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Start vert blank       index 15h  CRTC

Layout  b7-b0  VBS  Start vertical blanking

        The low-order 8 bits of the value that is compared to the
        horizontal scan line counter. When these two values are equal,
        the vertical blanking period begins.
        Bit 8 is in the VBS field of the Overflow register.
        Bit 9 is in the VBS field of the Max scan line register.