When access to proprietary data or sensitive information stored in a memory device of a programmable unit is attempted, a check is carried out to determine whether the requested access has been or could have been initiated by someone who is not authorized to do so, and in that the memory device outputs requested data, and/or stores data which is supplied to it only when the check shows that it can be assumed that the relevant access has not been initiated or could not have been initiated by someone who is not authorized to do so. Access is controlled, for example, by identifying the source of the requested access, or by associating the requested access with the execution of a secure command.