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| <h5 class="subsubsection">3.6.8.2 Output Section LMA</h5> |
| |
| <p><a name="index-AT_003e_0040var_007blma_005fregion_007d-438"></a><a name="index-AT_0028_0040var_007blma_007d_0029-439"></a><a name="index-load-address-440"></a><a name="index-section-load-address-441"></a>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 address expression which may appear in |
| an output section description sets the VMA (see <a href="Output-Section-Address.html#Output-Section-Address">Output Section Address</a>). |
| |
| <p>The expression <var>lma</var> that follows the <code>AT</code> keyword specifies |
| the load address of the section. |
| |
| <p>Alternatively, with ‘<samp><span class="samp">AT></span><var>lma_region</var></samp>’ expression, you may |
| specify a memory region for the section's load address. See <a href="MEMORY.html#MEMORY">MEMORY</a>. |
| Note that if the section has not had a VMA assigned to it then the |
| linker will use the <var>lma_region</var> as the VMA region as well. |
| |
| <p>If neither <code>AT</code> nor <code>AT></code> is specified for an allocatable |
| section, the linker will set the LMA such that the difference between |
| VMA and LMA for the section is the same as the preceding output |
| section in the same region. If there is no preceding output section |
| or the section is not allocatable, the linker will set the LMA equal |
| to the VMA. |
| See <a href="Output-Section-Region.html#Output-Section-Region">Output Section Region</a>. |
| |
| <p><a name="index-ROM-initialized-data-442"></a><a name="index-initialized-data-in-ROM-443"></a>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><span class="samp">.text</span></samp>’, which starts at <code>0x1000</code>, one called |
| ‘<samp><span class="samp">.mdata</span></samp>’, which is loaded at the end of the ‘<samp><span class="samp">.text</span></samp>’ section |
| even though its VMA is <code>0x2000</code>, and one called ‘<samp><span class="samp">.bss</span></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. |
| |
| <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> |
| <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. |
| |
| <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; |
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