745 lines
25 KiB
HTML
745 lines
25 KiB
HTML
<html>
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<head>
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<title>NXFLAT</title>
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</head>
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<body background="backgd.gif">
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<hr><hr>
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<table width ="100%">
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<tr align="center" bgcolor="#e4e4e4">
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<td>
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<h1><big><font color="#3c34ec"><i>NXFLAT</i></font></big></h1>
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<h2><font color="#dc143c">>>> Under Construction <<<</font></h2>
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<p>Last Updated: June 29, 2012</p>
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</td>
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</tr>
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</table>
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<hr><hr>
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<table width ="100%">
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<tr bgcolor="#e4e4e4">
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<td>
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<h1>Table of Contents</h1>
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</td>
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</tr>
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</table>
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<center><table width ="80%">
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<tr>
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<td>
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<table>
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<tr>
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<td valign="top" width="22"><img height="20" width="20" src="favicon.ico"></td>
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<td>
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<a href="#overview">1.0 Overview</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#functionality">1.1 Functionality</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#background">1.2 Background</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#limitations">1.3 Limitations</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#supported">1.4 Supported Processors</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#status">1.5 Development Status</a>
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</td>
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</tr>
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<tr>
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<td valign="top" width="22"><img height="20" width="20" src="favicon.ico"></td>
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<td>
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<a href="#toolchain">2.0 NXFLAT Toolchain</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#building">1.2 Building the NXFLAT Toolchain</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#mknxflat">1.2 mknxflat</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#ldnxflat">1.3 ldnxflat</a>
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</td>
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</tr>
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<tr>
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<td><br></td>
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<td>
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<a href="#making">1.4 Making an NXFLAT module</a>
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</td>
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</tr>
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<tr>
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<td valign="top" width="22"><img height="20" width="20" src="favicon.ico"></td>
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<td>
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<a href="#binfmt">3.0. Binary Loader APIs</a>
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</td>
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</tr>
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<tr>
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<td valign="top" width="22"><img height="20" width="20" src="favicon.ico"></td>
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<td>
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<a href="#nogot">Appendix A. No GOT Operation</a>
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</td>
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</tr>
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<tr>
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<td valign="top" width="22"><img height="20" width="20" src="favicon.ico"></td>
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<td>
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<a href="#pictext">Appendix B. PIC Text Workaround</a>
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</td>
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</tr>
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</table>
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</td>
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</tr>
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</table></center>
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<table width ="100%">
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<tr bgcolor="#e4e4e4">
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<td>
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<a name="overview"><h1>1.0 Overview</h1></a>
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</td>
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</tr>
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</table>f
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<a name="functionality"><h2>1.1 Functionality</h2></a>
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<p>
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NXFLAT is a customized and simplified version of binary format implemented a few years ago called
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<a HREF="http://xflat.sourceforge.net/">XFLAT</A>
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With the NXFLAT binary format you will be able to do the following:
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</p>
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<ul>
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<li>Place separately linked programs in a file system, and</li>
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<li>Execute those programs by dynamically linking them to the base NuttX code.</li>
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</ul>
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<p>
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This allows you to extend the NuttX base code after it has been written into FLASH.
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One motivation for implementing NXFLAT is support clean CGI under an HTTPD server.
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</p>
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<p>
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This feature is especially attractive when combined with the NuttX ROMFS support:
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ROMFS allows you to execute programs in place (XIP) in flash without copying anything
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other than the .data section to RAM.
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In fact, the initial NXFLAT release only worked on ROMFS.
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Later extensions also support execution NXFLAT binaries from an SRAM copy as well.
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</p>
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<p>
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This NuttX feature includes:
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</p>
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<ul>
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<li>
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A dynamic loader that is built into the NuttX core
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(See <a href="http://nuttx.svn.sourceforge.net/viewvc/nuttx/trunk/nuttx/binfmt/">SVN</a>).
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</li>
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<li>
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Minor changes to RTOS to support position independent code, and
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</li>
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<li>
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A linker to bind ELF binaries to produce the NXFLAT binary format
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(See <a href="http://nuttx.svn.sourceforge.net/viewvc/nuttx/trunk/misc/buildroot/toolchain/nxflat/">SVN).
