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Input status #1 reg.   read 03DAh  write  n/a

Layout  b5-b4       Used in EGA (DIA)
        b3     VR   Vertical retrace
        b2-b1       Used in EGA (LSW,LST)
        b0     DE   Display enable NOT

- Vertical retrace (bit 3)
        0 : Display is in display mode
        1 : Display is in vertical retrace mode

- Display enable NOT (bit 0)
        0 : Display is in display mode
        1 : Display is not in display mode. Either the horizontal or
            vertical retrace period is active.

                b3 b0   Display status
                0  0    Display mode
                1  1    Vertical retrace time
                0  1    Horizontal retrace time