A data processing system (10) allows an authorized user to unlock a security mode by providing a code stored in a plurality of mask registers (60,62,66) such that the system is selectively allowed to communicate with an external device. When an reset signal is received, a selector (48) selects a first mask register (60) and retrieves a first stored address value and a first stored data value therefrom. The first stored address and data values are respectively compared with a first address value and a first data value by a comparator (44). This process of selecting and comparing continues until a final match signal is asserted. When the final match signal is asserted, a secure signal is negated and the system may communicate with the external user.