An I/O port is a device that looks like a memory cell to the computer but contains connection to the outside world.
An I/O port typically uses a latch. When the CPU writes to the address associated with the latch, the latch device captures the data and makes it available on a set of wires external to the CPU and memory system.
The I/O ports can be read-only, write-only, or read/write. The write-only port is shown in the Figure 6.3.
Figure 6.3: The write only port