title: What is the structure of an application protocol data unit (APDU) command and response?
body: <p>I am trying to learn <strong>Java Card</strong>.  I have just started and have not found many resources.  My first question is how to understand APDU commands.  (such as those defined in ISO/IEC 7816-4)</p>
  
  <p>For example, if I see a byte pattern such as <strong><code>10101010</code></strong> how can I understand the meaning of this, and in particular determine the <strong><code>CLA</code></strong> or <strong><code>INS</code></strong>, for example?</p>