title: What do the abbriviations (Rn, Rd, ...) in the instruction set of ARM mean?
body: <p>recently i checked the Instruction Set for an ARM Cortex-M3 processor.
  For example:</p>
  
  <pre><code>ADD &lt;Rd&gt;, &lt;Rn&gt;, &lt;Rm&gt;
  </code></pre>
  
  <p>What do those abbriviations mean exactly? 
  I guess they mean different kinds of addresses, like directely addressed, relatively addressed or so.
  But what exactly?</p>
  
  <p>Thanks!</p>