Port 3B5  -  Mono/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!

        Register    Use                                         Contents
          0         Horizontal total characters - 1               61h
          1         Horizontal displayed characters               50h
          2         Horizontal synch position                     52h
          3         Horizontal synch width in chars.              0Fh
          4         Vertical total lines -1                       19h
                    ((lines*scan lines) div chars)-1
          5         Vertical total lines -1                       06h
                    ((lines*scan lines) mod chars)-1
          6         Vertical displayed rows                       19h
          7         Vertical synch position row                   19h
          8         Interlace mode                                02h
          9         Maximum scan line address                     0Dh
          A         Cursor starting scanline                      0Bh
                    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                        0Ch
                    111x xxxx Unused
                    xxx1 1111 ending scanline
                              0=top, 0Dh=button
          C         Memory address MSB                            00h
          D         Memory address LSB                            00h
          E         Cursor address MSB                            ??
          F         Cursor address LSB                            ??
         10         Reserved for light pen
         11         Reserved for light pen