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<a name="Target-Description-Format"></a>
<div class="header">
<p>
Next: <a href="Predefined-Target-Types.html#Predefined-Target-Types" accesskey="n" rel="next">Predefined Target Types</a>, Previous: <a href="Retrieving-Descriptions.html#Retrieving-Descriptions" accesskey="p" rel="previous">Retrieving Descriptions</a>, Up: <a href="Target-Descriptions.html#Target-Descriptions" accesskey="u" rel="up">Target Descriptions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Target-Description-Format-1"></a>
<h3 class="section">G.2 Target Description Format</h3>
<a name="index-target-descriptions_002c-XML-format"></a>

<p>A target description annex is an <a href="http://www.w3.org/XML/">XML</a>
document which complies with the Document Type Definition provided in
the <small>GDB</small> sources in <samp>gdb/features/gdb-target.dtd</samp>.  This
means you can use generally available tools like <code>xmllint</code> to
check that your feature descriptions are well-formed and valid.
However, to help people unfamiliar with XML write descriptions for
their targets, we also describe the grammar here.
</p>
<p>Target descriptions can identify the architecture of the remote target
and (for some architectures) provide information about custom register
sets.  They can also identify the OS ABI of the remote target.
<small>GDB</small> can use this information to autoconfigure for your
target, or to warn you if you connect to an unsupported target.
</p>
<p>Here is a simple target description:
</p>
<div class="smallexample">
<pre class="smallexample">&lt;target version=&quot;1.0&quot;&gt;
  &lt;architecture&gt;i386:x86-64&lt;/architecture&gt;
&lt;/target&gt;
</pre></div>

<p>This minimal description only says that the target uses
the x86-64 architecture.
</p>
<p>A target description has the following overall form, with [ ] marking
optional elements and &hellip; marking repeatable elements.  The elements
are explained further below.
</p>
<div class="smallexample">
<pre class="smallexample">&lt;?xml version=&quot;1.0&quot;?&gt;
&lt;!DOCTYPE target SYSTEM &quot;gdb-target.dtd&quot;&gt;
&lt;target version=&quot;1.0&quot;&gt;
  <span class="roman">[</span><var>architecture</var><span class="roman">]</span>
  <span class="roman">[</span><var>osabi</var><span class="roman">]</span>
  <span class="roman">[</span><var>compatible</var><span class="roman">]</span>
  <span class="roman">[</span><var>feature</var>&hellip;<span class="roman">]</span>
&lt;/target&gt;
</pre></div>

<p>The description is generally insensitive to whitespace and line
breaks, under the usual common-sense rules.  The XML version
declaration and document type declaration can generally be omitted
(<small>GDB</small> does not require them), but specifying them may be
useful for XML validation tools.  The &lsquo;<samp>version</samp>&rsquo; attribute for
&lsquo;<samp>&lt;target&gt;</samp>&rsquo; may also be omitted, but we recommend
including it; if future versions of <small>GDB</small> use an incompatible
revision of <samp>gdb-target.dtd</samp>, they will detect and report
the version mismatch.
</p>
<a name="Inclusion"></a>
<h4 class="subsection">G.2.1 Inclusion</h4>
<a name="index-target-descriptions_002c-inclusion"></a>
<a name="index-XInclude"></a>
<a name="index-_003cxi_003ainclude_003e"></a>

<p>It can sometimes be valuable to split a target description up into
several different annexes, either for organizational purposes, or to
share files between different possible target descriptions.  You can
divide a description into multiple files by replacing any element of
the target description with an inclusion directive of the form:
</p>
<div class="smallexample">
<pre class="smallexample">&lt;xi:include href=&quot;<var>document</var>&quot;/&gt;
</pre></div>

<p>When <small>GDB</small> encounters an element of this form, it will retrieve
the named XML <var>document</var>, and replace the inclusion directive with
the contents of that document.  If the current description was read
using &lsquo;<samp>qXfer</samp>&rsquo;, then so will be the included document;
<var>document</var> will be interpreted as the name of an annex.  If the
current description was read from a file, <small>GDB</small> will look for
<var>document</var> as a file in the same directory where it found the
original description.
</p>
<a name="Architecture"></a>
<h4 class="subsection">G.2.2 Architecture</h4>
<a name="index-_003carchitecture_003e"></a>

