Blitter
Operations
and
System
DMA
The
operations
of
the
blitter
affect
the
performance
of
the
rest
of
the
system.
The
fol-
lowing sections explain how
system
performance is affected by
blitter
direct
memory
access (DMA) priority, DMA time
slot
allocation,
bus
sharing
between
the
68000
and
the
bit-plane,
the
operations
of
the
blitter
and
Copper,
and
different playfield display sizes.
BLITTER
DMA
PRIORITY
The
blitter
performs
its
various
data-fetch,
modify,
and
store
operations
through
D11A
sequences,
and
it
shares
memory access
with
other
devices in
the
system.
Each
device
that
accesses
memory
has
a
priority
level assigned
to
it,
which
indicates
its
importance
relative
to
other
devices.
Disk DMA,
audio
DMA,
bit-plane
DMA,
and
sprite
DMA
all
have
the
highest priority
level.
Bit-plane
D11A
has
priority over
sprite
DMA
under
certain
circumstances.
Each
of
these four devices is allocated a
group
of
time
slots
during
each horizontal scan
of
the
video
beam.
If
a device does
not
request one of
its
allocated
time
slots,
the
slot
is open
for
other
uses.
These
devices are given first
priority
because missed D11A cycles can
cause lost
data,
noise in
the
sound
output,
or
on-screen
interruptions.
The
Copper
has
the
next
priority
because
it
has
to
perform
its
operations
at
the
~ame
time
during
each
display frame
to
remain synchronized
with
the
display
beam
sweeping
across
the
screen.
The
lowest priorities are assigned
to
the
blitter
and
the
68000, in
that
order.
The
blitter
is given
the
higher
priority
because
it
performs
data
copying, modifying,
and
line draw-
ing
operations operations
much
faster
than
the
68000.
DMA
TIME
SLOT
ALLOCATION
During
a
horizontal
scan line
(about
63 microseconds),
there
are 227.5 "color clocks",
or
memory
access cycles. A memory cycle is approximately 280
ns
in
duration.
The
total
of
227.5 cycles
per
horizontal
line includes
both
display
time
and
non-display time.
Of
this
total
time, 226 cycles are available
to
be allocated
to
the
various
devices
that
need
memory
access.
The
time-slot allocation
per
horizontal line is
186
Blitter
Hardware
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