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CRTC registers

         W A R N I N G 

        A programmer can design new video modes by manipulating the
        VGA registers directly. However it is not advised because of
        the complexity of the manipulations required to change a mode.

        A mistake could damage the monitor or the display adapter.

        The lower 8 registers in the CRTC contain sensitive horizontal
        and vertical timing settings. Software expecting the EGA may
        program these registers unwittingly. To un-protect the CRTC
        registers 0-7, first set the CR field in the End horizontal
        blanking register, then clear the PR field in the Vertical
        retrace end register.