<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2007</year>
      <year>2020</year>
      <holder>Ericsson AB, All Rights Reserved</holder>
    </copyright>
    <legalnotice>
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
 
      http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.

  The Initial Developer of the Original Code is Ericsson AB.
    </legalnotice>

    <title>External Term Format</title>
    <prepared>Kenneth</prepared>
    <docno></docno>
    <date>2007-09-21</date>
    <rev>PA1</rev>
    <file>erl_ext_dist.xml</file>
  </header>

  <section>
    <title>Introduction</title>
    <p>
      The external term format is mainly used in the distribution
      mechanism of Erlang.
    </p>
    <p>
      As Erlang has a fixed number of types, there is no need for a
      programmer to define a specification for the external format used
      within some application.
      All Erlang terms have an external representation and the interpretation
      of the different terms is application-specific.
    </p>
    <p>
      In Erlang the BIF <seemfa marker="erts:erlang#term_to_binary/1">
      <c>erlang:term_to_binary/1,2</c></seemfa> is used to convert a
      term into the external format.
      To convert binary data encoding to a term, the BIF
      <seemfa marker="erts:erlang#binary_to_term/1">
      <c>erlang:binary_to_term/1</c></seemfa> is used.
    </p>
    <p>
      The distribution does this implicitly when sending messages across
      node boundaries.
    </p>
    <marker id="overall_format"/>
    <p>
      The overall format of the term format is as follows:
    </p>
    <table align="left">
      <row>
        <cell align="center">1</cell>
        <cell align="center">1</cell>
        <cell align="center">N</cell>
      </row>
      <row>
        <cell align="center"><c>131</c></cell>
        <cell align="center"><c>Tag</c></cell>
        <cell align="center"><c>Data</c></cell>
      </row>
    <tcaption>Term Format</tcaption></table>
    <note>
      <p>
        When messages are
        <seeguide marker="erl_dist_protocol#connected_nodes">passed between
        connected nodes</seeguide> and a
        <seeguide marker="#distribution_header">distribution
        header</seeguide> is used, the first byte containing the version
        number (131) is omitted from the terms that follow the distribution
        header. This is because the version number is implied by the version
        number in the distribution header.
      </p>
    </note>
    <p>
      The compressed term format is as follows:
    </p>
    <table align="left">
      <row>
        <cell align="center">1</cell>
        <cell align="center">1</cell>
        <cell align="center">4</cell>
        <cell align="center">N</cell>
      </row>
      <row>
        <cell align="center"><c>131</c></cell>
        <cell align="center"><c>80</c></cell>
        <cell align="center"><c>UncompressedSize</c></cell>
        <cell align="center"><c>Zlib-compressedData</c></cell>
      </row>
    <tcaption>Compressed Term Format</tcaption></table>
    <p>
      Uncompressed size (unsigned 32-bit integer in big-endian byte order)
      is the size of the data before it was compressed.
      The compressed data has the following format when it has been expanded:
    </p>
    <table align="left">
      <row>
        <cell align="center">1</cell>
        <cell align="center">Uncompressed Size</cell>
      </row>
      <row>
        <cell align="center"><c>Tag</c></cell>
        <cell align="center"><c>Data</c></cell>
      </row>
    <tcaption>Compressed Data Format when Expanded</tcaption></table>
    <marker id="utf8_atoms"/>
    <note>
      <p>As from ERTS 9.0 (OTP 20), atoms may contain any Unicode
        characters and are always encoded using the UTF-8 external formats
	<seeguide marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seeguide>
        or <seeguide marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seeguide>.
	The old Latin-1 formats <seeguide marker="#ATOM_EXT"><c>ATOM_EXT</c></seeguide>
        and <seeguide marker="#SMALL_ATOM_EXT"><c>SMALL_ATOM_EXT</c></seeguide>
	are deprecated and are only kept for backward
	compatibility when decoding terms encoded by older nodes.</p>
      <p>Support for UTF-8 encoded atoms in the external format has been
        available since ERTS 5.10 (OTP R16). This ability allows such old nodes
	to decode, store and encode any Unicode atoms received from a new OTP 20
        node.</p>
      <p>The maximum number of allowed characters in an atom is 255. In the
        UTF-8 case, each character can need 4 bytes to be encoded.</p>
    </note>
  </section>