<p>An &lsquo;<samp>&lt;architecture&gt;</samp>&rsquo; element has this form:
</p>
<div class="smallexample">
<pre class="smallexample">  &lt;architecture&gt;<var>arch</var>&lt;/architecture&gt;
</pre></div>

<p><var>arch</var> is one of the architectures from the set accepted by
<code>set architecture</code> (see <a href="Targets.html#Targets">Specifying a Debugging Target</a>).
</p>
<a name="OS-ABI"></a>
<h4 class="subsection">G.2.3 OS ABI</h4>
<a name="index-_003cosabi_003e"></a>

<p>This optional field was introduced in <small>GDB</small> version 7.0.
Previous versions of <small>GDB</small> ignore it.
</p>
<p>An &lsquo;<samp>&lt;osabi&gt;</samp>&rsquo; element has this form:
</p>
<div class="smallexample">
<pre class="smallexample">  &lt;osabi&gt;<var>abi-name</var>&lt;/osabi&gt;
</pre></div>

<p><var>abi-name</var> is an OS ABI name from the same selection accepted by
<code>set&nbsp;osabi</code><!-- /@w --> (see <a href="ABI.html#ABI">Configuring the Current ABI</a>).
</p>
<a name="Compatible-Architecture"></a>
<h4 class="subsection">G.2.4 Compatible Architecture</h4>
<a name="index-_003ccompatible_003e"></a>

<p>This optional field was introduced in <small>GDB</small> version 7.0.
Previous versions of <small>GDB</small> ignore it.
</p>
<p>A &lsquo;<samp>&lt;compatible&gt;</samp>&rsquo; element has this form:
</p>
<div class="smallexample">
<pre class="smallexample">  &lt;compatible&gt;<var>arch</var>&lt;/compatible&gt;
</pre></div>

<p><var>arch</var> is one of the architectures from the set accepted by
<code>set architecture</code> (see <a href="Targets.html#Targets">Specifying a Debugging Target</a>).
</p>
<p>A &lsquo;<samp>&lt;compatible&gt;</samp>&rsquo; element is used to specify that the target
is able to run binaries in some other than the main target architecture
given by the &lsquo;<samp>&lt;architecture&gt;</samp>&rsquo; element.  For example, on the
Cell Broadband Engine, the main architecture is <code>powerpc:common</code>
or <code>powerpc:common64</code>, but the system is able to run binaries
in the <code>spu</code> architecture as well.  The way to describe this
capability with &lsquo;<samp>&lt;compatible&gt;</samp>&rsquo; is as follows:
</p>
<div class="smallexample">
<pre class="smallexample">  &lt;architecture&gt;powerpc:common&lt;/architecture&gt;
  &lt;compatible&gt;spu&lt;/compatible&gt;
</pre></div>

<a name="Features"></a>
<h4 class="subsection">G.2.5 Features</h4>
<a name="index-_003cfeature_003e"></a>

<p>Each &lsquo;<samp>&lt;feature&gt;</samp>&rsquo; describes some logical portion of the target
system.  Features are currently used to describe available CPU
registers and the types of their contents.  A &lsquo;<samp>&lt;feature&gt;</samp>&rsquo; element
has this form:
</p>
<div class="smallexample">
<pre class="smallexample">&lt;feature name=&quot;<var>name</var>&quot;&gt;
  <span class="roman">[</span><var>type</var>&hellip;<span class="roman">]</span>
  <var>reg</var>&hellip;
&lt;/feature&gt;
</pre></div>

<p>Each feature&rsquo;s name should be unique within the description.  The name
of a feature does not matter unless <small>GDB</small> has some special
knowledge of the contents of that feature; if it does, the feature
should have its standard name.  See <a href="Standard-Target-Features.html#Standard-Target-Features">Standard Target Features</a>.
</p>
<a name="Types"></a>
<h4 class="subsection">G.2.6 Types</h4>

