Input and Output

Programs communicate with the outside world via 4-bit input or output ports.

The operation of these ports is controlled by special I/O instructions described here. These ports are physically located on the same devices which hold ROMs and DATA RAMs; therefore, they are referred to as ROM ports or RAM ports. These are totally separate from the instruction or data locations provided in ROM or RAM, and should not be confused with them.
The ports associated with RAMs may be used only for output.