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145 lines
7.0 KiB
HTML
145 lines
7.0 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>MIPS Macros (Using as)</title>
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<meta name="description" content="MIPS Macros (Using as)">
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<meta name="keywords" content="MIPS Macros (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="MIPS_002dDependent.html#MIPS_002dDependent" rel="up" title="MIPS-Dependent">
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<link href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" rel="next" title="MIPS Symbol Sizes">
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<link href="MIPS-Options.html#MIPS-Options" rel="prev" title="MIPS Options">
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<style type="text/css">
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a.summary-letter {text-decoration: 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="MIPS-Macros"></a>
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<div class="header">
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<p>
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Next: <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" accesskey="n" rel="next">MIPS Symbol Sizes</a>, Previous: <a href="MIPS-Options.html#MIPS-Options" accesskey="p" rel="prev">MIPS Options</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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</div>
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<hr>
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<a name="High_002dlevel-assembly-macros"></a>
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<h4 class="subsection">9.26.2 High-level assembly macros</h4>
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<p>MIPS assemblers have traditionally provided a wider range of
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instructions than the MIPS architecture itself. These extra
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instructions are usually referred to as “macro” instructions
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<a name="DOCF2" href="#FOOT2"><sup>2</sup></a>.
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</p>
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<p>Some MIPS macro instructions extend an underlying architectural instruction
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while others are entirely new. An example of the former type is <code>and</code>,
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which allows the third operand to be either a register or an arbitrary
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immediate value. Examples of the latter type include <code>bgt</code>, which
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branches to the third operand when the first operand is greater than
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the second operand, and <code>ulh</code>, which implements an unaligned
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2-byte load.
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</p>
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<p>One of the most common extensions provided by macros is to expand
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memory offsets to the full address range (32 or 64 bits) and to allow
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symbolic offsets such as ‘<samp>my_data + 4</samp>’ to be used in place of
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integer constants. For example, the architectural instruction
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<code>lbu</code> allows only a signed 16-bit offset, whereas the macro
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<code>lbu</code> allows code such as ‘<samp>lbu $4,array+32769($5)</samp>’.
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The implementation of these symbolic offsets depends on several factors,
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such as whether the assembler is generating SVR4-style PIC (selected by
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<samp>-KPIC</samp>, see <a href="MIPS-Options.html#MIPS-Options">Assembler options</a>), the size of symbols
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(see <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes">Directives to override the size of symbols</a>),
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and the small data limit (see <a href="MIPS-Small-Data.html#MIPS-Small-Data">Controlling the use
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of small data accesses</a>).
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</p>
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<a name="index-_002eset-macro"></a>
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<a name="index-_002eset-nomacro"></a>
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<p>Sometimes it is undesirable to have one assembly instruction expand
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to several machine instructions. The directive <code>.set nomacro</code>
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tells the assembler to warn when this happens. <code>.set macro</code>
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restores the default behavior.
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</p>
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<a name="index-at-register_002c-MIPS"></a>
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<a name="index-_002eset-at_003dreg"></a>
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<p>Some macro instructions need a temporary register to store intermediate
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results. This register is usually <code>$1</code>, also known as <code>$at</code>,
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but it can be changed to any core register <var>reg</var> using
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<code>.set at=<var>reg</var></code>. Note that <code>$at</code> always refers
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to <code>$1</code> regardless of which register is being used as the
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temporary register.
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</p>
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<a name="index-_002eset-at"></a>
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<a name="index-_002eset-noat"></a>
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<p>Implicit uses of the temporary register in macros could interfere with
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explicit uses in the assembly code. The assembler therefore warns
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whenever it sees an explicit use of the temporary register. The directive
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<code>.set noat</code> silences this warning while <code>.set at</code> restores
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the default behavior. It is safe to use <code>.set noat</code> while
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<code>.set nomacro</code> is in effect since single-instruction macros
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never need a temporary register.
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</p>
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<p>Note that while the <small>GNU</small> assembler provides these macros for compatibility,
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it does not make any attempt to optimize them with the surrounding code.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT2" href="#DOCF2">(2)</a></h3>
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<p>The term “macro” is somewhat overloaded here, since
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these macros have no relation to those defined by <code>.macro</code>,
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see <a href="Macro.html#Macro"><code>.macro</code></a>.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="MIPS-Symbol-Sizes.html#MIPS-Symbol-Sizes" accesskey="n" rel="next">MIPS Symbol Sizes</a>, Previous: <a href="MIPS-Options.html#MIPS-Options" accesskey="p" rel="prev">MIPS Options</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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</div>
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</body>
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</html>
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