The DEFINE FILE statement establishes the size and structure of
  files with relative organization and associates them with a logical
  unit number.  The DEFINE FILE statement is comparable to the OPEN
  statement (in situations where you can use the OPEN statement, it
  is the preferable mechanism for creating and opening files).
  Statement format:

     DEFINE FILE u(m, n, U, asv) [,u(m, n, U, asv)]...

     u    Is an integer constant or variable that specifies the
          logical unit number.

     m    Is an integer constant or variable that specifies the
          number of records in the file.

     n    Is an integer constant or variable that specifies the
          length of each record in 16-bit words (2 bytes).

     U    Specifies that the file is unformatted (binary); this
          is the only acceptable entry in this position.

     asv  Is an integer variable, called the associated variable
          of the file.  At the end of each direct access I/O
          operation, the record number of the next higher numbered
          record in the file is assigned to "asv"; "asv" must not
          be a dummy argument.

  The DEFINE FILE statement specifies that a file containing "m"
  fixed-length records, each composed of n 16-bit words, exists (or
  is to exist) on the specified logical unit.  The records in the
  file are numbered sequentially from 1 through "m".

  A DEFINE FILE statement must be executed before the first direct
  access I/O statement referring to the specified file, even though
  the DEFINE FILE statement does not itself open the file.  The file
  is actually opened when the first direct access I/O statement for
  the unit is executed.

  If this I/O statement is a WRITE statement, a new relative
  organization file is created.  If it is a READ or FIND statement,
  an existing file is opened, unless the specified file does not
  exist.  If a file does not exist, an error occurs.