path: root/erts/doc/src/erl_ext_dist.xml

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2007</year>
      <year>2017</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 <seealso marker="erts:erlang#term_to_binary/1">
      <c>erlang:term_to_binary/1,2</c></seealso> is used to convert a
      term into the external format.
      To convert binary data encoding to a term, the BIF
      <seealso marker="erts:erlang#binary_to_term/1">
      <c>erlang:binary_to_term/1</c></seealso> 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
        <seealso marker="erl_dist_protocol#connected_nodes">passed between
        connected nodes</seealso> and a
        <seealso marker="#distribution_header">distribution
        header</seealso> 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
	<seealso marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seealso>
        or <seealso marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seealso>.
	The old Latin-1 formats <seealso marker="#ATOM_EXT"><c>ATOM_EXT</c></seealso>
        and <seealso marker="#SMALL_ATOM_EXT"><c>SMALL_ATOM_EXT</c></seealso>
	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 abillity 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>  
    <title>Distribution Header</title>
    <p>
      <marker id="distribution_header"/>
      As from ERTS 5.7.2 the old atom cache protocol was
      dropped and a new one was introduced. This protocol
      introduced the distribution header. Nodes with an ERTS version
      earlier than 5.7.2 can still communicate with new nodes,
      but no distribution header and no atom cache are used.</p>
    <p>
      The distribution header only contains an atom cache
      reference section, but can in the future contain more
      information. The distribution header precedes one or more Erlang
      terms on the external format. For more information, see the
      documentation of the
      <seealso marker="erl_dist_protocol#connected_nodes">protocol between
      connected nodes</seealso> in the
      <seealso marker="erl_dist_protocol">distribution protocol</seealso>
      documentation.
    </p>
    <p>
      <seealso marker="#ATOM_CACHE_REF">ATOM_CACHE_REF</seealso>
      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>
    <p>
      The 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>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>
      After flag fields for atom cache references, another half byte flag
      field is located with 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
      <seealso marker="erl_dist_protocol#dflags">
      <c>DFLAG_UTF8_ATOMS</c></seealso>
      has been exchanged between both nodes in the
      <seealso marker="erl_dist_protocol#distribution_handshake">
      distribution handshake</seealso>,
      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
      <seealso marker="#utf8_atoms">note on UTF-8 encoded atoms</seealso>
      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="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
        <seealso marker="#distribution_header">distribution header</seealso>.
     </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 float 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
      <seealso marker="#NEW_FLOAT_EXT"><c>NEW_FLOAT_EXT</c></seealso>.
    </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 <seealso marker="#NEW_PORT_EXT"><c>NEW_PORT_EXT</c></seealso>
	  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>
	  Encodes a port identifier (obtained from
          <seealso marker="erlang#open_port/2"><c>erlang:open_port/2</c></seealso>).
	  <c>Node</c> is an encoded atom, that is,
	  <seealso marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seealso>,
	  <seealso marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seealso>
	  or <seealso marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seealso>.
	  <c>ID</c> is a 32-bit big endian unsigned integer. Only 28 bits are
	  significant; the rest are to be 0. The <c>Creation</c> works just like in
	  <seealso marker="#NEW_PID_EXT"><c>NEW_PID_EXT</c></seealso>.
	  Port operations are not allowed across node boundaries.
	</p>
	<p>Introduced in OTP 19, but only to be decoded and echoed back. Not
	  encoded for local ports. Planned to supersede <seealso marker="#PORT_EXT">
	  <c>PORT_EXT</c></seealso> in OTP 23 when
	  <seealso marker="erl_dist_protocol#dflags"><c>DFLAG_BIG_CREATON</c></seealso>
	  becomes mandatory.
	</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 <seealso marker="#NEW_PID_EXT"><c>NEW_PID_EXT</c></seealso>
	  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
	    <seealso marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seealso>,
	    <seealso marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seealso>
	    or <seealso
	    marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seealso>.</p>
	  </item>
	  <tag><c>ID</c></tag>
	  <item><p>A 32-bit big endian unsigned integer. Only 15 bits are
	    significant; the rest are to be 0.</p>
	  </item>
	  <tag><c>Serial</c></tag>
	  <item><p>A 32-bit big endian unsigned integer. Only 13 bits are
	    significant; the rest are to 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 <seealso marker="erts:erlang#list_to_pid/1">
	    erlang:list_to_pid/1</seealso>).</p>
	  </item>
	</taglist>
	<p>Introduced in OTP 19, but only to be decoded and echoed back. Not
	  encoded for local processes. Planned to supersede <seealso marker="#PID_EXT">
	  <c>PID_EXT</c></seealso> in OTP 23 when
	  <seealso marker="erl_dist_protocol#dflags"><c>DFLAG_BIG_CREATON</c></seealso>
	  becomes mandatory.
	</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
	  <seealso marker="#SMALL_TUPLE_EXT"><c>SMALL_TUPLE_EXT</c></seealso>
	  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
	  <seealso marker="#LIST_EXT"><c>LIST_EXT</c></seealso>.
