Commodore Amiga A500 Technical Reference Manual page 200

B-BITPLANE (SIX (6)
CHANNELS)
C-COPPER(ONE(1)
CHANNEL)
D-AUDIO (FOUR (4)
CHANNELS)
197
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 asserting
the DMAL input on the DMA Controller.