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<!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml"><head><title>pycrc</title><link rel="stylesheet" type="text/css" href="docbook.css"></link><meta name="generator" content="DocBook XSL Stylesheets V1.79.2"></meta><meta xmlns="" name="viewport" content="width=device-width, initial-scale=1"></head><body><div class="refentry" id="pycrc"><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>pycrc — a free, easy to use Cyclic Redundancy Check (CRC) calculator and C source code generator.</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="cmdsynopsis"><p id="d0e35"><code class="command">python pycrc.py</code> [OPTIONS]</p></div></div><div class="refsect1" id="d0e40"><h2>Description</h2><p>
<a class="link" href="https://pycrc.org" target="_top">pycrc</a>
is a CRC reference implementation in Python and a C source code generator for parametrised CRC models.
The generated C source code can be optimised for simplicity,
speed or small memory footprint, as required on small embedded systems.
The following operations are implemented:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>calculate the checksum of a string (ASCII or hex)</p></li><li class="listitem"><p>calculate the checksum of a file</p></li><li class="listitem"><p>generate the header and source files for a C implementation.</p></li></ul></div><p>
</p><p>
pycrc supports the following variants of the CRC algorithm:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p><em class="replaceable"><code>bit-by-bit</code></em> or <em class="replaceable"><code>bbb</code></em>:
the basic algorithm which operates individually on every bit of the augmented message
(i.e. the input data with <em class="replaceable"><code>Width</code></em> zero bits added at the end).
This algorithm is a straightforward implementation of the basic polynomial division and
is the easiest one to understand, but it is also the slowest one among all possible
variants.
</p></li><li class="listitem"><p><em class="replaceable"><code>bit-by-bit-fast</code></em> or <em class="replaceable"><code>bbf</code></em>:
a variation of the simple <em class="replaceable"><code>bit-by-bit</code></em> algorithm.
This algorithm still iterates over every bit of the message, but does not augment
it (does not add <em class="replaceable"><code>Width</code></em> zero bits at the end).
It gives the same result as the <em class="replaceable"><code>bit-by-bit</code></em> method by
carefully choosing the initial value of the algorithm.
This method might be a good choice for embedded platforms, where code space is more
important than execution speed.
</p></li><li class="listitem"><p><em class="replaceable"><code>table-driven</code></em> or <em class="replaceable"><code>tbl</code></em>:
the standard table driven algorithm.
This is the fastest variant because it operates on one byte at a time, as opposed to one
bit at the time.
This method uses a look-up table (usually of 256 elements), which might not be acceptable
for small embedded systems. The number of elements in the look-up table can be reduced
with the <code class="option">--table-idx-width</code> command line switch.
The value of 4 bits for the table index (16 elements in the look-up table) can be a good
compromise between execution speed and code size.
</p><p>
The <code class="option">--slice-by</code> option enables a variant of the <em class="replaceable"><code>table-driven</code></em>
algorithm that operates on 32 bits of data or more at a time rather than 8 bits.
This can dramatically speed-up the calculation of the CRC, at the cost of
increased code and data size.
<span class="emphasis"><em>Note</em></span>: this option is experimental and not well-tested.
Check your results and please raise bugs if you find problems.
