title: X86 instruction format according to SDM
body: <p>I see the following instruction format description of <code>ADD</code> according to X86 SDM manual.</p>
  
  <p><a href="https://i.stack.imgur.com/7zw73.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/7zw73.png" alt="enter image description here"></a></p>
  
  <p>This is somehow different from what I have seen before in 8086 instruction format. There we have</p>
  
  <pre><code>OPCODE  |  DIRECTION | WIDTH         MOD | REG | R/M
    6           1          1            2     3     3
  </code></pre>
  
  <p>According to the figure, if we assume <code>04</code> as a one byte instruction code, then it will be <code>00000100</code>. I guess <code>Ib</code> is the immediate byte. Therefore, the second byte will be the immediate number.</p>
  
  <p>Sound confusing. Any comment?</p>