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

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

<chapter>
  <header>
    <copyright>
      <year>2007</year>
      <year>2013</year>
      <holder>Ericsson AB, All Rights Reserved</holder>
    </copyright>
    <legalnotice>
  The contents of this file are subject to the Erlang Public License,
  Version 1.1, (the "License"); you may not use this file except in
  compliance with the License. You should have received a copy of the
  Erlang Public License along with this software. If not, it can be
  retrieved online at http://www.erlang.org/.

  Software distributed under the License is distributed on an "AS IS"
  basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
  the License for the specific language governing rights 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>
      Since 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 has an external representation and the interpretation 
      of the different terms are application specific.
    </p>
    <p>
      In Erlang the BIF <seealso marker="erts:erlang#term_to_binary/1">term_to_binary/1,2</seealso> is used to convert a 
      term into the external format. 
      To convert binary data encoding a term the BIF
      <seealso marker="erts:erlang#binary_to_term/1">
	binary_to_term/1
      </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:
    </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></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 since
      the version number is implied by the version number in the
      distribution header.
    </p>
    </note>
    <p>
      A compressed term looks like this:
    </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">131</cell>
	<cell align="center">80</cell>
	<cell align="center">UncompressedSize</cell>
	<cell align="center">Zlib-compressedData</cell>
      </row>
    <tcaption></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">Tag</cell>
	<cell align="center">Data</cell>
      </row>
    <tcaption></tcaption></table>
    <marker id="utf8_atoms"/>
    <note>
    <p>As of ERTS version 5.10 (OTP-R16) support
    for UTF-8 encoded atoms has been introduced in the external format.
    However, only characters that can be encoded using Latin1 (ISO-8859-1)
    are currently supported in atoms. The support for UTF-8 encoded atoms
    in the external format has been implemented in order to be able to support
    all Unicode characters in atoms in <em>some future release</em>. Full
    support for Unicode atoms will not happen before OTP-R18, and might
    be introduced even later than that. Until full Unicode support for
    atoms has been introduced, it is an <em>error</em> to pass atoms containing
    characters that cannot be encoded in Latin1, and <em>the behavior is
    undefined</em>.</p>
    <p>When the
    <seealso marker="erl_dist_protocol#dflags"><c>DFLAG_UTF8_ATOMS</c></seealso>
    distribution flag has been exchanged between both nodes in the
    <seealso marker="erl_dist_protocol#distribution_handshake">distribution handshake</seealso>,
    all atoms in the distribution header will be encoded in UTF-8; otherwise,
    all atoms in the distribution header will be encoded in Latin1. The two
    new tags <seealso marker="#ATOM_UTF8_EXT">ATOM_UTF8_EXT</seealso>, and
    <seealso marker="#SMALL_ATOM_UTF8_EXT">SMALL_ATOM_UTF8_EXT</seealso>
    will only be used if the <c>DFLAG_UTF8_ATOMS</c> distribution flag has
    been exchanged between nodes, or if an atom containing characters
    that cannot be encoded in Latin1 is encountered.
    </p>
    <p>The maximum number of allowed characters in an atom is 255. In the
    UTF-8 case each character may need 4 bytes to be encoded.
    </p>
    </note>
  </section>

  <marker id="distribution_header"/>
  <section>  
    <title>Distribution header</title>
    <p>
      As of erts version 5.7.2 the old atom cache protocol was
      dropped and a new one was introduced. This atom cache protocol
      introduced the distribution header. Nodes with erts versions
      earlier than 5.7.2 can still communicate with new nodes,
      but no distribution header and no atom cache will be used.</p>
    <p>
      The distribution header currently only contains an atom cache
      reference section, but could 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 refers
      to the atom cache references made in the distribution header. The range
      is 0 &lt;= <c>AtomCacheReferenceIndex</c> &lt; 255, i.e., at most 255
      different atom cache references from the following terms can be made.
    </p>
    <p>
      The distribution header format is:
    </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></tcaption></table>
    <p>
      <c>Flags</c> consists 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 have
      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, i.e., an atom cache can contain 2048 entries.
    </p>
    <p>
      After flag fields for atom cache references, another half byte flag
      field is located which 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 the <c>LongAtoms</c>
      flag. 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 follows
      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 will follow:
    </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 number of bytes that <c>AtomText</c>
      consists of. Length is a two byte big endian integer
      if the <c>LongAtoms</c> flag has been set, otherwise a one byte
      integer. When the
      <seealso marker="erl_dist_protocol#dflags"><c>DFLAG_UTF8_ATOMS</c></seealso>
      distribution flag has been exchanged between both nodes in the
      <seealso marker="erl_dist_protocol#distribution_handshake">distribution handshake</seealso>,
      characters in <c>AtomText</c> is encoded in UTF-8; otherwise,
      encoded in Latin1. Subsequent <c>CachedAtomRef</c>s with the same
      <c>SegmentIndex</c> and <c>InternalSegmentIndex</c> as this
      <c>NewAtomCacheRef</c> will 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
      <seealso marker="#utf8_atoms">note on UTF-8 encoded atoms</seealso>
      in the beginning of this document.
    </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
      will follow:
    </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></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">97</cell>
	<cell align="center">Int</cell>
      </row>
    <tcaption></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">98</cell>
	<cell align="center">Int</cell>
      </row>
    <tcaption></tcaption></table>
    <p>
      Signed 32 bit integer in big-endian format (i.e. MSB first)
    </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">99</cell>
	<cell align="center">Float String</cell>
      </row>
    <tcaption></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">
	NEW_FLOAT_EXT
	</seealso>.
    </p>
  </section>

