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| <h3 class="section">3.7 MEMORY Command</h3> |
| |
| <p><a name="index-MEMORY-464"></a><a name="index-memory-regions-465"></a><a name="index-regions-of-memory-466"></a><a name="index-allocating-memory-467"></a><a name="index-discontinuous-memory-468"></a>The linker's default configuration permits allocation of all available |
| memory. You can override this by using the <code>MEMORY</code> command. |
| |
| <p>The <code>MEMORY</code> command describes the location and size of blocks of |
| memory in the target. You can use it to describe which memory regions |
| may be used by the linker, and which memory regions it must avoid. You |
| can then assign sections to particular memory regions. The linker will |
| set section addresses based on the memory regions, and will warn about |
| regions that become too full. The linker will not shuffle sections |
| around to fit into the available regions. |
| |
| <p>A linker script may contain at most one use of the <code>MEMORY</code> |
| command. However, you can define as many blocks of memory within it as |
| you wish. The syntax is: |
| <pre class="smallexample"> MEMORY |
| { |
| <var>name</var> [(<var>attr</var>)] : ORIGIN = <var>origin</var>, LENGTH = <var>len</var> |
| ... |
| } |
| </pre> |
| <p>The <var>name</var> is a name used in the linker script to refer to the |
| region. The region name has no meaning outside of the linker script. |
| Region names are stored in a separate name space, and will not conflict |
| with symbol names, file names, or section names. Each memory region |
| must have a distinct name within the <code>MEMORY</code> command. However you can |
| add later alias names to existing memory regions with the <a href="REGION_005fALIAS.html#REGION_005fALIAS">REGION_ALIAS</a> |
| command. |
| |
| <p><a name="index-memory-region-attributes-469"></a>The <var>attr</var> string is an optional list of attributes that specify |
| whether to use a particular memory region for an input section which is |
| not explicitly mapped in the linker script. As described in |
| <a href="SECTIONS.html#SECTIONS">SECTIONS</a>, if you do not specify an output section for some input |
| section, the linker will create an output section with the same name as |
| the input section. If you define region attributes, the linker will use |
| them to select the memory region for the output section that it creates. |
| |
| <p>The <var>attr</var> string must consist only of the following characters: |
| <dl> |
| <dt>‘<samp><span class="samp">R</span></samp>’<dd>Read-only section |
| <br><dt>‘<samp><span class="samp">W</span></samp>’<dd>Read/write section |
| <br><dt>‘<samp><span class="samp">X</span></samp>’<dd>Executable section |
| <br><dt>‘<samp><span class="samp">A</span></samp>’<dd>Allocatable section |
| <br><dt>‘<samp><span class="samp">I</span></samp>’<dd>Initialized section |
| <br><dt>‘<samp><span class="samp">L</span></samp>’<dd>Same as ‘<samp><span class="samp">I</span></samp>’ |
| <br><dt>‘<samp><span class="samp">!</span></samp>’<dd>Invert the sense of any of the preceding attributes |
| </dl> |
| |
| <p>If a unmapped section matches any of the listed attributes other than |
| ‘<samp><span class="samp">!</span></samp>’, it will be placed in the memory region. The ‘<samp><span class="samp">!</span></samp>’ |
| attribute reverses this test, so that an unmapped section will be placed |
| in the memory region only if it does not match any of the listed |
| attributes. |
| |
| <p><a name="index-ORIGIN-_003d-470"></a><a name="index-o-_003d-471"></a><a name="index-org-_003d-472"></a>The <var>origin</var> is an numerical expression for the start address of |
| the memory region. The expression must evaluate to a constant and it |
| cannot involve any symbols. The keyword <code>ORIGIN</code> may be |
| abbreviated to <code>org</code> or <code>o</code> (but not, for example, |
| <code>ORG</code>). |
| |
| <p><a name="index-LENGTH-_003d-473"></a><a name="index-len-_003d-474"></a><a name="index-l-_003d-475"></a>The <var>len</var> is an expression for the size in bytes of the memory |
| region. As with the <var>origin</var> expression, the expression must |
| be numerical only and must evaluate to a constant. The keyword |
| <code>LENGTH</code> may be abbreviated to <code>len</code> or <code>l</code>. |
| |
| <p>In the following example, we specify that there are two memory regions |
| available for allocation: one starting at ‘<samp><span class="samp">0</span></samp>’ for 256 kilobytes, |
| and the other starting at ‘<samp><span class="samp">0x40000000</span></samp>’ for four megabytes. The |
| linker will place into the ‘<samp><span class="samp">rom</span></samp>’ memory region every section which |
| is not explicitly mapped into a memory region, and is either read-only |
| or executable. The linker will place other sections which are not |
| explicitly mapped into a memory region into the ‘<samp><span class="samp">ram</span></samp>’ memory |
| region. |
| |
| <pre class="smallexample"> MEMORY |
| { |
| rom (rx) : ORIGIN = 0, LENGTH = 256K |
| ram (!rx) : org = 0x40000000, l = 4M |
| } |
| </pre> |
| <p>Once you define a memory region, you can direct the linker to place |
| specific output sections into that memory region by using the |
| ‘<samp><span class="samp">></span><var>region</var></samp>’ output section attribute. For example, if you have |
| a memory region named ‘<samp><span class="samp">mem</span></samp>’, you would use ‘<samp><span class="samp">>mem</span></samp>’ in the |
| output section definition. See <a href="Output-Section-Region.html#Output-Section-Region">Output Section Region</a>. If no address |
| was specified for the output section, the linker will set the address to |
| the next available address within the memory region. If the combined |
| output sections directed to a memory region are too large for the |
| region, the linker will issue an error message. |
| |
| <p>It is possible to access the origin and length of a memory in an |
| expression via the <code>ORIGIN(</code><var>memory</var><code>)</code> and |
| <code>LENGTH(</code><var>memory</var><code>)</code> functions: |
| |
| <pre class="smallexample"> _fstack = ORIGIN(ram) + LENGTH(ram) - 4; |
| </pre> |
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