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108 lines
4.6 KiB
HTML
108 lines
4.6 KiB
HTML
4 years ago
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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) 1991-2019 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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section entitled "GNU Free Documentation License".
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-->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>MMIX-Expand (Using as)</title>
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<meta name="description" content="MMIX-Expand (Using as)">
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<meta name="keywords" content="MMIX-Expand (Using as)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="MMIX_002dDependent.html#MMIX_002dDependent" rel="up" title="MMIX-Dependent">
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<link href="MMIX_002dSyntax.html#MMIX_002dSyntax" rel="next" title="MMIX-Syntax">
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<link href="MMIX_002dOpts.html#MMIX_002dOpts" rel="prev" title="MMIX-Opts">
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<style type="text/css">
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="MMIX_002dExpand"></a>
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<div class="header">
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<p>
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Next: <a href="MMIX_002dSyntax.html#MMIX_002dSyntax" accesskey="n" rel="next">MMIX-Syntax</a>, Previous: <a href="MMIX_002dOpts.html#MMIX_002dOpts" accesskey="p" rel="prev">MMIX-Opts</a>, Up: <a href="MMIX_002dDependent.html#MMIX_002dDependent" accesskey="u" rel="up">MMIX-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="Instruction-expansion-1"></a>
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<h4 class="subsection">9.27.2 Instruction expansion</h4>
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<a name="index-instruction-expansion_002c-MMIX"></a>
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<p>When <code>as</code> encounters an instruction with an operand that is
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either not known or does not fit the operand size of the instruction,
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<code>as</code> (and <code>ld</code>) will expand the instruction into
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a sequence of instructions semantically equivalent to the operand fitting
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the instruction. Expansion will take place for the following
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instructions:
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</p>
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<dl compact="compact">
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<dt>‘<samp>GETA</samp>’</dt>
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<dd><p>Expands to a sequence of four instructions: <code>SETL</code>, <code>INCML</code>,
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<code>INCMH</code> and <code>INCH</code>. The operand must be a multiple of four.
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</p></dd>
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<dt>Conditional branches</dt>
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<dd><p>A branch instruction is turned into a branch with the complemented
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condition and prediction bit over five instructions; four instructions
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setting <code>$255</code> to the operand value, which like with <code>GETA</code> must
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be a multiple of four, and a final <code>GO $255,$255,0</code>.
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</p></dd>
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<dt>‘<samp>PUSHJ</samp>’</dt>
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<dd><p>Similar to expansion for conditional branches; four instructions set
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<code>$255</code> to the operand value, followed by a <code>PUSHGO $255,$255,0</code>.
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</p></dd>
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<dt>‘<samp>JMP</samp>’</dt>
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<dd><p>Similar to conditional branches and <code>PUSHJ</code>. The final instruction
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is <code>GO $255,$255,0</code>.
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</p></dd>
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</dl>
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<p>The linker <code>ld</code> is expected to shrink these expansions for
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code assembled with ‘<samp>--relax</samp>’ (though not currently implemented).
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</p>
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</body>
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</html>
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