IBM Color Graphics Adapter

  3D8h (W): Mode Control register
    bit 0  80x25 Alpha mode if set, 40x25 else
        1  Graphics mode if set, alpha else.
        2  BW mode if set, color else
        3  Enable video signal if set
        4  640x200 Graphics mode if set, 320x200 else
        5  if set bit 7 of the attribute controlls background, else blink

  3D9h (W): Color Select register
           The function of this register depends on the active mode. 
           Text modes:      320x200 modes:         640x200 mode:
        0  Blue border      Blue background        Blue foreground
        1  Green border     Green background       Green foreground
        2  Red border       Red background         Red foreground
        3  Bright border    Bright background      Bright foreground
        4  Backgr. color    Alt. intens. colors
        5  No func.         Selects palette 

           Palette 0 is Green, red and brown, 
           palette 1 is Cyan, magenta and white.

  3DAh (R): Status register 
    bit 0  If set retrace active and the video memory may be
           accessed by the PC.
        1  Lightpen trigger has occured if set
        2  Lightpen switch is off if set  
        3  Vertical retrace active if set.
           Video RAM can be accessed for the next 1.25 ms.




  Video Modes:

  00h T   40  25  2   (8x8)
  01h T   40  25  2   (8x8)
  02h T   80  25  16  (8x8)
  03h T   80  25  16  (8x8)
  04h G  320 200  4
  05h G  320 200  4
  06h G  640 200  2