VMS Help PASCAL, Input Output, General, OPEN, History  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  The 'history' is a value that indicates whether the file  exists
  or  if  Compaq  Pascal must create the file.  If you specify OLD
  and if Compaq Pascal cannot find the file, an error occurs.   If
  you  specify  READONLY,  you can only read from the file; if you
  attempt to write to the file, an error occurs.  If  you  specify
  UNKNOWN,  Compaq Pascal looks for an existing file but creates a
  new file if there is no existing file.  If you  specify  OLD  or
  UNKNOWN  and  if  the  attempt to open the file generates a file
  protection error, Compaq Pascal tries again using READONLY.

  NEW is the default for OPEN/REWRITE openings, while OLD  is  the
  default for EXTEND/RESET openings.