Int 2F Fn 121A  U - Dos 3.0+ Internal - Get File's Drive                   [D]

   AX = 121Ah
   DS:SI -> filename

Return: AL = drive (0 = default, 1 = A:, etc, FFh = invalid)
   DS:SI -> filename without leading X: (if present)

See Also: INT 21/AH=19h,INT 21/AH=60h