  <section>
    <marker id="distribution_header"/>
    <title>Distribution Header</title>
    <p>
      The distribution header is sent by the erlang distribution to
      carry metadata about the coming
      <seeguide marker="erl_dist_protocol#control_message">control message</seeguide>
      and potential payload. It is primarily used to handle the atom cache
      in the Erlang distribution. Since OTP-22 it is also used to fragment
      large distribution messages into multiple smaller fragments.
      For more information about how the distribution uses the distribution header,
      see the documentation of the
      <seeguide marker="erl_dist_protocol#connected_nodes">protocol between
      connected nodes</seeguide> in the
      <seeguide marker="erl_dist_protocol">distribution protocol</seeguide>
      documentation.
    </p>
    <p>
      Any <seeguide marker="#ATOM_CACHE_REF">ATOM_CACHE_REF</seeguide>
      entries with corresponding <c>AtomCacheReferenceIndex</c> in terms
      encoded on the external format following a distribution header refer
      to the atom cache references made in the distribution header. The range
      is 0 &lt;= <c>AtomCacheReferenceIndex</c> &lt; 255, that is, at most 255
      different atom cache references from the following terms can be made.
    </p>
    <section>
      <title>Normal Distribution Header</title>
      <p>
        The non-fragmented distribution header format is as follows:
      </p>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">NumberOfAtomCacheRefs/2+1 | 0</cell>
          <cell align="center">N | 0</cell>
        </row>
        <row>
          <cell align="center"><c>131</c></cell>
          <cell align="center"><c>68</c></cell>
          <cell align="center"><c>NumberOfAtomCacheRefs</c></cell>
          <cell align="center"><c>Flags</c></cell>
          <cell align="center"><c>AtomCacheRefs</c></cell>
        </row>
      <tcaption>Normal Distribution Header Format</tcaption></table>
      <p>
        <c>Flags</c> consist of <c>NumberOfAtomCacheRefs/2+1</c> bytes,
        unless <c>NumberOfAtomCacheRefs</c> is <c>0</c>. If
        <c>NumberOfAtomCacheRefs</c> is <c>0</c>, <c>Flags</c> and
        <c>AtomCacheRefs</c> are omitted. Each atom cache reference has
        a half byte flag field. Flags corresponding to a specific
        <c>AtomCacheReferenceIndex</c> are located in flag byte number
        <c>AtomCacheReferenceIndex/2</c>. Flag byte 0 is the first byte
        after the <c>NumberOfAtomCacheRefs</c> byte. Flags for an even
        <c>AtomCacheReferenceIndex</c> are located in the least significant
        half byte and flags for an odd <c>AtomCacheReferenceIndex</c> are
        located in the most significant half byte.
      </p>
      <p>
        The flag field of an atom cache reference has the following
        format:
      </p>
      <table align="left">
        <row>
          <cell align="center">1 bit</cell>
          <cell align="center">3 bits</cell>
        </row>
        <row>
          <cell align="center"><c>NewCacheEntryFlag</c></cell>
          <cell align="center"><c>SegmentIndex</c></cell>
        </row>
      <tcaption></tcaption></table>
      <p>
        The most significant bit is the <c>NewCacheEntryFlag</c>. If set,
        the corresponding cache reference is new. The three least
        significant bits are the <c>SegmentIndex</c> of the corresponding
        atom cache entry. An atom cache consists of 8 segments, each of size
        256, that is, an atom cache can contain 2048 entries.
      </p>
      <p>
        Another half byte flag field is located along with flag fields for
        atom cache references. When <c>NumberOfAtomCacheRefs</c> is even,
        this half byte is the least significant half byte of the byte that
        follows the atom cache references. When <c>NumberOfAtomCacheRefs</c>
        is odd, this half byte is the most significant half byte of the last
        byte of the atom cache references (on the wire, it will appear before
        the last cache reference). It has the following format:
      </p>
      <table align="left">
        <row>
          <cell align="center">3 bits</cell>
          <cell align="center">1 bit</cell>
        </row>
        <row>
          <cell align="center"><c>CurrentlyUnused</c></cell>
          <cell align="center"><c>LongAtoms</c></cell>
        </row>
      <tcaption></tcaption></table>
      <p>
        The least significant bit in that half byte is flag <c>LongAtoms</c>.
        If it is set, 2 bytes are used for atom lengths instead of
        1 byte in the distribution header.
      </p>
      <p>
        After the <c>Flags</c> field follow the <c>AtomCacheRefs</c>. The
        first <c>AtomCacheRef</c> is the one corresponding to
        <c>AtomCacheReferenceIndex</c> 0. Higher indices follow
        in sequence up to index <c>NumberOfAtomCacheRefs - 1</c>.
      </p>
      <p>
        If the <c>NewCacheEntryFlag</c> for the next <c>AtomCacheRef</c> has
        been set, a <c>NewAtomCacheRef</c> on the following format follows:
      </p>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">1 | 2</cell>
          <cell align="center">Length</cell>
        </row>
        <row>
          <cell align="center"><c>InternalSegmentIndex</c></cell>
          <cell align="center"><c>Length</c></cell>
          <cell align="center"><c>AtomText</c></cell>
        </row>
      <tcaption></tcaption></table>
      <p>
        <c>InternalSegmentIndex</c> together with the <c>SegmentIndex</c>
        completely identify the location of an atom cache entry in the
        atom cache. <c>Length</c> is the number of bytes that <c>AtomText</c>
        consists of. Length is a 2 byte big-endian integer
        if flag <c>LongAtoms</c> has been set, otherwise a 1 byte
        integer. When distribution flag
        <seeguide marker="erl_dist_protocol#DFLAG_UTF8_ATOMS">
        <c>DFLAG_UTF8_ATOMS</c></seeguide>
        has been exchanged between both nodes in the
        <seeguide marker="erl_dist_protocol#distribution_handshake">
          distribution handshake</seeguide>,
          characters in <c>AtomText</c> are encoded in UTF-8, otherwise
          in Latin-1. The following <c>CachedAtomRef</c>s with the same
          <c>SegmentIndex</c> and <c>InternalSegmentIndex</c> as this
          <c>NewAtomCacheRef</c> refer to this atom until a new
          <c>NewAtomCacheRef</c> with the same <c>SegmentIndex</c>
          and <c>InternalSegmentIndex</c> appear.
      </p>
      <p>
        For more information on encoding of atoms, see the
        <seeguide marker="#utf8_atoms">note on UTF-8 encoded atoms</seeguide>
        in the beginning of this section.
      </p>
      <p>
        If the <c>NewCacheEntryFlag</c> for the next <c>AtomCacheRef</c>
        has not been set, a <c>CachedAtomRef</c> on the following format
        follows:
      </p>
      <table align="left">
        <row>
          <cell align="center">1</cell>
        </row>
        <row>
          <cell align="center"><c>InternalSegmentIndex</c></cell>
        </row>
      <tcaption></tcaption></table>
      <p>
        <c>InternalSegmentIndex</c> together with the <c>SegmentIndex</c>
        identify the location of the atom cache entry in the atom cache.
        The atom corresponding to this <c>CachedAtomRef</c> is the
        latest <c>NewAtomCacheRef</c> preceding this <c>CachedAtomRef</c>
        in another previously passed distribution header.
      </p>
    </section>
    <section>
      <marker id="fragments"/>
      <title>Distribution Header for fragmented messages</title>
      <p>Messages sent between Erlang nodes can sometimes be
      quite large. Since OTP-22 it is possible to split large messages
      into smaller fragments in order to allow smaller messages to be interleaved
      between larges messages. It is only the <c>message</c> part of each
      <seeguide marker="erl_dist_protocol#connected_nodes">distributed message</seeguide>
      that may be split using fragmentation. Therefore it is recommended to use the
      <seeguide marker="erl_dist_protocol#new-ctrlmessages-for-erlang-otp-22">
        PAYLOAD control messages</seeguide> introduced in OTP-22.
      </p>
      <p>Fragmented distribution messages are only used if the receiving node
      signals that it supports them via the
      <seeguide marker="erl_dist_protocol#DFLAG_FRAGMENTS">DFLAG_FRAGMENTS</seeguide>
      distribution flag.</p>
      <p>A process must complete the sending of a fragmented message before it
      can start sending any other message on the same distribution channel.</p>

      <p>The start of a sequence of fragmented messages looks like this:</p>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">8</cell>
          <cell align="center">8</cell>
          <cell align="center">1</cell>
          <cell align="center">NumberOfAtomCacheRefs/2+1 | 0</cell>
          <cell align="center">N | 0</cell>
        </row>
        <row>
          <cell align="center"><c>131</c></cell>
          <cell align="center"><c>69</c></cell>
          <cell align="center"><c>SequenceId</c></cell>
          <cell align="center"><c>FragmentId</c></cell>
          <cell align="center"><c>NumberOfAtomCacheRefs</c></cell>
          <cell align="center"><c>Flags</c></cell>
          <cell align="center"><c>AtomCacheRefs</c></cell>
        </row>
        <tcaption>Starting Fragmented Distribution Header Format</tcaption>
      </table>

