Function 15h (21)        Sequential Write, Using FCBs

    Writes the record addressed by the current block and current record
    fields from the Disk Transfer Address (DTA), then increments the block
    and record fields.

       On entry:      AH         15h
                      DX:DS      Pointer to an opened FCB

       Returns:       AL         00h       If write was successful
                                 01h       If disk full
                                 02h       IF DTA too small (segment wrap)

       Network:       Requires Write access rights

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       Notes:         The sequential record number is taken from the
                      values in the current block (offset 0Ch) and the
                      current record (offset 20h) fields of the FCB. These
                      fields are automatically updated after the write
                      operation. The number of bytes to write is taken
                      from the record size field at offset 0Eh of the FCB.

                      The data to be written to disk is taken from the
                      current Disk Transfer Address (DTA). Use Function
                      1Ah to set the DTA. If the transfer of data would
                      cause segment wraparound, an error 2 is generated.

                      If the record written doesn't fill an entire sector,
                      DOS buffers the data for an eventual write when a
                      sector's worth of data is accumulated.

                      If the file specified has the read-only attribute
                      set, the write is not performed.