Int 10 Fn F1  - EGA Register Interface Library - Write One Register        [V]

   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

See Also: AX=7F05h,AH=F0h,AH=F3h