03D0-03DF ---- CGA   (Color Graphics Adapter)


03D0       same as 03D4
03D1       same as 03D5
03D2       same as 03D4
03D3       same as 03D5
03D4   w   CRT (6845) index register   (EGA/VGA)
       selects which register (0-11h) is to be accessed through 3B5
03D5   w   CRT (6845) data register    (EGA/VGA)
       selected by port 3B4. registers C-F may be read
        (for registers see at 3B5)
03D6       same as 03D4
03D7       same as 03D5
03D8   r/w CGA mode control register  (except PCjr)
        bit 7-6      not used
        bit 5   = 1  blink enabled
        bit 4   = 1  640*200 graphics mode
        bit 3   = 1  video enabled
        bit 2   = 1  monochrome signal
        bit 1   = 0  text mode
            = 1  320*200 graphics mode
        bit 0   = 0  40*25 text mode
            = 1  80*25 text mode

03D9   r/w CGA palette register
        bit 7-6      not used
        bit 5   = 0 active color set: red, green brown
            = 1 active color set: cyan, magenta, white
        bit 4       intense colors in graphics, background colors text
        bit 3       intense border in 40*25, intense background
320*200
                intense foreground in 640*200
        bit 2       red border in 40*25, red background in 320*200,
                red foreground in 640*200
        bit 1       green border in 40*25, green background in
320*200,
                green foreground in 640*200
        bit 0       blue border in 40*25, blue background in 320*200,
                blue foreground in 640*200

03DA   r   CGA status register EGA/VGA: input status 1 register
        bit 7-4     not used
        bit 3 = 1   in vertical retrace
        bit 2 = 1   light pen switch is off
        bit 1 = 1   positive edge from light pen has set trigger
        bit 0 = 0   do not use memory
              = 1   memory access without interfering with display

03DA   w   EGA/VGA feature control register

03DB   w   clear light pen latch
03DC   r/w preset licht pen latch

03DF       CRT/CPU page register  PCjr only