    <section>
      <marker id="ATOM_EXT"/>
      <title>ATOM_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>100</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>AtomName</c></cell>
	  </row>
	<tcaption></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 Latin1 characters that forms
	the <c>AtomName</c>.
	<em>Note</em>: The maximum allowed value for <c>Len</c> is 255.
      </p>
    </section>

    <section>
      <marker id="REFERENCE_EXT"/>
      <title>REFERENCE_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>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></tcaption></table>
      <p>
	Encode a reference object (an object generated with <c>make_ref/0</c>).
	The <c>Node</c> term is an encoded atom, i.e. 
	<seealso marker="#ATOM_EXT">ATOM_EXT</seealso>, 
	<seealso marker="#SMALL_ATOM_EXT">SMALL_ATOM_EXT</seealso> or
	<seealso marker="#ATOM_CACHE_REF">ATOM_CACHE_REF</seealso>. 
	The <c>ID</c> field contains a big-endian 
	unsigned integer,
	but <em>should be regarded as uninterpreted data</em> 
	since this field is node specific. 
	<c>Creation</c> is a byte containing a node serial number that
	makes it possible to separate old (crashed) nodes from a new one.
      </p>
      <p>
	In <c>ID</c>, only 18 bits are significant; the rest should be 0.
	In <c>Creation</c>, only 2 bits are significant; the rest should be 0.

	See <seealso marker="#NEW_REFERENCE_EXT">NEW_REFERENCE_EXT</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></tcaption></table>
	<p>
	  Encode a port object (obtained form <c>open_port/2</c>).
	  The <c>ID</c> is a node specific identifier for a local port.
	  Port operations are not allowed across node boundaries.
	  The <c>Creation</c> works just like in
	  <seealso marker="#REFERENCE_EXT">REFERENCE_EXT</seealso>.
	</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></tcaption></table>
	<p>
	  Encode a process identifier object (obtained from <c>spawn/3</c> or
	  friends).
	  The <c>ID</c> and <c>Creation</c> fields works just like in 
	  <seealso marker="#REFERENCE_EXT">REFERENCE_EXT</seealso>, while
	  the <c>Serial</c> field is used to improve safety.
	  
	  In <c>ID</c>, only 15 bits are significant; the rest should be 0.
	</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">104</cell>
	    <cell align="center">Arity</cell>
	    <cell align="center">Elements</cell>
	  </row>
	<tcaption></tcaption></table>
	<p>
	  <c>SMALL_TUPLE_EXT</c> encodes a tuple. The <c>Arity</c>
	  field is an unsigned byte that determines how many element
	  that follows in the <c>Elements</c> section.
	</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">105</cell>
	    <cell align="center">Arity</cell>
	    <cell align="center">Elements</cell>
	  </row>
	<tcaption></tcaption></table>
	<p>
	  Same as 
	  <seealso marker="#SMALL_TUPLE_EXT">SMALL_TUPLE_EXT</seealso>
	  with the exception that <c>Arity</c> is an 
	  unsigned 4 byte integer in big endian format.
	</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">106</cell>
	</row>
      <tcaption></tcaption></table>
      <p>
	The representation for an empty list, i.e. 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">107</cell>
	    <cell align="center">Length</cell>
	    <cell align="center">Characters</cell>
	  </row>
	<tcaption></tcaption></table>
	<p>
	  String does NOT 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.
	  Since the <c>Length</c> field is an unsigned 2 byte integer 
	  (big endian), implementations must make sure that lists longer than 
	  65535 elements are encoded as 
	  <seealso marker="#LIST_EXT">LIST_EXT</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">108</cell>
	    <cell align="center">Length</cell>
	    <cell align="center">Elements</cell>
	    <cell align="center">Tail</cell>
	  </row>
	<tcaption></tcaption></table>

