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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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<head>
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<title>What This Module Does</title>
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<meta name="viewport" content="width=device-width initial-scale=1">
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</head>
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<body class="commentary-font">
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<nav role="navigation">
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<h1><a href="../index.html">
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<img src="../docs-assets/Inform.png" height=72">
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</a></h1>
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<ul><li><a href="../compiler.html">compiler tools</a></li>
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<li><a href="../other.html">other tools</a></li>
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<li><a href="../extensions.html">extensions and kits</a></li>
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<li><a href="../units.html">unit test tools</a></li>
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</ul><h2>Compiler Webs</h2><ul>
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<li><a href="../inbuild/index.html">inbuild</a></li>
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<li><a href="../inform7/index.html">inform7</a></li>
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<li><a href="../inter/index.html">inter</a></li>
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</ul><h2>Inbuild Modules</h2><ul>
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<li><a href="../supervisor-module/index.html">supervisor</a></li>
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</ul><h2>Inform7 Modules</h2><ul>
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<li><a href="../core-module/index.html">core</a></li>
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<li><a href="../assertions-module/index.html">assertions</a></li>
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<li><a href="../values-module/index.html">values</a></li>
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<li><a href="../knowledge-module/index.html">knowledge</a></li>
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<li><a href="../imperative-module/index.html">imperative</a></li>
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<li><a href="../runtime-module/index.html">runtime</a></li>
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<li><a href="../if-module/index.html">if</a></li>
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<li><a href="../multimedia-module/index.html">multimedia</a></li>
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<li><a href="../index-module/index.html">index</a></li>
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</ul><h2>Inter Modules</h2><ul>
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<li><a href="../bytecode-module/index.html">bytecode</a></li>
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<li><a href="../building-module/index.html">building</a></li>
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<li><a href="../pipeline-module/index.html">pipeline</a></li>
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<li><a href="../final-module/index.html">final</a></li>
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</ul><h2>Services</h2><ul>
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<li><a href="index.html"><span class="selectedlink">arch</span></a></li>
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<li><a href="../calculus-module/index.html">calculus</a></li>
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<li><a href="../html-module/index.html">html</a></li>
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<li><a href="../inflections-module/index.html">inflections</a></li>
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<li><a href="../kinds-module/index.html">kinds</a></li>
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<li><a href="../linguistics-module/index.html">linguistics</a></li>
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<li><a href="../problems-module/index.html">problems</a></li>
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<li><a href="../syntax-module/index.html">syntax</a></li>
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<li><a href="../words-module/index.html">words</a></li>
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<li><a href="../../../inweb/docs/foundation-module/index.html">foundation</a></li>
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</ul>
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</nav>
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<main role="main">
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<!--Weave of 'What This Module Does' generated by Inweb-->
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<div class="breadcrumbs">
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<ul class="crumbs"><li><a href="../index.html">Home</a></li><li><a href="../compiler.html">Services</a></li><li><a href="index.html">arch</a></li><li><a href="index.html#P">Preliminaries</a></li><li><b>What This Module Does</b></li></ul></div>
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<p class="purpose">An overview of the arch module's role and abilities.</p>
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<ul class="toc"><li><a href="P-wtmd.html#SP1">§1. Prerequisites</a></li><li><a href="P-wtmd.html#SP2">§2. Architecture versus VM</a></li><li><a href="P-wtmd.html#SP4">§4. Compatibility</a></li></ul><hr class="tocbar">
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<p class="commentary firstcommentary"><a id="SP1" class="paragraph-anchor"></a><b>§1. Prerequisites. </b>The arch module is a part of the Inform compiler toolset. It is
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presented as a literate program or "web". Before diving in:
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</p>
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<ul class="items"><li>(a) It helps to have some experience of reading webs: see <a href="../../../inweb/docs/index.html" class="internal">inweb</a> for more.
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</li><li>(b) The module is written in C, in fact ANSI C99, but this is disguised by the
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fact that it uses some extension syntaxes provided by the <a href="../../../inweb/docs/index.html" class="internal">inweb</a> literate
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programming tool, making it a dialect of C called InC. See <a href="../../../inweb/docs/index.html" class="internal">inweb</a> for
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full details, but essentially: it's C without predeclarations or header files,
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and where functions have names like <span class="extract"><span class="extract-syntax">Tags::add_by_name</span></span> rather than <span class="extract"><span class="extract-syntax">add_by_name</span></span>.
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</li><li>(c) This module uses other modules drawn from the <a href="../compiler.html" class="internal">compiler</a>, and also
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uses a module of utility functions called <a href="../../../inweb/docs/foundation-module/index.html" class="internal">foundation</a>.
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For more, see <a href="../../../inweb/docs/foundation-module/P-abgtf.html" class="internal">A Brief Guide to Foundation (in foundation)</a>.
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</li></ul>
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<p class="commentary firstcommentary"><a id="SP2" class="paragraph-anchor"></a><b>§2. Architecture versus VM. </b>The Inform 7 build process ultimately wants to make code for some target
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virtual machine — traditionally, the Z or Glulx machines. But it does this
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in two stages: first generating abstract Inter code, then further generating
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VM code from that.