</p></li></ul></div><p>
</p></div><div class="refsect1" id="d0e113"><h2>Options</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term">
<code class="option">--version</code>
</span></dt><dd><p>show the program version number and exit.</p></dd><dt><span class="term">
<code class="option">-h</code>
, </span><span class="term">
<code class="option">--help</code>
</span></dt><dd><p>show this help message and exit.</p></dd><dt><span class="term">
<code class="option">--verbose</code>
</span></dt><dd><p>be more verbose; in particular, print the value of the parameters and the chosen model to <code class="filename">stdout</code>.</p></dd><dt><span class="term">
<code class="option">--check-string=</code><em class="replaceable"><code>STRING</code></em>
</span></dt><dd><p>calculate the checksum of a string (default: <span class="quote">“<span class="quote"><em class="replaceable"><code>123456789</code></em></span>”</span>). If the string contains non-ASCII characters then it will be UTF-8 decoded.</p></dd><dt><span class="term">
<code class="option">--check-hexstring=</code><em class="replaceable"><code>STRING</code></em>
</span></dt><dd><p>calculate the checksum of a hexadecimal number string.</p></dd><dt><span class="term">
<code class="option">--check-file=</code><em class="replaceable"><code>FILE</code></em>
</span></dt><dd><p>calculate the checksum of a file. If the file contains non-ASCII characters then it will be UTF-8 decoded.</p></dd><dt><span class="term">
<code class="option">--generate=</code><em class="replaceable"><code>CODE</code></em>
</span></dt><dd><p>generate C source code; choose the type from {<em class="replaceable"><code>h</code></em>,
<em class="replaceable"><code>c</code></em>, <em class="replaceable"><code>c-main</code></em>, <em class="replaceable"><code>table</code></em>}.</p></dd><dt><span class="term">
<code class="option">--std=</code><em class="replaceable"><code>STD</code></em>
</span></dt><dd><p>specify the C dialect of the generated code from {C89, ANSI, C99}.</p></dd><dt><span class="term">
<code class="option">--algorithm=</code><em class="replaceable"><code>ALGO</code></em>
</span></dt><dd><p>choose an algorithm from {<em class="replaceable"><code>bit-by-bit</code></em>, <em class="replaceable"><code>bbb</code></em>,
<em class="replaceable"><code>bit-by-bit-fast</code></em>, <em class="replaceable"><code>bbf</code></em>,
<em class="replaceable"><code>table-driven</code></em>, <em class="replaceable"><code>tbl</code></em>,
<em class="replaceable"><code>all</code></em>}.</p></dd><dt><span class="term">
<code class="option">--model=</code><em class="replaceable"><code>MODEL</code></em>
</span></dt><dd><p>choose a parameter set from
{<em class="replaceable"><code>crc-5</code></em>,
<em class="replaceable"><code>crc-8</code></em>,
<em class="replaceable"><code>dallas-1-wire</code></em>,
<em class="replaceable"><code>crc-12-3gpp</code></em>,
<em class="replaceable"><code>crc-15</code></em>,
<em class="replaceable"><code>crc-16</code></em>,
<em class="replaceable"><code>crc-16-usb</code></em>,
<em class="replaceable"><code>crc-16-modbus</code></em>,
<em class="replaceable"><code>crc-16-genibus</code></em>,
<em class="replaceable"><code>crc-16-ccitt</code></em>,
<em class="replaceable"><code>r-crc-16</code></em>,
<em class="replaceable"><code>kermit</code></em>,
<em class="replaceable"><code>x-25</code></em>,
<em class="replaceable"><code>xmodem</code></em>,
<em class="replaceable"><code>zmodem</code></em>,
<em class="replaceable"><code>crc-24</code></em>,
<em class="replaceable"><code>crc-32</code></em>,
<em class="replaceable"><code>crc-32c</code></em>,
<em class="replaceable"><code>crc-32-mpeg</code></em>,
<em class="replaceable"><code>crc-32-bzip2</code></em>,
<em class="replaceable"><code>posix</code></em>,
<em class="replaceable"><code>jam</code></em>,
<em class="replaceable"><code>xfer</code></em>,
<em class="replaceable"><code>crc-64</code></em>,
<em class="replaceable"><code>crc-64-jones</code></em>,
