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        Port 3D5  -  Color/VGA Data register

        Data to be placed in the register selected by port 3B4.
        Only registers 0Ch-0Fh may be retrieved; all others are
        write-only.

        Warning: don't play around with register 0 to 9. It can cause
                 permanent damage to the hardware!


        Reg   Use                                 A40  A80  Grap
          0   Horizontal total characters - 1     38h  71h  38h
          1   Horizontal displayed characters     28h  50h  28h
          2   Horizontal synch position           2Dh  5Ah  2Dh
          3   Horizontal synch width in chars.    0Ah  0Ah  0Ah
          4   Vertical total lines -1             1Fh  1Fh  1Fh
              ((lines*scan lines) div chars)-1
          5   Vertical total lines -1             06h  06h  06h
              ((lines*scan lines) mod chars)-1
          6   Vertical displayed rows             19h  19h  19h
          7   Vertical synch position row         1Ch  1Ch  70h
          8   Interlace mode                      02h  02h  02h
          9   Maximum scan line address           07h  07h  01h
          A   Cursor starting scanline            06h  06h  06h
              1xxx xxxx Unused
              x1xx xxxx Blink rate
              xx1x xxxx 0=display, 1=no display
              xxx1 1111 starting scanline
                        0=top, 0Dh=button
          B   Cursor ending scanline              07h  07h  07h
              111x xxxx Unused
              xxx1 1111 ending scanline
                        0=top, 0Dh=button
          C   Memory address MSB                  00h  00h  00h
          D   Memory address LSB                  00h  00h  00h
          E   Cursor address MSB                  ??   ??   ??
          F   Cursor address LSB                  ??   ??   ??
         10   Light pen MSB                       ??   ??   ??
              Vertical Retrace Start (VGA)
         11   Light pen LSB                       ??   ??   ??
              Vertical Retrace End
         12   Vertical Display Enable End (VGA)
         13   Offset (VGA)
         14   Underline location (VGA)
         15   Start Vertical Blanking (VGA)
         16   End Vertical blanking (VGA)
         17   CRT mode control (VGA)
         18   Line Compare (VGA)