is shown and the picture scrolls downward. On an NTSC system, with a display that has
200 vertical lines, each step can be as little as 1/200th of the screen. In interlaced mode
each step could be 1/400th of the screen if clever manipulation of the pointers is used,
but it is recommended that scrolling be done two lines at a time to maintain the odd/even
field relationship. Using a PAL system with 256 lines on the display, the step can be
1/256th of a screen, or 1/512th of a screen in interlace.
Figure 3-23: Vertical Scrolling
To set up a playfield 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.
- 76 Playfield Hardware -
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