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93 lines
3.9 KiB
HTML
93 lines
3.9 KiB
HTML
<!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) 1991-2019 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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<head>
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<title>i386-Bugs (Using as)</title>
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<meta name="description" content="i386-Bugs (Using as)">
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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="i386_002dDependent.html#i386_002dDependent" rel="up" title="i386-Dependent">
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<link href="i386_002dNotes.html#i386_002dNotes" rel="next" title="i386-Notes">
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</head>
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<a name="i386_002dBugs"></a>
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<div class="header">
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<p>
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Next: <a href="i386_002dNotes.html#i386_002dNotes" accesskey="n" rel="next">i386-Notes</a>, Previous: <a href="i386_002dArch.html#i386_002dArch" accesskey="p" rel="prev">i386-Arch</a>, Up: <a href="i386_002dDependent.html#i386_002dDependent" accesskey="u" rel="up">i386-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="AT_0026T-Syntax-bugs"></a>
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<h4 class="subsection">9.15.16 AT&T Syntax bugs</h4>
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<p>The UnixWare assembler, and probably other AT&T derived ix86 Unix
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assemblers, generate floating point instructions with reversed source
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and destination registers in certain cases. Unfortunately, gcc and
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possibly many other programs use this reversed syntax, so we’re stuck
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with it.
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</p>
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<p>For example
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</p>
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<div class="smallexample">
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<pre class="smallexample"> fsub %st,%st(3)
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</pre></div>
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<p>results in ‘<samp>%st(3)</samp>’ being updated to ‘<samp>%st - %st(3)</samp>’ rather
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than the expected ‘<samp>%st(3) - %st</samp>’. This happens with all the
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non-commutative arithmetic floating point operations with two register
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operands where the source register is ‘<samp>%st</samp>’ and the destination
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register is ‘<samp>%st(i)</samp>’.
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</p>
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</body>
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</html>
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