Bitfields for 65C12 memory access control:
Bit(s)	Description	)
 7	unused
 6	always use main screen memory if I/O addr 3000h-7FFFh (overrides bit 5)
 5	use shadow screen memory if screen address specified
 4	use current ROM rather than ROM selected by bits 3-0 (only if I/O
	  address between 8000h and BFFFh)
 3-0	paged ROM number