title: Why do x86-64 systems have only a 48 bit virtual address space?
body: <p>In a book I read the following:</p>
  
  <blockquote>
    <p>32-bit processors have 2^32 possible addresses, while current 64-bit processors have a 48-bit address space</p>
  </blockquote>
  
  <p>My expectation was that if it's a 64-bit processor, the address space should also be 2^64. </p>
  
  <p>So I was wondering what is the reason for this limitation?</p>