title: What's the point of instructions with only the REX prefix in 64bit mode?
body: <p>For example one of the MOV has 2 versions, one with REX, one without (from Intel's doc) :</p>
  
  <pre><code>88 /r MOV r/m8, r8
  REX + 88 /r MOV r/m8***, r8***
  
  ***In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH.
  </code></pre>
  
  <p>From what I understand, the 2 instructions are identical except the second one uses an additional byte and provides less options ... So basically it is useless.</p>
  
  <p>What am I missing ?</p>