<!DOCTYPE html>
<html lang="en" data-content_root="../../../">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="viewport" content="width=device-width, initial-scale=1" />
<title>Notes — The Linux Kernel documentation</title>
<link rel="stylesheet" type="text/css" href="../../../_static/pygments.css?v=fa44fd50" />
<link rel="stylesheet" type="text/css" href="../../../_static/alabaster.css?v=3918102e" />
<script src="../../../_static/documentation_options.js?v=5929fcd5"></script>
<script src="../../../_static/doctools.js?v=9bcbadda"></script>
<script src="../../../_static/sphinx_highlight.js?v=dc90522c"></script>
<link rel="index" title="Index" href="../../../genindex.html" />
<link rel="search" title="Search" href="../../../search.html" />
<link rel="next" title="TODO LIST" href="todo.html" />
<link rel="prev" title="Current State" href="netwinder-fpe.html" />
<link rel="stylesheet" href="../../../_static/custom.css" type="text/css" />
</head><body>
<div class="document">
<div class="sphinxsidebar" role="navigation" aria-label="Main">
<div class="sphinxsidebarwrapper">
<p class="logo"><a href="../../../index.html">
<img class="logo" src="../../../_static/logo.svg" alt="Logo of The Linux Kernel"/>
</a></p>
<h1 class="logo"><a href="../../../index.html">The Linux Kernel</a></h1>
<p class="blurb">6.18.50</p>
<search id="searchbox" style="display: none" role="search">
<h3 id="searchlabel">Quick search</h3>
<div class="searchformwrapper">
<form class="search" action="../../../search.html" method="get">
<input type="text" name="q" aria-labelledby="searchlabel" autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"/>
<input type="submit" value="Go" />
</form>
</div>
</search>
<script>document.getElementById('searchbox').style.display = "block"</script>
<p>
<h3 class="kernel-toc-contents">Contents</h3>
<input type="checkbox" class="kernel-toc-toggle" id = "kernel-toc-toggle" checked>
<label class="kernel-toc-title" for="kernel-toc-toggle"></label>
<div class="kerneltoc" id="kerneltoc">
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../../process/development-process.html">Development process</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../process/submitting-patches.html">Submitting patches</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../process/code-of-conduct.html">Code of conduct</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../maintainer/index.html">Maintainer handbook</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../process/index.html">All development-process docs</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../../core-api/index.html">Core API</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../driver-api/index.html">Driver APIs</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../subsystem-apis.html">Subsystems</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../locking/index.html">Locking</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../../process/license-rules.html">Licensing rules</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../doc-guide/index.html">Writing documentation</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../dev-tools/index.html">Development tools</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../dev-tools/testing-overview.html">Testing guide</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../kernel-hacking/index.html">Hacking guide</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../trace/index.html">Tracing</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../fault-injection/index.html">Fault injection</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../livepatch/index.html">Livepatching</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../rust/index.html">Rust</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../../admin-guide/index.html">Administration</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../kbuild/index.html">Build system</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../admin-guide/reporting-issues.html">Reporting issues</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../tools/index.html">Userspace tools</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../userspace-api/index.html">Userspace API</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../../firmware-guide/index.html">Firmware</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../../devicetree/index.html">Firmware and Devicetree</a></li>
</ul>
<ul class="current">
<li class="toctree-l1 current"><a class="reference internal" href="../../index.html">CPU architectures</a><ul class="current">
<li class="toctree-l2"><a class="reference internal" href="../../arc/index.html">ARC architecture</a></li>
<li class="toctree-l2 current"><a class="reference internal" href="../index.html">ARM Architecture</a><ul class="current">
<li class="toctree-l3"><a class="reference internal" href="../arm.html">ARM Linux 2.6 and upper</a></li>
<li class="toctree-l3"><a class="reference internal" href="../booting.html">Booting ARM Linux</a></li>
<li class="toctree-l3"><a class="reference internal" href="../cluster-pm-race-avoidance.html">Cluster-wide Power-up/power-down race avoidance algorithm</a></li>
<li class="toctree-l3"><a class="reference internal" href="../firmware.html">Interface for registering and calling firmware-specific operations for ARM</a></li>
<li class="toctree-l3"><a class="reference internal" href="../interrupts.html">Interrupts</a></li>
<li class="toctree-l3"><a class="reference internal" href="../kernel_mode_neon.html">Kernel mode NEON</a></li>
