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Node:<a name="Overlay%20Description">Overlay Description</a>,

Previous:<a rel="previous" accesskey="p" href="Output-Section-Attributes.html#Output%20Section%20Attributes">Output Section Attributes</a>,

Up:<a rel="up" accesskey="u" href="SECTIONS.html#SECTIONS">SECTIONS</a>

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<h4 class="subsection">Overlay description</h4>



   <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>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:

<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>]

         ...

       } [&gt;<var>region</var>] [:<var>phdr</var>...] [=<var>fill</var>]

     </pre>



   <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> contruct (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>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>If the <code>NOCROSSREFS</code> keyword is used, and there 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%20Commands">NOCROSSREFS</a>.



   <p>For each section within the <code>OVERLAY</code>, the linker automatically

defines two symbols.  The symbol <code>__load_start_</code><var>secname</var><code></code> is

defined as the starting load address of the section.  The symbol

<code>__load_stop_</code><var>secname</var><code></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>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>Here is an example.  Remember that this would appear inside a

<code>SECTIONS</code> construct.

<pre class="smallexample">       OVERLAY 0x1000 : AT (0x4000)

        {

          .text0 { o1/*.o(.text) }

          .text1 { o2/*.o(.text) }

        }

     </pre>



<p>This will define both <code>.text0</code> and <code>.text1</code> to start at

address 0x1000.  <code>.text0</code> will be loaded at address 0x4000, and

<code>.text1</code> will be loaded immediately after <code>.text0</code>.  The

following symbols will be defined: <code>__load_start_text0</code>,

<code>__load_stop_text0</code>, <code>__load_start_text1</code>,

<code>__load_stop_text1</code>.



   <p>C code to copy overlay <code>.text1</code> into the overlay area might look

like the following.



<pre class="smallexample">       extern char __load_start_text1, __load_stop_text1;

       memcpy ((char *) 0x1000, &amp;__load_start_text1,

               &amp;__load_stop_text1 - &amp;__load_start_text1);

     </pre>



   <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.



<pre class="smallexample">       .text0 0x1000 : AT (0x4000) { o1/*.o(.text) }

       __load_start_text0 = LOADADDR (.text0);

       __load_stop_text0 = LOADADDR (.text0) + SIZEOF (.text0);

       .text1 0x1000 : AT (0x4000 + SIZEOF (.text0)) { o2/*.o(.text) }

       __load_start_text1 = LOADADDR (.text1);

       __load_stop_text1 = LOADADDR (.text1) + SIZEOF (.text1);

       . = 0x1000 + MAX (SIZEOF (.text0), SIZEOF (.text1));

     </pre>



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