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