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220 lines
10 KiB
HTML
220 lines
10 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>Symbol Names (Using as)</title>
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<meta name="description" content="Symbol Names (Using as)">
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<meta name="keywords" content="Symbol Names (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="Symbols.html#Symbols" rel="up" title="Symbols">
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<link href="Dot.html#Dot" rel="next" title="Dot">
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<link href="Setting-Symbols.html#Setting-Symbols" rel="prev" title="Setting Symbols">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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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="Symbol-Names"></a>
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<div class="header">
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<p>
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Next: <a href="Dot.html#Dot" accesskey="n" rel="next">Dot</a>, Previous: <a href="Setting-Symbols.html#Setting-Symbols" accesskey="p" rel="prev">Setting Symbols</a>, Up: <a href="Symbols.html#Symbols" accesskey="u" rel="up">Symbols</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="Symbol-Names-1"></a>
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<h3 class="section">5.3 Symbol Names</h3>
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<a name="index-symbol-names"></a>
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<a name="index-names_002c-symbol"></a>
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<p>Symbol names begin with a letter or with one of ‘<samp>._</samp>’. On most
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machines, you can also use <code>$</code> in symbol names; exceptions are
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noted in <a href="Machine-Dependencies.html#Machine-Dependencies">Machine Dependencies</a>. That character may be followed by any
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string of digits, letters, dollar signs (unless otherwise noted for a
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particular target machine), and underscores.
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</p>
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<p>Case of letters is significant: <code>foo</code> is a different symbol name
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than <code>Foo</code>.
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</p>
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<p>Symbol names do not start with a digit. An exception to this rule is made for
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Local Labels. See below.
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</p>
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<p>Multibyte characters are supported. To generate a symbol name containing
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multibyte characters enclose it within double quotes and use escape codes. cf
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See <a href="Strings.html#Strings">Strings</a>. Generating a multibyte symbol name from a label is not
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currently supported.
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</p>
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<p>Each symbol has exactly one name. Each name in an assembly language program
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refers to exactly one symbol. You may use that symbol name any number of times
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in a program.
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</p>
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<a name="Local-Symbol-Names"></a>
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<h4 class="subheading">Local Symbol Names</h4>
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<a name="index-local-symbol-names"></a>
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<a name="index-symbol-names_002c-local"></a>
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<p>A local symbol is any symbol beginning with certain local label prefixes.
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By default, the local label prefix is ‘<samp>.L</samp>’ for ELF systems or
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‘<samp>L</samp>’ for traditional a.out systems, but each target may have its own
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set of local label prefixes.
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On the HPPA local symbols begin with ‘<samp>L$</samp>’.
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</p>
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<p>Local symbols are defined and used within the assembler, but they are
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normally not saved in object files. Thus, they are not visible when debugging.
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You may use the ‘<samp>-L</samp>’ option (see <a href="L.html#L">Include Local Symbols</a>)
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to retain the local symbols in the object files.
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</p>
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<a name="Local-Labels-1"></a>
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<h4 class="subheading">Local Labels</h4>
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<a name="index-local-labels"></a>
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<a name="index-temporary-symbol-names"></a>
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<a name="index-symbol-names_002c-temporary"></a>
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<p>Local labels are different from local symbols. Local labels help compilers and
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programmers use names temporarily. They create symbols which are guaranteed to
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be unique over the entire scope of the input source code and which can be
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referred to by a simple notation. To define a local label, write a label of
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the form ‘<samp><b>N</b>:</samp>’ (where <b>N</b> represents any non-negative integer).
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To refer to the most recent previous definition of that label write
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‘<samp><b>N</b>b</samp>’, using the same number as when you defined the label. To refer
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to the next definition of a local label, write ‘<samp><b>N</b>f</samp>’. The ‘<samp>b</samp>’
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stands for “backwards” and the ‘<samp>f</samp>’ stands for “forwards”.
