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<title>Debugging with GDB: Memory Region Attributes</title>

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<a name="Memory-Region-Attributes"></a>
<div class="header">
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<hr>
<a name="Memory-Region-Attributes-1"></a>
<h3 class="section">10.17 Memory Region Attributes</h3>
<a name="index-memory-region-attributes"></a>

<p><em>Memory region attributes</em> allow you to describe special handling
required by regions of your target&rsquo;s memory.  <small>GDB</small> uses
attributes to determine whether to allow certain types of memory
accesses; whether to use specific width accesses; and whether to cache
target memory.  By default the description of memory regions is
fetched from the target (if the current target supports this), but the
user can override the fetched regions.
</p>
<p>Defined memory regions can be individually enabled and disabled.  When a
memory region is disabled, <small>GDB</small> uses the default attributes when
accessing memory in that region.  Similarly, if no memory regions have
been defined, <small>GDB</small> uses the default attributes when accessing
all memory.
</p>
<p>When a memory region is defined, it is given a number to identify it;
to enable, disable, or remove a memory region, you specify that number.
</p>
<dl compact="compact">
<dd><a name="index-mem"></a>
</dd>
<dt><code>mem <var>lower</var> <var>upper</var> <var>attributes</var>&hellip;</code></dt>
<dd><p>Define a memory region bounded by <var>lower</var> and <var>upper</var> with
attributes <var>attributes</var>&hellip;, and add it to the list of regions
monitored by <small>GDB</small>.  Note that <var>upper</var> == 0 is a special
case: it is treated as the target&rsquo;s maximum memory address.
(0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.)
</p>
</dd>
<dt><code>mem auto</code></dt>
<dd><p>Discard any user changes to the memory regions and use target-supplied
regions, if available, or no regions if the target does not support.
</p>
<a name="index-delete-mem"></a>
</dd>
<dt><code>delete mem <var>nums</var>&hellip;</code></dt>
<dd><p>Remove memory regions <var>nums</var>&hellip; from the list of regions
monitored by <small>GDB</small>.
</p>
<a name="index-disable-mem"></a>
</dd>
<dt><code>disable mem <var>nums</var>&hellip;</code></dt>
<dd><p>Disable monitoring of memory regions <var>nums</var>&hellip;.
A disabled memory region is not forgotten.
It may be enabled again later.
</p>
<a name="index-enable-mem"></a>
</dd>
<dt><code>enable mem <var>nums</var>&hellip;</code></dt>
<dd><p>Enable monitoring of memory regions <var>nums</var>&hellip;.
</p>
<a name="index-info-mem"></a>
</dd>
<dt><code>info mem</code></dt>
<dd><p>Print a table of all defined memory regions, with the following columns
for each region:
</p>
<dl compact="compact">
<dt><em>Memory Region Number</em></dt>
<dt><em>Enabled or Disabled.</em></dt>
<dd><p>Enabled memory regions are marked with &lsquo;<samp>y</samp>&rsquo;.
Disabled memory regions are marked with &lsquo;<samp>n</samp>&rsquo;.
</p>
</dd>
<dt><em>Lo Address</em></dt>
<dd><p>The address defining the inclusive lower bound of the memory region.
</p>
</dd>
<dt><em>Hi Address</em></dt>
<dd><p>The address defining the exclusive upper bound of the memory region.
</p>
</dd>
<dt><em>Attributes</em></dt>
<dd><p>The list of attributes set for this memory region.
</p></dd>
</dl>
</dd>
</dl>


<a name="Attributes"></a>
<h4 class="subsection">10.17.1 Attributes</h4>

<a name="Memory-Access-Mode"></a>
<h4 class="subsubsection">10.17.1.1 Memory Access Mode</h4>
<p>The access mode attributes set whether <small>GDB</small> may make read or
write accesses to a memory region.
</p>
<p>While these attributes prevent <small>GDB</small> from performing invalid
memory accesses, they do nothing to prevent the target system, I/O DMA,
etc. from accessing memory.
</p>
<dl compact="compact">
<dt><code>ro</code></dt>
<dd><p>Memory is read only.
</p></dd>
<dt><code>wo</code></dt>
<dd><p>Memory is write only.
</p></dd>
<dt><code>rw</code></dt>
<dd><p>Memory is read/write.  This is the default.
</p></dd>
</dl>

<a name="Memory-Access-Size"></a>
<h4 class="subsubsection">10.17.1.2 Memory Access Size</h4>
<p>The access size attribute tells <small>GDB</small> to use specific sized
accesses in the memory region.  Often memory mapped device registers
require specific sized accesses.  If no access size attribute is
specified, <small>GDB</small> may use accesses of any size.
</p>
<dl compact="compact">
<dt><code>8</code></dt>
<dd><p>Use 8 bit memory accesses.
</p></dd>
<dt><code>16</code></dt>
<dd><p>Use 16 bit memory accesses.
</p></dd>
<dt><code>32</code></dt>
<dd><p>Use 32 bit memory accesses.
</p></dd>
<dt><code>64</code></dt>
<dd><p>Use 64 bit memory accesses.
</p></dd>
</dl>


<a name="Data-Cache"></a>
<h4 class="subsubsection">10.17.1.3 Data Cache</h4>
<p>The data cache attributes set whether <small>GDB</small> will cache target
memory.  While this generally improves performance by reducing debug
protocol overhead, it can lead to incorrect results because <small>GDB</small>
does not know about volatile variables or memory mapped device
registers.
</p>
<dl compact="compact">
<dt><code>cache</code></dt>
<dd><p>Enable <small>GDB</small> to cache target memory.
</p></dd>
<dt><code>nocache</code></dt>
<dd><p>Disable <small>GDB</small> from caching target memory.  This is the default.
</p></dd>
</dl>

<a name="Memory-Access-Checking"></a>
<h4 class="subsection">10.17.2 Memory Access Checking</h4>
<p><small>GDB</small> can be instructed to refuse accesses to memory that is
not explicitly described.  This can be useful if accessing such
regions has undesired effects for a specific target, or to provide
better error checking.  The following commands control this behaviour.
</p>
<dl compact="compact">
<dd><a name="index-set-mem-inaccessible_002dby_002ddefault"></a>
</dd>
<dt><code>set mem inaccessible-by-default [on|off]</code></dt>
<dd><p>If <code>on</code> is specified, make  <small>GDB</small> treat memory not
explicitly described by the memory ranges as non-existent and refuse accesses
to such memory.  The checks are only performed if there&rsquo;s at least one
memory range defined.  If <code>off</code> is specified, make <small>GDB</small>
treat the memory not explicitly described by the memory ranges as RAM.
The default value is <code>on</code>.
<a name="index-show-mem-inaccessible_002dby_002ddefault"></a>
</p></dd>
<dt><code>show mem inaccessible-by-default</code></dt>
<dd><p>Show the current handling of accesses to unknown memory.
</p></dd>
</dl>


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