Alternate function select (PS, EGA, VGA, MCGA)

        AH = 12h
        BL = 10h: return EGA information
             Returns:
             BH = 0: color mode in effect (3Dx)
                  1: mono mode in effect (3Bx)
             BL = 0:  64k bytes memory installed
                  1: 128k bytes memory installed
                  2: 192k bytes memory installed
                  3: 256k bytes memory installed
             CH = feature bits
             CL = switch settings
        BL = 20h: select alternate print screen routine
        BL = 30h: select vertical resolution for alphanumeric modes (VGA only)
             ???
        BL = 31h: enable/disable default palette loading (VGA/MCGA)
             ??? = on/off
        BL = 32h: enable/disable video addressing (VGA/MCGA)
             ??? = on/off
        BL = 33h: enable/disable default gray-scale summing (VGA/MCGA)
             ??? = on/off
        BL = 34h: enable/disable alphanumeric cursor emulation (VGA only)
             ??? = on/off
        BL = 35h: display-switch interface
             ???