title: How does SAS distinguish missing from non-missing values?
body: <p>It's commonly said that "SAS missing values equal minus infinity". But There is a problem with that statement, since there can be 27 or 28 "flavors" of missing values (the default <code>.</code> and <code>.a</code> to <code>.z</code> and <code>._</code>), each having a predefined sort order. </p>
  
  <p>Since it can't be that some infinities are larger than others, I came to understand that:</p>
  
  <ol>
  <li>Missing values are <em>treated like</em> minus infinity when compared to valid numerical data, and that </li>
  <li>When compared to other missing values, they are ranked with another set of predefined rules.</li>
  </ol>
  
  <p>So my question is: at the lowest level, how does SAS store numerical data in a way that it can distinguish the missing from the non-missing numerical values? Is there a "missingness bit" like there is a "sign bit"?</p>