<em class="replaceable"><code>crc-64-xz</code></em>}.</p></dd><dt><span class="term">
<code class="option">--width=</code><em class="replaceable"><code>NUM</code></em>
</span></dt><dd><p>use <em class="replaceable"><code>NUM</code></em> bits in the <em class="replaceable"><code>Polynomial</code></em>.</p></dd><dt><span class="term">
<code class="option">--poly=</code><em class="replaceable"><code>HEX</code></em>
</span></dt><dd><p>use <em class="replaceable"><code>HEX</code></em> as <em class="replaceable"><code>Polynomial</code></em>.</p></dd><dt><span class="term">
<code class="option">--reflect-in=</code><em class="replaceable"><code>BOOL</code></em>
</span></dt><dd><p>reflect the octets in the input message.</p></dd><dt><span class="term">
<code class="option">--xor-in=</code><em class="replaceable"><code>HEX</code></em>
</span></dt><dd><p>use <em class="replaceable"><code>HEX</code></em> as initial value.</p></dd><dt><span class="term">
<code class="option">--reflect-out=</code><em class="replaceable"><code>BOOL</code></em>
</span></dt><dd><p>reflect the resulting checksum before applying the XorOut value.</p></dd><dt><span class="term">
<code class="option">--xor-out=</code><em class="replaceable"><code>HEX</code></em>
</span></dt><dd><p>xor the final CRC value with <em class="replaceable"><code>HEX</code></em>.</p></dd><dt><span class="term">
<code class="option">--slice-by=</code><em class="replaceable"><code>NUM</code></em>
</span></dt><dd><p>speed-up the table-driven calculation by operating on
<em class="replaceable"><code>NUM</code></em> octets of data rather than a
single octet at a time.
<em class="replaceable"><code>NUM</code></em> must be one of the values
{<em class="replaceable"><code>4</code></em>, <em class="replaceable"><code>8</code></em>,
<em class="replaceable"><code>16</code></em>}.</p></dd><dt><span class="term">
<code class="option">--table-idx-width=</code><em class="replaceable"><code>NUM</code></em>
</span></dt><dd><p>use <em class="replaceable"><code>NUM</code></em> bits to index the CRC table;
<em class="replaceable"><code>NUM</code></em> must be one of the values
{<em class="replaceable"><code>1</code></em>, <em class="replaceable"><code>2</code></em>,
<em class="replaceable"><code>4</code></em>, <em class="replaceable"><code>8</code></em>}.</p></dd><dt><span class="term">
<code class="option">--force-poly</code>
</span></dt><dd><p>override any errors about possibly unsuitable
polynoms. pycrc does not allow even polynoms or
polynoms that are wider than Width. Use this option
to override the error, if you know what you are
doing.</p></dd><dt><span class="term">
<code class="option">--symbol-prefix=</code><em class="replaceable"><code>STRING</code></em>
</span></dt><dd><p>when generating source code, use <em class="replaceable"><code>STRING</code></em>
as prefix to the exported C symbols.</p></dd><dt><span class="term">
<code class="option">--crc-type=</code><em class="replaceable"><code>STRING</code></em>
</span></dt><dd><p>when generating source code, use <em class="replaceable"><code>STRING</code></em> as crc_t type.</p></dd><dt><span class="term">
<code class="option">--include-file=</code><em class="replaceable"><code>FILE</code></em>
</span></dt><dd><p>when generating source code, include also <em class="replaceable"><code>FILE</code></em> as header file.
This option can be specified multiple times.</p></dd><dt><span class="term">
<code class="option">-o</code><em class="replaceable"><code>FILE</code></em>
, </span><span class="term">
<code class="option">--output=</code><em class="replaceable"><code>FILE</code></em>
</span></dt><dd><p>write the generated code to <em class="replaceable"><code>FILE</code></em> instead of <code class="filename">stdout</code>.</p></dd></dl></div></div><div class="refsect1" id="d0e558"><h2>The CRC Parametric Model</h2><p>
The parametric model follows Ross N. Williams' convention described in
<a class="link" href="http://www.ross.net/crc/crcpaper.html" target="_top">A Painless Guide to CRC Error Detection Algorithms</a>,
often called the Rocksoft Model.
