A system accessible by a test access port controller via a test access port interface includes a data register. The data register is selectable based on an instruction register signal in the test access port interface. The instruction register signal is derived form an instruction register in the test access port controller. A shift register is connected to a data input and a data output in the test access port interface and to the data register. The operation of the shift register is controlled based on an indication of a state of a test access port controller state machine that is received over the test access port interface.