Programming Input/Output

Abstract

In order to transfer data between the microprocessor and external devices, hardware is needed to provide temporary storage, to decode some address lines, to communicate with the data bus, to control interrupts, and to synchronise data movement with the processor clock cycles. This can be assembled from small-scale integrated circuits but the package count can be reduced from a dozen or more to one or two by using the special interface packages designed by the microprocessor manufacturer. The most widely used is the parallel interface which generally provides two 8-bit ports and a few signalling lines for connecting to external devices. In order to afford maximum flexibility in a single design, the package is made programmable. This means that before any data can be transferred one or more internal registers must be loaded.