Since most people are familiar with this kind of parameters, pycrc follows this convention, described as follows:
</p><div class="glosslist"><dl><dt><span class="glossterm"><em class="replaceable"><code>Width</code></em></span></dt><dd class="glossdef"><p>
The number of significant bits in the CRC <em class="replaceable"><code>Polynomial</code></em>,
excluding the most significant 1.
This will also be the number of bits in the final CRC result.
In previous versions of pycrc only multiples of 8 could be used as
<em class="replaceable"><code>Width</code></em> for the <em class="replaceable"><code>table-driven</code></em> algorithm.
As of version 0.7.5 any value is accepted for <em class="replaceable"><code>Width</code></em> for all algorithms.
</p></dd><dt><span class="glossterm"><em class="replaceable"><code>Polynomial</code></em></span></dt><dd class="glossdef"><p>
The unreflected polynomial of the CRC algorithm.
</p><p>
The <em class="replaceable"><code>Polynomial</code></em> may be specified in its standard form,
i.e. with bit <em class="replaceable"><code>Width</code></em>+1 set to 1, but the most significant
bit may also be omitted.
For example, both numbers 0x18005 and 0x8005 are accepted for a 16-bit
<em class="replaceable"><code>Polynomial</code></em>.
</p><p>
Most polynomials used in real world applications are odd (the least significant
bit is 1), but there are some good even ones.
pycrc allows the use of even polynomials with the <code class="option">--force-poly</code>
option.
Some even polynomials may yield incorrect checksums depending on the used algorithm.
Use at your own risk and if at all possible use a well-known <em class="replaceable"><code>MODEL</code></em> above.
</p></dd><dt><span class="glossterm"><em class="replaceable"><code>ReflectIn</code></em></span></dt><dd class="glossdef"><p>
Reflect the octets of the message before processing them.
</p><p>
A word is reflected or reversed by <span class="quote">“<span class="quote">flipping</span>”</span> its bits around the
mid-point of the word.
The most significant bit of the word is moved to the least significant position,
the second-most significant bit is moved to the second-least significant position
and so on.
The reflected value of 0xa2 (10100010b) is 0x45 (01000101b), for example.
</p><p>
Some CRC algorithms can be implemented more efficiently in a bit reversed version,
that's why many of the standard CRC models use reflected input octets.
</p></dd><dt><span class="glossterm"><em class="replaceable"><code>XorIn</code></em></span></dt><dd class="glossdef"><p>
The initial value (usually all 0 or all 1) for algorithms which operate on the
non-augmented message, that is, any algorithm other than the
<em class="replaceable"><code>bit-by-bit</code></em> one.
This value can be interpreted as a value which will be XOR-ed into the CRC register
after <em class="replaceable"><code>Width</code></em> iterations of the
<em class="replaceable"><code>bit-by-bit</code></em> algorithm.
This implies that the simple <em class="replaceable"><code>bit-by-bit</code></em> algorithm must
calculate the initial value using some sort of reverse CRC algorithm on the
<em class="replaceable"><code>XorIn</code></em> value.
</p></dd><dt><span class="glossterm"><em class="replaceable"><code>ReflectOut</code></em></span></dt><dd class="glossdef"><p>
Reflect the final CRC result. This operation takes place before XOR-ing the final CRC
value with the <em class="replaceable"><code>XorOut</code></em> parameter.
</p></dd><dt><span class="glossterm"><em class="replaceable"><code>XorOut</code></em></span></dt><dd class="glossdef"><p>
A value (usually all bits 0 or all 1) which will be XOR-ed to the final CRC value.
</p></dd><dt><span class="glossterm"><em class="replaceable"><code>Check</code></em></span></dt><dd class="glossdef"><p>
This value is not exactly a parameter of a model but it is sometimes given together
with the Rocksoft Model parameters.
It is the CRC value of the parametrised model over the string
<span class="quote">“<span class="quote"><em class="replaceable"><code>123456789</code></em></span>”</span> and
can be used as a sanity check for a particular CRC implementation.