      <p>The continuation of a sequence of fragmented messages looks like this:</p>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">1</cell>
          <cell align="center">8</cell>
          <cell align="center">8</cell>
        </row>
        <row>
          <cell align="center"><c>131</c></cell>
          <cell align="center"><c>70</c></cell>
          <cell align="center"><c>SequenceId</c></cell>
          <cell align="center"><c>FragmentId</c></cell>
        </row>
        <tcaption>Continuing Fragmented Distribution Header Format</tcaption>
      </table>

      <p>
        The starting distribution header is very similar to a non-fragmented distribution
        header. The atom cache works the same as for normal distribution header and
        is the same for the entire sequence. The additional fields added are the
        sequence id and fragment id.
      </p>

      <taglist>
        <tag>Sequence ID</tag>
        <item>
          <p>
            The sequence id is used to uniquely identify a fragmented message sent
            from one process to another on the same distributed connection. This is used
            to identify which sequence a fragment is a part of as the same process can
            be in the process of receiving multiple sequences at the same time.
          </p>
          <p>
            As one process can only be sending one fragmented message at once,
            it can be convenient to use the local PID as the sequence id.
          </p>
        </item>
        <tag>Fragments ID</tag>
        <item>
          <p>
            The Fragment ID is used to number the fragments in a sequence.
            The id starts at the total number of fragments and then decrements to 1
            (which is the final fragment). So if a sequence consists of 3 fragments
            the fragment id in the starting header will be 3, and then fragments 2 and 1
            are sent.
          </p>
          <p>
            The fragments must be delivered in the correct order, so if an unordered
            distribution carrier is used, they must be ordered before delivered to the
            Erlang run-time.
          </p>
        </item>
      </taglist>

      <section>
        <title>Example:</title>
        <p>
          As an example, let say that we want to send
          <c>{call, &lt;0.245.2>, {set_get_state, &lt;&lt;0:1024>>}}</c> to
          registered process <c>reg</c> using a fragment size of 128. To send
          this message we need a distribution header, atom cache updates,
          the control message (which would be <c>{6, &lt;0.245.2>, [], reg}</c> in this case)
          and finally the actual message. This would all be encoded into:
        </p>

        <code>
131,69,0,0,2,168,0,0,5,83,0,0,0,0,0,0,0,2,               %% Header with seq and frag id
5,4,137,9,10,5,236,3,114,101,103,9,4,99,97,108,108,      %% Atom cache updates
238,13,115,101,116,95,103,101,116,95,115,116,97,116,101,
104,4,97,6,103,82,0,0,0,0,85,0,0,0,0,2,82,1,82,2,        %% Control message
104,3,82,3,103,82,0,0,0,0,245,0,0,0,2,2,                 %% Actual message using cached atoms
104,2,82,4,109,0,0,0,128,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

131,70,0,0,2,168,0,0,5,83,0,0,0,0,0,0,0,1,               %% Cont Header with seq and frag id
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,               %% Rest of payload
0,0,0,0</code>

       <p>
         Let us break that apart into its components. First we have the
         distribution header tags together with the sequence id and
         a fragment id of 2.
       </p>
       <code>
131,69,                   %% Start fragment header
0,0,2,168,0,0,5,83,       %% The sequence ID
0,0,0,0,0,0,0,2,           %% The fragment ID
</code>
       <p>Then we have the updates to the atom cache:</p>
       <code>
5,4,137,9,  %% 5 atoms and their flags
10,5,       %% The already cached atom ids
236,3,114,101,103,  %% The atom 'reg'
9,4,99,97,108,108,  %% The atom 'call'
238,13,115,101,116,95,103,101,116,95,115,116,97,116,101, %% The atom 'set_get_state'
       </code>
       <p>
         The first byte says that we have 5 atoms that are part
         of the cache. Then follows three bytes that are the
         atom cache ref flags. Each of the flags uses 4 bits so
         they are a bit hard to read in decimal byte form. In
         binary half-byte form they look like this:
       </p>
       <code>0000, 0100, 1000, 1001, 1001</code>
       <p>
         As the high bit of the first two atoms in the
         cache are not set we know that they are already in the cache,
         so they do not have to be sent again (this is the node name of the
         receiving and sending node). Then follows the atoms that have to be sent,
         together with their segment ids.
       </p>
       <p>
         Then the listing of the atoms comes, starting with 10 and 5
         which are the atom refs of the already cached atoms. Then the
         new atoms are sent.
       </p>
       <p>
         When the atom cache is setup correctly the control message is sent.
       </p>
       <code>104,4,97,6,103,82,0,0,0,0,85,0,0,0,0,2,82,1,82,2,</code>
       <p>
         Note that up until here it is not allowed to fragments the message.
         The entire atom cache and control message has to be part of the
         starting fragment. After the control message the payload of the message
         is sent using 128 bytes:
       </p>
       <code>
104,3,82,3,103,82,0,0,0,0,245,0,0,0,2,2,
104,2,82,4,109,0,0,0,128,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
       </code>
       <p>
         Since the payload is larger than 128-bytes it is split into two
         fragments. The second fragment does not have any atom cache update
         instructions so it is a lot simpler:
       </p>
       <code>
131,70,0,0,2,168,0,0,5,83,0,0,0,0,0,0,0,1, %% Continuation dist header 70 with seq and frag id
0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0, %% remaining payload
0,0,0,0
       </code>
       <note>
         <p>
           The fragment size of 128 is only used as an example.
           Any fragments size may be used when sending fragmented messages.
         </p>
       </note>
      </section>
    </section>
  </section>

  <section>
    <marker id="ATOM_CACHE_REF"/>
    <title>ATOM_CACHE_REF</title>
    <table align="left">
      <row>
        <cell align="center">1</cell>
        <cell align="center">1</cell>
      </row>
      <row>
        <cell align="center"><c>82</c></cell>
        <cell align="center"><c>AtomCacheReferenceIndex</c></cell>
      </row>
      <tcaption>ATOM_CACHE_REF</tcaption></table>
      <p>
        Refers to the atom with <c>AtomCacheReferenceIndex</c> in the
        <seeguide marker="#distribution_header">distribution header</seeguide>.
     </p>
  </section>