<li class="toctree-l3"><a class="reference internal" href="../kernel_user_helpers.html">Kernel-provided User Helpers</a></li>
<li class="toctree-l3"><a class="reference internal" href="../memory.html">Kernel Memory Layout on ARM Linux</a></li>
<li class="toctree-l3"><a class="reference internal" href="../mem_alignment.html">Memory alignment</a></li>
<li class="toctree-l3"><a class="reference internal" href="../tcm.html">ARM TCM (Tightly-Coupled Memory) handling in Linux</a></li>
<li class="toctree-l3"><a class="reference internal" href="../setup.html">Kernel initialisation parameters on ARM Linux</a></li>
<li class="toctree-l3"><a class="reference internal" href="../swp_emulation.html">Software emulation of deprecated SWP instruction (CONFIG_SWP_EMULATE)</a></li>
<li class="toctree-l3"><a class="reference internal" href="../uefi.html">The Unified Extensible Firmware Interface (UEFI)</a></li>
<li class="toctree-l3"><a class="reference internal" href="../vlocks.html">vlocks for Bare-Metal Mutual Exclusion</a></li>
<li class="toctree-l3"><a class="reference internal" href="../porting.html">Porting</a></li>
<li class="toctree-l3"><a class="reference internal" href="../features.html">Feature status on arm architecture</a></li>
<li class="toctree-l3 current"><a class="reference internal" href="../index.html#soc-specific-documents">SoC-specific documents</a></li>
</ul>
</li>
<li class="toctree-l2"><a class="reference internal" href="../../arm64/index.html">ARM64 Architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../loongarch/index.html">LoongArch Architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../m68k/index.html">m68k Architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../mips/index.html">MIPS-specific Documentation</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../nios2/index.html">Nios II Specific Documentation</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../openrisc/index.html">OpenRISC Architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../parisc/index.html">PA-RISC Architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../powerpc/index.html">powerpc</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../riscv/index.html">RISC-V architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../s390/index.html">s390 Architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../sh/index.html">SuperH Interfaces Guide</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../sparc/index.html">Sparc Architecture</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../x86/index.html">x86-specific Documentation</a></li>
<li class="toctree-l2"><a class="reference internal" href="../../xtensa/index.html">Xtensa Architecture</a></li>
</ul>
</li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../../staging/index.html">Unsorted documentation</a></li>
</ul>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../../translations/index.html">Translations</a></li>
</ul>
</div>
<script type="text/javascript"> <!--
var sbar = document.getElementsByClassName("sphinxsidebar")[0];
let currents = document.getElementsByClassName("current")
if (currents.length) {
sbar.scrollTop = currents[currents.length - 1].offsetTop;
}
--> </script>
<div role="note" aria-label="source link">
<h3>This Page</h3>
<ul class="this-page-menu">
<li><a href="../../../_sources/arch/arm/nwfpe/notes.rst.txt"
rel="nofollow">Show Source</a></li>
</ul>
</div>
</div>
</div>
<div class="documentwrapper">
<div class="bodywrapper">
<div class="body" role="main">
<section id="notes">
<h1>Notes<a class="headerlink" href="#notes" title="Link to this heading">¶</a></h1>
<p>There seems to be a problem with exp(double) and our emulator. I haven’t
been able to track it down yet. This does not occur with the emulator
supplied by Russell King.</p>
<p>I also found one oddity in the emulator. I don’t think it is serious but
will point it out. The ARM calling conventions require floating point
registers f4-f7 to be preserved over a function call. The compiler quite
often uses an stfe instruction to save f4 on the stack upon entry to a
function, and an ldfe instruction to restore it before returning.</p>
<p>I was looking at some code, that calculated a double result, stored it in f4
then made a function call. Upon return from the function call the number in
f4 had been converted to an extended value in the emulator.</p>
<p>This is a side effect of the stfe instruction. The double in f4 had to be
converted to extended, then stored. If an lfm/sfm combination had been used,
then no conversion would occur. This has performance considerations. The
result from the function call and f4 were used in a multiplication. If the
emulator sees a multiply of a double and extended, it promotes the double to
extended, then does the multiply in extended precision.</p>
<p>This code will cause this problem:</p>
<p>double x, y, z;
z = log(x)/log(y);</p>
<p>The result of log(x) (a double) will be calculated, returned in f0, then
moved to f4 to preserve it over the log(y) call. The division will be done
in extended precision, due to the stfe instruction used to save f4 in log(y).</p>
</section>
</div>
</div>
</div>
<div class="clearer"></div>
</div>
<div class="footer">
©The kernel development community.
|
Powered by <a href="https://www.sphinx-doc.org/">Sphinx 8.1.3</a>
& <a href="https://alabaster.readthedocs.io">Alabaster 0.7.16</a>
|
<a href="../../../_sources/arch/arm/nwfpe/notes.rst.txt"
rel="nofollow">Page source</a>
</div>
</body>
</html>