Vertical display end   index 12h  CRTC

Layout  b7-b0  VDE  Vertical display enable end

        The low-order eight bits of the value in the Vertical display
        enable end register. This determines the last horizontal line
        to be displayed on the bottom of the monitor, including the
        overscan region. Bits 8 and 9 of this value are found in the
        Overflow register.