The relationship between the mass storage units and the five basic components is pictured in fig 5-3. The reader shoud note the similarity between fig.5-3 and fig 1-2, which schematizes the expanded data processing cycle. Although a mass storage device feeds data into and retrieves data from the CPU, we reserve the term I/O device for those devices by means of which an opertor can input/output data to-from the CPU in a from which can be easily read and understood, e.g. typewriter terminal, card punch, card reader. I/O devices are considerably shower than mass storage devices (see Section 5.5). Hence, the latter have become essentialfor the efficient processing of any large volume of stored data (e.g. master inventory files) and for storing programs which must be quickly accessed.