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| <h4 class="subsection">3.4.4 Assign alias names to memory regions</h4> |
| |
| <p><a name="index-REGION_005fALIAS_0028_0040var_007balias_007d_002c-_0040var_007bregion_007d_0029-368"></a><a name="index-region-alias-369"></a><a name="index-region-names-370"></a> |
| 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. |
| |
| <pre class="smallexample"> REGION_ALIAS(<var>alias</var>, <var>region</var>) |
| </pre> |
| <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>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: |
| |
| <ul> |
| <li><code>.text</code> program code; |
| <li><code>.rodata</code> read-only data; |
| <li><code>.data</code> read-write initialized data; |
| <li><code>.bss</code> read-write zero initialized data. |
| </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 summary=""><tr align="left"><td valign="top" width="25%">Section </td><td valign="top" width="25%">Variant A </td><td valign="top" width="25%">Variant B </td><td valign="top" width="25%">Variant C |
| <br></td></tr><tr align="left"><td valign="top" width="25%">.text </td><td valign="top" width="25%">RAM </td><td valign="top" width="25%">ROM </td><td valign="top" width="25%">ROM |
| <br></td></tr><tr align="left"><td valign="top" width="25%">.rodata </td><td valign="top" width="25%">RAM </td><td valign="top" width="25%">ROM </td><td valign="top" width="25%">ROM2 |
| <br></td></tr><tr align="left"><td valign="top" width="25%">.data </td><td valign="top" width="25%">RAM </td><td valign="top" width="25%">RAM/ROM </td><td valign="top" width="25%">RAM/ROM2 |
| <br></td></tr><tr align="left"><td valign="top" width="25%">.bss </td><td valign="top" width="25%">RAM </td><td valign="top" width="25%">RAM </td><td valign="top" width="25%">RAM |
| <br></td></tr></table> |
| 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>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: |
| <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> |
| <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>: |
| <dl> |
| <dt><code>A</code><dd>Here everything goes into the <code>RAM</code>. |
| <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> |
| <br><dt><code>B</code><dd>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>. |
| <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> |
| <br><dt><code>C</code><dd>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>. |
| <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> |
| </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: |
| <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> |
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