  <section>
    <marker id="SMALL_INTEGER_EXT"/>
    <title>SMALL_INTEGER_EXT</title>
    <table align="left">
      <row>
        <cell align="center">1</cell>
        <cell align="center">1</cell>
      </row>
      <row>
        <cell align="center"><c>97</c></cell>
        <cell align="center"><c>Int</c></cell>
      </row>
    <tcaption>SMALL_INTEGER_EXT</tcaption></table>
    <p>
      Unsigned 8-bit integer.
    </p>
  </section>

  <section>
    <marker id="INTEGER_EXT"/>
    <title>INTEGER_EXT</title>
    <table align="left">
      <row>
        <cell align="center">1</cell>
        <cell align="center">4</cell>
      </row>
      <row>
        <cell align="center"><c>98</c></cell>
        <cell align="center"><c>Int</c></cell>
      </row>
    <tcaption>INTEGER_EXT</tcaption></table>
    <p>
      Signed 32-bit integer in big-endian format.
    </p>
  </section>

  <section>
    <marker id="FLOAT_EXT"/>
    <title>FLOAT_EXT</title>
    <table align="left">
      <row>
        <cell align="center">1</cell>
        <cell align="center">31</cell>
      </row>
      <row>
        <cell align="center"><c>99</c></cell>
        <cell align="center"><c>Float string</c></cell>
      </row>
    <tcaption>FLOAT_EXT</tcaption></table>
    <p>
      A finite float (i.e. not inf, -inf or NaN) is stored in
      string format. The format used in sprintf to format the
      float is "%.20e"
      (there are more bytes allocated than necessary).
      To unpack the float, use sscanf with format "%lf".
    </p>
    <p>
      This term is used in minor version 0 of the external format;
      it has been superseded by
      <seeguide marker="#NEW_FLOAT_EXT"><c>NEW_FLOAT_EXT</c></seeguide>.
    </p>
  </section>

    <section>
      <marker id="PORT_EXT"/>
      <title>PORT_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">N</cell>
	    <cell align="center">4</cell>
	    <cell align="center">1</cell>
	  </row>
	  <row>
	    <cell align="center"><c>102</c></cell>
	    <cell align="center"><c>Node</c></cell>
	    <cell align="center"><c>ID</c></cell>
	    <cell align="center"><c>Creation</c></cell>
	  </row>
	<tcaption>PORT_EXT</tcaption></table>
	<p>
	  Same as <seeguide marker="#NEW_PORT_EXT"><c>NEW_PORT_EXT</c></seeguide>
	  except the <c>Creation</c> field is only one byte and only two
	  bits are significant, the rest are to be 0.
	</p>
    </section>

    <section>
      <marker id="NEW_PORT_EXT"/>
      <title>NEW_PORT_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">N</cell>
	    <cell align="center">4</cell>
	    <cell align="center">4</cell>
	  </row>
	  <row>
	    <cell align="center"><c>89</c></cell>
	    <cell align="center"><c>Node</c></cell>
	    <cell align="center"><c>ID</c></cell>
	    <cell align="center"><c>Creation</c></cell>
	  </row>
	<tcaption>NEW_PORT_EXT</tcaption></table>
	<p>
	  Same as <seeguide marker="#V4_PORT_EXT"><c>V4_PORT_EXT</c></seeguide>
	  except the <c>ID</c> field is only four bytes. Only 28 bits are
	  significant; the rest are to be 0. 
	</p>
	<p><c>NEW_PORT_EXT</c> was introduced in OTP 19, but only to be decoded
	  and echoed back. Not encoded for local ports.
	</p>
	<p>In OTP 23 distribution flag
	  <seeguide marker="erl_dist_protocol#DFLAG_BIG_CREATION"><c>DFLAG_BIG_CREATION</c></seeguide>
	  became mandatory. All ports are now
	  encoded using <c>NEW_PORT_EXT</c>, even external ports received as <seeguide
	  marker="#PORT_EXT"><c>PORT_EXT</c></seeguide> from older nodes.
	</p>
    </section>

    <section>
      <marker id="V4_PORT_EXT"/>
      <title>V4_PORT_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">N</cell>
	    <cell align="center">8</cell>
	    <cell align="center">4</cell>
	  </row>
	  <row>
	    <cell align="center"><c>89</c></cell>
	    <cell align="center"><c>Node</c></cell>
	    <cell align="center"><c>ID</c></cell>
	    <cell align="center"><c>Creation</c></cell>
	  </row>
	<tcaption>NEW_PORT_EXT</tcaption></table>
	<p>
	  Encodes a port identifier (obtained from
          <seemfa marker="erlang#open_port/2"><c>erlang:open_port/2</c></seemfa>).
	  <c>Node</c> is an encoded atom, that is,
	  <seeguide marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seeguide>,
	  <seeguide marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seeguide>
	  or <seeguide marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seeguide>.
	  <c>ID</c> is a 64-bit big endian unsigned integer. The <c>Creation</c>
	  works just like in <seeguide marker="#NEW_PID_EXT"><c>NEW_PID_EXT</c></seeguide>.
	  Port operations are not allowed across node boundaries.
	</p>
	<p><c>V4_PORT_EXT</c> was introduced in OTP 24, but only to be decoded
	  and echoed back. Not encoded for local ports.
	</p>
    </section>

    <section>
      <marker id="PID_EXT"/>
      <title>PID_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">N</cell>
	    <cell align="center">4</cell>
	    <cell align="center">4</cell>
	    <cell align="center">1</cell>
	  </row>
	  <row>
	    <cell align="center"><c>103</c></cell>
	    <cell align="center"><c>Node</c></cell>
	    <cell align="center"><c>ID</c></cell>
	    <cell align="center"><c>Serial</c></cell>
	    <cell align="center"><c>Creation</c></cell>
	  </row>
	<tcaption>PID_EXT</tcaption></table>
	<p>
	  Same as <seeguide marker="#NEW_PID_EXT"><c>NEW_PID_EXT</c></seeguide>
	  except the <c>Creation</c> field is only one byte and only two
	  bits are significant, the rest are to be 0.
	</p>
    </section>

