120 lines
5.9 KiB
HTML
120 lines
5.9 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This file documents the GNU Assembler "as".
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Copyright (C) 1991-2016 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License".
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-->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using as: MIPS NaN Encodings</title>
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<meta name="description" content="Using as: MIPS NaN Encodings">
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<meta name="keywords" content="Using as: MIPS NaN Encodings">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="MIPS_002dDependent.html#MIPS_002dDependent" rel="up" title="MIPS-Dependent">
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<link href="MIPS-Option-Stack.html#MIPS-Option-Stack" rel="next" title="MIPS Option Stack">
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<link href="MIPS-FP-ABI-Compatibility.html#MIPS-FP-ABI-Compatibility" rel="prev" title="MIPS FP ABI Compatibility">
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="MIPS-NaN-Encodings"></a>
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<div class="header">
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<p>
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Next: <a href="MIPS-Option-Stack.html#MIPS-Option-Stack" accesskey="n" rel="next">MIPS Option Stack</a>, Previous: <a href="MIPS-FP-ABIs.html#MIPS-FP-ABIs" accesskey="p" rel="prev">MIPS FP ABIs</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Directives-to-record-which-NaN-encoding-is-being-used"></a>
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<h4 class="subsection">9.27.10 Directives to record which NaN encoding is being used</h4>
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<a name="index-MIPS-IEEE-754-NaN-data-encoding-selection"></a>
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<a name="index-_002enan-directive_002c-MIPS"></a>
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<p>The IEEE 754 floating-point standard defines two types of not-a-number
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(NaN) data: “signalling” NaNs and “quiet” NaNs. The original version
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of the standard did not specify how these two types should be
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distinguished. Most implementations followed the i387 model, in which
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the first bit of the significand is set for quiet NaNs and clear for
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signalling NaNs. However, the original MIPS implementation assigned the
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opposite meaning to the bit, so that it was set for signalling NaNs and
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clear for quiet NaNs.
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</p>
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<p>The 2008 revision of the standard formally suggested the i387 choice
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and as from Sep 2012 the current release of the MIPS architecture
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therefore optionally supports that form. Code that uses one NaN encoding
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would usually be incompatible with code that uses the other NaN encoding,
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so MIPS ELF objects have a flag (<code>EF_MIPS_NAN2008</code>) to record which
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encoding is being used.
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</p>
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<p>Assembly files can use the <code>.nan</code> directive to select between the
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two encodings. ‘<samp>.nan 2008</samp>’ says that the assembly file uses the
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IEEE 754-2008 encoding while ‘<samp>.nan legacy</samp>’ says that the file uses
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the original MIPS encoding. If several <code>.nan</code> directives are given,
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the final setting is the one that is used.
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</p>
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<p>The command-line options <samp>-mnan=legacy</samp> and <samp>-mnan=2008</samp>
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can be used instead of ‘<samp>.nan legacy</samp>’ and ‘<samp>.nan 2008</samp>’
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respectively. However, any <code>.nan</code> directive overrides the
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command-line setting.
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</p>
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<p>‘<samp>.nan legacy</samp>’ is the default if no <code>.nan</code> directive or
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<samp>-mnan</samp> option is given.
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</p>
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<p>Note that <small>GNU</small> <code>as</code> does not produce NaNs itself and
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therefore these directives do not affect code generation. They simply
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control the setting of the <code>EF_MIPS_NAN2008</code> flag.
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</p>
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<p>Traditional MIPS assemblers do not support these directives.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="MIPS-Option-Stack.html#MIPS-Option-Stack" accesskey="n" rel="next">MIPS Option Stack</a>, Previous: <a href="MIPS-FP-ABIs.html#MIPS-FP-ABIs" accesskey="p" rel="prev">MIPS FP ABIs</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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