225 lines
9.5 KiB
HTML
225 lines
9.5 KiB
HTML
<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>5. Introduction to Symbols</title>
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<link rel="stylesheet" type="text/css" href="Frontpage.css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="home" href="sleigh.html" title="SLEIGH">
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<link rel="up" href="sleigh.html" title="SLEIGH">
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<link rel="prev" href="sleigh_definitions.html" title="4. Basic Definitions">
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<link rel="next" href="sleigh_tokens.html" title="6. Tokens and Fields">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr><th colspan="3" align="center">5. Introduction to Symbols</th></tr>
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<tr>
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<td width="20%" align="left">
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<a accesskey="p" href="sleigh_definitions.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="sleigh_tokens.html">Next</a>
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</td>
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</tr>
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</table>
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<hr>
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</div>
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<div class="sect1">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="sleigh_symbols"></a>5. Introduction to Symbols</h2></div></div></div>
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<p>
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After the definition section, we are prepared to start writing the
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body of the specification. This part of the specification shows how
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the bits in an instruction break down into opcodes, operands,
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immediate values, and the other pieces of an instruction. Then once
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this is figured out, the specification must also describe exactly how
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the processor would manipulate the data and operands if this
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particular instruction were executed. All of SLEIGH revolves around
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these two major tasks of disassembling and following semantics. It
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should come as no surprise then that the primary symbols defined and
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manipulated in the specification all have two key properties.
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</p>
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<div class="informalexample"><div class="orderedlist"><ol class="orderedlist compact" type="1">
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<li class="listitem">
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How does the symbol get displayed as part of the disassembly?
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</li>
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<li class="listitem">
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What semantic variable is associated with the symbol, and how is it constructed?
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</li>
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</ol></div></div>
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<p>
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Formally a <span class="emphasis"><em>Specific Symbol</em></span> is defined as an identifier associated with
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</p>
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<div class="informalexample"><div class="orderedlist"><ol class="orderedlist compact" type="1">
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<li class="listitem">
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A string displayed in disassembly.
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</li>
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<li class="listitem">
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varnode used in semantic actions, and any p-code used to construct that varnode.
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</li>
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</ol></div></div>
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<p>
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The named registers that we defined earlier are the simplest examples
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of specific symbols (see
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<a class="xref" href="sleigh_definitions.html#sleigh_naming_registers" title="4.4. Naming Registers">Section 4.4, “Naming Registers”</a>). The symbol identifier
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itself is the string that will get printed in disassembly and the
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varnode associated with the symbol is the one constructed by the
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define statement.
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</p>
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<p>
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The other crucial part of the specification is how to map from the
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bits of a particular instruction to the specific symbols that
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apply. To this end we have the <span class="emphasis"><em>Family Symbol</em></span>,
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which is defined as an identifier associated with a map from machine
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instructions to specific symbols.
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</p>
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<div class="informalexample">
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<span class="bold"><strong>Family Symbol:</strong></span> Instruction Encodings => Specific Symbols
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</div>
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<p>
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The set of instruction encodings that map to a single specific symbol
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is called an <span class="emphasis"><em>instruction pattern</em></span> and is described
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more fully in <a class="xref" href="sleigh_constructors.html#sleigh_bit_pattern" title="7.4. The Bit Pattern Section">Section 7.4, “The Bit Pattern Section”</a>. In most cases, this
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can be thought of as a mask on the bits of the instruction and a value
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that the remaining unmasked bits must match. At any rate, the family
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symbol identifier, when taken out of context, represents the entire
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collection of specific symbols involved in this map. But in the
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context of a specific instruction, the identifier represents the one
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specific symbol associated with the encoding of that instruction by
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the family symbol map.
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</p>
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<p>
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Given these maps, the idea of the specification is to build up more
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and more complicated family symbols until we have a single root
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symbol. This gives us a single map from the bits of an instruction to
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the full disassembly of it and to the sequence of p-code instructions
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that simulate the instruction.
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</p>
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<p>
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The symbol responsible for combining smaller family symbols is called
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a <span class="emphasis"><em>table</em></span>, which is fully described in
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<a class="xref" href="sleigh_constructors.html#sleigh_tables" title="7.8. Tables">Section 7.8, “Tables”</a>. Any <span class="emphasis"><em>table</em></span> symbol
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can be used in the definition of other <span class="emphasis"><em>table</em></span>
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symbols until the root symbol is fully described. The root symbol has
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the predefined identifier <span class="emphasis"><em>instruction</em></span>.