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</li>
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</ul>
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<a name="background"><h2>1.2 Background</h2></a>
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<p>
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NXFLAT is derived from <a href="http://xflat.sourceforge.net/">XFLAT</a>.
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XFLAT is a toolchain add that provides full shared library and XIP executable
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support for processors that have no Memory Management Unit (MMU<sup>1</sup>).
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NXFLAT is greatly simplified for the deeply embedded environment targeted by Nuttx:
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</p>
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<ul>
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<li>NXFLAT does not support shared libraries, because</li>
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<li>NXFLAT does not support <i>exportation</i> of symbol values from a module</li>
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</ul>
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<p>
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Rather, the NXFLAT module only <i>imports</i> symbol values.
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In the NXFLAT model, the (PIC<sup>2</sup>) NXFLAT module resides in a FLASH file system and
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when it is loaded at run time, it is dynamically linked only to the (non-PIC) base NuttX
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code:
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The base NuttX <i>exports</i> a symbol table; the NXFLAT module <i>imports</i> those symbol value
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to dynamically bind the module to the base code.
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</p>
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<ul>
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<p>
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<sup>1</sup><small>MMU: "Memory Management Unit"</small><br>
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<sup>2</sup><small>PIC: "Position Independent Code"</small>
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</p>
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</ul>
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<a name="limitations"><h2>1.3 Limitations</h2></a>
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<ul>
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<li><b>ROMFS (or RAM mapping) Only</b>
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<ul>
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<p>
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The current NXFLAT release will work only with either (1) NXFLAT executable modules residing on a ROMFS file system, or (2) executables residing on other file systems provided that <code>CONFIG_FS_RAMMAP</code> is defined.
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This limitation is because the loader depends on the capability to <code>mmap()</code> the code segment.
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See the <a href="NuttxUserGuide.html#mmapxip">NuttX User Guide</a> for further information.
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</p>
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<p>
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NUTTX does not provide any general kind of file mapping capability.
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In fact, <i>true</i> file mapping is only possible with MCUs that provide an MMU<sup>1</sup>.
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Without an MMU, file system may support eXecution In Place (XIP) to mimic file mapping.
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Only the ROMFS file system supports that kind of XIP execution need by NXFLAT.
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</p>
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<p>
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It is also possible to <i>simulate</i> file mapping by allocating memory, copying the NXFLAT binary file into memory, and executing from the copy of the executable file in RAM.
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That capability can be enabled with the <code>CONFIG_FS_RAMMAP</code> configuration option.
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With that option enabled, NXFLAT will work that kind of file system but will require copying of all NXFLAT executables to RAM.
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</p>
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</ul>
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</li>
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<li><b>GCC/ARM/Cortex-M3/4 Only</b>
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<ul>
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<p>
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At present, the NXFLAT toolchain is only available for ARM and Cortex-M3/4 (thumb2) targets.
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</p>
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</ul>
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</li>
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<li><b>Read-Only Data in RAM</b>
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<ul>
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<p>
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Read-only data must reside in RAM.
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In code generated by GCC, all data references are indexed by the PIC<sup>2</sup> base register (that is usually R10 or <i>sl</i> for the ARM processors).
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The includes read-only data (<code>.rodata</code>).
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Embedded firmware developers normally like to keep <code>.rodata</code> in FLASH with the code sections.
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But because all data is referenced with the PIC base register, all of that data must lie in RAM.
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A NXFLAT change to work around this is under investigation<sup>3</sup>.
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</p>
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</ul>
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</li>
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<li><b>Globally Scoped Function Function Pointers</b>
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<ul>
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<p>
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If a function pointer is taken to a statically defined function, then (at least for ARM) GCC will generate a relocation that NXFLAT cannot handle.
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The workaround is make all such functions global in scope.
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A fix would involve a change to the GCC compiler as described in <a href="#pictext">Appendix B</a>.
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</p>
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</ul>
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</li>
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<li><b>Special Handling of Callbacks</b>
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<ul>
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<p>
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Callbacks through function pointers must be avoided or, when then cannot be avoided, handled very specially.
