A port is a special kind of register that sits at the "edge" of a chip. The word "port" comes from the Latin word for window, as it provides a window into the state of the chip. Each port has a number of flip-flops that latch a value into it, keeping it stable while the values are propagated over to another device, the reader. The reader will latch these signals into its copy of the port, namely the flip-flops on its end that will hold stable the values.

One thing that makes ports different from internal registers of the CPU is that there are timing considerations that must be taken into account. Since the various chips or devices in a communications system have their own internal clock, we cannot count on them being synchronized. Thus, they have to obey a protocol for when to read the values from the port's flip-flops or when to change them. This timing protocol is one of the trickiest aspects of all computer hardware.