title: how to get last n bits by bit-op?
body: <p>I want to get last n bit of number, for example:</p>
  
  <pre><code>num = 0b111111111
  # if I want to get last 8 bits, it will be 0b11111111
  # if I want to get last 2 bits, it will be 0b11
  </code></pre>
  
  <p>I thought this can be ok:</p>
  
  <pre><code>bits = 1 &lt;&lt; n
  little = num &amp; (~bits)
  </code></pre>
  
  <p>but this is wrong, if n = 8, it get <code>0b110111111</code></p>