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The reason for this is that the PIC module requires setting of a special value in a PIC register.
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If the callback does not set the PIC register, then the called back function will fail because it will be unable to correctly access data memory.
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Special logic is in place to handle some NuttX callbacks: Signal callbacks and watchdog timer callbacks.
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But other callbacks (like those used with <code>qsort()</code> must be avoided in an NXFLAT module.
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</p>
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</ul>
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</li>
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</ul>
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<ul><p>
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<sup>1</sup><small>MMU: "Memory Management Unit"</small><br>
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<sup>2</sup><small>PIC: "Position Independent Code"</small><br>
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<sup>3</sup><small>A work around is under consideration:
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At run time, the <code>.rodata</code> offsets will be indexed by a RAM address.
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If the dynamic loader were to offset those <code>.rodata</code> offsets properly, it
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still might be possible to reference <code>.rodata</code> in ROM.
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That work around is still a topic of investigation at this time.</small>
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</p></ul>
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<a name="supported"><h2>1.4 Supported Processors</h2></a>
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<p>
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As mentioned <a href="#limitations">above</a>, the NXFLAT toolchain is only available for ARM and
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Cortex-M3 (thumb2) targets.
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Furthermore, NXFLAT has only been tested on the Eagle-100 LMS6918 Cortex-M3 board.
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</p>
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<a name="status"><h2>1.5 Development Status</h2></a>
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<p>
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The initial release of NXFLAT was made in NuttX version 0.4.9.
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Testing is limited to the tests found under <code>apps/examples/nxflat</code> in the source tree.
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Some known problems exist
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(see the <a href="http://nuttx.svn.sourceforge.net/viewvc/nuttx/trunk/nuttx/TODO?view=log">TODO</a> list).
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As such, NXFLAT is currently in an early alpha phase.
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</p>
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<table width ="100%">
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<tr bgcolor="#e4e4e4">
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<td>
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<a name="toolchain"><h1>2.0 NXFLAT Toolchain</h1></a>
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</td>
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</tr>
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</table>
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<a name="building"><h2>1.2 Building the NXFLAT Toolchain</h2></a>
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<p>
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In order to use NXFLAT, you must use special NXFLAT tools to create the binary module in FLASH.
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To do this, you will need to download the buildroot package and build it on your Linux or Cygwin machine.
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The buildroot can be downloaded from
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<a https://sourceforge.net/project/showfiles.php?group_id=189573&package_id=224585">Sourceforge</a>.
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You will need version 0.1.7 or later.
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</p>
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<p>
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Here are some general build instructions:
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</p>
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<ul>
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<li>
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You must have already configured Nuttx in <code><some-dir>/nuttx</code>
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</li>
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<li>
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Download the buildroot package <code>buildroot-0.x.y</code> into <code><some-dir></code>
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<li>
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</li>
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Unpack <code><some-dir>/buildroot-0.x.y.tar.gz</code> using a command like <code>tar zxf buildroot-0.x.y</code>.
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This will result in a new directory like <code><some-dir>/misc/buildroot-0.x.y</code>
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</li>
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<li>
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Move this into position: <code>mv <some-dir>/misc/buildroot-0.x.y</code><some-dir>/buildroot</code>
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</li>
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<li>
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<code>cd </code><some-dir>/buildroot</code>
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</li>
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<li>
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Copy a configuration file into the top buildroot directory: <code>cp configs/abc-defconfig-x.y.z .config</code>.
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</li>
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<li>
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Enable building of the NXFLAT tools by <code>make menuconfig</code>.
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Select to build the NXFLAT toolchain with GCC (you can also select omit building GCC with and only build the
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NXFLAT toolchain for use with your own GCC toolchain).
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</li>
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<li>
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Make the toolchain: <code>make</code>.
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When the make completes, the tool binaries will be available under
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<code><some-dir>/buildroot/build_abc/staging_dir/bin</code>
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</li>
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</ul>
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<a name="mknxflat"><h2>1.2 mknxflat</h2></a>
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<p>
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<code>mknxflat</code> is used to build a <i>thunk</i> file.
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See <a href="#making">below</a> for usage.
