INT 21 - DOS 1+ - RANDOM BLOCK WRITE TO FCB FILE
        AH = 28h
        CX = number of records to write
        DS:DX -> opened FCB (see AH=0Fh)
        [DTA] = records to write
Return: AL = status
            00h successful
            01h disk full or file read-only
            02h segment wrap in DTA
        CX = number of records written
Notes:  write begins at current file position as specified in FCB; the file
          position is updated after writing
        if CX = 0000h on entry, no data is written; instead the file size is
          adjusted to be the same as the file position specified by the random
          record and record size fields of the FCB
        if the data to be written is less than a disk sector, it is copied into
          a DOS disk buffer, to be written out to disk at a later time
        not supported by MS Windows 3.0 DOSX.EXE DOS extender
SeeAlso: AH=27h,AH=40h,AH=59h