108 lines
5.4 KiB
HTML
108 lines
5.4 KiB
HTML
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<title>Output Section LMA - Untitled</title>
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<!--
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This file documents the GNU linker LD
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(GNU Binutils)
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version 2.19.
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Copyright (C) 1991, 92, 93, 94, 95, 96, 97, 98, 99, 2000,
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2001, 2002, 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
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<p>
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<a name="Output-Section-LMA"></a>Next: <a rel="next" accesskey="n" href="Forced-Output-Alignment.html#Forced-Output-Alignment">Forced Output Alignment</a>,
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Previous: <a rel="previous" accesskey="p" href="Output-Section-Type.html#Output-Section-Type">Output Section Type</a>,
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Up: <a rel="up" accesskey="u" href="Output-Section-Attributes.html#Output-Section-Attributes">Output Section Attributes</a>
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<hr><br>
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</div>
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<h5 class="subsubsection">3.6.8.2 Output Section LMA</h5>
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<p><a name="index-AT_003e_0040var_007blma_005fregion_007d-408"></a><a name="index-AT_0028_0040var_007blma_007d_0029-409"></a><a name="index-load-address-410"></a><a name="index-section-load-address-411"></a>Every section has a virtual address (VMA) and a load address (LMA); see
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<a href="Basic-Script-Concepts.html#Basic-Script-Concepts">Basic Script Concepts</a>. The address expression which may appear in
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an output section description sets the VMA (see <a href="Output-Section-Address.html#Output-Section-Address">Output Section Address</a>).
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<p>The expression <var>lma</var> that follows the <code>AT</code> keyword specifies
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the load address of the section.
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<p>Alternatively, with <span class="samp">AT></span><var>lma_region</var> expression, you may
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specify a memory region for the section's load address. See <a href="MEMORY.html#MEMORY">MEMORY</a>.
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Note that if the section has not had a VMA assigned to it then the
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linker will use the <var>lma_region</var> as the VMA region as well.
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<p>If neither <code>AT</code> nor <code>AT></code> is specified for an allocatable
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section, the linker will set the LMA such that the difference between
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VMA and LMA for the section is the same as the preceding output
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section in the same region. If there is no preceding output section
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or the section is not allocatable, the linker will set the LMA equal
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to the VMA.
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See <a href="Output-Section-Region.html#Output-Section-Region">Output Section Region</a>.
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<p><a name="index-ROM-initialized-data-412"></a><a name="index-initialized-data-in-ROM-413"></a>This feature is designed to make it easy to build a ROM image. For
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example, the following linker script creates three output sections: one
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called <span class="samp">.text</span>, which starts at <code>0x1000</code>, one called
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<span class="samp">.mdata</span>, which is loaded at the end of the <span class="samp">.text</span> section
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even though its VMA is <code>0x2000</code>, and one called <span class="samp">.bss</span> to hold
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uninitialized data at address <code>0x3000</code>. The symbol <code>_data</code> is
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defined with the value <code>0x2000</code>, which shows that the location
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counter holds the VMA value, not the LMA value.
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<pre class="smallexample"> SECTIONS
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{
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.text 0x1000 : { *(.text) _etext = . ; }
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.mdata 0x2000 :
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AT ( ADDR (.text) + SIZEOF (.text) )
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{ _data = . ; *(.data); _edata = . ; }
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.bss 0x3000 :
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{ _bstart = . ; *(.bss) *(COMMON) ; _bend = . ;}
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}
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</pre>
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<p>The run-time initialization code for use with a program generated with
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this linker script would include something like the following, to copy
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the initialized data from the ROM image to its runtime address. Notice
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how this code takes advantage of the symbols defined by the linker
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script.
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<pre class="smallexample"> extern char _etext, _data, _edata, _bstart, _bend;
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char *src = &_etext;
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char *dst = &_data;
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/* ROM has data at end of text; copy it. */
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while (dst < &_edata) {
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*dst++ = *src++;
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}
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/* Zero bss */
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for (dst = &_bstart; dst< &_bend; dst++)
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*dst = 0;
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</pre>
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</body></html>
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