a scan - register having first and second data input ports , a data output port , and inputs for at least first , second , third , and fourth control signals . the scan - register comprises the following elements : first means having inputs coupled to the first and second data input ports for selectively storing data appearing on one of the said data input ports in accordance with the occurrence of a predefined combination of states of at least the first and second control signals ; second menas having at least one input coupled to the second data input port for selectively storing data appearing on the second data input port in accordance with the occurrence of a predefined state of at least the third control signal ; and third means having at least one input port coupled to an output of one of the first and second means , and further having an output coupled to the data output port , for selectively storing data stored in either the first or second means in accordance with the occurrence of a predefined state of at least the fourth control signal and providing the data stored therein to the data output port . the data appearing on one of the first and second data input ports is therefore sequentially shiftable through selected ones of the first , second and third means , to the output port in accordance with the occurrence of predefined sequences of the predefined states of selected ones of the control signals .