Beam Position Detection; Using The Beam Position Counter - Commodore Amiga Hardware Reference Manual

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