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151 lines
8.5 KiB
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<title>MEMORY - Untitled</title>
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<!--
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This file documents the GNU linker LD
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(GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10))
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version 2.33.1.
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Copyright (C) 1991-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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</head>
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<body>
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<div class="node">
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<a name="MEMORY"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="PHDRS.html#PHDRS">PHDRS</a>,
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Previous: <a rel="previous" accesskey="p" href="SECTIONS.html#SECTIONS">SECTIONS</a>,
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Up: <a rel="up" accesskey="u" href="Scripts.html#Scripts">Scripts</a>
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<hr>
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</div>
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<h3 class="section">3.7 MEMORY Command</h3>
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<p><a name="index-MEMORY-526"></a><a name="index-memory-regions-527"></a><a name="index-regions-of-memory-528"></a><a name="index-allocating-memory-529"></a><a name="index-discontinuous-memory-530"></a>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>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>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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<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>
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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><a name="index-memory-region-attributes-531"></a>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>The <var>attr</var> string must consist only of the following characters:
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<dl>
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<dt>‘<samp><span class="samp">R</span></samp>’<dd>Read-only section
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<br><dt>‘<samp><span class="samp">W</span></samp>’<dd>Read/write section
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<br><dt>‘<samp><span class="samp">X</span></samp>’<dd>Executable section
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<br><dt>‘<samp><span class="samp">A</span></samp>’<dd>Allocatable section
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<br><dt>‘<samp><span class="samp">I</span></samp>’<dd>Initialized section
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<br><dt>‘<samp><span class="samp">L</span></samp>’<dd>Same as ‘<samp><span class="samp">I</span></samp>’
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<br><dt>‘<samp><span class="samp">!</span></samp>’<dd>Invert the sense of any of the attributes that follow
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</dl>
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<p>If an unmapped section matches any of the listed attributes other than
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‘<samp><span class="samp">!</span></samp>’, it will be placed in the memory region. The ‘<samp><span class="samp">!</span></samp>’
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attribute reverses the test for the characters that follow, so that an
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unmapped section will be placed in the memory region only if it does
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not match any of the attributes listed afterwards. Thus an attribute
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string of ‘<samp><span class="samp">RW!X</span></samp>’ will match any unmapped section that has either
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or both of the ‘<samp><span class="samp">R</span></samp>’ and ‘<samp><span class="samp">W</span></samp>’ attributes, but only as long as
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the section does not also have the ‘<samp><span class="samp">X</span></samp>’ attribute.
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<p><a name="index-ORIGIN-_003d-532"></a><a name="index-o-_003d-533"></a><a name="index-org-_003d-534"></a>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><a name="index-LENGTH-_003d-535"></a><a name="index-len-_003d-536"></a><a name="index-l-_003d-537"></a>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>In the following example, we specify that there are two memory regions
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available for allocation: one starting at ‘<samp><span class="samp">0</span></samp>’ for 256 kilobytes,
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and the other starting at ‘<samp><span class="samp">0x40000000</span></samp>’ for four megabytes. The
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linker will place into the ‘<samp><span class="samp">rom</span></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><span class="samp">ram</span></samp>’ memory
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region.
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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>
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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><span class="samp">></span><var>region</var></samp>’ output section attribute. For example, if you have
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a memory region named ‘<samp><span class="samp">mem</span></samp>’, you would use ‘<samp><span class="samp">>mem</span></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>It is possible to access the origin and length of a memory in an
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expression via the <code>ORIGIN(</code><var>memory</var><code>)</code> and
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<code>LENGTH(</code><var>memory</var><code>)</code> functions:
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<pre class="smallexample"> _fstack = ORIGIN(ram) + LENGTH(ram) - 4;
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</pre>
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</body></html>
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