Horizontal Scrolling; Figure 3-23 Vertical Scrolling - Commodore Amiga Hardware Reference Manual

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