Commodore Amiga A500 Technical Reference Manual page 203

B-BITPLANE (SIX (6)
CHANNELS)
C-COPPER (ONE (1)
CHANNEL)
D-AUDIO (FOUR (4)
CHANNELS)
The bitplane controller continuously
(during display) transfers display data
from memory to display buffer
registers. There are six
DMA
channels to
handle the data from six independent bit
planes. The buffers convert this bitplane
data into pixel data for the display.
Each bitplane can be a full image or a
window into an image that is up to four
times the screen size. They can be
grouped into two separate images, each
with its own color registers.
The Copper is a coprocessor that uses
one of the
DMA
channels to fetch its
instructions. The DMA pointer is the
instruction counter and must be
preloaded with the starting address of
Copper's instructions.
The Copper can move (write) data into
chip registers. It can skip, jump, and
wait (halt). These simple instructions
give great power and flexibility because
of the following features.
When the Copper is halted, it is off the
data bus, using no bus cycles until the
wait is over. The programmed wait
value is compared to a counter that
keeps track of the
TV
beam position
(Beam Counter) and when they are
equal, the Copper will resume fetching
instructions.
It can cause interrupts, reload the color
registers, start the Blitter or service the
audio. It can modify almost any register
inside or outside the IC device, based on
the TV screen coordinates given by the
Beam Counter and the actual address
encoded on the RGA bus.
There are four audio channels, all of
which are located outside of the
DMA
Controller
IC.
Each controller is
independent and uses one DMA channel
from the
DMA
Controller IC and fetches
its data during a dedicated timing slot
within horizontal blanking. This is
accomplished by a controller asserbng
the
DMAL
input on the
DMA
Controller.