</p></dd></dl></div><p>
</p></div><div class="refsect1" id="d0e677"><h2>Code generation</h2><p>
In the default configuration, the generated code is strict ISO C99.
A minimal set of three functions are defined for each algorithm:
<code class="function">crc_init()</code>, <code class="function">crc_update()</code> and <code class="function">crc_finalize()</code>.
Depending on the number of parameters given to pycrc, a different interface will be defined.
A fully parametrised model has a simpler API, while the generated code for a runtime-specified
implementation requires a pointer to a configuration structure as first parameter to all functions.
</p><p>
The generated source code uses the type <span class="type">crc_t</span>, which is used throughout the code
to hold intermediate results and also the final CRC value.
It is defined in the generated header file and its type may be overridden with the
<code class="option">--crc-type</code> option.
</p><div class="refsect2" id="d0e699"><h3>Fully parametrised models</h3><p>
The prototypes of the CRC functions are normally generated by pycrc using the
<em class="replaceable"><code>--generate h</code></em> option.
The CRC functions for a fully parametrised model will look like:
</p><div class="funcsynopsis"><pre class="funcsynopsisinfo">
#include <stdlib.h>
typedef uint16_t crc_t; /* pycrc will use an appropriate size here */
</pre><table style="border: 0; cellspacing: 0; cellpadding: 0;" class="funcprototype-table"><tr><td><code class="funcdef">crc_t <strong class="fsfunc">crc_init</strong>(</code></td><td><code>void)</code>;</td><td> </td></tr></table><div class="funcprototype-spacer"> </div><table style="border: 0; cellspacing: 0; cellpadding: 0;" class="funcprototype-table"><tr><td><code class="funcdef">crc_t <strong class="fsfunc">crc_update</strong>(</code></td><td>crc_t <var class="pdparam">crc</var>, </td></tr><tr><td> </td><td>const unsigned char *<var class="pdparam">data</var>, </td></tr><tr><td> </td><td>size_t <var class="pdparam">data_len</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table style="border: 0; cellspacing: 0; cellpadding: 0;" class="funcprototype-table"><tr><td><code class="funcdef">crc_t <strong class="fsfunc">crc_finalize</strong>(</code></td><td>crc_t <var class="pdparam">crc</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div><p>
The code snippet below shows how to use the generated functions.
</p><pre class="programlisting">
#include "pycrc_generated_crc.h"
#include <stdio.h>
int main(void)
{
static const unsigned char str1[] = "1234";
static const unsigned char str2[] = "56789";
crc_t crc;
crc = crc_init();
crc = crc_update(crc, str1, sizeof(str1) - 1);
crc = crc_update(crc, str2, sizeof(str2) - 1);
/* more calls to crc_update... */
crc = crc_finalize(crc);
printf("0x%lx\n", (long)crc);
return 0;
}
</pre><p>
</p></div><div class="refsect2" id="d0e750"><h3>Models with runtime-configurable parameters</h3><p>
When the model is not fully defined then the missing parameters are stored in a structure of
type <span class="type">crc_cfg_t</span>.
If a CRC function requires a value from the <span class="type">crc_cfg_t</span> structure, then the first
function argument is always a pointer to that structure.
All fields of the configuration structure must be properly initialised before the first call
to any CRC function.
</p><p>
If the <em class="replaceable"><code>Width</code></em> was not specified when the code was generated, then
the <span class="type">crc_cfg_t</span> structure will contain three more fields:
<em class="parameter"><code>msb_mask</code></em>, <em class="parameter"><code>crc_mask</code></em> and <em class="parameter"><code>crc_shift</code></em>.
They are defined for performance reasons and must be initialised to the value given next to the
field definition.
