UART_CONTROL
uartaddr[UART_CONTROL / 4] = CR_TXEN;
bus_space_write_4(iot, ioh, UART_CONTROL, sc->sc_cr);
sc->sc_cr = bus_space_read_4(iot, ioh, UART_CONTROL);
bus_space_write_4(iot, ioh, UART_CONTROL, sc->sc_cr);