After the first line is fetched, the bit-plane pointers BPLxPTH and BPLxPTL contain the
value START+40. The modulo (in this case, 0) is added to the current value of the pointer,
so when the pointer begins the data fetch for the next line, it fetches the data you want
on that line. The data for the next line begins at memory location START+40.
Data for Line 2:
Location: START+40 START+42 START+44 .....START+78
Leftmost Next Word Next Word Last Display
Display Word Word
Figure 3-11: Data Fetched for the Second Line When Modulo = 0
Note that the pointers always contain an even number, because data is fetched from the
display a word at a time.
There are two modulo registers, BPL1MOD for the odd-numbered bit-planes and BPL2MOD
for the even-numbered bit-planes. This allows for differing modules for each playfield in
dual-playfield mode. For normal applications, both BPL1MOD and BPL2MOD will be the
same.
The following example sets the modulo to 0 for a low-resolution playfield with one bit-
plane. The bit-plane is odd-numbered.
MOVE.W #0,BPL1MOD+CUSTOM ; Set modulo to 0
DATA FETCH IN HIGH-RESOLUTION MODE
When you are using high-resolution mode to display the basic playfield, you need to fetch
80 bytes for each line, instead of 40.
MODULO IN INTERLACED MODE
For interlaced mode, you must redefine the modulo, because interlaced mode uses two
separate scanning’s of the video screen for a single display of the playfield. During the
first scanning, the odd-numbered lines are fetched to the screen; and during the second
scanning, the even-numbered lines are fetched.
- Playfield Hardware 55 -
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