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<ul><pre>
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Usage: mknxflat [options] <bfd-filename>
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Where options are one or more of the following. Note
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that a space is always required between the option and
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any following arguments.
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-d Use dynamic symbol table. [symtab]
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-f <cmd-filename>
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Take next commands from <cmd-filename> [cmd-line]
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-o <out-filename>
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Output to <out-filename> [stdout]
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-v Verbose output [no output]
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-w Import weakly declared functions, i.e., weakly
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declared functions are expected to be provided at
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load-time [not imported]
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</pre></ul>
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<a name="ldnxflat"><h2>1.3 ldnxflat</h2></a>
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<p>
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<code>ldnxflat</code> is use to link your object files along with the <i>thunk</i> file
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generated by <a href="#mknxflat"><code>mknxflat</code></a> to produce the NXFLAT binary module.
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See <a href="#making">below</a> for usage.
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</p>
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<ul><pre>
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Usage: ldnxflat [options] <bfd-filename>
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Where options are one or more of the following. Note
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that a space is always required between the option and
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any following arguments.
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-d Use dynamic symbol table [Default: symtab]
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-e <entry-point>
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Entry point to module [Default: _start]
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-o <out-filename>
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Output to <out-filename> [Default: <bfd-filename>.nxf]
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-s <stack-size>
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Set stack size to <stack-size> [Default: 4096]
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-v Verbose output. If -v is applied twice, additional
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debug output is enabled [Default: no verbose output].
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</pre></ul>
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<a name="making"><h2>1.4 Making an NXFLAT module</h2></a>
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<p>
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Below is a snippet from an NXFLAT make file (simplified from NuttX
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<a href="http://nuttx.svn.sourceforge.net/viewvc/nuttx/trunk/apps/examples/nxflat/tests/hello/Makefile?view=log">
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Hello, World!</a> example.
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<p>
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<ul><table width="50%">
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<ul><table>
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<tr>
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<th>Target 1</th>
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<td><code>hello.r1:</code></td>
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<td><code>hello.o</code></td>
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</tr>
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<tr>
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<td><br></td>
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<td><br></td>
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<td><code>abc-elf-ld -r -d -warn-common -o $@ $^</code></td>
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</tr>
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<tr>
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<th>Target 2</th>
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<td><code>hello-thunk.S:</code></td>
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<td><code>hello.r1</code></td>
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</tr>
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<tr>
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<td><br></td>
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<td><br></td>
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<td><code>mknxflat -o $@ $^</code></td>
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</tr>
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<tr>
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<th>Target 3</th>
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<td><code>hello.r2:</code></td>
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<td><code>hello-thunk.S</code></td>
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</tr>
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<tr>
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<td><br></td>
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<td><br></td>
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<td>
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<code>abc-elf-ld -r -d -warn-common -T binfmt/libnxflat/gnu-nxflat.ld -no-check-sections -o $@ hello.o hello-thunk.o</code>
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</td>
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</tr>
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<tr>
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<th>Target 4</th>
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<td><code>hello:</code></td>
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<td><code>hello.r2</code></td>
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</tr>
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<tr>
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<td><br></td>
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<td><br></td>
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<td><code>ldnxflat -e main -s 2048 -o $@ $^</code></td>
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</tr>
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<tr>
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</table></ul>
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<p><b>Target 1</b>.
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This target links all of the module's object files together into one relocatable object.
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Two relocatable objects will be generated; this is the first one (hence, the suffic <code>.r1</code>).
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In this "Hello, World!" case, there is only a single object file, <code>hello.o</code>,
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that is linked to produce the <code>hello.r1</code> object.
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</p>
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<p>
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When the module's object files are compiled, some special compiler CFLAGS must be provided.
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First, the option <code>-fpic</code> is required to tell the compiler to generate position independent code (other
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GCC options, like <code>-fno-jump-tables</code> might also be desirable).
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For ARM compilers, two additional compilation options are required: <code>-msingle-pic-base</code>
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and <code>-mpic-register=r10</code>.