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</p>
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<div class="sect2">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="idm140526920845152"></a>5.1. Notes on Namespaces</h3></div></div></div>
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<p>
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Almost all identifiers live in the same global "scope". The global scope includes
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</p>
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<div class="informalexample"><div class="itemizedlist"><ul class="itemizedlist compact" style="list-style-type: bullet; ">
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<li class="listitem" style="list-style-type: disc">
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Names of address spaces
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</li>
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<li class="listitem" style="list-style-type: disc">
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Names of tokens
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</li>
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<li class="listitem" style="list-style-type: disc">
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Names of fields
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</li>
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<li class="listitem" style="list-style-type: disc">
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Names of user-defined p-code ops
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</li>
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<li class="listitem" style="list-style-type: disc">
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Names of registers
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</li>
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<li class="listitem" style="list-style-type: disc">
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Names of macros (see <a class="xref" href="sleigh_constructors.html#sleigh_macros" title="7.9. P-code Macros">Section 7.9, “P-code Macros”</a>)
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</li>
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<li class="listitem" style="list-style-type: disc">
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Names of tables (see <a class="xref" href="sleigh_constructors.html#sleigh_tables" title="7.8. Tables">Section 7.8, “Tables”</a>)
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</li>
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</ul></div></div>
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<p>
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All of the names in this scope must be unique. Each
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individual <span class="emphasis"><em>constructor</em></span> (defined in <a class="xref" href="sleigh_constructors.html" title="7. Constructors">Section 7, “Constructors”</a>)
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defines a local scope for operand names. As with most languages, a
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local symbol with the same name as a global
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symbol <span class="emphasis"><em>hides</em></span> the global symbol while that scope
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is in effect.
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</p>
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</div>
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<div class="sect2">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="sleigh_predefined_symbols"></a>5.2. Predefined Symbols</h3></div></div></div>
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<p>
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We list all of the symbols that are predefined by SLEIGH.
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</p>
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<div class="informalexample">
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<div class="table">
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<a name="predefine.htmltable"></a><p class="title"><b>Table 2. Predefined Symbols</b></p>
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<div class="table-contents"><table width="80%" frame="box" rules="all">
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<col width="30%">
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<col width="70%">
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<thead><tr>
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<td><span class="bold"><strong>Identifier</strong></span></td>
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<td><span class="bold"><strong>Meaning</strong></span></td>
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</tr></thead>
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<tbody>
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<tr>
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<td><code class="code">instruction</code></td>
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<td>The root instruction table.</td>
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</tr>
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<tr>
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<td><code class="code">const</code></td>
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<td>Special address space for building constant varnodes.</td>
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</tr>
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<tr>
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<td><code class="code">unique</code></td>
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<td>Address space for allocating temporary registers.</td>
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</tr>
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<tr>
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<td><code class="code">inst_start</code></td>
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<td>Offset of the address of the current instruction.</td>
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</tr>
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<tr>
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<td><code class="code">inst_next</code></td>
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<td>Offset of the address of the next instruction.</td>
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</tr>
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<tr>
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<td><code class="code">epsilon</code></td>
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<td>A special identifier indicating an empty bit pattern.</td>
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</tr>
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</tbody>
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</table></div>
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</div>
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<br class="table-break">
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</div>
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<p>
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The most important of these to be aware of
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are <span class="emphasis"><em>inst_start</em></span>
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and <span class="emphasis"><em>inst_next</em></span>. These are family symbols which map
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in the context of particular instruction to the integer offset of
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either the address of the instruction or the address of the next
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instruction respectively. These are used in any relative branching
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situation. The other symbols are rarely
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used. The <span class="emphasis"><em>const</em></span> and <span class="emphasis"><em>unique</em></span>
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identifiers are address spaces. The <span class="emphasis"><em>epsilon</em></span>
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identifier is inherited from SLED and is a specific symbol equivalent
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to the constant zero. The <span class="emphasis"><em>instruction</em></span> identifier
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is the root instruction table.
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</p>
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</div>
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</div>
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<div class="navfooter">
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<hr>
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left">
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<a accesskey="p" href="sleigh_definitions.html">Prev</a> </td>
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<td width="20%" align="center"> </td>
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<td width="40%" align="right"> <a accesskey="n" href="sleigh_tokens.html">Next</a>
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</td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">4. Basic Definitions </td>
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<td width="20%" align="center"><a accesskey="h" href="sleigh.html">Home</a></td>
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<td width="40%" align="right" valign="top"> 6. Tokens and Fields</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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