title: Why MOV AH,1 is not supported in 64 bit mode of intel microprocessor?
body: <p>In the book "THE INTEL MICROPROCESSORS" of Barry B. Brey, it is written that </p>
  
  <pre><code>MOV AH, 1
  </code></pre>
  
  <p>is not allowed in 64 bit mode, but allowed in 32 bit or 16 bit mode. If <code>MOV AL, 1</code> can be allowed in 64 bit mode, what is the problem with <code>MOV AH, 1</code> ?</p>