Implement page register not the same size as the number of address bits

A register map could conceivably be a subset of the memory space.

Want to be able to specify the size of the page register. Such a change must raise an exception if the existing size of the register map cannot be addressed by the revised page register size.

eg. In a 32 bit memory system where the register map is only a subset of that memory, pages in the register map might be indexable using only 8 bits if there were no more than 256 pages.