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158 lines
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This file documents the GNU Assembler "as".
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<p>
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<a name="Symbol-Names"></a>Next: <a rel="next" accesskey="n" href="Dot.html#Dot">Dot</a>,
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Previous: <a rel="previous" accesskey="p" href="Setting-Symbols.html#Setting-Symbols">Setting Symbols</a>,
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Up: <a rel="up" accesskey="u" href="Symbols.html#Symbols">Symbols</a>
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<h3 class="section">5.3 Symbol Names</h3>
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<p><a name="index-symbol-names-213"></a><a name="index-names_002c-symbol-214"></a>Symbol names begin with a letter or with one of <span class="samp">._</span>. On most
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machines, you can also use <code>$</code> in symbol names; exceptions are
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noted in <a href="Machine-Dependencies.html#Machine-Dependencies">Machine Dependencies</a>. That character may be followed by any
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string of digits, letters, dollar signs (unless otherwise noted for a
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particular target machine), and underscores.
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<p>Case of letters is significant: <code>foo</code> is a different symbol name
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than <code>Foo</code>.
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<p>Each symbol has exactly one name. Each name in an assembly language program
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refers to exactly one symbol. You may use that symbol name any number of times
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in a program.
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<h4 class="subheading">Local Symbol Names</h4>
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<p><a name="index-local-symbol-names-215"></a><a name="index-symbol-names_002c-local-216"></a>A local symbol is any symbol beginning with certain local label prefixes.
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By default, the local label prefix is <span class="samp">.L</span> for ELF systems or
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<span class="samp">L</span> for traditional a.out systems, but each target may have its own
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set of local label prefixes.
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On the HPPA local symbols begin with <span class="samp">L$</span>.
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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 <span class="samp">-L</span> option (see <a href="L.html#L">Include Local Symbols: <span class="option">-L</span></a>) to retain the local symbols in the object files.
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<h4 class="subheading">Local Labels</h4>
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<p><a name="index-local-labels-217"></a><a name="index-temporary-symbol-names-218"></a><a name="index-symbol-names_002c-temporary-219"></a>Local labels help compilers and programmers use names temporarily.
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They create symbols which are guaranteed to be unique over the entire scope of
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the input source code and which can be referred to by a simple notation.
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To define a local label, write a label of the form <b>N</b><span class="samp">:</span> (where <b>N</b>
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represents any positive integer). To refer to the most recent previous
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definition of that label write <b>N</b><span class="samp">b</span>, using the same number as when
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you defined the label. To refer to the next definition of a local label, write
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<b>N</b><span class="samp">f</span>—the <span class="samp">b</span> stands for “backwards” and the <span class="samp">f</span> stands
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for “forwards”.
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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 <b>N</b>), 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 (<b>0:</b><small class="dots">...</small><b>9:</b>) are
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implemented in a slightly more efficient manner than the others.
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<p>Here is an example:
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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>
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<p>Which is the equivalent of:
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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>
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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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<dl>
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<dt><em>local label prefix</em><dd>All local symbols begin with the system-specific local label prefix.
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Normally both <span class="command">as</span> 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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<span class="samp">-L</span> option then <span class="command">as</span> 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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<br><dt><var>number</var><dd>This is the number that was used in the local label definition. So if the
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label is written <span class="samp">55:</span> then the number is <span class="samp">55</span>.
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<br><dt><kbd>C-B</kbd><dd>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 <span class="samp">\002</span> (control-B).
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<br><dt><em>ordinal number</em><dd>This is a serial number to keep the labels distinct. The first definition of
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<span class="samp">0:</span> gets the number <span class="samp">1</span>. The 15th definition of <span class="samp">0:</span> gets the
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number <span class="samp">15</span>, and so on. Likewise the first definition of <span class="samp">1:</span> gets
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the number <span class="samp">1</span> and its 15th definition gets <span class="samp">15</span> as well.
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</dl>
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<p>So for example, the first <code>1:</code> may be named <code>.L1</code><kbd>C-B</kbd><code>1</code>, and
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the 44th <code>3:</code> may be named <code>.L3</code><kbd>C-B</kbd><code>44</code>.
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<h4 class="subheading">Dollar Local Labels</h4>
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<p><a name="index-dollar-local-symbols-220"></a>
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<code>as</code> also supports an even more local form of local labels called
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dollar labels. These labels go out of scope (i.e., they become undefined) as
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soon as a non-local label is defined. Thus they remain valid for only a small
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region of the input source code. Normal local labels, by contrast, remain in
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scope for the entire file, or until they are redefined by another occurrence of
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the same local label.
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<p>Dollar labels are defined in exactly the same way as ordinary local labels,
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except that instead of being terminated by a colon, they are terminated by a
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dollar sign, e.g., <b>55$</b>.
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<p>They can also be distinguished from ordinary local labels by their transformed
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names which use ASCII character <span class="samp">\001</span> (control-A) as the magic character
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to distinguish them from ordinary labels. For example, the fifth definition of
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<span class="samp">6$</span> may be named <span class="samp">.L6</span><kbd>C-A</kbd><span class="samp">5</span>.
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