BLINKER OVERLAY PAGEFRAME
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  Purpose : Enable use of the EMS pageframe for overlays

  Syntax  : BLINKER OVERLAY PAGEFRAME ON|OFF

  Default : OFF

  Example : # use the EMS pageframe
            BLINKER OVERLAY PAGEFRAME ON

  NOTE: Please read this section fully before using this command.

  This command will allow the overlay manager to utilize the expanded
  memory pageframe for the overlay pool (maximum size 64kb), on machines
  equipped with LIM EMS (3.2 or higher) expanded memory boards or
  emulators.

  To use this option there must be 64kb or more of available expanded
  memory, and the program's overlay OPSIZE must be less than or equal to
  64kb.  At runtime, if expanded memory is available and this option is
  enabled, the overlay manager will allocate 4 expanded memory pages, map
  them into the pageframe, and use the pageframe for overlay execution.
  The overlay OPSIZE will automatically be expanded to use all 64kb
  available in the pageframe, resulting in increased execution speed of
  overlaid code.

  There is no advantage to using this option if the EMS pageframe is
  located at an address below 640kb.

  When using this command, your program MUST NOT use ANY functions which
  access expanded memory via the EMS pageframe UNLESS the pageframe
  mapping state is preserved, and you make sure no overlaid functions are
  called prior to restoring the mapping state. Additionally, overlaid
  functions will not be able to access expanded memory via the EMS
  pageframe UNLESS they take particular care to determine within which
  page(s) they are executing, otherwise they may map themselves out of
  memory. Failure to adhere to the se guidelines will result in program
  failure, corruption of the program's expanded memory, or both.

  If both this command and the UMB command are used, and both the
  pageframe and a UMB are available, Blinker will use the pageframe.

  If either or both of these are specified and are not available on the
  end user's system, Blinker will automatically ignore these commands and
  allocate the overlay area from conventional memory. This use of the
  pageframe may be adjusted at runtime through the use of the BLINKER
  environment variable (see the 'Environment Variable' section in chapter
  2).

  With Clipper Summer'87 this command is only effective when the fixed
  overlay allocation method is selected using the command BLINKER OVERLAY
  FIXED.

  When this command is activated with Clipper Summer'87, Blinker will
  disable Clipper's use of expanded memory by automatically burning in the
  E0 setting to the CLIPPER environment variable.

  When this command is activated with CLIPPER 5.01, Blinker will
  automatically burn in the BADCACHE setting to the CLIPPER environment
  variable, causing Clipper 5.01 to save the pageframe mapping state
  between each access to expanded memory.  This will protect the overlay
  area in the pageframe from corruption.