Beam
Position
Detection
Sometimes you might want
to
synchronize the 68000 processor
to
the video beam
that
is
creating the screen display. In some cases, you may also wish
to
update a
part
of the
display memory
after the system has already accessed the
data
from the memory for the
display area.
The
address for accessing the beam counter is provided so
that
you can determine the
value of the video beam counter and perform certain operations based on the beam posi·
tion. Note, however,
that
the Copper
is
already capable of watching the display position
for you and doing certain register-based operations automatically. Refer
to
"Copper
Interrupts" below and chapter
2,
"Coprocessor Hardware/' for further information.
In addition, when you are using a light pen with this system, this same address
is
used
to
read the light pen position rather than the beam position. This is described fully in
chapter 8, "Interface Hardware."
USING
THE
BEAM
POSITION
COUNTER
There are four addresses
that
access the beam position counter. Their usage
is
described
in table
7-5.
System Control Hardware
205
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