking interval, COPILC will
be
automatically reset
to
point
to
the original
list.
For
more information about interlaced displays,
see
chapter 3, "Playfield Hardware."
USING
THE
COPPER
WITH
THE
BLITTER
If
the Copper
is
used
to
start
up a sequence of blitter operations,
it
must wait for the
blitter-finished interrupt before starting another blitter operation. Changing blitter
registers while the blitter
is
operating causes unpredictable results. For
just
this pur-
pose, the WAlT instruction includes an additional control bit, called BFD (for blitter
finished disable). Normally, this bit is a I and only the beam counter comparisons con-
trol the WAlT.
Coprocessor Hardware
23
To Next Page
Loading...
