NI16550_PCR_OFFSET
serial_port_out(port, NI16550_PCR_OFFSET, pcr);
pcr = serial_port_in(port, NI16550_PCR_OFFSET);
serial_port_out(port, NI16550_PCR_OFFSET, pcr);
pcr = port->serial_in(port, NI16550_PCR_OFFSET);
port->serial_out(port, NI16550_PCR_OFFSET, pcr);
pcr = serial_port_in(port, NI16550_PCR_OFFSET);