Control of access to semiconductor memories - uses access controller requiring validating inputs before opening and gate linking address bus and address decoder circuits

摘要

Unauthorised access to electronic memories containing confidential information is prevented by placing an access control circuit between the address bus and the address decoder circuits. The access control circuit (19) requires validating input signals (28,29) before it will supply a signal (17) to one input (16) of an AND gate (15) whose other input (14) is from the address bus (11). Thus, the required address will only be passed to the address decoder (26) when the AND gate (15) has signals from both the address bus and the access controller. This prevents access to the memory for either reading or writing. The required access code may be extracted by the controller from the memory in a preliminary cycle. The access controller program is stored in fusible link semiconductor read only memory (ROM).