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.
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