<p>Any register&rsquo;s value is a collection of bits which <small>GDB</small> must
interpret.  The default interpretation is a two&rsquo;s complement integer,
but other types can be requested by name in the register description.
Some predefined types are provided by <small>GDB</small> (see <a href="Predefined-Target-Types.html#Predefined-Target-Types">Predefined Target Types</a>), and the description can define additional composite
and enum types.
</p>
<p>Each type element must have an &lsquo;<samp>id</samp>&rsquo; attribute, which gives
a unique (within the containing &lsquo;<samp>&lt;feature&gt;</samp>&rsquo;) name to the type.
Types must be defined before they are used.
</p>
<a name="index-_003cvector_003e"></a>
<p>Some targets offer vector registers, which can be treated as arrays
of scalar elements.  These types are written as &lsquo;<samp>&lt;vector&gt;</samp>&rsquo; elements,
specifying the array element type, <var>type</var>, and the number of elements,
<var>count</var>:
</p>
<div class="smallexample">
<pre class="smallexample">&lt;vector id=&quot;<var>id</var>&quot; type=&quot;<var>type</var>&quot; count=&quot;<var>count</var>&quot;/&gt;
</pre></div>

<a name="index-_003cunion_003e"></a>
<p>If a register&rsquo;s value is usefully viewed in multiple ways, define it
with a union type containing the useful representations.  The
&lsquo;<samp>&lt;union&gt;</samp>&rsquo; element contains one or more &lsquo;<samp>&lt;field&gt;</samp>&rsquo; elements,
each of which has a <var>name</var> and a <var>type</var>:
</p>
<div class="smallexample">
<pre class="smallexample">&lt;union id=&quot;<var>id</var>&quot;&gt;
  &lt;field name=&quot;<var>name</var>&quot; type=&quot;<var>type</var>&quot;/&gt;
  &hellip;
&lt;/union&gt;
</pre></div>

<a name="index-_003cstruct_003e"></a>
<a name="index-_003cflags_003e"></a>
<p>If a register&rsquo;s value is composed from several separate values, define
it with either a structure type or a flags type.
A flags type may only contain bitfields.
A structure type may either contain only bitfields or contain no bitfields.
If the value contains only bitfields, its total size in bytes must be
specified.
</p>
<p>Non-bitfield values have a <var>name</var> and <var>type</var>.
</p>
<div class="smallexample">
<pre class="smallexample">&lt;struct id=&quot;<var>id</var>&quot;&gt;
  &lt;field name=&quot;<var>name</var>&quot; type=&quot;<var>type</var>&quot;/&gt;
  &hellip;
&lt;/struct&gt;
</pre></div>

<p>Both <var>name</var> and <var>type</var> values are required.
No implicit padding is added.
</p>
<p>Bitfield values have a <var>name</var>, <var>start</var>, <var>end</var> and <var>type</var>.
</p>
<div class="smallexample">
<pre class="smallexample">&lt;struct id=&quot;<var>id</var>&quot; size=&quot;<var>size</var>&quot;&gt;
  &lt;field name=&quot;<var>name</var>&quot; start=&quot;<var>start</var>&quot; end=&quot;<var>end</var>&quot; type=&quot;<var>type</var>&quot;/&gt;
  &hellip;
&lt;/struct&gt;
</pre></div>

<div class="smallexample">
<pre class="smallexample">&lt;flags id=&quot;<var>id</var>&quot; size=&quot;<var>size</var>&quot;&gt;
  &lt;field name=&quot;<var>name</var>&quot; start=&quot;<var>start</var>&quot; end=&quot;<var>end</var>&quot; type=&quot;<var>type</var>&quot;/&gt;
  &hellip;
&lt;/flags&gt;
</pre></div>

<p>The <var>name</var> value is required.
Bitfield values may be named with the empty string, &lsquo;<samp>&quot;&quot;</samp>&rsquo;,
in which case the field is &ldquo;filler&rdquo; and its value is not printed.
Not all bits need to be specified, so &ldquo;filler&rdquo; fields are optional.
</p>
<p>The <var>start</var> and <var>end</var> values are required, and <var>type</var>
is optional.
The field&rsquo;s <var>start</var> must be less than or equal to its <var>end</var>,
and zero represents the least significant bit.
</p>
<p>The default value of <var>type</var> is <code>bool</code> for single bit fields,
and an unsigned integer otherwise.
</p>
<p>Which to choose?  Structures or flags?
</p>
<p>Registers defined with &lsquo;<samp>flags</samp>&rsquo; have these advantages over
defining them with &lsquo;<samp>struct</samp>&rsquo;:
</p>
<ul>
<li> Arithmetic may be performed on them as if they were integers.
</li><li> They are printed in a more readable fashion.
</li></ul>

