title: Fix "NASM error: Invalid combination of opcode and operands" for add on Real Mode
body: <p>I want to increase the DS Register by 200h on Real Mode but it gives me the "invalid combination of opcode" error when doing it like:</p>
  
  <pre><code>add ds,200h
  </code></pre>
  
  <p>I tried with ax and there was no problem. I guess that it happens because the ds register itself but I have no explanation for this.</p>
  
  <p>It's not like I MUST do it that way, I fixed it by doing this instead:</p>
  
  <pre><code>mov  ax,200h
  mov  ds,ax 
  </code></pre>
  
  <p>But I would like to know the exact reason for this. Thanks for your time.</p>