"A special purpose microcomputer" which controls voice modules in a computer music instrument is designed and implemented. Also, "MIDI protocol" is investigated and implemented, which represents a formal set of hardware and software rules for sending and receiving musical-event data between computer music instruments. Both hardware and software aspects are considered. The keyboard with 88 notes is used to cover the frequency ranges of various voices. "Sequencer" and "Tempo" are implemented. Sequencer allows one to record and play back songs. Tempo supplies a tempo adjustment which is added to or subtracted from the current tempo of the sequence. Some control effects such as sustain, mute and instrument selection are studied. Experimental results show the effectiveness of the designed microcomputer.