INT 21 - DOS 1+ - READ RANDOM RECORD FROM FCB FILE
        AH = 21h
        DS:DX -> opened FCB (see AH=0Fh)
Return: AL = status
            00h successful
            01h end of file, no data read
            02h segment wrap in DTA, no data read
            03h end of file, partial record read
        [DTA] = record read from file
Notes:  the record is read from the current file position as specified by the
          random record and record size fields of the FCB
        the file position is not updated after reading the record
        if a partial record is read, it is zero-padded to the full size
        not supported by MS Windows 3.0 DOSX.EXE DOS extender
SeeAlso: AH=22h,AH=27h,AH=3Fh