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<title>LD: REGION_ALIAS</title>
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<a name="REGION_005fALIAS"></a>
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<div class="header">
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<p>
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Next: <a href="Miscellaneous-Commands.html#Miscellaneous-Commands" accesskey="n" rel="next">Miscellaneous Commands</a>, Previous: <a href="Format-Commands.html#Format-Commands" accesskey="p" rel="prev">Format Commands</a>, Up: <a href="Simple-Commands.html#Simple-Commands" accesskey="u" rel="up">Simple Commands</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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</div>
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<hr>
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<a name="Assign-alias-names-to-memory-regions"></a>
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<h4 class="subsection">3.4.4 Assign alias names to memory regions</h4>
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<a name="index-REGION_005fALIAS_0028alias_002c-region_0029"></a>
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<a name="index-region-alias"></a>
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<a name="index-region-names"></a>
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<p>Alias names can be added to existing memory regions created with the
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<a href="MEMORY.html#MEMORY">MEMORY</a> command. Each name corresponds to at most one memory region.
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</p>
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<div class="smallexample">
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<pre class="smallexample">REGION_ALIAS(<var>alias</var>, <var>region</var>)
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</pre></div>
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<p>The <code>REGION_ALIAS</code> function creates an alias name <var>alias</var> for the
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memory region <var>region</var>. This allows a flexible mapping of output sections
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to memory regions. An example follows.
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</p>
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<p>Suppose we have an application for embedded systems which come with various
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memory storage devices. All have a general purpose, volatile memory <code>RAM</code>
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that allows code execution or data storage. Some may have a read-only,
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non-volatile memory <code>ROM</code> that allows code execution and read-only data
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access. The last variant is a read-only, non-volatile memory <code>ROM2</code> with
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read-only data access and no code execution capability. We have four output
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sections:
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</p>
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<ul>
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<li> <code>.text</code> program code;
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</li><li> <code>.rodata</code> read-only data;
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</li><li> <code>.data</code> read-write initialized data;
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</li><li> <code>.bss</code> read-write zero initialized data.
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</li></ul>
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<p>The goal is to provide a linker command file that contains a system independent
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part defining the output sections and a system dependent part mapping the
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output sections to the memory regions available on the system. Our embedded
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systems come with three different memory setups <code>A</code>, <code>B</code> and
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<code>C</code>:
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</p><table>
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<tr><td width="25%">Section</td><td width="25%">Variant A</td><td width="25%">Variant B</td><td width="25%">Variant C</td></tr>
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<tr><td width="25%">.text</td><td width="25%">RAM</td><td width="25%">ROM</td><td width="25%">ROM</td></tr>
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<tr><td width="25%">.rodata</td><td width="25%">RAM</td><td width="25%">ROM</td><td width="25%">ROM2</td></tr>
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<tr><td width="25%">.data</td><td width="25%">RAM</td><td width="25%">RAM/ROM</td><td width="25%">RAM/ROM2</td></tr>
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<tr><td width="25%">.bss</td><td width="25%">RAM</td><td width="25%">RAM</td><td width="25%">RAM</td></tr>
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</table>
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<p>The notation <code>RAM/ROM</code> or <code>RAM/ROM2</code> means that this section is
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loaded into region <code>ROM</code> or <code>ROM2</code> respectively. Please note that
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the load address of the <code>.data</code> section starts in all three variants at
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the end of the <code>.rodata</code> section.
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</p>
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<p>The base linker script that deals with the output sections follows. It
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includes the system dependent <code>linkcmds.memory</code> file that describes the
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memory layout:
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</p><div class="smallexample">
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<pre class="smallexample">INCLUDE linkcmds.memory
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SECTIONS
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{
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.text :
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{
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*(.text)
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} > REGION_TEXT
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.rodata :
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{
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*(.rodata)
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rodata_end = .;
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} > REGION_RODATA
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.data : AT (rodata_end)
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{
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data_start = .;
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*(.data)
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} > REGION_DATA
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data_size = SIZEOF(.data);
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data_load_start = LOADADDR(.data);
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.bss :
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{
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*(.bss)
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} > REGION_BSS
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}
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</pre></div>
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<p>Now we need three different <code>linkcmds.memory</code> files to define memory
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regions and alias names. The content of <code>linkcmds.memory</code> for the three
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variants <code>A</code>, <code>B</code> and <code>C</code>:
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</p><dl compact="compact">
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<dt><code>A</code></dt>
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<dd><p>Here everything goes into the <code>RAM</code>.
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</p><div class="smallexample">
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<pre class="smallexample">MEMORY
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{
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RAM : ORIGIN = 0, LENGTH = 4M
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}
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REGION_ALIAS("REGION_TEXT", RAM);
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REGION_ALIAS("REGION_RODATA", RAM);
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REGION_ALIAS("REGION_DATA", RAM);
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REGION_ALIAS("REGION_BSS", RAM);
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</pre></div>
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</dd>
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<dt><code>B</code></dt>
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<dd><p>Program code and read-only data go into the <code>ROM</code>. Read-write data goes
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into the <code>RAM</code>. An image of the initialized data is loaded into the
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<code>ROM</code> and will be copied during system start into the <code>RAM</code>.
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</p><div class="smallexample">
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<pre class="smallexample">MEMORY
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{
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ROM : ORIGIN = 0, LENGTH = 3M
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RAM : ORIGIN = 0x10000000, LENGTH = 1M
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}
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REGION_ALIAS("REGION_TEXT", ROM);
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REGION_ALIAS("REGION_RODATA", ROM);
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REGION_ALIAS("REGION_DATA", RAM);
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REGION_ALIAS("REGION_BSS", RAM);
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</pre></div>
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</dd>
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<dt><code>C</code></dt>
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<dd><p>Program code goes into the <code>ROM</code>. Read-only data goes into the
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<code>ROM2</code>. Read-write data goes into the <code>RAM</code>. An image of the
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initialized data is loaded into the <code>ROM2</code> and will be copied during
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system start into the <code>RAM</code>.
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</p><div class="smallexample">
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<pre class="smallexample">MEMORY
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{
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ROM : ORIGIN = 0, LENGTH = 2M
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ROM2 : ORIGIN = 0x10000000, LENGTH = 1M
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RAM : ORIGIN = 0x20000000, LENGTH = 1M
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}
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REGION_ALIAS("REGION_TEXT", ROM);
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REGION_ALIAS("REGION_RODATA", ROM2);
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REGION_ALIAS("REGION_DATA", RAM);
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REGION_ALIAS("REGION_BSS", RAM);
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</pre></div>
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</dd>
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</dl>
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<p>It is possible to write a common system initialization routine to copy the
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<code>.data</code> section from <code>ROM</code> or <code>ROM2</code> into the <code>RAM</code> if
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necessary:
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</p><div class="smallexample">
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<pre class="smallexample">#include <string.h>
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extern char data_start [];
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extern char data_size [];
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extern char data_load_start [];
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void copy_data(void)
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{
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if (data_start != data_load_start)
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{
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memcpy(data_start, data_load_start, (size_t) data_size);
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}
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}
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</pre></div>
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Next: <a href="Miscellaneous-Commands.html#Miscellaneous-Commands" accesskey="n" rel="next">Miscellaneous Commands</a>, Previous: <a href="Format-Commands.html#Format-Commands" accesskey="p" rel="prev">Format Commands</a>, Up: <a href="Simple-Commands.html#Simple-Commands" accesskey="u" rel="up">Simple Commands</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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