nct_read_reg
sc->cache.inv[gp->grpnum] = nct_read_reg(sc, REG_INV, gp->grpnum);
sc->cache.ior[gp->grpnum] = nct_read_reg(sc, REG_IOR, gp->grpnum);
sc->cache.out[gp->grpnum] = nct_read_reg(sc, REG_DAT, gp->grpnum);
val = nct_read_reg(sc, reg, group);