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<head>
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<title>What This Module Does</title>
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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 works in two stages: first it generates abstract
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Inter code, then it further generates "final 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 universal: 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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eventual build product needed, and that only the second stage would vary
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according to this.
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</p>
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<p class="commentary">Which is very nearly true, but not quite. Here's 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) kits of Inter code normally used in compilation make certain other
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architectural assumptions based on the integer size (for example, the
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assembly-language syntax and semantics are different in these cases);
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</li><li>(c) it also generates different code with debugging enabled than without.
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</li></ul>
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<p class="commentary">Reason (c) could be avoided, at some cost in complexity, but reasons (a) and (b)
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are 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#SP2" class="internal">target_vm</a> object, on the other hand, expresses the choices made at
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the second stage too. The term "VM" is traditional here, and stands for
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"virtual machine", because until 2021, Inform could only generate code which
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would ultimately run on virtual machines called Glulx and the Z-machine. But it
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can now, for example, also generate C.
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</p>
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<p class="commentary">As a result, "VM" now has a more general meaning, and really means "form of
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final code generation". The Glulx format used to be specified by supplying the
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command-line option <span class="extract"><span class="extract-syntax">-format=ulx</span></span> to <a href="../inform7/index.html" class="internal">inform7</a> or <a href="../inter/index.html" class="internal">inter</a>: that still works,
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though it is deprecated, and <span class="extract"><span class="extract-syntax">-format=Inform6/32d</span></span> is better. But equally
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possible now would be <span class="extract"><span class="extract-syntax">-format=C/32d</span></span>. Here the target is a native executable
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to be compiled with a C compiler.
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</p>
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<p class="commentary">As these new-style <span class="extract"><span class="extract-syntax">-format</span></span> options suggest, the compilation process thus
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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">depends</span><span class="plain-syntax"> </span><span class="element-syntax">on</span><span class="plain-syntax"> </span><span class="element-syntax">architecture</span><span class="plain-syntax">: </span><span class="element-syntax">depends</span><span class="plain-syntax"> </span><span class="element-syntax">on</span><span class="plain-syntax"> </span><span class="element-syntax">target</span><span class="plain-syntax">:</span>
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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">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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<span class="plain-syntax"> ----------------> </span><span class="element-syntax">Executable</span>
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<span class="plain-syntax"> </span><span class="element-syntax">via</span><span class="plain-syntax"> </span><span class="function-syntax">CLANG</span><span class="plain-syntax">/</span><span class="function-syntax">GCC</span>
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<span class="plain-syntax"> ...</span>
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</pre>
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<p class="commentary">Note that a single <a href="2-tvm.html#SP2" class="internal">target_vm</a> object can be used with just one architecture:
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use the function <a href="2-tvm.html#SP11" class="internal">TargetVMs::get_architecture</a> to obtain this. If a target supports
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multiple architectures, then there will be multiple <a href="2-tvm.html#SP2" class="internal">target_vm</a> objects for it,
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one for each architecture it supports. For example, the Glulx VM can be reached
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by <span class="extract"><span class="extract-syntax">Inform6/32</span></span> or <span class="extract"><span class="extract-syntax">Inform6/32d</span></span>. There can also be multiple versions: for example,
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<span class="extract"><span class="extract-syntax">Inform6/16/v8</span></span> is a valid target. The function <a href="2-tvm.html#SP6" class="internal">TargetVMs::find</a> finds the
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<a href="2-tvm.html#SP2" class="internal">target_vm</a> object associated with a given textual form like <span class="extract"><span class="extract-syntax">"C/32d"</span></span>, if
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the toolchain supports this.
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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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