The OPEN procedure opens a file and allows you to  specify  file
  characteristics.

  Syntax:

            OPEN( file_variable
                  ,[[file_name]]
                  ,[[history]]
                  ,[[record_length]]
                  ,[[access_method]]
                  ,[[record_type]]
                  ,[[carriage_control]]
                  ,[[organization]]
                  ,[[disposition]]
                  ,[[file_sharing]]
                  ,[[user_action]]
                  ,[[default_file_name]]
                  ,[[ERROR := error_recovery]] )

            OPEN( FILE_VARIABLE     := file_variable
               [[,FILE_NAME         := file_name]]
               [[,HISTORY           := history]]
               [[,RECORD_LENGTH     := record_length]]
               [[,ACCESS_METHOD     := access_method]]
               [[,RECORD_TYPE       := record_type]]
               [[,CARRIAGE_CONTROL  := carriage_control]]
               [[,ORGANIZATION      := organization]]
               [[,DISPOSITION       := disposition]]
               [[,SHARING           := file_sharing]]
               [[,USER_ACTION       := user_action]]
               [[,DEFAULT           := default_file_name]]
               [[,ERROR             := error_recovery]] )

  Before the OPEN procedure is called, the file  is  in  undefined
  mode; its mode does not change after OPEN has been executed.

  You cannot use OPEN on a file variable that is already open.

  If you use INPUT or OUTPUT, VSI  Pascal  implicitly  opens  them
  just  before their first use.  VSI Pascal implicitly opens INPUT
  with a history of READONLY.  If you choose, you  can  explicitly
  open INPUT or OUTPUT; to do this, call the OPEN procedure at any
  point in your compilation unit before  you  use  the  first  I/O
  routine on that file.

  Because the RESET, REWRITE,  and  EXTEND  procedures  implicitly
  open  files, you need not always use the OPEN procedure.  RESET,
  REWRITE, and EXTEND impose the same  defaults  as  OPEN,  except
  where noted (in the HISTORY parameter).

  You must use the OPEN procedure  to  create  a  TEXT  file  with
  fixed-length  components,  to  create  a  file  with relative or
  indexed organization, to open a file for direct or keyed access,
  or to specify a line length other than the default for a line in
  a TEXT file.

  See the "HP Pascal Language Reference Manual" for  the  complete
  description of the OPEN procedure.