<p>Registers defined with &lsquo;<samp>struct</samp>&rsquo; have one advantage over
defining them with &lsquo;<samp>flags</samp>&rsquo;:
</p>
<ul>
<li> One can fetch individual fields like in &lsquo;<samp>C</samp>&rsquo;.

<div class="smallexample">
<pre class="smallexample">(gdb) print $my_struct_reg.field3
$1 = 42
</pre></div>

</li></ul>

<a name="Registers-2"></a>
<h4 class="subsection">G.2.7 Registers</h4>
<a name="index-_003creg_003e"></a>

<p>Each register is represented as an element with this form:
</p>
<div class="smallexample">
<pre class="smallexample">&lt;reg name=&quot;<var>name</var>&quot;
     bitsize=&quot;<var>size</var>&quot;
     <span class="roman">[</span>regnum=&quot;<var>num</var>&quot;<span class="roman">]</span>
     <span class="roman">[</span>save-restore=&quot;<var>save-restore</var>&quot;<span class="roman">]</span>
     <span class="roman">[</span>type=&quot;<var>type</var>&quot;<span class="roman">]</span>
     <span class="roman">[</span>group=&quot;<var>group</var>&quot;<span class="roman">]</span>/&gt;
</pre></div>

<p>The components are as follows:
</p>
<dl compact="compact">
<dt><var>name</var></dt>
<dd><p>The register&rsquo;s name; it must be unique within the target description.
</p>
</dd>
<dt><var>bitsize</var></dt>
<dd><p>The register&rsquo;s size, in bits.
</p>
</dd>
<dt><var>regnum</var></dt>
<dd><p>The register&rsquo;s number.  If omitted, a register&rsquo;s number is one greater
than that of the previous register (either in the current feature or in
a preceding feature); the first register in the target description
defaults to zero.  This register number is used to read or write
the register; e.g. it is used in the remote <code>p</code> and <code>P</code>
packets, and registers appear in the <code>g</code> and <code>G</code> packets
in order of increasing register number.
</p>
</dd>
<dt><var>save-restore</var></dt>
<dd><p>Whether the register should be preserved across inferior function
calls; this must be either <code>yes</code> or <code>no</code>.  The default is
<code>yes</code>, which is appropriate for most registers except for
some system control registers; this is not related to the target&rsquo;s
ABI.
</p>
</dd>
<dt><var>type</var></dt>
<dd><p>The type of the register.  It may be a predefined type, a type
defined in the current feature, or one of the special types <code>int</code>
and <code>float</code>.  <code>int</code> is an integer type of the correct size
for <var>bitsize</var>, and <code>float</code> is a floating point type (in the
architecture&rsquo;s normal floating point format) of the correct size for
<var>bitsize</var>.  The default is <code>int</code>.
</p>
</dd>
<dt><var>group</var></dt>
<dd><p>The register group to which this register belongs.  It can be one of the
standard register groups <code>general</code>, <code>float</code>, <code>vector</code> or an
arbitrary string.  Group names should be limited to alphanumeric characters.
If a group name is made up of multiple words the words may be separated by
hyphens; e.g. <code>special-group</code> or <code>ultra-special-group</code>.  If no
<var>group</var> is specified, <small>GDB</small> will not display the register in
<code>info registers</code>.
</p>
</dd>
</dl>

<hr>
<div class="header">
<p>
Next: <a href="Predefined-Target-Types.html#Predefined-Target-Types" accesskey="n" rel="next">Predefined Target Types</a>, Previous: <a href="Retrieving-Descriptions.html#Retrieving-Descriptions" accesskey="p" rel="previous">Retrieving Descriptions</a>, Up: <a href="Target-Descriptions.html#Target-Descriptions" accesskey="u" rel="up">Target Descriptions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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