BIT-PLANES AND DISPLAY WINDOWS OF ALL SIZES
You have seen how to form single and dual playfields in which the playfield in memory is
the same size as the display window. This section shows you how to define and use a
playfield whose big picture in memory is larger than the display window, how to define
display windows that are larger or smaller than the normal playfield size, and how to
move the display window in the big picture.
WHEN THE BIG PICTURE IS LARGER THAN THE DISPLAY WINDOW
If you design a memory picture larger than the display window, you must choose which
part of it to display. Displaying a portion of a larger playfield differs in the following ways
from displaying the basic playfields described up to now:
o If the big picture in memory is larger than the display window, you must respecify the
modules. The modulo must be some value other than 0.
o You must allocate more memory for the larger memory picture.
SPECIFYING THE MODULO
For a memory picture wider than the display window, you need to respecify the modulo so
that the correct data words are fetched for each line of the display. As an example,
assume the display window is the standard 320 pixels wide, so 40 bytes are to be
displayed on each line. The big picture in memory, however, is exactly twice as wide as
the display window, or 80 bytes wide. Also, assume that you wish to display the left half
of the big picture. Figure 3-14 shows the relationship between the big picture and the
picture to be displayed.
- 68 Playfield Hardware -
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