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</p>
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|
|
|
<p><b>Target 2</b>.
|
|
Given the <code>hello.r1</code> relocatable object, this target will invoke
|
|
<a href="#mknxflat"><code>mknxflat</code></a>
|
|
to make the <i>thunk</i> file, <code>hello-thunk.S</code>.
|
|
This <i>thunk</i> file contains all of the information needed to create the imported function list.
|
|
</p>
|
|
|
|
<p><b>Target 3</b>
|
|
This target is similar to <b>Target 1</b>.
|
|
In this case, it will link together the module's object files (only <code>hello.o</code> here)
|
|
along with the assembled <i>thunk</i> file, <code>hello-thunk.o</code> to create the second relocatable object,
|
|
<code>hello.r2</code>.
|
|
The linker script, <code>gnu-nxflat.ld</code> is required at this point to correctly position the sections.
|
|
This linker script produces two segments:
|
|
An <i>I-Space</i> (Instruction Space) segment containing mostly <code>.text</code> and a <i>D-Space</i> (Data Space) segment
|
|
containing <code>.got</code>, <code>.data</code>, and <code>.bss</code> sections.
|
|
The I-Space section must be origined at address 0 (so that the segment's addresses are really offsets into
|
|
the I-Space segment)
|
|
and the D-Space section must also be origined at address 0 (so that segment's addresses are really offsets into
|
|
the I-Space segment).
|
|
The option <code>-no-check-sections</code> is required to prevent the linker from failing because these segments overlap.
|
|
</p>
|
|
|
|
<p><b>Target 4</b>.
|
|
Finally, this target will use the <code>hello.r2</code> relocatable object to create the final, NXFLAT module
|
|
<code>hello</code> by executing <a href="#ldnxflat"><code>ldnxflat</code></a>.
|
|
</p>
|
|
|
|
<table width ="100%">
|
|
<tr bgcolor="#e4e4e4">
|
|
<td>
|
|
<a name="binfmt"><h1>3.0 Binary Loader APIs</h1></a>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p><b>Relevant Header Files:</b></p>
|
|
<ul>
|
|
<li>
|
|
The interface to the binary loader is described in the header file
|
|
<a href="http://nuttx.svn.sourceforge.net/viewvc/nuttx/trunk/nuttx/include/nuttx/binfmt.h?view=log">
|
|
<code>include/nuttx/binfmt.h</code></a>.
|
|
A brief summary of the APIs prototyped in that header file are listed below.
|
|
</li>
|
|
<li>
|
|
NXFLAT APIs needed to register NXFLAT as a binary loader appear in the header file
|
|
<a href="http://nuttx.svn.sourceforge.net/viewvc/nuttx/trunk/nuttx/include/nuttx/nxflat.h?view=log">
|
|
<code>include/nuttx/nxflat.h</code></a>.
|
|
</li>
|
|
<li>
|
|
The format of an NXFLAT object itself is described in the header file:
|
|
<a href="http://nuttx.svn.sourceforge.net/viewvc/nuttx/trunk/nuttx/include/nuttx/nxflat.h?view=log">
|
|
<code>include/nuttx/nxflat.h</code></a>.
|
|
</li>
|
|
</ul>
|
|
|
|
<p><b>binfmt Registration</b>
|
|
These first interfaces are used only by a binary loader module, such as NXFLAT itself.
|
|
NXFLAT (or any future binary loader) calls <code>register_binfmt()</code> to incorporate
|
|
itself into the system.
|
|
In this way, the binary loader logic is dynamically extensible to support any kind of loader.
|
|
Normal application code should not be concerned with these interfaces.
|
|
</p>
|
|
|
|
<ul>
|
|
<p><b><code>int register_binfmt(FAR struct binfmt_s *binfmt)</code></b>
|
|
<ul>
|
|
<p><b>Description:</b>
|
|
Register a loader for a binary format
|
|
</p>
|
|
<p><b>Returned Value:</b>
|
|
This is a NuttX internal function so it follows the convention that
|
|
0 (<code>OK</code>) is returned on success and a negated errno is returned on
|
|
failure.
