Figure 2-1 Interlaced Bit-Plane In Ram - 400 Lines Long - Commodore Amiga Hardware Reference Manual

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Data on
the Screen
Odd field - line 1
Even field - line 1
Odd field - line 2
Even field - line 2
Odd field - last line
Even field - last line
Data in
Memory
Line 1
Line 2
Line 3
Line 4
Line 399
Line 400
Figure 2-1: Interlaced Bit-Plane in RAM - 400 Lines Long
The system retrieves data for bit-plane displays by using pointers to the starting address
of the data in memory. As you can see, the starting address for the even-numbered
fields is one line greater than the starting address for the odd-numbered fields. There-
fore, the bit-plane pointer must contain a different value for alternate fields of the inter-
laced display. This means that two separate Copper instruction lists are required.
To get the Copper to execute the correct list, you set an interrupt to the 68000 just
after the first line of the display. When the interrupt is executed, you change the con-
tents of the COPILC location register to point to the second list. Then, during the
vertical blanking interval, COPILC will be automatically reset to point to the original
list.
For more information about interlaced displays, see chapter 3, "Playfield Hardware."
USING THE COPPER WITH THE BLITTER
If the Copper is used to start up a sequence of blitter operations, it must wait for the
blitter-finished interrupt before starting another blitter operation. Changing blitter
registers while the blitter is operating causes unpredictable results. For just this pur-
pose, the WAlT instruction includes an additional control bit, called BFD (for blitter
finished disable). Normally, this bit is a I and only the beam counter comparisons con-
trol the WAlT.
Coprocessor Hardware 23