INT 10 - EGA Register Interface Library - WRITE ONE REGISTER
        AH = F1h
        DX = group index (see AH=F0h)
            if single register:
                BL = value to write
            otherwise
                BL = register number
                BH = value to write
Return: BL = data
Note:   the RIL is provided by EGA.SYS, the Microsoft Mouse driver, the OS/2
          compatibility box, and others
SeeAlso: AH=F0h,AH=F3h