|
|
</p>
|
|
</ul>
|
|
|
|
<p><b><code>int unregister_binfmt(FAR struct binfmt_s *binfmt)</code></b>
|
|
<ul>
|
|
<p><b>Description:</b>
|
|
Register a loader for a binary format
|
|
</p>
|
|
<p><b>Returned Value:</b>
|
|
This is a NuttX internal function so it follows the convention that
|
|
0 (<code>OK</code>) is returned on success and a negated errno is returned on
|
|
failure.
|
|
</p>
|
|
</ul>
|
|
</ul>
|
|
|
|
<p><b>NXFLAT Initialization</b>
|
|
These interfaces are specific to NXFLAT.
|
|
Normally, an application needs only call <code>nxflat_initialize()</code> during its initialization
|
|
to have full NXFLAT support.
|
|
</p>
|
|
|
|
<ul>
|
|
<p><b><code>int nxflat_initialize(void)</code></b>
|
|
<ul>
|
|
<p><b>Description:</b>
|
|
NXFLAT support is built unconditionally. However, it order to
|
|
use this binary format, this function must be called during system
|
|
format in order to register the NXFLAT binary format.
|
|
This function calls <code>register_binfmt()</code> appropriately.
|
|
</p>
|
|
<p><b>Returned Value:</b>
|
|
This is a NuttX internal function so it follows the convention that
|
|
0 (OK) is returned on success and a negated errno is returned on
|
|
failure.
|
|
</p>
|
|
</ul>
|
|
|
|
<p><b><code>void nxflat_uninitialize(void)</code></b>
|
|
<ul>
|
|
<p><b>Description:</b>
|
|
Unregister the NXFLAT binary loader
|
|
</p>
|
|
<p><b>Returned Value:</b>
|
|
None
|
|
</p>
|
|
</ul>
|
|
</ul>
|
|
|
|
<p><b>Binary Loader Interfaces</b>.
|
|
The remaining APIs are called by user applications to maintain modules in the file system.
|
|
</p>
|
|
|
|
<ul>
|
|
<p><b><code>int load_module(FAR struct binary_s *bin)</code></b>
|
|
<ul>
|
|
<p><b>Description:</b>
|
|
Load a module into memory, bind it to an exported symbol take, and
|
|
prep the module for execution.
|
|
</p>
|
|
<p><b>Returned Value:</b>
|
|
This is an end-user function, so it follows the normal convention:
|
|
Returns 0 (<code>OK</code>) on success. On failure, it returns -1 (<code>ERROR</code>) with
|
|
errno set appropriately.
|
|
</p>
|
|
</ul>
|
|
|
|
<p><b><code>int unload_module(FAR const struct binary_s *bin)</code></b>
|
|
<ul>
|
|
<p><b>Description:</b>
|
|
Unload a (non-executing) module from memory. If the module has
|
|
been started (via <code>exec_module()</code>), calling this will be fatal.
|
|
</p>
|
|
<p><b>Returned Value:</b>
|
|
This is a NuttX internal function so it follows the convention that
|
|
0 (<code>OK</code>) is returned on success and a negated errno is returned on
|
|
failure.
|
|
</p>
|
|
</ul>
|
|
|
|
<p><b><code>int exec_module(FAR const struct binary_s *bin, int priority)</code></b>
|
|
<ul>
|
|
<p><b>Description:</b>
|
|
Execute a module that has been loaded into memory by load_module().
|
|
</p>
|
|
<p><b>Returned Value:</b>
|
|
This is an end-user function, so it follows the normal convention:
|
|
Returns the PID of the exec'ed module. On failure, it.returns
|
|
-1 (<code>ERROR</code>) and sets errno appropriately.
|
|
</p>
|
|
</ul>
|
|
</ul>
|
|
|
|
<table width ="100%">
|
|
<tr bgcolor="#e4e4e4">
|
|
<td>
|
|
<a name="nogot"><h1>Appendix A. No GOT Operation</h1></a>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p>
|
|
When GCC generate position independent code, new code sections will appear in your programs.
|
|
One of these is the GOT (Global Offset Table) and, in ELF environments, another is the PLT (Procedure
|
|
Lookup Table.
|
|
For example, if your C code generated (ARM) assembly language like this without PIC:
|
|
<p>
|
|
<ul><pre>
|
|
ldr r1, .L0 <-- Fetch the offset to 'x'
|
|
ldr r0, [r10, r1] <-- Load the value of 'x' with PIC
|
|
offset
|
|
...
