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  Function 15H (decimal 21) writes a sequential record and is the companion
  to function 14H. As with function 14H, DOS tracks its current position in
  the file by updating the FCB whose address you pass in DS:DX. DOS copies
  the data from the current DTA to the file and reports the status of the
  write operation in AL.

  If AL = 00H, the write operation was a success. If AL = 01H, the disk was
  full and the record was not written. If AL = 02H, the amount of memory
  remaining in the DTA's segment was less than the record size, so DOS
  aborted the write operation.

  It's important to note that data is logically written by this function,
  but not necessarily physically written. DOS buffers output data until it
  has a complete disk sector to write--only then does DOS actually transfer
  the data to the disk.