3 ports are used to control the OPL3 soundcard: &FEBC and &FEBE are register select ports, and &FEBD is the data port.
OPL3 is register compatible with OPL2, with twice the number of signal sources, 4 new operator modes, 4 new selectable waveforms (including square wave) and stereo output.
OPL3 offers 18 FM 2-op chans in its standard mode, but that can be configured: