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102 lines
5.5 KiB
HTML
102 lines
5.5 KiB
HTML
4 years ago
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<title>Bundle directives - Using as</title>
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<meta name="description" content="Using as">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Pseudo-Ops.html#Pseudo-Ops" title="Pseudo Ops">
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<link rel="prev" href="Balign.html#Balign" title="Balign">
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<link rel="next" href="Byte.html#Byte" title="Byte">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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This file documents the GNU Assembler "as".
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Copyright (C) 1991-2015 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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Bundle-directives"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Byte.html#Byte">Byte</a>,
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Previous: <a rel="previous" accesskey="p" href="Balign.html#Balign">Balign</a>,
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Up: <a rel="up" accesskey="u" href="Pseudo-Ops.html#Pseudo-Ops">Pseudo Ops</a>
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<hr>
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</div>
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<h3 class="section">7.8 Bundle directives</h3>
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<h4 class="subsection">7.8.1 <code>.bundle_align_mode </code><var>abs-expr</var></h4>
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<p><a name="index-g_t_0040code_007bbundle_005falign_005fmode_007d-directive-290"></a><a name="index-bundle-291"></a><a name="index-instruction-bundle-292"></a><a name="index-aligned-instruction-bundle-293"></a><code>.bundle_align_mode</code> enables or disables <dfn>aligned instruction
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bundle</dfn> mode. In this mode, sequences of adjacent instructions are grouped
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into fixed-sized <dfn>bundles</dfn>. If the argument is zero, this mode is
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disabled (which is the default state). If the argument it not zero, it
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gives the size of an instruction bundle as a power of two (as for the
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<code>.p2align</code> directive, see <a href="P2align.html#P2align">P2align</a>).
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<p>For some targets, it's an ABI requirement that no instruction may span a
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certain aligned boundary. A <dfn>bundle</dfn> is simply a sequence of
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instructions that starts on an aligned boundary. For example, if
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<var>abs-expr</var> is <code>5</code> then the bundle size is 32, so each aligned
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chunk of 32 bytes is a bundle. When aligned instruction bundle mode is in
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effect, no single instruction may span a boundary between bundles. If an
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instruction would start too close to the end of a bundle for the length of
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that particular instruction to fit within the bundle, then the space at the
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end of that bundle is filled with no-op instructions so the instruction
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starts in the next bundle. As a corollary, it's an error if any single
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instruction's encoding is longer than the bundle size.
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<h4 class="subsection">7.8.2 <code>.bundle_lock</code> and <code>.bundle_unlock</code></h4>
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<p><a name="index-g_t_0040code_007bbundle_005flock_007d-directive-294"></a><a name="index-g_t_0040code_007bbundle_005funlock_007d-directive-295"></a>The <code>.bundle_lock</code> and directive <code>.bundle_unlock</code> directives
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allow explicit control over instruction bundle padding. These directives
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are only valid when <code>.bundle_align_mode</code> has been used to enable
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aligned instruction bundle mode. It's an error if they appear when
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<code>.bundle_align_mode</code> has not been used at all, or when the last
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directive was <code>.bundle_align_mode 0</code><!-- /@w -->.
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<p><a name="index-bundle_002dlocked-296"></a>For some targets, it's an ABI requirement that certain instructions may
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appear only as part of specified permissible sequences of multiple
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instructions, all within the same bundle. A pair of <code>.bundle_lock</code>
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and <code>.bundle_unlock</code> directives define a <dfn>bundle-locked</dfn>
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instruction sequence. For purposes of aligned instruction bundle mode, a
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sequence starting with <code>.bundle_lock</code> and ending with
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<code>.bundle_unlock</code> is treated as a single instruction. That is, the
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entire sequence must fit into a single bundle and may not span a bundle
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boundary. If necessary, no-op instructions will be inserted before the
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first instruction of the sequence so that the whole sequence starts on an
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aligned bundle boundary. It's an error if the sequence is longer than the
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bundle size.
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<p>For convenience when using <code>.bundle_lock</code> and <code>.bundle_unlock</code>
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inside assembler macros (see <a href="Macro.html#Macro">Macro</a>), bundle-locked sequences may be
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nested. That is, a second <code>.bundle_lock</code> directive before the next
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<code>.bundle_unlock</code> directive has no effect except that it must be
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matched by another closing <code>.bundle_unlock</code> so that there is the
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same number of <code>.bundle_lock</code> and <code>.bundle_unlock</code> directives.
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</body></html>
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