These registers point to the current word of data
that is being displayed from each bitplane. They
are set up by the programmer, but increment as each
word of data is fetched from memory. Therefore,
these registers must be reset for each screen. This
is usually done by the copper list, but may also be
written directly to by the programmer during the
vertical blank interrupt. The initial value stored
here should be the starting address of the bitplane.
The value stored here must begin on a word boundary.