The CPC can choose which ROM chip is associated with the 16K area starting at &C000. This is usually the reserved for the contents of the screen (RAM), but the address range is shared with ROM Memory.
On start up, the CPC checks whether any ROMs are installedusing the "KL_ROM_WALK" firmware routine. The CPC464 only checks for ROMs in positions 7-0, but the 6128 checks for ROMs in positions 15-0. The ROMs are scanned in the reverse order starting from 7 or 15 depending on the CPC type. If a ROM is present, the CPC reads what commands the ROM offers and saves them in a table in RAM.
Further detailed information on the ROM bank numbering can be found [[Upper_ROM_Bank_Number|here]]