Port 3FD  -  Line status register

        1xxx xxxx  Forced to 0
        x1xx xxxx  1=TEMT (Transmitter Empty)
                   The transmit shift and hold registers are empty
        xx1x xxxx  1=THRE (Transmitter Holding Register Empty)
                   Indicates that a new character can be send.
                   Set when character send.
        xxx1 xxxx  1=BI (Break Interrupt)
                   Set when connection could be lost. Reset when register
                   read.
        xxxx 1xxx  1=FE (Framing Error)
                   Set when last character didn't have a valid stop bit.
                   Reset when register read.
        xxxx x1xx  1=PE (Parity Error)
                   Reset when register read.
        xxxx xx1x  1=OE (Overrun Error)
                   Set when data is received and buffer (port 3D8)
                   isn't empty. Reset when register is read.
        xxxx xxx1  1=DR (Data Ready)
                   Set when data is received
                   Reset when port 3F8 is read.

        Note: Bit 1 through 4 are error conditions that produce a Receive
        Line Status Interrupt whenever any of the corresponding conditions
        are detected.

        Note: This is a Read-Only register. Writing is not recommended,
        as this operation is used for factory testing.