	</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
	  <seealso marker="#NIL_EXT"><c>NIL_EXT</c></seealso>
	  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
	  <seealso marker="erts:erlang#list_to_binary/1">
	  <c>erlang:list_to_binary/1</c></seealso>,
	  <seealso marker="erts:erlang#term_to_binary/1">
	  <c>erlang:term_to_binary/1</c></seealso>,
	  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 binum 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 <seealso marker="#SMALL_BIG_EXT">
	  <c>SMALL_BIG_EXT</c></seealso> 
	  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 <seealso marker="#NEW_REFERENCE_EXT">
	<c>NEW_REFERENCE_EXT</c></seealso> 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 <seealso marker="#NEWER_REFERENCE_EXT">
	  <c>NEWER_REFERENCE_EXT</c></seealso> <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
	  <seealso marker="erts:erlang#make_ref/0">erlang:make_ref/0</seealso>.
	</p>
	<taglist>
	  <tag><c>Node</c></tag>
	  <item><p>The name of the originating node, encoded using
	    <seealso marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seealso>,
	    <seealso marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seealso>
	    or <seealso marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seealso>.</p>
	  </item>
	  <tag><c>Len</c></tag>
	  <item><p>A 16-bit big endian unsigned integer 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
	  <seealso marker="#NEW_PID_EXT"><c>NEW_PID_EXT</c></seealso>.</p>
	  </item>
	</taglist>
	<p>Introduced in OTP 19, but only to be decoded and echoed back. Not
	  encoded for local references. Planned to supersede <seealso marker="#NEW_REFERENCE_EXT">
	  <c>NEW_REFERENCE_EXT</c></seealso> in OTP 23 when
	  <seealso marker="erl_dist_protocol#dflags"><c>DFLAG_BIG_CREATON</c></seealso>
	  becomes mandatory.
	</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
	      <seealso marker="#PID_EXT"><c>PID_EXT</c></seealso>.
	      Represents the process in which the fun was created.
	    </p>
	  </item>
	<tag><c>Module</c></tag>
	<item>
	  <p>Encoded as an atom, using
	    <seealso marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seealso>,
	    <seealso marker="#SMALL_ATOM_UTF8_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seealso>,
	    or <seealso marker="#ATOM_CACHE_REF">
	    <c>ATOM_CACHE_REF</c></seealso>.
	    This is the module that the fun is implemented in.
	  </p>
	</item>
	<tag><c>Index</c></tag>
	<item>
	  <p>An integer encoded using
	    <seealso marker="#SMALL_INTEGER_EXT">
	    <c>SMALL_INTEGER_EXT</c></seealso> 
	    or <seealso marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seealso>.
	    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
	    <seealso marker="#SMALL_INTEGER_EXT">
	    <c>SMALL_INTEGER_EXT</c></seealso> or 
	    <seealso marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seealso>.
	    <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
	      <seealso marker="#ATOM_EXT"><c>ATOM_UTF8_EXT</c></seealso>, 
	      <seealso marker="#SMALL_ATOM_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seealso>,
	      or <seealso marker="#ATOM_CACHE_REF">
	      <c>ATOM_CACHE_REF</c></seealso>. 
	      Is the module that the fun is implemented in.
	    </p>
	  </item>
	  <tag><c>OldIndex</c></tag>
	  <item>
	    <p>An integer encoded using
	      <seealso marker="#SMALL_INTEGER_EXT">
	      <c>SMALL_INTEGER_EXT</c></seealso> or
	      <seealso marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seealso>.
	      Is typically a small index into the module's fun table.
	    </p>
	  </item>
	  <tag><c>OldUniq</c></tag>
	  <item>
	    <p>An integer encoded using
	      <seealso marker="#SMALL_INTEGER_EXT">
	      <c>SMALL_INTEGER_EXT</c></seealso> or 
	      <seealso marker="#INTEGER_EXT"><c>INTEGER_EXT</c></seealso>.
	      <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
	      <seealso marker="#PID_EXT"><c>PID_EXT</c></seealso>.
	      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 <seealso marker="#ATOM_EXT"><c>ATOM_UTF8_EXT</c></seealso>, 
	  <seealso marker="#SMALL_ATOM_EXT"><c>SMALL_ATOM_UTF8_EXT</c></seealso>, or
	  <seealso marker="#ATOM_CACHE_REF"><c>ATOM_CACHE_REF</c></seealso>).
	</p>
	<p>
	  <c>Arity</c> is an integer encoded using
	  <seealso marker="#SMALL_INTEGER_EXT">
	  <c>SMALL_INTEGER_EXT</c></seealso>.
	</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 float 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
	<seealso marker="#utf8_atoms">note on UTF-8 encoded atoms</seealso>
	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
	<seealso marker="#ATOM_UTF8_EXT"><c>ATOM_UTF8_EXT</c></seealso>.
      </p>
      <p>
	For more information on encoding of atoms, see the
	<seealso marker="#utf8_atoms">note on UTF-8 encoded atoms</seealso>
	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
	  <seealso marker="erl_dist_protocol#dflags">
	  <c>DFLAG_SMALL_ATOM_TAGS</c></seealso> in the
	  <seealso marker="erl_dist_protocol#distribution_handshake">
	  distribution handshake</seealso>.
	</p>
      </note>
    </section>

  </chapter>