title: How ARM system mode is useful in nested interrupt?
body: <p>Can someone explain me, how exactly system mode works when we have nested interrupt ?</p>
  
  <p>As per ARM site </p>
  
  <blockquote>
    <p>ARMv4 and later architectures include a privileged mode called System mode, to overcome this problem. System mode shares the same registers as User mode, it can run tasks that require privileged access, and exceptions no longer overwrite the link register.</p>
  </blockquote>
  
  <p>So how system mode make use of registers of User mode ?</p>
  
  <p>And upto what hierarchy level nested interrupts can be handled?</p>