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</p>
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<p>There is no restriction on how you can use these labels, and you can reuse them
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too. So that it is possible to repeatedly define the same local label (using
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the same number ‘<samp><b>N</b></samp>’), although you can only refer to the most recently
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defined local label of that number (for a backwards reference) or the next
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definition of a specific local label for a forward reference. It is also worth
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noting that the first 10 local labels (‘<samp><b>0:</b></samp>’…‘<samp><b>9:</b></samp>’) are
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implemented in a slightly more efficient manner than the others.
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</p>
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<p>Here is an example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">1: branch 1f
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2: branch 1b
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1: branch 2f
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2: branch 1b
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</pre></div>
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<p>Which is the equivalent of:
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</p>
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<div class="smallexample">
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<pre class="smallexample">label_1: branch label_3
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label_2: branch label_1
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label_3: branch label_4
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label_4: branch label_3
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</pre></div>
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<p>Local label names are only a notational device. They are immediately
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transformed into more conventional symbol names before the assembler uses them.
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The symbol names are stored in the symbol table, appear in error messages, and
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are optionally emitted to the object file. The names are constructed using
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these parts:
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</p>
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<dl compact="compact">
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<dt><code><em>local label prefix</em></code></dt>
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<dd><p>All local symbols begin with the system-specific local label prefix.
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Normally both <code>as</code> and <code>ld</code> forget symbols
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that start with the local label prefix. These labels are
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used for symbols you are never intended to see. If you use the
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‘<samp>-L</samp>’ option then <code>as</code> retains these symbols in the
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object file. If you also instruct <code>ld</code> to retain these symbols,
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you may use them in debugging.
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</p>
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</dd>
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<dt><code><var>number</var></code></dt>
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<dd><p>This is the number that was used in the local label definition. So if the
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label is written ‘<samp>55:</samp>’ then the number is ‘<samp>55</samp>’.
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</p>
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</dd>
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<dt><code><kbd>C-B</kbd></code></dt>
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<dd><p>This unusual character is included so you do not accidentally invent a symbol
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of the same name. The character has ASCII value of ‘<samp>\002</samp>’ (control-B).
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</p>
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</dd>
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<dt><code><em>ordinal number</em></code></dt>
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<dd><p>This is a serial number to keep the labels distinct. The first definition of
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‘<samp>0:</samp>’ gets the number ‘<samp>1</samp>’. The 15th definition of ‘<samp>0:</samp>’ gets the
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number ‘<samp>15</samp>’, and so on. Likewise the first definition of ‘<samp>1:</samp>’ gets
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the number ‘<samp>1</samp>’ and its 15th definition gets ‘<samp>15</samp>’ as well.
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</p></dd>
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</dl>
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<p>So for example, the first <code>1:</code> may be named <code>.L1<kbd>C-B</kbd>1</code>, and
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the 44th <code>3:</code> may be named <code>.L3<kbd>C-B</kbd>44</code>.
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</p>
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<a name="Dollar-Local-Labels"></a>
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<h4 class="subheading">Dollar Local Labels</h4>
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<a name="index-dollar-local-symbols"></a>
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<p>On some targets <code>as</code> also supports an even more local form of
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local labels called dollar labels. These labels go out of scope (i.e., they
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become undefined) as soon as a non-local label is defined. Thus they remain
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valid for only a small region of the input source code. Normal local labels,
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by contrast, remain in scope for the entire file, or until they are redefined
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by another occurrence of the same local label.
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</p>
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<p>Dollar labels are defined in exactly the same way as ordinary local labels,
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except that they have a dollar sign suffix to their numeric value, e.g.,
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‘<samp><b>55$:</b></samp>’.
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</p>
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<p>They can also be distinguished from ordinary local labels by their transformed
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names which use ASCII character ‘<samp>\001</samp>’ (control-A) as the magic character
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to distinguish them from ordinary labels. For example, the fifth definition of
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‘<samp>6$</samp>’ may be named ‘<samp>.L6<kbd>C-A</kbd>5</samp>’.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Dot.html#Dot" accesskey="n" rel="next">Dot</a>, Previous: <a href="Setting-Symbols.html#Setting-Symbols" accesskey="p" rel="prev">Setting Symbols</a>, Up: <a href="Symbols.html#Symbols" accesskey="u" rel="up">Symbols</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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