ENVY_MT_RBUFSZ
sc->intrs[sc->nintr].ipos = envy_mt_read_2(sc, ENVY_MT_RBUFSZ);
reg = envy_mt_read_2(sc, ENVY_MT_RBUFSZ);
envy_mt_write_2(sc, ENVY_MT_RBUFSZ, bufsz / 4 - 1);