    <section>
      <marker id="NEW_PID_EXT"/>
      <title>NEW_PID_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">N</cell>
	    <cell align="center">4</cell>
	    <cell align="center">4</cell>
	    <cell align="center">4</cell>
	  </row>
	  <row>
	    <cell align="center"><c>88</c></cell>
	    <cell align="center"><c>Node</c></cell>
	    <cell align="center"><c>ID</c></cell>
	    <cell align="center"><c>Serial</c></cell>
	    <cell align="center"><c>Creation</c></cell>
	  </row>
	<tcaption>NEW_PID_EXT</tcaption></table>
	<p>
	  Encodes an Erlang process identifier object.
	</p>
	<taglist>
	  <tag><c>Node</c></tag>
	  <item><p>The name of the originating node, encoded using
	    <seeguide marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seeguide>,
	    <seeguide marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seeguide>
	    or <seeguide
	    marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seeguide>.</p>
	  </item>
	  <tag><c>ID</c></tag>
	  <item><p>A 32-bit big endian unsigned integer. If distribution flag
	  <seeguide marker="erl_dist_protocol#DFLAG_V4_NC"><c>DFLAG_V4_NC</c></seeguide>
	  is not set, only 15 bits may be used and the rest must be 0.</p>
	  </item>
	  <tag><c>Serial</c></tag>
	  <item><p>A 32-bit big endian unsigned integer. If distribution flag
	  <seeguide marker="erl_dist_protocol#DFLAG_V4_NC"><c>DFLAG_V4_NC</c></seeguide>
	  is not set, only 13 bits may be used and the rest must be 0.</p>
	  </item>
	  <tag><c>Creation</c></tag>
	  <item><p>A 32-bit big endian unsigned integer. All identifiers
	    originating from the same node incarnation must have identical <c>Creation</c>
	    values. This makes it possible to separate identifiers from old
	    (crashed) nodes from a new one. The value zero should be avoided for
	    normal operations as it is used as a wild card for debug purpose
	    (like a pid returned by <seemfa marker="erts:erlang#list_to_pid/1">
	    erlang:list_to_pid/1</seemfa>).</p>
	  </item>
	</taglist>
	<p><c>NEW_PID_EXT</c> was introduced in OTP 19, but only to be decoded
	  and echoed back. Not encoded for local processes.
	</p>
	<p>In OTP 23 distribution flag
	  <seeguide marker="erl_dist_protocol#DFLAG_BIG_CREATION"><c>DFLAG_BIG_CREATION</c></seeguide>
	  became mandatory. All pids are now encoded using <c>NEW_PID_EXT</c>,
	  even external pids received as
	  <seeguide marker="#PID_EXT"><c>PID_EXT</c></seeguide> from older nodes.
	</p>
	<p>In OTP 24 distribution flag
	  <seeguide marker="erl_dist_protocol#DFLAG_V4_NC"><c>DFLAG_V4_NC</c></seeguide>
	  was introduced, accepting full 64-bit pids to be decoded and echoed back.
	</p>
    </section>

    <section>
      <marker id="SMALL_TUPLE_EXT"/>
      <title>SMALL_TUPLE_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">1</cell>
	    <cell align="center">N</cell>
	  </row>
	  <row>
	    <cell align="center"><c>104</c></cell>
	    <cell align="center"><c>Arity</c></cell>
	    <cell align="center"><c>Elements</c></cell>
	  </row>
	<tcaption>SMALL_TUPLE_EXT</tcaption></table>
	<p>
	  Encodes a tuple. The <c>Arity</c>
	  field is an unsigned byte that determines how many elements
	  that follows in section <c>Elements</c>.
	</p>
    </section>

    <section>
      <marker id="LARGE_TUPLE_EXT"/>
      <title>LARGE_TUPLE_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">N</cell>
	  </row>
	  <row>
	    <cell align="center"><c>105</c></cell>
	    <cell align="center"><c>Arity</c></cell>
	    <cell align="center"><c>Elements</c></cell>
	  </row>
	<tcaption>LARGE_TUPLE_EXT</tcaption></table>
	<p>
	  Same as
	  <seeguide marker="#SMALL_TUPLE_EXT"><c>SMALL_TUPLE_EXT</c></seeguide>
	  except that <c>Arity</c> is an
          unsigned 4 byte integer in big-endian format.
	</p>
    </section>

    <section>
      <marker id="MAP_EXT"/>
      <title>MAP_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">N</cell>
	  </row>
	  <row>
	    <cell align="center"><c>116</c></cell>
	    <cell align="center"><c>Arity</c></cell>
	    <cell align="center"><c>Pairs</c></cell>
	  </row>
	<tcaption>MAP_EXT</tcaption></table>
	<p>
	  Encodes a map. The <c>Arity</c> field is an unsigned
	  4 byte integer in big-endian format that determines the number of
	  key-value pairs in the map. Key and value pairs (<c>Ki => Vi</c>)
	  are encoded in section <c>Pairs</c> in the following order:
	  <c>K1, V1, K2, V2,..., Kn, Vn</c>.
	  Duplicate keys are <em>not allowed</em> within the same map.
	</p>
	<p><em>As from </em>Erlang/OTP 17.0</p>
    </section>

    <section>
      <marker id="NIL_EXT"/>
      <title>NIL_EXT</title>
      <table align="left">
	<row>
	  <cell align="center">1</cell>
	</row>
	<row>
	  <cell align="center"><c>106</c></cell>
	</row>
      <tcaption>NIL_EXT</tcaption></table>
      <p>
	The representation for an empty list, that is, the Erlang syntax
        <c>[]</c>.
      </p>
    </section>

    <section>
      <marker id="STRING_EXT"/>
      <title>STRING_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">2</cell>
	    <cell align="center">Len</cell>
	  </row>
	  <row>
	    <cell align="center"><c>107</c></cell>
	    <cell align="center"><c>Length</c></cell>
	    <cell align="center"><c>Characters</c></cell>
	  </row>
	<tcaption>STRING_EXT</tcaption></table>
	<p>
	  String does <em>not</em> have a corresponding Erlang representation,
	  but is an optimization for sending lists of bytes (integer in
	  the range 0-255) more efficiently over the distribution.
	  As field <c>Length</c> is an unsigned 2 byte integer
	  (big-endian), implementations must ensure that lists longer than
	  65535 elements are encoded as
	  <seeguide marker="#LIST_EXT"><c>LIST_EXT</c></seeguide>.
	</p>
    </section>

    <section>
      <marker id="LIST_EXT"/>
      <title>LIST_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">&nbsp;</cell>
	    <cell align="center">&nbsp;</cell>
	  </row>
	  <row>
	    <cell align="center"><c>108</c></cell>
	    <cell align="center"><c>Length</c></cell>
	    <cell align="center"><c>Elements</c></cell>
	    <cell align="center"><c>Tail</c></cell>
	  </row>
	<tcaption>LIST_EXT</tcaption></table>
	<p>
	  <c>Length</c> is the number of elements that follows in section
	  <c>Elements</c>. <c>Tail</c> is the final tail of the list; it is
	  <seeguide marker="#NIL_EXT"><c>NIL_EXT</c></seeguide>
	  for a proper list, but can be any type if the list is
	  improper (for example, <c>[a|b]</c>).
	</p>
    </section>

