Abstract: A message waiting light control system for use in cooperation with a message storage and retrieval computer includes a microprocessor, a memory, address decoders, and an organization of message waiting lights, relays, and latches. The lights, relays and latches are organized in subgroups as memory mapped input/output words which correspond to the words of bits in the memory. Groups of the latches and relays are addressable. The microprocessor is responsive to an identification code number and a message waiting command to address the proper group and to write a data word to the proper subgroup to activate the selected light. In a similar manner, the lights may be deactivated.