title: Reserved bit meaning
body: <p>In the file <a href="http://lxr.linux.no/#linux+v3.9.4/arch/x86/mm/fault.c#L29" rel="nofollow">http://lxr.linux.no/#linux+v3.9.4/arch/x86/mm/fault.c#L29</a>, there are some information about page fault error code:</p>
  
  <pre><code>Page fault error code bits:
  bit 0 ==    0: no page found       1: protection fault
  bit 1 ==    0: read access         1: write access
  bit 2 ==    0: kernel-mode access  1: user-mode access
  bit 3 ==                           1: use of reserved bit detected
  bit 4 ==                           1: fault was an instruction fetch
  </code></pre>
  
  <p>In the case "bit 3" "1: use of reserved bit detected", which are the possible scenario that can make kernel set this bit to 1? Is there any documents about it?</p>
  
  <p>Thank you</p>