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The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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▀ The records are written (logically, not necessarily physically) to disk
  from memory at the current disk transfer area address, specified by the
  most recent call to Int 21H Function 1AH. If the size and location of
  the buffer are such that a segment overflow or wraparound would occur,
  the function fails with a return code of 02H.
 
▀ The file location of the data to be written is determined by the combi-
  nation of the relative-record field (offset 21H) and the record size
  field (offset 0EH) of the FCB. The default record size is 128 bytes.
 
▀ After the disk transfer is performed, the current block (offset 0CH),
  current record (offset 020H), and relative-record (offset 21H) fields
  of the FCB are updated to point to the next record in the file.
 
▀ If this function is called with CX = 00H, no data is written to the
  disk but the file is extended or truncated to the length specified by
  combination of the record size (offset 0EH) and the relative-record
  (offset 21H) fields of the FCB.
 
▀ Compare with Int 21H Function 22H, which transfers only one record per
  function call and does not update the FCB relative-record field.
 
▀ [3.0+] If the program is running on a network, the user must have Write
  access rights to the directory containing the file to be written.
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