EFFECTS
OF
DIFFERENT
DISPLAY
SIZES
Each horizontal line in a normal, full-sized display contains 320 pixels
in
low-resolution
mode or 640 pixels
in
high-resolution mode. Thus, either
20
or 40 words will be fetched
during the horizontal line display time.
If
you want
to
scroll a playfield, one extra
data
word per line must be fetched from the memory.
Display size is adjustable (see chapter
3,
"Playfield Hardware"),
~nd
bit-plane DMA
takes precedence over sprite
DMA.
As shown
in
figure 6-11, larger displays may block
out
one or more of the highest-numbered sprites, especially with scrolling.
EFFECTS
OF
BLITTER
OPERATION
As mentioned above, the blitter normally has a higher priority
than
the processor for
DMA cycles. There are certain cases, however, when the blitter and the 68000 can share
memory cycles. If given the chance, the blitter would steal every available memory
cycle. Display, disk, and audio DMA take precedence over the blitter, so
it
cannot block
them from bus access. Depending on the setting of the blitter DMA mode bit, com-
monly referred
to
as the "blitter-nasty" bit, the processor may be blocked from bus
access. This
bit
is called BLTPRI (for "blitter has priority over processor") and
is
in
register DMACONW.
If
BLTPRI is a
1,
the blitter will keep the bus for every available memory cycle. This
could potentially be every cycle.
If
BLTPRI is a 0, the DMA manager will monitor the 68000 cycle requests.
If
the 68000
is unsatisfied for three consecutive memory cycles, the blitter will release the bus for one
cycle.
Table 6-4 shows all of the possible operating modes of the blitter, along with the distri-
bution of its memory access windows.
The
table shows three words of a blit (the first
word, any middle word, and the last word)
and·
how bus activity occurs for this
sequence.
The
following conventions are used
in
this table:
o
A,
B,
and C stand for the sources.
o D
stands
for the destination.
o Numerical suffixes indicate which word within a blit is being fetched.
For
example
AO
is the first memory word fetch;
Al
is any middle memory word
fetch; and A2 is the last memory word fetch.
Blitter Hardware
191
To Next Page
Loading...