    <section>
      <marker id="BINARY_EXT"/>
      <title>BINARY_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">Len</cell>
	  </row>
	  <row>
	    <cell align="center"><c>109</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>Data</c></cell>
	  </row>
	<tcaption>BINARY_EXT</tcaption></table>
	<p>
	  Binaries are generated with bit syntax expression or with
	  <seemfa marker="erts:erlang#list_to_binary/1">
	  <c>erlang:list_to_binary/1</c></seemfa>,
	  <seemfa marker="erts:erlang#term_to_binary/1">
	  <c>erlang:term_to_binary/1</c></seemfa>,
	  or as input from binary ports.
	  The <c>Len</c> length field is an unsigned 4 byte integer
	  (big-endian).
	</p>
    </section>

    <section>
      <marker id="SMALL_BIG_EXT"/>
      <title>SMALL_BIG_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">1</cell>
	    <cell align="center">1</cell>
	    <cell align="center">n</cell>
	  </row>
	  <row>
	    <cell align="center"><c>110</c></cell>
	    <cell align="center"><c>n</c></cell>
	    <cell align="center"><c>Sign</c></cell>
	    <cell align="center"><c>d(0)</c> ... <c>d(n-1)</c></cell>
	  </row>
	<tcaption>SMALL_BIG_EXT</tcaption></table>
	<p>
	  Bignums are stored in unary form with a <c>Sign</c> byte,
	  that is, 0 if the bignum is positive and 1 if it is negative. The
	  digits are stored with the least significant byte stored first. To
	  calculate the integer, the following formula can be used:
	</p>
	<p><c>B</c> = 256<br/>
	  <c>(d0*B^0 + d1*B^1 + d2*B^2 + ... d(N-1)*B^(n-1))</c>
	</p>
    </section>

    <section>
      <marker id="LARGE_BIG_EXT"/>
      <title>LARGE_BIG_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">1</cell>
	    <cell align="center">n</cell>
	  </row>
	  <row>
	    <cell align="center"><c>111</c></cell>
	    <cell align="center"><c>n</c></cell>
	    <cell align="center"><c>Sign</c></cell>
	    <cell align="center"><c>d(0)</c> ... <c>d(n-1)</c></cell>
	  </row>
	<tcaption>LARGE_BIG_EXT</tcaption></table>
	<p>
	  Same as <seeguide marker="#SMALL_BIG_EXT">
	  <c>SMALL_BIG_EXT</c></seeguide> 
	  except that the length field is an unsigned 4 byte integer.
	</p>
    </section>

    <section>
      <marker id="REFERENCE_EXT"/>
      <title>REFERENCE_EXT (deprecated)</title>
      <table align="left">
        <row>
          <cell align="center">1</cell>
          <cell align="center">N</cell>
          <cell align="center">4</cell>
          <cell align="center">1</cell>
        </row>
        <row>
          <cell align="center"><c>101</c></cell>
          <cell align="center"><c>Node</c></cell>
          <cell align="center"><c>ID</c></cell>
          <cell align="center"><c>Creation</c></cell>
        </row>
      <tcaption>REFERENCE_EXT</tcaption></table>
      <p>
	The same as <seeguide marker="#NEW_REFERENCE_EXT">
	<c>NEW_REFERENCE_EXT</c></seeguide> except <c>ID</c> is only one word
	(<c>Len</c> = 1).
      </p>
    </section>

    <section>
      <marker id="NEW_REFERENCE_EXT"/>
      <title>NEW_REFERENCE_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">2</cell>
	    <cell align="center">N</cell>
	    <cell align="center">1</cell>
	    <cell align="center">N'</cell>
	  </row>
	  <row>
	    <cell align="center"><c>114</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>Node</c></cell>
	    <cell align="center"><c>Creation</c></cell>
	    <cell align="center"><c>ID ...</c></cell>
	  </row>
	<tcaption>NEW_REFERENCE_EXT</tcaption></table>
	<p>
	  The same as <seeguide marker="#NEWER_REFERENCE_EXT">
	  <c>NEWER_REFERENCE_EXT</c></seeguide> <em>except</em>:
	</p>
	<taglist>
	  <tag><c>ID</c></tag>
	  <item><p>In the first word (4 bytes) of <c>ID</c>, only 18 bits are
	    significant, the rest must be 0.</p>
	  </item>
	  <tag><c>Creation</c></tag>
	  <item><p>Only one byte long and only two bits are significant, the rest must be 0.</p>
	  </item>
	</taglist>
    </section>

    <section>
      <marker id="NEWER_REFERENCE_EXT"/>
      <title>NEWER_REFERENCE_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">2</cell>
	    <cell align="center">N</cell>
	    <cell align="center">4</cell>
	    <cell align="center">N'</cell>
	  </row>
	  <row>
	    <cell align="center"><c>90</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>Node</c></cell>
	    <cell align="center"><c>Creation</c></cell>
	    <cell align="center"><c>ID ...</c></cell>
	  </row>
	<tcaption>NEWER_REFERENCE_EXT</tcaption></table>
	<p>
	  Encodes a reference term generated with
	  <seemfa marker="erts:erlang#make_ref/0">erlang:make_ref/0</seemfa>.
	</p>
	<taglist>
	  <tag><c>Node</c></tag>
	  <item><p>The name of the originating node, encoded using
	    <seeguide marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seeguide>,
	    <seeguide marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seeguide>
	    or <seeguide marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seeguide>.</p>
	  </item>
	  <tag><c>Len</c></tag>
	  <item><p>A 16-bit big endian unsigned integer not larger than 5 when the
	  <seeguide marker="erl_dist_protocol#DFLAG_V4_NC"><c>DFLAG_V4_NC</c></seeguide>
	  has been set; otherwise not larger than 3.</p>
	  </item>
	  <tag><c>ID</c></tag>
	  <item><p>A sequence of <c>Len</c> big-endian unsigned integers
	    (4 bytes each, so <c>N'</c>&nbsp;=&nbsp;4&nbsp;*&nbsp;<c>Len</c>),
	    but is to be regarded as uninterpreted data.</p>
	  </item>
	  <tag><c>Creation</c></tag>
	  <item><p>Works just like in
	  <seeguide marker="#NEW_PID_EXT"><c>NEW_PID_EXT</c></seeguide>.</p>
	  </item>
	</taglist>
	<p><c>NEWER_REFERENCE_EXT</c> was introduced in OTP 19, but only to be decoded
	  and echoed back. Not encoded for local references.
	</p>
	<p>In OTP 23 distribution flag
	  <seeguide marker="erl_dist_protocol#DFLAG_BIG_CREATION"><c>DFLAG_BIG_CREATION</c></seeguide>
	  became mandatory. All references are now encoded using
	  <c>NEWER_REFERENCE_EXT</c>, even external references received as
	  <seeguide marker="#NEW_REFERENCE_EXT"><c>NEW_REFERENCE_EXT</c></seeguide>
	  from older nodes.
	</p>
    </section>

