Data for Line 2:
Location: START+80 START+82 START+84... START+118
Leftmost Next Word Next Word Last Display
Display Word Word
Figure 3-16: Data Fetch for the Second Line When Modulo = 40
To display the right half of the big picture, you set up a vertical blanking routine to start
the bit-plane pointers at location START+40 rather than START with the modulo remaining
at 40. The data layout is shown in Figures 3-17 and 3-18.
Data for Line 1:
Location START+40 START+42 START-44... START+78
Leftmost Next Word Next Word Last Display
Display Word Word
Figure 3-17: Data Layout for First Line Right Half of Big Picture
Now, the bit-plane pointers contain the value START+80. The modulo (40) is added to the
pointers so that when they begin the data fetch for the second line, the correct data is
fetched.
Data for Line 2:
Location: START+120 START+122 START+124... START+158
Leftmost Next Word Next Word Last Display
Display Word Word
Figure 3-18: Data Layout for Second Line Right Half of Big Picture
Remember, in high-resolution mode, you need to fetch twice as many bytes as in low-
resolution mode. For a normal-sized display, you fetch 80 bytes for each horizontal line
instead of 40.
- 70 Playfield Hardware -
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