INT 21 - DOS 1+ - CLOSE FILE USING FCB
        AH = 10h
        DS:DX -> File Control Block (see AH=0Fh)
Return: AL = status
            00h successful
            FFh failed
Notes:  a successful close forces all disk buffers used by the file to be
          written and the directory entry to be updated
        not supported by MS Windows 3.0 DOSX.EXE DOS extender
SeeAlso: AH=16h,AH=3Eh