Feature control reg.   read 03CAh  write 03DAh

Layout  b3     VSS  Vertical sync select
        b1     FC1  (EGA only)
        b0     FC0  (EGA only)

- Vertical sync select (bit 3)
        Controls the signal output to the vertical sync line that
        controls the monitor.

        0 : Normal vert. sync output to the monitor
        1 : Vertical sync output to the monitor is the logical OR of
            the vertical sync and the vertical display enable.