Bit-Plane
Pointer
Start
Address
.
........................................
As
the
value
of
the
bit-plane
pointer increases, more
of
the
lower part
of
the
picture
is
shown.
As
it decreases, more
of
the
upper
part
is
shown .
Figure
3-23:
Vertical Scrolling
To
set up a play
field
for vertical scrolling, you need
to
form bit-planes tall enough
to
allow for the amount of scrolling you want, write software
to
calculate the bit-plane
pointers for the scrolling you want, and allow for the Copper
to
use the resultant
pointers.
Assume you wish
to
scroll a playfield upward one line
at
a time.
To
accomplish this,
before each
field
is displayed, the bit-plane pointers have
to
increase by enough
to
ensure
that
the pointers begin one line lower each time.
For
a normal-sized, low-resolution
display in which the modulo
is
0,
the pointers would be incremented by 40 bytes each
time.
HORIZONTAL
SCROLLING
You can scroll playfields horizontally from left
to
right or right
to
left on the screen.
You control the speed of scrolling by specifying the amount of delay in pixels. Delay
means
that
an extra word of
data
is
fetched
but
not immediately displayed. The extra
word is placed
just
to
the left of the window's leftmost edge and before normal
data
fetch. As the display shifts
to
the
right,the
bits
in
this extra word appear on-screen
at
74 Playfield Hardware
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