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<h3 class="section">3.7 MEMORY Command</h3>
<p><a name="index-MEMORY-431"></a><a name="index-memory-regions-432"></a><a name="index-regions-of-memory-433"></a><a name="index-allocating-memory-434"></a><a name="index-discontinuous-memory-435"></a>The linker's default configuration permits allocation of all available
memory. You can override this by using the <code>MEMORY</code> command.
<p>The <code>MEMORY</code> command describes the location and size of blocks of
memory in the target. You can use it to describe which memory regions
may be used by the linker, and which memory regions it must avoid. You
can then assign sections to particular memory regions. The linker will
set section addresses based on the memory regions, and will warn about
regions that become too full. The linker will not shuffle sections
around to fit into the available regions.
<p>A linker script may contain at most one use of the <code>MEMORY</code>
command. However, you can define as many blocks of memory within it as
you wish. The syntax is:
<pre class="smallexample"> MEMORY
{
<var>name</var> [(<var>attr</var>)] : ORIGIN = <var>origin</var>, LENGTH = <var>len</var>
...
}
</pre>
<p>The <var>name</var> is a name used in the linker script to refer to the
region. The region name has no meaning outside of the linker script.
Region names are stored in a separate name space, and will not conflict
with symbol names, file names, or section names. Each memory region
must have a distinct name.
<p><a name="index-memory-region-attributes-436"></a>The <var>attr</var> string is an optional list of attributes that specify
whether to use a particular memory region for an input section which is
not explicitly mapped in the linker script. As described in
<a href="SECTIONS.html#SECTIONS">SECTIONS</a>, if you do not specify an output section for some input
section, the linker will create an output section with the same name as
the input section. If you define region attributes, the linker will use
them to select the memory region for the output section that it creates.
<p>The <var>attr</var> string must consist only of the following characters:
<dl>
<dt><span class="samp">R</span><dd>Read-only section
<br><dt><span class="samp">W</span><dd>Read/write section
<br><dt><span class="samp">X</span><dd>Executable section
<br><dt><span class="samp">A</span><dd>Allocatable section
<br><dt><span class="samp">I</span><dd>Initialized section
<br><dt><span class="samp">L</span><dd>Same as <span class="samp">I</span>
<br><dt><span class="samp">!</span><dd>Invert the sense of any of the preceding attributes
</dl>
<p>If a unmapped section matches any of the listed attributes other than
<span class="samp">!</span>, it will be placed in the memory region. The <span class="samp">!</span>
attribute reverses this test, so that an unmapped section will be placed
in the memory region only if it does not match any of the listed
attributes.
<p><a name="index-ORIGIN-_003d-437"></a><a name="index-o-_003d-438"></a><a name="index-org-_003d-439"></a>The <var>origin</var> is an numerical expression for the start address of
the memory region. The expression must evaluate to a constant and it
cannot involve any symbols. The keyword <code>ORIGIN</code> may be
abbreviated to <code>org</code> or <code>o</code> (but not, for example,
<code>ORG</code>).
<p><a name="index-LENGTH-_003d-440"></a><a name="index-len-_003d-441"></a><a name="index-l-_003d-442"></a>The <var>len</var> is an expression for the size in bytes of the memory
region. As with the <var>origin</var> expression, the expression must
be numerical only and must evaluate to a constant. The keyword
<code>LENGTH</code> may be abbreviated to <code>len</code> or <code>l</code>.
<p>In the following example, we specify that there are two memory regions
available for allocation: one starting at <span class="samp">0</span> for 256 kilobytes,
and the other starting at <span class="samp">0x40000000</span> for four megabytes. The
linker will place into the <span class="samp">rom</span> memory region every section which
is not explicitly mapped into a memory region, and is either read-only
or executable. The linker will place other sections which are not
explicitly mapped into a memory region into the <span class="samp">ram</span> memory
region.
<pre class="smallexample"> MEMORY
{
rom (rx) : ORIGIN = 0, LENGTH = 256K
ram (!rx) : org = 0x40000000, l = 4M
}
</pre>
<p>Once you define a memory region, you can direct the linker to place
specific output sections into that memory region by using the
<span class="samp">></span><var>region</var> output section attribute. For example, if you have
a memory region named <span class="samp">mem</span>, you would use <span class="samp">>mem</span> in the
output section definition. See <a href="Output-Section-Region.html#Output-Section-Region">Output Section Region</a>. If no address
was specified for the output section, the linker will set the address to
the next available address within the memory region. If the combined
output sections directed to a memory region are too large for the
region, the linker will issue an error message.
<p>It is possible to access the origin and length of a memory in an
expression via the <code>ORIGIN(</code><var>memory</var><code>)</code> and
<code>LENGTH(</code><var>memory</var><code>)</code> functions:
<pre class="smallexample"> _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
</pre>
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