Int 10 Fn 12  - Video - Alternate Function Select (ps, EGA, VGA, Mcga) -   [V]

   AH = 12h
   BL = 10h

Return: BH = 00h color mode in effect (I/O port 3Dxh)
        01h mono mode in effect (I/O port 3Bxh)
   BL = installed memory (00h = 64K, 01h = 128K, 02h = 192K, 03h = 256K)
   CH = feature bits
   CL = switch settings
       bit 0: switch 1 OFF
       bit 1: switch 2 OFF
       bit 2: switch 3 OFF
       bit 3: switch 4 OFF
       bits 4-7: unused

Note:  one possible check for the presence of an EGA or later display card
     is to call this function with BH=FFh; if not present, BH will be
     unchanged on return