    <section>
      <marker id="FUN_EXT"/>
      <title>FUN_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">N1</cell>
	    <cell align="center">N2</cell>
	    <cell align="center">N3</cell>
	    <cell align="center">N4</cell>
	    <cell align="center">N5</cell>
	  </row>
	  <row>
	    <cell align="center"><c>117</c></cell>
	    <cell align="center"><c>NumFree</c></cell>
	    <cell align="center"><c>Pid</c></cell>
	    <cell align="center"><c>Module</c></cell>
	    <cell align="center"><c>Index</c></cell>
	    <cell align="center"><c>Uniq</c></cell>
	    <cell align="center"><c>Free vars ...</c></cell>
	  </row>
	<tcaption>FUN_EXT</tcaption></table>
	<taglist>
	  <tag><c>Pid</c></tag>
	  <item>
	    <p>A process identifier as in
	      <seeguide marker="#PID_EXT"><c>PID_EXT</c></seeguide>.
	      Represents the process in which the fun was created.
	    </p>
	  </item>
	<tag><c>Module</c></tag>
	<item>
	  <p>Encoded as an atom, using
	    <seeguide marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seeguide>,
	    <seeguide marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seeguide>,
	    or <seeguide marker="#ATOM_CACHE_REF">
	    <c>ATOM_CACHE_REF</c></seeguide>.
	    This is the module that the fun is implemented in.
	  </p>
	</item>
	<tag><c>Index</c></tag>
	<item>
	  <p>An integer encoded using
	    <seeguide marker="#SMALL_INTEGER_EXT">
	    <c>SMALL_INTEGER_EXT</c></seeguide> 
	    or <seeguide marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seeguide>.
	    It is typically a small index into the module's fun table.
	  </p>
	</item>
	<tag><c>Uniq</c></tag>
	<item>
	  <p>An integer encoded using
	    <seeguide marker="#SMALL_INTEGER_EXT">
	    <c>SMALL_INTEGER_EXT</c></seeguide> or 
	    <seeguide marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seeguide>.
	    <c>Uniq</c> is the hash value of the parse for the fun.
	  </p>
	</item>
	<tag><c>Free vars</c></tag>
	<item>
	  <p><c>NumFree</c> number of terms, each one encoded according
	    to its type.
	  </p>
	</item>
	</taglist>
    </section>

    <section>
      <marker id="NEW_FUN_EXT"/>
      <title>NEW_FUN_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">1</cell>
	    <cell align="center">16</cell>
	    <cell align="center">4</cell>
	    <cell align="center">4</cell>
	    <cell align="center">N1</cell>
	    <cell align="center">N2</cell>
	    <cell align="center">N3</cell>
	    <cell align="center">N4</cell>
	    <cell align="center">N5</cell>
	  </row>
	  <row>
	    <cell align="center"><c>112</c></cell>
	    <cell align="center"><c>Size</c></cell>
	    <cell align="center"><c>Arity</c></cell>
	    <cell align="center"><c>Uniq</c></cell>
	    <cell align="center"><c>Index</c></cell>
	    <cell align="center"><c>NumFree</c></cell>
	    <cell align="center"><c>Module</c></cell>
	    <cell align="center"><c>OldIndex</c></cell>
	    <cell align="center"><c>OldUniq</c></cell>
	    <cell align="center"><c>Pid</c></cell>
	    <cell align="center"><c>Free Vars</c></cell>
	  </row>
	<tcaption>NEW_FUN_EXT</tcaption></table>
	<p>
	  This is the new encoding of internal funs: <c>fun F/A</c> and
	  <c>fun(Arg1,..) -> ... end</c>.
	</p>
	<taglist>
	  <tag><c>Size</c></tag> 
	  <item>
	    <p>The total number of bytes, including field <c>Size</c>.</p>
	  </item>
	  <tag><c>Arity</c></tag> 
	  <item>
	    <p>The arity of the function implementing the fun.</p>
	  </item>
	  <tag><c>Uniq</c></tag>
	  <item>
	    <p>The 16 bytes MD5 of the significant parts of the Beam file.</p>
	  </item>
	  <tag><c>Index</c></tag> 
	  <item>
	    <p>An index number. Each fun within a module has an unique
	      index. <c>Index</c> is stored in big-endian byte order.
	    </p>
	  </item>
	  <tag><c>NumFree</c></tag> 
	  <item>
	    <p>The number of free variables.</p>
	  </item>
	  <tag><c>Module</c></tag>
	  <item>
	    <p>Encoded as an atom, using
	      <seeguide marker="#ATOM_EXT"><c>ATOM_UTF8_EXT</c></seeguide>, 
	      <seeguide marker="#SMALL_ATOM_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seeguide>,
	      or <seeguide marker="#ATOM_CACHE_REF">
	      <c>ATOM_CACHE_REF</c></seeguide>. 
	      Is the module that the fun is implemented in.
	    </p>
	  </item>
	  <tag><c>OldIndex</c></tag>
	  <item>
	    <p>An integer encoded using
	      <seeguide marker="#SMALL_INTEGER_EXT">
	      <c>SMALL_INTEGER_EXT</c></seeguide> or
	      <seeguide marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seeguide>.
	      Is typically a small index into the module's fun table.
	    </p>
	  </item>
	  <tag><c>OldUniq</c></tag>
	  <item>
	    <p>An integer encoded using
	      <seeguide marker="#SMALL_INTEGER_EXT">
	      <c>SMALL_INTEGER_EXT</c></seeguide> or 
	      <seeguide marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seeguide>.
	      <c>Uniq</c> is the hash value of the parse tree for the fun.
	    </p>
	  </item>
	  <tag><c>Pid</c></tag>
	  <item>
	    <p>A process identifier as in
	      <seeguide marker="#PID_EXT"><c>PID_EXT</c></seeguide>.
	      Represents the process in which the fun was created.
	    </p>
	  </item>
	  <tag><c>Free vars</c></tag>
	  <item>
	    <p><c>NumFree</c> number of terms, each one encoded according
	      to its type.
	    </p>
	  </item>
	</taglist>
    </section>

