Displaying And Redisplaying The Playfield; Enabling The Color Display - Commodore Amiga Hardware Reference Manual

The bit-planes for a full-screen-sized, interlaced display are 400, rather than 200, lines
long. Assuming that the playfield in memory is the normal 320 pixels wide, data for the
interlaced picture begins at the following locations (these are all byte addresses):
Line 1 START
Line 2 START+40
Line 3 START+80
Line 4 START+120
and so on. Therefore, you use a modulo of 40 to skip the lines in the other field. For
odd fields, the bit-plane pointers begin at START. For even fields, the bit-plane
pointers begin at START+40.
You can use the Copper to handle resetting of the bit-plane pointers for interlaced
displays.
DISPLAYING AND REDISPLAYING THE PLAYFIELD
You start playfield display by making certain that the bit-plane pointers are set and
bit-plane DMA is turned on. You turn on bit-plane DMA by writing a 1 to bit BPLEN
in the DMACON (for DMA control) register. See chapter 7, "System Control
Hardware," for instructions on setting this register.
Each time the playfield is redisplayed, you have to reset the bit-plane pointers. Reset-
ting is necessary because the pointers have been incremented to point to each successive
word in memory and must be repointed to the first word for the next display. You write
Copper instructions to handle the redisplay or perform this operation as part of a verti-
cal blanking task.
ENABLING THE COLOR DISPLAY
To enable color rather than black and white display, you need to set bit 9 in BPLCONO.
Doing so enables the color burst signal on composite video; it does not affect RGB video.
52 Playfield Hardware