INT 21 - DOS 1+ - SEQUENTIAL WRITE TO FCB FILE
        AH = 15h
        DS:DX -> opened FCB (see AH=0Fh)
        [DTA] = record to write
Return: AL = status
            00h successful
            01h disk full
            02h segment wrap in DTA
Notes:  writes a record of the size specified in the FCB beginning at the
          current file position, then updates the current block and current
          record fields in the FCB
        if less than a full sector is written, the data is placed in a DOS
          buffer to be written out at a later time
        not supported by MS Windows 3.0 DOSX.EXE DOS extender
SeeAlso: AH=14h,AH=1Ah,AH=40h,INT 2F/AX=1109h