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  Function 10H (decimal 16) closes a file and updates the file's directory
  entry. Call this function with the segmented address of the file's FCB in
  DS:DX. DOS returns AL = 00H if the function successfully closed the file
  or AL = FFH if an error occurred.

  It is good practice to use function 10H to explicitly close all files you
  opened with function 0FH or 16H. This ensures that the file contents are
  updated from DOS internal file buffers and that the corresponding
  directory entries are current.