FCR_MTRON
fdc->sc_fcr |= FCR_DSEL(drive) | FCR_MTRON;
fdc->sc_fcr &= ~(FCR_DSEL(drive)|FCR_MTRON);
fdc->sc_fcr |= FCR_DSEL(0)|FCR_MTRON; /* XXX */
fdc->sc_fcr &= ~(FCR_DSEL(0)|FCR_MTRON); /* XXX */