4.
For each subsequent display field, during the vertical blanking interval, rewrite
the sprite pointers.
CAUTION
If
sprite DMA
is
turned
off
while a sprite is being displayed
(that
is,
after
VSTART
but
before VSTOP), the system will continue
to
display the line of
sprite
data
that
was most recently fetched. This causes a vertical
bar
to
appear on the screen.
It
is
recommended
that
sprite
DMA
be turned
off
only
during vertical blanking or during some portion of the display where you are
sure
that
no sprite
is
being displayed.
SELECTING
A
DMA
CHANNEL
AND
SETTING
THE
POINTERS
In deciding which DMA channel
to
use, you should take into consideration the colors
assigned to the sprite and the sprite's video priority.
The
sprite
DMA
channel uses two pointers to read
in
sprite
data
and control words.
During the vertical blanking interval before the first display of the sprite, you need to
write the sprite's memory address into these pointers.
The
pointers for each sprite are
called SPRxPTH and SPRxPTL, where "x" is the number of the sprite DMA channel.
SPRxPTH points to the high three bits of the memory address of the first word
in
the
sprite and SPRxPTL points to the
low
fifteen bits. As usual, you can write a long word
into SPRxPTH.
In the following example the processor initializes the
data
pointers for sprite
O.
Nor-
mally, this
is
done by the Copper. The sprite
is
at
address $20000.
SPROPTH
EQU
SPROPTL EQU
MOVE.L
$DFF120
$DFF122
#$20000,SPROPTH ;
Write $20000 to sprite 0 pointer
These pointers are dynamic; they are incremented by the sprite DMA channel to point
first to the control words, then to the
data
words, and finally to the end-of-data words.
After reading
in
the sprite control information and storing
it
in
other registers, they
proceed to read
in
the color descriptor words. The color descriptor words are stored
in
sprite
data
registers, which are used by the sprite DMA channel to display the
data
on
screen. For more information about
how
the sprite DMA channels handle the display,
see the "Hardware Details" section below.
Sprite Hardware
105
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