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| <p> |
| Previous: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="p" rel="prev">Output Section Attributes</a>, Up: <a href="SECTIONS.html#SECTIONS" accesskey="u" rel="up">SECTIONS</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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| <a name="Overlay-Description-1"></a> |
| <h4 class="subsection">3.6.9 Overlay Description</h4> |
| <a name="index-OVERLAY"></a> |
| <a name="index-overlays"></a> |
| <p>An overlay description provides an easy way to describe sections which |
| are to be loaded as part of a single memory image but are to be run at |
| the same memory address. At run time, some sort of overlay manager will |
| copy the overlaid sections in and out of the runtime memory address as |
| required, perhaps by simply manipulating addressing bits. This approach |
| can be useful, for example, when a certain region of memory is faster |
| than another. |
| </p> |
| <p>Overlays are described using the <code>OVERLAY</code> command. The |
| <code>OVERLAY</code> command is used within a <code>SECTIONS</code> command, like an |
| output section description. The full syntax of the <code>OVERLAY</code> |
| command is as follows: |
| </p><div class="smallexample"> |
| <pre class="smallexample">OVERLAY [<var>start</var>] : [NOCROSSREFS] [AT ( <var>ldaddr</var> )] |
| { |
| <var>secname1</var> |
| { |
| <var>output-section-command</var> |
| <var>output-section-command</var> |
| … |
| } [:<var>phdr</var>…] [=<var>fill</var>] |
| <var>secname2</var> |
| { |
| <var>output-section-command</var> |
| <var>output-section-command</var> |
| … |
| } [:<var>phdr</var>…] [=<var>fill</var>] |
| … |
| } [><var>region</var>] [:<var>phdr</var>…] [=<var>fill</var>] [,] |
| </pre></div> |
| |
| <p>Everything is optional except <code>OVERLAY</code> (a keyword), and each |
| section must have a name (<var>secname1</var> and <var>secname2</var> above). The |
| section definitions within the <code>OVERLAY</code> construct are identical to |
| those within the general <code>SECTIONS</code> construct (see <a href="SECTIONS.html#SECTIONS">SECTIONS</a>), |
| except that no addresses and no memory regions may be defined for |
| sections within an <code>OVERLAY</code>. |
| </p> |
| <p>The comma at the end may be required if a <var>fill</var> is used and |
| the next <var>sections-command</var> looks like a continuation of the expression. |
| </p> |
| <p>The sections are all defined with the same starting address. The load |
| addresses of the sections are arranged such that they are consecutive in |
| memory starting at the load address used for the <code>OVERLAY</code> as a |
| whole (as with normal section definitions, the load address is optional, |
| and defaults to the start address; the start address is also optional, |
| and defaults to the current value of the location counter). |
| </p> |
| <p>If the <code>NOCROSSREFS</code> keyword is used, and there are any |
| references among the sections, the linker will report an error. Since |
| the sections all run at the same address, it normally does not make |
| sense for one section to refer directly to another. |
| See <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">NOCROSSREFS</a>. |
| </p> |
| <p>For each section within the <code>OVERLAY</code>, the linker automatically |
| provides two symbols. The symbol <code>__load_start_<var>secname</var></code> is |
| defined as the starting load address of the section. The symbol |
| <code>__load_stop_<var>secname</var></code> is defined as the final load address of |
| the section. Any characters within <var>secname</var> which are not legal |
| within C identifiers are removed. C (or assembler) code may use these |
| symbols to move the overlaid sections around as necessary. |
| </p> |
| <p>At the end of the overlay, the value of the location counter is set to |
| the start address of the overlay plus the size of the largest section. |
| </p> |
| <p>Here is an example. Remember that this would appear inside a |
| <code>SECTIONS</code> construct. |
| </p><div class="smallexample"> |
| <pre class="smallexample"> OVERLAY 0x1000 : AT (0x4000) |
| { |
| .text0 { o1/*.o(.text) } |
| .text1 { o2/*.o(.text) } |
| } |
| </pre></div> |
| <p>This will define both ‘<samp>.text0</samp>’ and ‘<samp>.text1</samp>’ to start at |
| address 0x1000. ‘<samp>.text0</samp>’ will be loaded at address 0x4000, and |
| ‘<samp>.text1</samp>’ will be loaded immediately after ‘<samp>.text0</samp>’. The |
| following symbols will be defined if referenced: <code>__load_start_text0</code>, |
| <code>__load_stop_text0</code>, <code>__load_start_text1</code>, |
| <code>__load_stop_text1</code>. |
| </p> |
| <p>C code to copy overlay <code>.text1</code> into the overlay area might look |
| like the following. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> extern char __load_start_text1, __load_stop_text1; |
| memcpy ((char *) 0x1000, &__load_start_text1, |
| &__load_stop_text1 - &__load_start_text1); |
| </pre></div> |
| |
| <p>Note that the <code>OVERLAY</code> command is just syntactic sugar, since |
| everything it does can be done using the more basic commands. The above |
| example could have been written identically as follows. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> .text0 0x1000 : AT (0x4000) { o1/*.o(.text) } |
| PROVIDE (__load_start_text0 = LOADADDR (.text0)); |
| PROVIDE (__load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0)); |
| .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) } |
| PROVIDE (__load_start_text1 = LOADADDR (.text1)); |
| PROVIDE (__load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1)); |
| . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1)); |
| </pre></div> |
| |
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| <div class="header"> |
| <p> |
| Previous: <a href="Output-Section-Attributes.html#Output-Section-Attributes" accesskey="p" rel="prev">Output Section Attributes</a>, Up: <a href="SECTIONS.html#SECTIONS" accesskey="u" rel="up">SECTIONS</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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