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<title>LD: MEMORY</title>
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<a name="MEMORY"></a>
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<div class="header">
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<p>
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Next: <a href="PHDRS.html#PHDRS" accesskey="n" rel="next">PHDRS</a>, Previous: <a href="SECTIONS.html#SECTIONS" accesskey="p" rel="prev">SECTIONS</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="MEMORY-Command"></a>
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<h3 class="section">3.7 MEMORY Command</h3>
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<a name="index-MEMORY"></a>
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<a name="index-memory-regions"></a>
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<a name="index-regions-of-memory"></a>
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<a name="index-allocating-memory"></a>
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<a name="index-discontinuous-memory"></a>
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<p>The linker’s default configuration permits allocation of all available
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memory. You can override this by using the <code>MEMORY</code> command.
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</p>
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<p>The <code>MEMORY</code> command describes the location and size of blocks of
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memory in the target. You can use it to describe which memory regions
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may be used by the linker, and which memory regions it must avoid. You
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can then assign sections to particular memory regions. The linker will
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set section addresses based on the memory regions, and will warn about
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regions that become too full. The linker will not shuffle sections
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around to fit into the available regions.
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</p>
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<p>A linker script may contain many uses of the <code>MEMORY</code> command,
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however, all memory blocks defined are treated as if they were
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specified inside a single <code>MEMORY</code> command. The syntax for
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<code>MEMORY</code> is:
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</p><div class="smallexample">
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<pre class="smallexample">MEMORY
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{
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<var>name</var> [(<var>attr</var>)] : ORIGIN = <var>origin</var>, LENGTH = <var>len</var>
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…
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}
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</pre></div>
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<p>The <var>name</var> is a name used in the linker script to refer to the
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region. The region name has no meaning outside of the linker script.
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Region names are stored in a separate name space, and will not conflict
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with symbol names, file names, or section names. Each memory region
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must have a distinct name within the <code>MEMORY</code> command. However you can
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add later alias names to existing memory regions with the <a href="REGION_005fALIAS.html#REGION_005fALIAS">REGION_ALIAS</a>
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command.
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</p>
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<a name="index-memory-region-attributes"></a>
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<p>The <var>attr</var> string is an optional list of attributes that specify
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whether to use a particular memory region for an input section which is
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not explicitly mapped in the linker script. As described in
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<a href="SECTIONS.html#SECTIONS">SECTIONS</a>, if you do not specify an output section for some input
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section, the linker will create an output section with the same name as
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the input section. If you define region attributes, the linker will use
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them to select the memory region for the output section that it creates.
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</p>
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<p>The <var>attr</var> string must consist only of the following characters:
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</p><dl compact="compact">
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<dt>‘<samp>R</samp>’</dt>
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<dd><p>Read-only section
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</p></dd>
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<dt>‘<samp>W</samp>’</dt>
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<dd><p>Read/write section
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</p></dd>
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<dt>‘<samp>X</samp>’</dt>
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<dd><p>Executable section
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</p></dd>
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<dt>‘<samp>A</samp>’</dt>
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<dd><p>Allocatable section
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</p></dd>
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<dt>‘<samp>I</samp>’</dt>
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<dd><p>Initialized section
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</p></dd>
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<dt>‘<samp>L</samp>’</dt>
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<dd><p>Same as ‘<samp>I</samp>’
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</p></dd>
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<dt>‘<samp>!</samp>’</dt>
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<dd><p>Invert the sense of any of the attributes that follow
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</p></dd>
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</dl>
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<p>If a unmapped section matches any of the listed attributes other than
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‘<samp>!</samp>’, it will be placed in the memory region. The ‘<samp>!</samp>’
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attribute reverses this test, so that an unmapped section will be placed
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in the memory region only if it does not match any of the listed
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attributes.
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</p>
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<a name="index-ORIGIN-_003d"></a>
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<a name="index-o-_003d"></a>
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<a name="index-org-_003d"></a>
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<p>The <var>origin</var> is an numerical expression for the start address of
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the memory region. The expression must evaluate to a constant and it
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cannot involve any symbols. The keyword <code>ORIGIN</code> may be
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abbreviated to <code>org</code> or <code>o</code> (but not, for example,
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<code>ORG</code>).
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</p>
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<a name="index-LENGTH-_003d"></a>
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<a name="index-len-_003d"></a>
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<a name="index-l-_003d"></a>
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<p>The <var>len</var> is an expression for the size in bytes of the memory
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region. As with the <var>origin</var> expression, the expression must
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be numerical only and must evaluate to a constant. The keyword
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<code>LENGTH</code> may be abbreviated to <code>len</code> or <code>l</code>.
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</p>
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<p>In the following example, we specify that there are two memory regions
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available for allocation: one starting at ‘<samp>0</samp>’ for 256 kilobytes,
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and the other starting at ‘<samp>0x40000000</samp>’ for four megabytes. The
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linker will place into the ‘<samp>rom</samp>’ memory region every section which
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is not explicitly mapped into a memory region, and is either read-only
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or executable. The linker will place other sections which are not
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explicitly mapped into a memory region into the ‘<samp>ram</samp>’ memory
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region.
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</p>
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<div class="smallexample">
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<pre class="smallexample">MEMORY
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{
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rom (rx) : ORIGIN = 0, LENGTH = 256K
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ram (!rx) : org = 0x40000000, l = 4M
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}
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</pre></div>
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<p>Once you define a memory region, you can direct the linker to place
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specific output sections into that memory region by using the
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‘<samp>><var>region</var></samp>’ output section attribute. For example, if you have
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a memory region named ‘<samp>mem</samp>’, you would use ‘<samp>>mem</samp>’ in the
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output section definition. See <a href="Output-Section-Region.html#Output-Section-Region">Output Section Region</a>. If no address
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was specified for the output section, the linker will set the address to
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the next available address within the memory region. If the combined
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output sections directed to a memory region are too large for the
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region, the linker will issue an error message.
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</p>
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<p>It is possible to access the origin and length of a memory in an
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expression via the <code>ORIGIN(<var>memory</var>)</code> and
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<code>LENGTH(<var>memory</var>)</code> functions:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
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</pre></div>
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Next: <a href="PHDRS.html#PHDRS" accesskey="n" rel="next">PHDRS</a>, Previous: <a href="SECTIONS.html#SECTIONS" accesskey="p" rel="prev">SECTIONS</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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