title: Difference between addq and movl, then addq
body: <p>What is the functional difference between </p>
  
  <p><code>addq    (%rbx), %rax
  </code></p>
  
  <p>and</p>
  
  <p><code>movl    (%rbx), %ecx
   addq    %rcx, %rax</code></p>
  
  <p>in assembly?</p>
  
  <p>I think they do the same thing, but what is the functional difference? </p>