3 ports are used to control the OPL3 soundcard: &FEBC and &FEBE are register select ports, and &FEBD is the data port.
OPL3 offers 18 FM 2-op chans in its basic standard mode, but that can be configured:
* One setting converts 3 of the FM channels into a 5-channel percussion set
* Another setting pairs 12 of the FM channels into 6 FM 4-op channels