The output of a ROM (2) is coupled to a data output circuit (4) via a bus (11) and connects to a programme counter (7) and a -security code address comparator (8) over an address bus (10). A further comparator (3) connects to a status output circuit (9) to check on the security code data status bit. The security code information is included in the use programme and is located at a specific address in the memory. Checks are made using the comparators to determine correct form of address and the data. ADVANTAGE - Double check on security data and address.