MLB_USIO_REG_DR
writew(ch, port->membase + MLB_USIO_REG_DR);
ch = readw(port->membase + MLB_USIO_REG_DR);
writew(c, port->membase + MLB_USIO_REG_DR);
writew(port->x_char, port->membase + MLB_USIO_REG_DR);