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| <title>Using as: MIPS NaN Encodings</title> |
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| <a name="MIPS-NaN-Encodings"></a> |
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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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| <a name="Directives-to-record-which-NaN-encoding-is-being-used"></a> |
| <h4 class="subsection">9.27.10 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> |
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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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