title: Why TSC needed when there is PIT available
body: <p>I was reading <a href="http://www.kerneltravel.net/downloads/UnderstandLinuxKernel2nd.pdf" rel="nofollow">"Understanding Linux Kernel"</a>, where it says about three clocks</p>
  
  <ol>
  <li><p>Real Time Clock</p></li>
  <li><p>Time Stamp Counter (TSC)</p></li>
  <li><p>Programmable Interval timer (PIT)</p></li>
  </ol>
  
  <p>I have few questions regarding these. </p>
  
  <ol>
  <li><p>Why Real Time clock is not used after computer is powered on?</p></li>
  <li><p>Is it running while the system is running?</p></li>
  <li><p>TSC register uses PIT, then why we need TSC register based timing?</p></li>
  <li><p>The value of IRQ8 from the /proc/interrupts gives 1 for cpu0 and 0
  for others even after system is running for some amount of time. Why
  only one interrupt from IRQ8, which is the real time clock.</p></li>
  </ol>
  
  <p>This is NOT a homework question</p>