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 <a name="MIPS-NaN-Encodings"></a>
 <div class="header">
 <p>
 Next: <a href="MIPS-Option-Stack.html#MIPS-Option-Stack" accesskey="n" rel="next">MIPS Option Stack</a>, Previous: <a href="MIPS-insn.html#MIPS-insn" accesskey="p" rel="prev">MIPS insn</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-Dependent</a> &nbsp; [<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>
 </div>
 <hr>
 <a name="Directives-to-record-which-NaN-encoding-is-being-used"></a>
 <h4 class="subsection">9.27.9 Directives to record which NaN encoding is being used</h4>

 <a name="index-MIPS-IEEE-754-NaN-data-encoding-selection"></a>
 <a name="index-_002enan-directive_002c-MIPS"></a>
 <p>The IEEE 754 floating-point standard defines two types of not-a-number
 (NaN) data: &ldquo;signalling&rdquo; NaNs and &ldquo;quiet&rdquo; NaNs.  The original version
 of the standard did not specify how these two types should be
 distinguished.  Most implementations followed the i387 model, in which
 the first bit of the significand is set for quiet NaNs and clear for
 signalling NaNs.  However, the original MIPS implementation assigned the
 opposite meaning to the bit, so that it was set for signalling NaNs and
 clear for quiet NaNs.
 </p>
 <p>The 2008 revision of the standard formally suggested the i387 choice
 and as from Sep 2012 the current release of the MIPS architecture
 therefore optionally supports that form.  Code that uses one NaN encoding
 would usually be incompatible with code that uses the other NaN encoding,
 so MIPS ELF objects have a flag (<code>EF_MIPS_NAN2008</code>) to record which
 encoding is being used.
 </p>
 <p>Assembly files can use the <code>.nan</code> directive to select between the
 two encodings.  &lsquo;<samp>.nan 2008</samp>&rsquo; says that the assembly file uses the
 IEEE 754-2008 encoding while &lsquo;<samp>.nan legacy</samp>&rsquo; says that the file uses
 the original MIPS encoding.  If several <code>.nan</code> directives are given,
 the final setting is the one that is used.
 </p>
 <p>The command-line options <samp>-mnan=legacy</samp> and <samp>-mnan=2008</samp>
 can be used instead of &lsquo;<samp>.nan legacy</samp>&rsquo; and &lsquo;<samp>.nan 2008</samp>&rsquo;
 respectively.  However, any <code>.nan</code> directive overrides the
 command-line setting.
 </p>
 <p>&lsquo;<samp>.nan legacy</samp>&rsquo; is the default if no <code>.nan</code> directive or
 <samp>-mnan</samp> option is given.
 </p>
 <p>Note that <small>GNU</small> <code>as</code> does not produce NaNs itself and
 therefore these directives do not affect code generation.  They simply
 control the setting of the <code>EF_MIPS_NAN2008</code> flag.
 </p>
 <p>Traditional MIPS assemblers do not support these directives.
 </p>
 <hr>
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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-insn.html#MIPS-insn" accesskey="p" rel="prev">MIPS insn</a>, Up: <a href="MIPS_002dDependent.html#MIPS_002dDependent" accesskey="u" rel="up">MIPS-Dependent</a> &nbsp; [<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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	<a name="MIPS-NaN-Encodings"></a>
	<div class="header">
	<p>
	Next: <a href="MIPS-Option-Stack.html#MIPS-Option-Stack" accesskey="n" rel="next">MIPS Option Stack</a>, Previous: <a href="MIPS-insn.html#MIPS-insn" accesskey="p" rel="prev">MIPS insn</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>
	</div>
	<hr>
	<a name="Directives-to-record-which-NaN-encoding-is-being-used"></a>
	<h4 class="subsection">9.27.9 Directives to record which NaN encoding is being used</h4>

	<a name="index-MIPS-IEEE-754-NaN-data-encoding-selection"></a>
	<a name="index-_002enan-directive_002c-MIPS"></a>
	<p>The IEEE 754 floating-point standard defines two types of not-a-number
	(NaN) data: “signalling” NaNs and “quiet” NaNs. The original version
	of the standard did not specify how these two types should be
	distinguished. Most implementations followed the i387 model, in which
	the first bit of the significand is set for quiet NaNs and clear for
	signalling NaNs. However, the original MIPS implementation assigned the
	opposite meaning to the bit, so that it was set for signalling NaNs and
	clear for quiet NaNs.
	</p>
	<p>The 2008 revision of the standard formally suggested the i387 choice
	and as from Sep 2012 the current release of the MIPS architecture
	therefore optionally supports that form. Code that uses one NaN encoding
	would usually be incompatible with code that uses the other NaN encoding,
	so MIPS ELF objects have a flag (<code>EF_MIPS_NAN2008</code>) to record which
	encoding is being used.
	</p>
	<p>Assembly files can use the <code>.nan</code> directive to select between the
	two encodings. ‘<samp>.nan 2008</samp>’ says that the assembly file uses the
	IEEE 754-2008 encoding while ‘<samp>.nan legacy</samp>’ says that the file uses
	the original MIPS encoding. If several <code>.nan</code> directives are given,
	the final setting is the one that is used.
	</p>
	<p>The command-line options <samp>-mnan=legacy</samp> and <samp>-mnan=2008</samp>
	can be used instead of ‘<samp>.nan legacy</samp>’ and ‘<samp>.nan 2008</samp>’
	respectively. However, any <code>.nan</code> directive overrides the
	command-line setting.
	</p>
	<p>‘<samp>.nan legacy</samp>’ is the default if no <code>.nan</code> directive or
	<samp>-mnan</samp> option is given.
	</p>
	<p>Note that <small>GNU</small> <code>as</code> does not produce NaNs itself and
	therefore these directives do not affect code generation. They simply
	control the setting of the <code>EF_MIPS_NAN2008</code> flag.
	</p>
	<p>Traditional MIPS assemblers do not support these directives.
	</p>
	<hr>
	<div class="header">
	<p>
	Next: <a href="MIPS-Option-Stack.html#MIPS-Option-Stack" accesskey="n" rel="next">MIPS Option Stack</a>, Previous: <a href="MIPS-insn.html#MIPS-insn" accesskey="p" rel="prev">MIPS insn</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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