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| <title>LD: Output Section LMA</title> |
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| <a name="Output-Section-LMA"></a> |
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| <p> |
| Next: <a href="Forced-Output-Alignment.html#Forced-Output-Alignment" accesskey="n" rel="next">Forced Output Alignment</a>, Previous: <a href="Output-Section-Type.html#Output-Section-Type" accesskey="p" rel="prev">Output Section Type</a>, Up: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="u" rel="up">Output Section Attributes</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="Output-Section-LMA-1"></a> |
| <h4 class="subsubsection">3.6.8.2 Output Section LMA</h4> |
| <a name="index-AT_003elma_005fregion"></a> |
| <a name="index-AT_0028lma_0029"></a> |
| <a name="index-load-address"></a> |
| <a name="index-section-load-address"></a> |
| <p>Every section has a virtual address (VMA) and a load address (LMA); see |
| <a href="Basic-Script-Concepts.html#Basic-Script-Concepts">Basic Script Concepts</a>. The virtual address is specified by the |
| see <a href="Output-Section-Address.html#Output-Section-Address">Output Section Address</a> described earlier. The load address is |
| specified by the <code>AT</code> or <code>AT></code> keywords. Specifying a load |
| address is optional. |
| </p> |
| <p>The <code>AT</code> keyword takes an expression as an argument. This |
| specifies the exact load address of the section. The <code>AT></code> keyword |
| takes the name of a memory region as an argument. See <a href="MEMORY.html#MEMORY">MEMORY</a>. The |
| load address of the section is set to the next free address in the |
| region, aligned to the section’s alignment requirements. |
| </p> |
| <p>If neither <code>AT</code> nor <code>AT></code> is specified for an allocatable |
| section, the linker will use the following heuristic to determine the |
| load address: |
| </p> |
| <ul> |
| <li> If the section has a specific VMA address, then this is used as |
| the LMA address as well. |
| |
| </li><li> If the section is not allocatable then its LMA is set to its VMA. |
| |
| </li><li> Otherwise if a memory region can be found that is compatible |
| with the current section, and this region contains at least one |
| section, then the LMA is set so the difference between the |
| VMA and LMA is the same as the difference between the VMA and LMA of |
| the last section in the located region. |
| |
| </li><li> If no memory regions have been declared then a default region |
| that covers the entire address space is used in the previous step. |
| |
| </li><li> If no suitable region could be found, or there was no previous |
| section then the LMA is set equal to the VMA. |
| </li></ul> |
| |
| <a name="index-ROM-initialized-data"></a> |
| <a name="index-initialized-data-in-ROM"></a> |
| <p>This feature is designed to make it easy to build a ROM image. For |
| example, the following linker script creates three output sections: one |
| called ‘<samp>.text</samp>’, which starts at <code>0x1000</code>, one called |
| ‘<samp>.mdata</samp>’, which is loaded at the end of the ‘<samp>.text</samp>’ section |
| even though its VMA is <code>0x2000</code>, and one called ‘<samp>.bss</samp>’ to hold |
| uninitialized data at address <code>0x3000</code>. The symbol <code>_data</code> is |
| defined with the value <code>0x2000</code>, which shows that the location |
| counter holds the VMA value, not the LMA value. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">SECTIONS |
| { |
| .text 0x1000 : { *(.text) _etext = . ; } |
| .mdata 0x2000 : |
| AT ( ADDR (.text) + SIZEOF (.text) ) |
| { _data = . ; *(.data); _edata = . ; } |
| .bss 0x3000 : |
| { _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;} |
| } |
| </pre></div> |
| |
| <p>The run-time initialization code for use with a program generated with |
| this linker script would include something like the following, to copy |
| the initialized data from the ROM image to its runtime address. Notice |
| how this code takes advantage of the symbols defined by the linker |
| script. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">extern char _etext, _data, _edata, _bstart, _bend; |
| char *src = &_etext; |
| char *dst = &_data; |
| |
| /* ROM has data at end of text; copy it. */ |
| while (dst < &_edata) |
| *dst++ = *src++; |
| |
| /* Zero bss. */ |
| for (dst = &_bstart; dst< &_bend; dst++) |
| *dst = 0; |
| </pre></div> |
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| <p> |
| Next: <a href="Forced-Output-Alignment.html#Forced-Output-Alignment" accesskey="n" rel="next">Forced Output Alignment</a>, Previous: <a href="Output-Section-Type.html#Output-Section-Type" accesskey="p" rel="prev">Output Section Type</a>, Up: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="u" rel="up">Output Section Attributes</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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