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  This section is only intended for those wanting to use the
  Extend System, or wishing to understand more about the
  restrictions imposed by protected mode operation (of 286, 386,
  486 chips) and/or Windows.
  
  The full Extend System is available, but the following rules
  apply:
  
  (a)  programs must not modify anything stored in a CODE
  segment, as such segments are not writable (this is enforced
  by 286, 386, 486 and Pentium chips)
  
  (b)  direct access to hardware is not a good idea, as this is
  often incompatible with Windows (in particular, don't try to
  access video ram; accessing interrupt controllers or comms
  ports may work)
  
  (c)  never do arithmetic on segment register values or segment
  addresses (they are selectors which the protected mode
  hardware and Windows control; you can load/store/compare
  selectors, of course)
  
  (d)  don't try to load a segment register with a value which
  is not a valid selector (e.g. don't use these registers as
  temporaries)
  
  (e)  don't try to control the features of the cpu which are
  only available to protected mode programs (e.g. the interrupt
  flag bit)
  
  (f)  using the VM API is allowed, but rule (c) still applies
  (this should not cause any problems)

  You can, of course, use Windows API functions, such as
  GlobalAlloc(), if you wish: see the next section.