title: Set address pointer to pointer in PL/I
body: <p>I don't understand source code PL/I:</p>
  
  <pre><code>      DCL AA CHAR(10)   BASED(P1);
        DCL BB CHAR(10)  INIT ("BBBBBBBBBB");
        DCL CC CHAR(10)  INIT ("CCCCCCCCCC");
        DCL DD CHAR(10)  INIT ("DDDDDDDDDDD");
        DCL ADDR BUILTIN;
        DCL P1 PTR;
        DCL P2 PTR;
  
          P1 = ADDR(BB);
          Display(AA);
  
          P1 = ADDR(P2);
          Display(AA);
  
          P2 = ADDR(DD);
          Display(AA);
  </code></pre>
  
  <p>help me understand display variable AA. Many thanks</p>