o Custom bit-blitter used for high speed
data
movement, adaptable
to
bit-plane ani-
mation.
The
blitter has been designed
to
efficiently retrieve
data
from up
to
three
sources, combine the
data
in one of
256
different possible ways, and optionally store
the combined
data
in
a destination area. This is one of the situations where the
68000 gives up memory cycles
to
a DMA channel
that
can do the job more
efficiently.
The
bit-blitter, in a special mode, draws patterned lines into rectangu-
larly organized memory regions
at
a speed of about 1 million dots per second; and
it
can efficiently handle area
fill.
o Audio consisting of four low-noise digital channels with independently programmable
volume and sampling rate.
The
audio channels retrieve their control and
data
via
direct memory access. Once started, each channel can automatically
playa
specified
waveform without further processor interaction. Two channels are directed into
each of the two stereo audio outputs.
The
audio channels may be linked together if
desired
to
provide amplitude or frequency modulation or both forms of modulation
simultaneously.
o DMA-controlled floppy disk read and write on a full-track basis. This means
that
the built-in disk can read something over 5.6K bytes of
data
in a single disk revolu-
tion
(11
sectors of
512
bytes each).
All of the special functions described above are produced by three custom-designed VLSI
circuits, which work in concert with the 68000. These circuits and the 68000 use the
shared memory on a fully interleaved basis. Since the 68000 only needs
to
access the
memory bus during each alternate clock cycle in order
to
run full-speed, the rest of the
time the memory bus is free for other activities.
The
special-purpose hardware uses the memory bus during these free cycles, effectively
allowing the 68000
to
run
at
full rated speed most of the time. We say
"most
of the
time" because there are some occasions when the special-purpose hardware steals
memory cycles from the 68000,
but
with good reason. Specifically, the coprocessor and
the data-moving DMA channel called the blitter can each steal time from the 68000 for
jobs they can do better than the 68000. Thus, the system DMA channels are designed
with maximum performance
in
mind; the job
to
be done is performed by the most
efficient hardware element available. In addition, sprites, audio, and disk
DMA
also
steal cycles when
in
operation.
Another primary feature of the Amiga hardware is the ability
to
dynamically control
which
part
of memory is used for the background display, audio, and sprites.
The
Amiga is not limited
to
a small, specific area of RAM for a frame buffer. Instead, the
system allows display bit-planes, sprite-processor control lists, coprocessor instruction
lists, or audio channel control lists to be located anywhere within the lowest 512K of the
memory map.
4 Introduction
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