DEFINE_FILE

 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.