A number of MIDI interfaces exist for the C64. Most of them take the form of cartridges.

• C64 MIDI Interfaces - Description of C64 Midi interfaces and registers, by GRG.
• C64 MIDI Data - Short Description of data, by GRG.

• TinyMidi - by GRG. A working example program in tass format.
• Triad MidiSlave Manager V1.1 - by King Fisher/TRIAD. (Docs here)

• http://www.buchty.net/articles/midikurs.html - C64 MIDI tutorial in German from GO64! (5-8/98)

Worth a read if you don't know what MIDI is.

• http://en.wikipedia.org/wiki/MIDI
• http://www.midi.org/