NaCl SDK doc: Recover sandbox internal doc for x86-64

native_client_sdk/doc_generated/reference/sandbox_internals/index.html

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 <section id="sandbox-internals">
 <span id="sandbox-internals-index"></span><h1 id="sandbox-internals"><span id="sandbox-internals-index"></span>Sandbox Internals</h1>
 <p>The sandbox internals documentation describes implementation details for
 Native Client sandboxing, which is also used by Portable Native
 Client. These details can be useful to reimplement a sandbox, or to
 write assembly code that follows sandboxing rules for Native Client
 (Portable Native Client does not allow platform-specific assembly code).</p>
 <p>As an implementation detail, the Native Client sandboxes described here
 are currently used by Portable Native Client to execute code on the
 corresponding machines in a safe manner. The portable bitcode contained
 in a <strong>pexe</strong> is translated to a machine-specific <strong>nexe</strong> before
 execution. This may change at a point in time: Portable Native Client
 doesn&#8217;t necessarily need these sandboxes to execute code on these
 machines. Note that the Portable Native Client compiler itself is also
 untrusted: it too runs in a Native Client sandbox described below.</p>
 <p>Native Client has sandboxes for:</p>
 <ul class="small-gap">
 <li><a class="reference internal" href="/native-client/reference/sandbox_internals/arm-32-bit-sandbox.html#arm-32-bit-sandbox"><em>ARM 32-bit</em></a>.</li>
 <li>x86-32: the original design is described in <a class="reference external" href="http://research.google.com/pubs/archive/34913.pdf">Native Client: A Sandbox
 for Portable, Untrusted x86 Native Code</a>, the current
 design has changed slightly since then.</li>
-<li>x86-64: the original design is described in <a class="reference external" href="http://research.google.com/pubs/archive/35649.pdf">Adapting Software Fault
-Isolation to Contemporary CPU Architectures</a>, the current
-design has changed slightly since then.</li>
+<li><a class="reference internal" href="/native-client/reference/sandbox_internals/x86-64-sandbox.html#x86-64-sandbox"><em>x86-64</em></a>.</li>
 <li>MIPS32, described in the <a class="reference external" href="https://code.google.com/p/nativeclient/issues/attachmentText?id=2275&amp;aid=22750018000&amp;name=native-client-mips-0.4.txt">overview of Native Client for MIPS</a>,
 and <a class="reference external" href="https://code.google.com/p/nativeclient/issues/detail?id=2275">bug 2275</a>.</li>
 </ul>
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