    <section>
      <marker id="EXPORT_EXT"/>
      <title>EXPORT_EXT</title>	
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">N1</cell>
	    <cell align="center">N2</cell>
	    <cell align="center">N3</cell>
	  </row>
	  <row>
	    <cell align="center"><c>113</c></cell>
	    <cell align="center"><c>Module</c></cell>
	    <cell align="center"><c>Function</c></cell>
	    <cell align="center"><c>Arity</c></cell>
	  </row>
	<tcaption>EXPORT_EXT</tcaption></table>
	<p>
	  This term is the encoding for external funs: <c>fun M:F/A</c>.
	</p>
	<p>
	  <c>Module</c> and <c>Function</c> are atoms
	  (encoded using <seeguide marker="#ATOM_EXT"><c>ATOM_UTF8_EXT</c></seeguide>, 
	  <seeguide marker="#SMALL_ATOM_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seeguide>, or
	  <seeguide marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seeguide>).
	</p>
	<p>
	  <c>Arity</c> is an integer encoded using
	  <seeguide marker="#SMALL_INTEGER_EXT">
	  <c>SMALL_INTEGER_EXT</c></seeguide>.
	</p>
    </section>

    <section>
      <marker id="BIT_BINARY_EXT"/>
      <title>BIT_BINARY_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">4</cell>
	    <cell align="center">1</cell>
	    <cell align="center">Len</cell>
	  </row>
	  <row>
	    <cell align="center"><c>77</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>Bits</c></cell>
	    <cell align="center"><c>Data</c></cell>
	  </row>
	<tcaption>BIT_BINARY_EXT</tcaption></table>
	<p>
	  This term represents a bitstring whose length in bits does
	  not have to be a multiple of 8.
	  The <c>Len</c> field is an unsigned 4 byte integer (big-endian).
	  The <c>Bits</c> field is the number of bits (1-8) that are used
	  in the last byte in the data field,
	  counting from the most significant bit to the least significant.
	</p>
    </section>

    <section>
      <marker id="NEW_FLOAT_EXT"/>
      <title>NEW_FLOAT_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">8</cell>
	  </row>
	  <row>
	    <cell align="center"><c>70</c></cell>
	    <cell align="center"><c>IEEE float</c></cell>
	  </row>
	<tcaption>NEW_FLOAT_EXT</tcaption></table>
	<p>
	  A finite float (i.e. not inf, -inf or NaN) is stored as 8 bytes
	  in big-endian IEEE format.
	</p>
	<p>
	  This term is used in minor version 1 of the external format.
	</p>
    </section>

    <section>
      <marker id="ATOM_UTF8_EXT"/>
      <title>ATOM_UTF8_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">2</cell>
	    <cell align="center">Len</cell>
	  </row>
	  <row>
	    <cell align="center"><c>118</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>AtomName</c></cell>
	  </row>
	<tcaption>ATOM_UTF8_EXT</tcaption></table>
      <p>
	An atom is stored with a 2 byte unsigned length in big-endian order,
	followed by <c>Len</c> bytes containing the <c>AtomName</c> encoded
	in UTF-8.
      </p>
      <p>
	For more information on encoding of atoms, see the
	<seeguide marker="#utf8_atoms">note on UTF-8 encoded atoms</seeguide>
	in the beginning of this section.
      </p>
    </section>

    <section>
      <marker id="SMALL_ATOM_UTF8_EXT"/>
      <title>SMALL_ATOM_UTF8_EXT</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">1</cell>
	    <cell align="center">Len</cell>
	  </row>
	  <row>
	    <cell align="center"><c>119</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>AtomName</c></cell>
	  </row>
	<tcaption>SMALL_ATOM_UTF8_EXT</tcaption></table>
      <p>
	An atom is stored with a 1 byte unsigned length,
	followed by <c>Len</c> bytes containing the <c>AtomName</c> encoded
	in UTF-8. Longer atoms encoded in UTF-8 can be represented using
	<seeguide marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seeguide>.
      </p>
      <p>
	For more information on encoding of atoms, see the
	<seeguide marker="#utf8_atoms">note on UTF-8 encoded atoms</seeguide>
	in the beginning of this section.
      </p>
    </section>

    <section>
      <marker id="ATOM_EXT"/>
      <title>ATOM_EXT (deprecated)</title>
	<table align="left">
	  <row>
            <cell align="center">1</cell>
            <cell align="center">2</cell>
            <cell align="center">Len</cell>
	  </row>
	  <row>
            <cell align="center"><c>100</c></cell>
            <cell align="center"><c>Len</c></cell>
            <cell align="center"><c>AtomName</c></cell>
	  </row>
	<tcaption>ATOM_EXT</tcaption></table>
      <p>
        An atom is stored with a 2 byte unsigned length in big-endian order,
        followed by <c>Len</c> numbers of 8-bit Latin-1 characters that forms
        the <c>AtomName</c>. The maximum allowed value for <c>Len</c> is 255.
      </p>
    </section>

    <section>
      <marker id="SMALL_ATOM_EXT"/>
      <title>SMALL_ATOM_EXT (deprecated)</title>
	<table align="left">
	  <row>
	    <cell align="center">1</cell>
	    <cell align="center">1</cell>
	    <cell align="center">Len</cell>
	  </row>
	  <row>
	    <cell align="center"><c>115</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>AtomName</c></cell>
	  </row>
	<tcaption>SMALL_ATOM_EXT</tcaption></table>
      <p>
	An atom is stored with a 1 byte unsigned length,
	followed by <c>Len</c> numbers of 8-bit Latin-1 characters that
	forms the <c>AtomName</c>.
      </p>
      <note>
	<p>
	  <c>SMALL_ATOM_EXT</c> was introduced in ERTS 5.7.2 and
	  require an exchange of distribution flag
	  <seeguide marker="erl_dist_protocol#DFLAG_SMALL_ATOM_TAGS">
	  <c>DFLAG_SMALL_ATOM_TAGS</c></seeguide> in the
	  <seeguide marker="erl_dist_protocol#distribution_handshake">
	  distribution handshake</seeguide>.
	</p>
      </note>
    </section>

  </chapter>


