BLINKER OVERLAY UMB
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  Purpose : Enable use of the XMS UMBs for overlays

  Syntax  : BLINKER OVERLAY UMB ON|OFF

  Default : ON

  Example : # do not use XMS UMB's for the overlay area
            BLINKER OVERLAY UMB OFF

  This command used in conjunction with a suitable UMB driver allows the
  Blinker overlay manager to execute overlays in upper memory blocks above
  640kb.  Blinker's use of UMBs is dependent on there being a suitably
  large contiguous block available.  The size of the UMB allocated will be
  the same as the program's overlay opsize.

  Use of this command requires a 386 processor (or better), and an XMS 2.0
  extended memory driver that supports UMB's (upper memory blocks).  UMB's
  are blocks of memory which are mapped into the area above 640kb, using
  the memory management capabilities of the 386/486 family of processors.
  Upper memory blocks are sometimes referred to as 'high memory' or LOADHI
  areas.

  If your XMS driver supports the loading of drivers into high memory, it
  is likely that you can use this feature. Blinker will need a UMB at
  least as large as the overlay area, so you may need to alter the
  placement of your programs in high memory to facilitate this.  Most 386
  memory managers provide utilities to help optimize this area.

  If both this command and the PAGEFRAME command are used, and both the
  pageframe and a UMB are available, then 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 UMB's
  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 with the command BLINKER OVERLAY
  FIXED.