</p><p>
For example, a completely undefined CRC implementation will generate a <span class="type">crc_cfg_t</span>
structure as below:
</p><pre class="programlisting">
typedef struct {
unsigned int width;
crc_t poly;
bool reflect_in;
crc_t xor_in;
bool reflect_out;
crc_t xor_out;
// internal parameters
crc_t msb_mask; // initialise as (crc_t)1u << (cfg->width - 1)
crc_t crc_mask; // initialise as (cfg->msb_mask - 1) | cfg->msb_mask
unsigned int crc_shift; // initialise as cfg->width < 8 ? 8 - cfg->width : 0
} crc_cfg_t;
</pre><p>
</p><p>
<em class="parameter"><code>msb_mask</code></em> is a bitmask with the most significant bit of a
<em class="replaceable"><code>Width</code></em> bits wide data type set to 1.
<em class="parameter"><code>crc_mask</code></em> is a bitmask with all bits of a
<em class="replaceable"><code>Width</code></em> bits wide data type set to 1.
<em class="parameter"><code>crc_shift</code></em> is a shift counter that is used when
<em class="replaceable"><code>Width</code></em> is less than 8.
It is the number of bits to shift the CRC register to align its top bit to a byte boundary.
</p><p>
The file <code class="filename">test/main.c</code> in the source package of pycrc
contains a fully featured example of how to use the generated source code.
A shorter, more compact <code class="code">main()</code> function can be generated with the
<em class="replaceable"><code>--generate c-main</code></em> option.
This second variant is the better option as it will always output valid code when
some of the CRC parameters are known and some are unknown during code generation.
</p></div></div><div class="refsect1" id="d0e817"><h2>Examples</h2><p>
</p><div class="glosslist"><dl><dt><span class="glossterm">Calculate the CRC-32 checksum of the string <span class="quote">“<span class="quote">123456789</span>”</span>:</span></dt><dd class="glossdef"><p>
<strong class="userinput"><code>python pycrc.py --model crc-32 --check-string 123456789</code></strong>
</p></dd><dt><span class="glossterm">Generate the source code of the table-driven algorithm for an embedded application.</span></dt><dd class="glossdef"><p>
The table index width of 4 bits ensures a moderate memory usage.
To be precise, the size of the resulting table will be <code class="code">16 * sizeof(crc_t)</code>.
</p><p>
<strong class="userinput"><code>python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate h -o crc.h</code></strong>
</p><p>
<strong class="userinput"><code>python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate c -o crc.c</code></strong>
</p><p>
A variant of the <em class="replaceable"><code>c</code></em> target is <em class="replaceable"><code>c-main</code></em>:
this target will generate a simple <em class="replaceable"><code>main()</code></em> function in addition to
the CRC functions:
</p><p>
<strong class="userinput"><code>python pycrc.py --model crc-16 --algorithm table-driven --table-idx-width 4 --generate c-main -o crc.c</code></strong>
</p></dd><dt><span class="glossterm">Generate the CRC table only:</span></dt><dd class="glossdef"><p>
<strong class="userinput"><code>python pycrc.py --model kermit --generate table -o crc-table.txt</code></strong>
</p></dd></dl></div><p>
</p></div><div class="refsect1" id="d0e880"><h2>See Also</h2><p>
The homepage of pycrc is <a class="link" href="https://pycrc.org" target="_top">https://pycrc.org</a>.
</p><p>
A list of common CRC models is at <a class="link" href="https://pycrc.org/models.html" target="_top">https://pycrc.org/models.html</a>.
For a long list of known CRC models, see Greg Cook's
<a class="link" href="http://reveng.sourceforge.net/crc-catalogue/" target="_top">Catalogue of Parameterised CRC Algorithms</a>.
</p></div><div class="refsect1" id="d0e896"><h2>Copyright</h2><p>
This work is licensed under a
<a class="link" href="https://creativecommons.org/licenses/by-sa/4.0/" target="_top">Creative Commons Attribution-ShareAlike 4.0 International</a>.
</p></div></div></body></html>
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