	<p>
	  <c>Length</c> is the number of elements that follows in the
	  <c>Elements</c> section. <c>Tail</c> is the final tail of
	  the list; it is
	  <seealso marker="#NIL_EXT">NIL_EXT</seealso>
	  for a proper list, but may be anything type if the list is
	  improper (for instance <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">109</cell>
	    <cell align="center">Len</cell>
	    <cell align="center">Data</cell>
	  </row>
	<tcaption></tcaption></table>
	<p>
	  Binaries are generated with bit syntax expression or with
	  <seealso marker="erts:erlang#list_to_binary/1">list_to_binary/1</seealso>,
	  <seealso marker="erts:erlang#term_to_binary/1">term_to_binary/1</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">110</cell>
	    <cell align="center">n</cell>
	    <cell align="center">Sign</cell>
	    <cell align="center">d(0) ... d(n-1)</cell>
	  </row>
	<tcaption></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 is negative. The
	  digits are stored with the LSB byte stored first. To
	  calculate the integer the following formula can be used:<br/>
	  
	  B = 256<br/>
	  (d0*B^0 + d1*B^1 + d2*B^2 + ... d(N-1)*B^(n-1))
	</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">111</cell>
	    <cell align="center">n</cell>
	    <cell align="center">Sign</cell>
	    <cell align="center">d(0) ... d(n-1)</cell>
	  </row>
	<tcaption></tcaption></table>
	<p>
	  Same as <seealso marker="#SMALL_BIG_EXT">SMALL_BIG_EXT</seealso> 
	  with the difference that the length field 
	  is an unsigned 4 byte integer.
	</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">114</cell>
	    <cell align="center">Len</cell>
	    <cell align="center">Node</cell>
	    <cell align="center">Creation</cell>
	    <cell align="center">ID ...</cell>
	  </row>
	<tcaption></tcaption></table>
	<p>
	  Node and Creation are as in 
	  <seealso marker="#REFERENCE_EXT">REFERENCE_EXT</seealso>.
	</p>
	<p>
	  <c>ID</c> contains a sequence of big-endian unsigned integers 
	  (4 bytes each, so <c>N'</c> is a multiple of 4), 
	  but should be regarded as uninterpreted data.
	</p>
	<p>
	  <c>N'</c> = 4 * <c>Len</c>.
	</p>
	<p>
	  In the first word (four bytes) of <c>ID</c>, only 18 bits are 
	  significant, the rest should be 0.
	  In <c>Creation</c>, only 2 bits are significant, 
	  the rest should be 0.
	</p>
	<p>
	  NEW_REFERENCE_EXT was introduced with distribution version 4.
	  In version 4, <c>N'</c> should be at most 12.
	</p>
	<p>
	  See <seealso marker="#REFERENCE_EXT">REFERENCE_EXT</seealso>).
	</p>
    </section>