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</p>
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<p class="commentary">It's an appealing notion that this first stage might be VM-independent: that
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is, that <a href="../inform7/index.html" class="internal">inform7</a> could generate the same Inter code regardless of the
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final VM, and that only the second stage would vary according to target.
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And this is nearly true, but not quite. There are (currently) two reasons
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why not:
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</p>
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<ul class="items"><li>(a) <a href="../inform7/index.html" class="internal">inform7</a> has to generate different code if integers are 16 rather
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than 32 bits wide, and
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</li><li>(b) it also generates different code with debugging enabled than without.
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</li></ul>
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<p class="commentary">Reason (b) could be avoided, at some cost in complexity, but reason (a) is
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something we cannot sensibly avoid without making Inter a much higher-level
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form of bytecode. Instead, we have "architectures" for Inter: for example,
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32-bit with debugging enabled is the <span class="extract"><span class="extract-syntax">32d</span></span> architecture. See <a href="2-arc.html" class="internal">Architectures</a>;
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if ever we introduce a 64-bit VM, that will need new architectures, and
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this is where they would go.
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</p>
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<p class="commentary firstcommentary"><a id="SP3" class="paragraph-anchor"></a><b>§3. </b>A <a href="2-tvm.html#SP1" class="internal">target_vm</a> object, on the other hand, represents an actual choice of
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virtual machine. For example, Glulx is a <a href="2-tvm.html#SP1" class="internal">target_vm</a>. The compilation
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process thus involves a combination of both architecture and target:
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</p>
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<pre class="displayed-code all-displayed-code code-font">
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<span class="plain-syntax"> </span><span class="element-syntax">Source</span><span class="plain-syntax"> </span><span class="element-syntax">text</span><span class="plain-syntax"> -----------> </span><span class="element-syntax">Inter</span><span class="plain-syntax"> </span><span class="element-syntax">code</span><span class="plain-syntax"> --------------> </span><span class="element-syntax">Bytecode</span><span class="plain-syntax"> </span><span class="element-syntax">for</span>
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<span class="plain-syntax"> </span><span class="function-syntax">INFORM7</span><span class="plain-syntax"> </span><span class="element-syntax">for</span><span class="plain-syntax"> </span><span class="element-syntax">architecture</span><span class="plain-syntax"> </span><span class="element-syntax">via</span><span class="plain-syntax"> </span><span class="function-syntax">INFORM6</span><span class="plain-syntax"> </span><span class="element-syntax">target</span><span class="plain-syntax"> </span><span class="element-syntax">virtual</span><span class="plain-syntax"> </span><span class="element-syntax">machine</span>
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</pre>
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<p class="commentary">Each VM can be used with just one architecture: use the function
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<a href="2-tvm.html#SP10" class="internal">TargetVMs::get_architecture</a> to obtain this. It might seem reasonable
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to say that Glulx ought to be viable with both <span class="extract"><span class="extract-syntax">32</span></span> and <span class="extract"><span class="extract-syntax">32d</span></span> architectures,
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but in fact "Glulx" is not a single virtual machine but a family of them.
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A specific member of this family would be the <a href="2-tvm.html#SP1" class="internal">target_vm</a> representing
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Glulx version 3.1.2 with debugging enabled, and that can be used with the
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<span class="extract"><span class="extract-syntax">32d</span></span> but not the <span class="extract"><span class="extract-syntax">32</span></span> architecture.
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</p>
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<p class="commentary">There can in principle be numerous VMs in any given family; see
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<a href="2-tvm.html#SP6" class="internal">TargetVMs::find_in_family</a> to obtain family members with given behaviour,
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and in general see <a href="2-tvm.html" class="internal">Target Virtual Machines</a> for more.
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</p>
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<p class="commentary firstcommentary"><a id="SP4" class="paragraph-anchor"></a><b>§4. Compatibility. </b>Not all software in the Inform stack — source text from the user, extensions,
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kits of Inter code — will be compatible with every architecture, or with
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every VM. We represent that by giving something a <a href="2-cmp.html#SP1" class="internal">compatibility_specification</a>
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object to say what it can work with: the function <a href="2-cmp.html#SP11" class="internal">Compatibility::test</a>
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determines whether any given VM is allowed with this specification.
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</p>
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<p class="commentary">A specification can be converted to or from text: see <a href="2-cmp.html#SP5" class="internal">Compatibility::write</a>
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and <a href="2-cmp.html#SP6" class="internal">Compatibility::from_text</a>. Typically, such text might read "for 32d only".
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</p>
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<p class="commentary">Lastly, <a href="2-cmp.html#SP2" class="internal">Compatibility::all</a> returns a specification meaning "works with
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anything". This should be the default; <a href="2-cmp.html#SP11" class="internal">Compatibility::test_universal</a> tests
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whether a specification is equivalent to this.
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</p>
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<ul class="progressbar"><li class="progressprevoff">❮</li><li class="progresscurrentchapter">P</li><li class="progresscurrent">wtmd</li><li class="progresssection"><a href="P-htitm.html">htitm</a></li><li class="progresschapter"><a href="1-am.html">1</a></li><li class="progresschapter"><a href="2-arc.html">2</a></li><li class="progressnext"><a href="P-htitm.html">❯</a></li></ul></div>
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