Overflow reg.          index 07h  CRTC  protected

Layout  b7     VRS  Vertical retrace start (bit 9)
        b6     VDE  Vertical display enable end (bit 9)
        b5     VT1  Vertical total (bit 9)
        b4     LC   Line compare (bit 8)
        b3     VBS  Start vertical blank (bit 8)
        b2     VRS  Vertical retrace start (bit 8)
        b1     VDE  Vertical display enable end (bit 8)
        b0     VT   Vertical total (bit 8)

        Certain registers associated with the vertical counter require
        field sizes larger than eight bits. The high-order bits of
        several of these registers are included in this register;
        others are found in the Max scan line register.