title: How to use a 64bit call gate
body: <p>In intel software developer maunal, the call instruction support:</p>
  
  <pre><code>call r/m64
  call m16:32
  call m16:64
  </code></pre>
  
  <p>In the user code, if I want to transfer to ring 0 via a 64-bit call-gate with selector 47.
  How to write the instruction in assembly using intel syntax?</p>
  
  <p>Tested:</p>
  
  <pre><code>call far [mem]; wrong
  call qword ptr [mem]; wrong
  </code></pre>
  
  <p>memory content:</p>
  
  <pre><code>[mem + 0]: qword 0x00
  [mem + 8]: word 47
  </code></pre>