03B0-03BF ---- MDA  (Monochrome Display Adapter based on 6845)


03B0       same as 03B4
03B1       same as 03B5
03B2       same as 03B4
03B3       same as 03B5
03B4   w   MDA CRT index register   (EGA/VGA)
       selects which register (0-11h) is to be accessed through 3B5
03B5   r/w MDA CRT data register    (EGA/VGA)
       selected by port 3B4. registers C-F may be read
        00  horizontal total
        01  horizontal displayed
        02  horizontal sync position
        03  horizontal sync pulse width
        04  vertical total
        05  vertical displayed
        06  vertical sync position
        07  vertical sunc pulse width
        08  interlace mode
        09  maximum scan lines
        0A  cursor start
        0B  cursor end
        0C  start address high
        0D  start address low
        0E  cursor location high
        0F  cursor location low
        10  light pen high
        11  light pen low
03B6       same as 03B4
03B7       same as 03B5
03B8   r/w MDA mode control register
        bit 7 not used
        bit 6 not used
        bit 5 enable blink
        bit 4 not used
        bit 3 video enable
        bit 2 not used
        bit 1 not used
        bit 0 high resolution mode

03B9       reserved for color select register on color adapter

03BA   r   CRT status register  EGA/VGA: input status 1 register
        bit 7-4 reserved
        bit 3   black/white video
        bit 2-1 reserved
        bit 0   horizontal drive

03BA   w   EGA/VGA feature control register

03BB       reserved for light pen strobe reset