Data
for
Line 1:
Location:
START
Leftmost
Display Word
START+2
Next Word
START+4
Next
Word
Screen
data fetch stops (DDFSTOP)
for
each
horizontal line after the last word
START+38
Last Display
Word
on
the line
has
been
fetched.
-------
.....
Figure
3-15:
Data Fetch for the First Line When Modulo = 40
At
this point, BPLxPTH and BPLxPTL contain the value START+40.
The
modulo,
which
is
40,
is
added
to
the current value of the pointer so
that
when
it
begins the
data
fetch for the next line,
it
fetches the
data
that
you intend for
that
line.
The
data
fetch
for line 2
is
shown
in
figure 3-16.
Data
for
Line 2:
Location: START+80
Leftmost
Display Word
START+82
Next Word
START+84
Next Word
START+118
Last Display
Word
Figure
3-16:
Data Fetch for the Second Line When Modulo = 40
To
display the right half of the big picture, you set up a vertical blanking routine
to
start
the bit-plane pointers
at
location START+40 rather than START with the modulo
remaining
at
40.
The
data
layout
is
shown
in
figures
3-17
and 3-18.
Playfield Hardware
67
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