A circuit is operable in a normal operating mode and a test mode. The circuit contains a privileged information supply circuit (12) coupled to the testable circuit (10). A test access circuit (19) provides access to the testable circuit (10). A test control circuit (18) controls switching of the test access circuit (19) to the test mode. A multiplex circuit (16) couples the privileged information supply circuit (12) to the testable circuit (10) for access to privileged information in the normal mode. In the test mode the shadow information supply circuit (14) is coupled to the testable circuit (10) instead.