6   Dynamic Overlaying
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     Clipper 5.0, using the supplied special version of .RTLink, manages
     compiled Clipper code using a technique called dynamic overlaying.
     This technique eliminates the need for complicated overlay structures
     and allows flexible runtime management of compiled code.

     During linking, .RTLink breaks compiled Clipper modules into fixed-size
     pages.  These pages are stored either in the executable file or in
     separate overlay files.  The paging architecture eliminates
     restrictions and memory calculations based on the size of compiled
     functions or modules.  Large modules are broken into multiple pages;
     small functions are grouped together in a single page.

     At execution time, the dynamic overlay manager loads pages based on
     information embedded in the .EXE by the linker.  The dynamic pages are
     loaded into VM segments, allowing the VMM to manage the overlay pages
     on a competitive basis with other uses of memory.  The paging
     architecture allows the system to discard low-use sections of code even
     if the code is associated with a pending active function; overlays are
     not required to nest in parallel with function activations.  Code
     pages which are being heavily used are maintained in memory by the
     VMM's LRU swapping policy.

     When possible, the VMM will place dynamic overlay pages in EMM,
     reducing overlay reads.  Overlay pages are never written to the VMM
     disk swap file, however.  If a VM segment containing an overlay page is
     to be removed from memory altogether, it is simply discarded.  If it is
     needed subsequently, it is re-read from the overlay file.

     In addition to virtual memory, the dynamic overlay manager uses a
     dedicated area of real memory to cache the most active dynamic overlay
     pages.

     Configuration:  The dynamic overlay manager will hold overlay pages
     in virtual memory only as long as it can maintain an active file handle
     for the disk file containing the pages.  If the file is closed, the
     pages are discarded from virtual memory.  The maximum number of file
     handles used by the dynamic overlay manager can be controlled using the
     DYNF setting of the CLIPPER environment variable.  Valid settings are
     from 1 to 8.  The default setting is 2.  For applications which use
     many separate overlay files, increasing this setting can improve
     performance.

     Example:  SET CLIPPER=DYNF:4