Note
that
fetching an extra word for scrolling will disable some sprites.
To
set up a play
field
for horizontal scrolling, you need to
o Define bit-planes wide enough to allow for the scrolling you need.
o Set the data-fetch registers to correctly place each horizontal line, including the
extra word, on the screen.
o Set the delay bits.
o Set the modulo so
that
the bit-plane pointers begin
at
the correct word for each
line.
o Write Copper instructions to handle the changes during the vertical blanking
interval.
Specifying
Data
Fetch
in
Horizontal
Scrolling
The
normal data-fetch
start
for non-scrolled displays is ($38).
If
horizontal scrolling
is
desired, then the
data
fetch must
start
one word sooner (DDFSTRT = $0030). Inciden-
tally, this will disable sprite
7.
DDFSTOP remains unchanged. Remember
that
the
set--
tings of the data-fetch registers affect both playfields.
Specifying
the
Modulo
in
Horizontal
Scrolling
As always, the modulo is two counts less than the difference between the address of the
next word you want
to
fetch and the address of the last word
that
was fetched. As an
example for horizontal scrolling, let us assume a 40-byte display in an 80-byte "big pic-
ture." Because horizontal scrolling requires a
data
fetch of two extra bytes, the
data
for
each line will
be
42
bytes long.
76
Play
field
Hardware
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