title: how does multilevel page table save memory spaces
body: <p>So I have learned that <a href="http://en.wikipedia.org/wiki/Page_table#Multilevel_page_table" rel="nofollow">Multilevel page table</a> is good at saving memory spaces during memory paging/mapping period.<br>
  Suppose I have a <code>page directory</code> with <strong>n</strong> items, so I will have <strong>n</strong> <code>page table</code>, so it costs just as large as space as single-level page table.<br>
  Is there anything I missed?</p>