ROMs are selected using the normal ROM selection I/O port (DFxx).
* If 'rom ROM read' is active then CPC will see 16KB 16 KB pages from the X-MEM flash at rom ROM slots 0-156 and 8-31. If boot from CPC, then ROM 0 will be from CPC.
* If 'free' is active, the CPC will also see 16KB 16 KB pages from the X-MEM flash at &40-&4f. All are visible regardless of booting from CPC or X-MEM.
* When boot from CPC is active, OS and BASIC are from CPC and not from X-MEM but they can still be accessed at &40-&4f if free is set.