14.8.12 WINDOWSIZE Clause
  The WINDOWSIZE clause specifies the number of block re-
  trieval pointers in memory for the file. WINDOWSIZE is not
  a file attribute, and therefore can be changed any time you
  open a file.

  Retrieval pointers are associated with the file header and
  point to contiguous blocks on disk. By keeping retrieval
  pointers in memory, you can reduce the I/O associated with
  locating a record because the operating system does not
  have to access the file header for pointers as frequently. The
  number of retrieval pointers in memory at any one time is
  determined by the system default or by the value you supply
  in the WINDOWSIZE clause. The usual default number of
  retrieval pointers is 7.

  A value of zero specifies the default number of retrieval
  pointers. A value of -1 specifies mapping the entire file, if
  possible. Values from -128 to -2 are reserved.