Port 3C2-R  -  VGA Input Status Register 0

            1xxx xxxx  CRT interrupt
                         When set to 1, this bit indicates a vertical
                         retrace interrupt is pending.
            x11x xxxx  Reserved
            xxx1 xxxx  Switch Sense Bit
                         Used by BIOS to determine the type of display
                         attached.
            xxxx 1111  Reserved