Commodore Amiga - Coding the Bit-Planes for Correct Coloring

Note that the memory requirements given here are for the playfield only. You may need
to allocate additional memory for other parts of the display - sprites, audio, animation
- and for your application programs. Memory allocation for other
parts of the display
is discussed in the chapters describing those topics.
CODING THE BIT-PLANES FOR CORRECT COLORING
After you have specified the number of bit-planes and set the bit-plane pointers, you can
actually write the color register codes into the bit-planes.
A One- or Two-Color Playfield
For a one-color playfield, all you need do is write Os in all the bits of the single bit-plane
as shown in the example below. This code
fills a low-resolution bit-plane with the back-
ground color
(COLOROO) by writing all Os into its memory area. The bit-plane starts at
$21000 and is 8,000 bytes long.
LOOP:
LEA
$21000,A-0
MOVE.W #2000,DO
MOVE.L #O,(AO)+
SUBQ.W #1,DO
BNE LOOP
jPoint at bit-plane
j Write 2000 longwords = 8000 bytes
; Write
out a zero
jDecrement counter
jLoop
until bit-plane is filled with Os
For a two-color playfield, you define a bit-plane that has Os where you want the back-
ground color and
Is where you want the color in register 1. The following example code
is identical
to the last example, except the bit-plane is filled with $FFOOFFOO instead of
all
O's. This will produce two colors.
LOOP:
LEA
$21000,AO
MOVE.W #2000,DO
MOVE.L
#$FFOOFFOO,(AO)+
SUBQ.W #1,DO
BNE LOOP
44 Playfield Hardware
jPoint at bit-plane
j Write 2000 longwords = 8000 bytes
j Write out $FFOOFFOO
jDecrement counter
jLoop until bit-plane is full