Vert retrace start     index 10h  CRTC    (protected)

Layout  b7-b0  VRS  Vertical retrace start

        Note
        This register is protected by the CR field in the End
        horizontal blanking register.

        The low-order 8 bits of the value that determines when a
        vertical retrace pulse begins. The scan line counter is
        compared to this register at each horizontal retrace time.
        When the scan-line counter equals this value, the vertical
        retrace period begins.

        Bits 8 and 9 are found in the Overflow register.