    <section>
      <marker id="SMALL_ATOM_EXT"/>
      <title>SMALL_ATOM_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>115</c></cell>
	    <cell align="center"><c>Len</c></cell>
	    <cell align="center"><c>AtomName</c></cell>
	  </row>
	<tcaption></tcaption></table>
      <p>
	An atom is stored with a 1 byte unsigned length, 
	followed by <c>Len</c> numbers of 8 bit Latin1 characters that
	forms the <c>AtomName</c>. Longer atoms can be represented
	by <seealso marker="#ATOM_EXT">ATOM_EXT</seealso>. <em>Note</em>
	the <c>SMALL_ATOM_EXT</c> was introduced in erts version 5.7.2 and
	require an exchange of the
	<seealso marker="erl_dist_protocol#dflags"><c>DFLAG_SMALL_ATOM_TAGS</c></seealso>
	distribution flag in the
	<seealso marker="erl_dist_protocol#distribution_handshake">distribution handshake</seealso>.
      </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">117</cell>
	    <cell align="center">NumFree</cell>
	    <cell align="center">Pid</cell>
	    <cell align="center">Module</cell>
	    <cell align="center">Index</cell>
	    <cell align="center">Uniq</cell>
	    <cell align="center">Free vars ...</cell>
	  </row>
	<tcaption></tcaption></table>
	<taglist>
	  <tag><c>Pid</c></tag> 
	  <item>
	    is a process identifier as in 
	    <seealso marker="#PID_EXT">PID_EXT</seealso>. 
	    It represents the process in which the fun was created.
	  </item>
	<tag><c>Module</c></tag>
	<item>
	  is an encoded as an atom, using 
	  <seealso marker="#ATOM_EXT">ATOM_EXT</seealso>, 
	  <seealso marker="#SMALL_ATOM_EXT">SMALL_ATOM_EXT</seealso>
	  or <seealso marker="#ATOM_CACHE_REF">ATOM_CACHE_REF</seealso>.
	  This is the module that the fun is implemented in.
	</item>
	<tag><c>Index</c></tag>
	<item>
	  is an integer encoded using 
	  <seealso marker="#SMALL_INTEGER_EXT">SMALL_INTEGER_EXT</seealso> 
	  or <seealso marker="#INTEGER_EXT">INTEGER_EXT</seealso>.
	  It is typically a small index into the module's fun table.
	</item>
	<tag><c>Uniq</c></tag>
	<item>
	  is an integer encoded using 
	  <seealso marker="#SMALL_INTEGER_EXT">SMALL_INTEGER_EXT</seealso> or 
	  <seealso marker="#INTEGER_EXT">INTEGER_EXT</seealso>.
	  <c>Uniq</c> is the hash value of the parse for the fun.
	</item>
	<tag><c>Free vars</c></tag>
	<item>
	  is <c>NumFree</c> number of terms, each one encoded according 
	  to its type.
	</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">112</cell>
	    <cell align="center">Size</cell>
	    <cell align="center">Arity</cell>
	    <cell align="center">Uniq</cell>
	    <cell align="center">Index</cell>
	    <cell align="center">NumFree</cell>
	    <cell align="center">Module</cell>
	    <cell align="center">OldIndex</cell>
	    <cell align="center">OldUniq</cell>
	    <cell align="center">Pid</cell>
	    <cell align="center">Free Vars</cell>
	  </row>
	<tcaption></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>
	    is the total number of bytes, including the <c>Size</c> field.
	  </item>
	  <tag><c>Arity</c></tag> 
	  <item>
	    is the arity of the function implementing the fun.
	  </item>
	  <tag><c>Uniq</c></tag>
	  <item>
	    is the 16 bytes MD5 of the significant parts of the Beam file.
	  </item>
	  <tag><c>Index</c></tag> 
	  <item>
	    is an index number. Each fun within a module has an unique
	    index. <c>Index</c> is stored in big-endian byte order.
	  </item>
	  <tag><c>NumFree</c></tag> 
	  <item>
	    is the number of free variables.
	  </item>
	  <tag><c>Module</c></tag>
	  <item>
	    is an encoded as an atom, using 
	    <seealso marker="#ATOM_EXT">ATOM_EXT</seealso>, 
	    <seealso marker="#SMALL_ATOM_EXT">SMALL_ATOM_EXT</seealso> or
	    <seealso marker="#ATOM_CACHE_REF">ATOM_CACHE_REF</seealso>. 
	    This is the module that the fun is implemented in.
	  </item>
	  <tag><c>OldIndex</c></tag>
	  <item>
	    is an integer encoded using 
	    <seealso marker="#SMALL_INTEGER_EXT">SMALL_INTEGER_EXT</seealso> 
	    or <seealso marker="#INTEGER_EXT">INTEGER_EXT</seealso>.
	    It is typically a small index into the module's fun table.
	  </item>
	  <tag><c>OldUniq</c></tag>
	  <item>
	    is an integer encoded using 
	    <seealso marker="#SMALL_INTEGER_EXT">SMALL_INTEGER_EXT</seealso> 
	    or 
	    <seealso marker="#INTEGER_EXT">INTEGER_EXT</seealso>.
	    <c>Uniq</c> is the hash value of the parse tree for the fun.
	  </item>
	  <tag><c>Pid</c></tag>
	  <item>
	    is a process identifier as in 
	    <seealso marker="#PID_EXT">PID_EXT</seealso>. 
	    It represents the process in which
	    the fun was created.
	  </item>

	  <tag><c>Free vars</c></tag>
	  <item>
	    is <c>NumFree</c> number of terms, each one encoded according 
	    to its type.
	  </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">113</cell>
	    <cell align="center">Module</cell>
	    <cell align="center">Function</cell>
	    <cell align="center">Arity</cell>
	  </row>
	<tcaption></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">ATOM_EXT</seealso>, 
	  <seealso marker="#SMALL_ATOM_EXT">SMALL_ATOM_EXT</seealso> or 
	  <seealso marker="#ATOM_CACHE_REF">ATOM_CACHE_REF</seealso>).
	</p>
	<p>
	  <c>Arity</c> is an integer encoded using 
	  <seealso marker="#SMALL_INTEGER_EXT">SMALL_INTEGER_EXT</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">77</cell>
	    <cell align="center">Len</cell>
	    <cell align="center">Bits</cell>
	    <cell align="center">Data</cell>
	  </row>
	<tcaption></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 towards 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">70</cell>
	    <cell align="center">IEEE float</cell>
	  </row>
	<tcaption></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></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
	<seealso marker="#utf8_atoms">note on UTF-8 encoded atoms</seealso>
	in the beginning of this document.
      </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></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">ATOM_UTF8_EXT</seealso>.
      </p>
      <p>
	For more information on encoding of atoms, see
	<seealso marker="#utf8_atoms">note on UTF-8 encoded atoms</seealso>
	in the beginning of this document.
      </p>
    </section>

  </chapter>