|
|
.L0: .word x <-- Offset to 'x'
|
|
</pre></ul>
|
|
|
|
<p>
|
|
Then when PIC is enabled (say with the -fpic compiler option), it will generate code like
|
|
this:
|
|
</p>
|
|
|
|
<ul><pre>
|
|
ldr r1, .L0 <-- Fetch the offset to the GOT entry
|
|
ldr r1, [r10,r1] <-- Fetch the (relocated) address
|
|
of 'x' from the GOT
|
|
ldr r0, [r1, #0] <-- Fetch the value of 'x'
|
|
...
|
|
.L1 .word x(GOT) <-- Offset to entry in the GOT
|
|
</pre></ul>
|
|
|
|
<p>See <a href="http://xflat.sourceforge.net/NoMMUSharedLibs.html#shlibsgot">reference</a></p>
|
|
|
|
<p>
|
|
Notice that the generates an extra level of indirection through the GOT.
|
|
This indirection is not needed by NXFLAT and only adds more RAM usage and
|
|
execution time.
|
|
</p>
|
|
<p>
|
|
NXFLAT (like <a href="http://xflat.sourceforge.net/">XFLAT</a>) can work even better with
|
|
the GOT.
|
|
Patches again older version of GCC exist to eliminate the GOT indirections.
|
|
Several are available <a href="http://xflat.cvs.sourceforge.net/viewvc/xflat/xflat/gcc/">here</a>
|
|
if you are inspired to port them to a new GCC version.
|
|
</p>
|
|
|
|
|
|
<table width ="100%">
|
|
<tr bgcolor="#e4e4e4">
|
|
<td>
|
|
<a name="pictext"><h1>Appendix B. PIC Text Workaround</h1></a>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
|
|
<p>
|
|
There is a problem with the memory model in GCC that prevents it from
|
|
being used as you need to use it in the NXFLAT context.
|
|
The problem is that GCC PIC model assumes that the executable lies in a flat, contiguous (virtual) address space like:
|
|
<p>
|
|
<ul><table width="15%">
|
|
<tr><th>Virtual</th></td>
|
|
<tr>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.text</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.got</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.data</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.bss</code></td>
|
|
</tr>
|
|
</table></ul>
|
|
<p>
|
|
It assumes that the PIC base register (usually r10 for ARM) points to the base of <code>.text</code>
|
|
so that any address in <code>.text</code>, <code>.got</code>, <code>.data</code>, <code>.bss</code>
|
|
can be found with an offset from the same base address.
|
|
But that is not the memory arrangement that we need in the XIP embedded environment.
|
|
We need two memory regions, one in FLASH containing shared code and on per task in RAM containing task-specific data:
|
|
</p>
|
|
|
|
<ul><table width="30%">
|
|
<tr><th>Flash</th><th>RAM</th></td>
|
|
<tr>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.text</code></td>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.got</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td bgcolor="#e4e4e4"><br></td>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.data</code></td>
|
|
</tr>
|
|
<tr>
|
|
<td bgcolor="#e4e4e4"><br></td>
|
|
<td align="center" bgcolor="#e4e4e4"><code>.bss</code></td>
|
|
</tr>
|
|
</table></ul>
|
|
|
|
<p>
|
|
The PIC base register needs to point to the base of the <code>.got</code> and only
|
|
addresses in the <code>.got</code>, <code>.data</code>, and <code>.bss</code>
|
|
sections can be accessed as an offset from the PIC base register.
|
|
See also this
|
|
<a href="http://xflat.cvs.sourceforge.net/viewvc/*checkout*/xflat/xflat/gcc/README?revision=1.1.1.1">XFLAT discussion</a>.
|
|
</p>
|
|
<p>
|
|
Patches again older version of GCC exist to correct this GCC behavior.
|
|
Several are available <a href="http://xflat.cvs.sourceforge.net/viewvc/xflat/xflat/gcc/">here</a>
|
|
if you are inspired to port them to a new GCC version.
|
|
</p>
|
|
|
|
</body>
|
|
</html>
|