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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Register transfer language</span></span>
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</style><div role="note" class="hatnote navigation-not-searchable">Not to be confused with <a href="Register-transfer_level" title="Register-transfer level">Register-transfer level</a>.</div>
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<p>In <a href="Computer_science" title="Computer science">computer science</a>, <b> register transfer language</b> (<b>RTL</b>) is a kind of <a href="Intermediate_representation" title="Intermediate representation">intermediate representation</a> (IR) that is very close to <a href="Assembly_language" title="Assembly language">assembly language</a>, such as that which is used in a <a href="Compiler" title="Compiler">compiler</a>. It is used to describe data flow at the <a href="Register-transfer_level" title="Register-transfer level">register-transfer level</a> of an <a href="Computer_architecture" title="Computer architecture">architecture</a>.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Academic papers and textbooks often use a form of RTL as an architecture-neutral assembly language. <b>RTL</b> is used as the name of a specific intermediate representation in several compilers, including the <a href="GNU_Compiler_Collection" title="GNU Compiler Collection">GNU Compiler Collection</a> (GCC), Zephyr, and the European compiler projects CerCo and <a href="CompCert" title="CompCert">CompCert</a>.
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<div class="mw-heading mw-heading2"><h2 id="History">History</h2></div>
<p>The idea behind RTL was first described in <i>The Design and Application of a Retargetable Peephole Optimizer</i>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading3"><h3 id="GCC">GCC</h3></div>
<p>In GCC, RTL is generated from the <a href="GIMPLE" class="mw-redirect" title="GIMPLE">GIMPLE</a> representation, transformed by various passes in the GCC <a href="Compiler_middle_end" class="mw-redirect" title="Compiler middle end">middle-end</a>, and then converted to assembly language.
</p><p>GCC's RTL is usually written in a form that looks like a <a href="Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a> <a href="S-expression" title="S-expression">S-expression</a>:
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<div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre><span class="p">(</span><span class="nf">set</span><span class="w"> </span><span class="p">(</span><span class="nf">reg:</span><span class="w"> </span><span class="nv">SI</span><span class="w"> </span><span class="mi">140</span><span class="p">)</span>
<span class="w"> </span><span class="p">(</span><span class="nf">plus:SI</span><span class="w"> </span><span class="p">(</span><span class="nf">reg:SI</span><span class="w"> </span><span class="mi">138</span><span class="p">)</span>
<span class="w"> </span><span class="p">(</span><span class="nf">reg:</span><span class="w"> </span><span class="nv">SI</span><span class="w"> </span><span class="mi">139</span><span class="p">)))</span>
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<p>This <a href="Side_effect_(computer_science)" title="Side effect (computer science)">side-effect expression</a> says "sum the contents of register 138 with the contents of register 139 and store the result in register 140". The SI specifies the access mode for each register. In the example, it is "SImode", i.e. "access the register as 32-bit integer".
</p><p>The sequence of RTL generated has some dependency on the characteristics of the processor for which GCC is generating code. However, the meaning of the RTL is more or less independent of the target: it would usually be possible to read and understand a piece of RTL without knowing what processor it was generated for. Similarly, the meaning of the RTL doesn't usually depend on the program's original high-level language.
</p><p>A register transfer language is a system for expressing in symbolic form the microoperation sequences among the registers of a digital module. It is a convenient tool for describing the internal organization of digital computers in a concise and precise manner. It can also be used to facilitate the design process of digital systems.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">
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</style><cite id="CITEREFMahani2013" class="citation journal cs1">Mahani, Negin (2013-01-16). <span class="id-lock-subscription" title="Paid subscription required"><a rel="nofollow" class="external text" href="https://dl.acm.org/doi/abs/10.1145/2429574.2429576">"Making alive register transfer level and transaction level modeling in Ada"</a></span>. <i>ACM SIGAda Ada Letters</i>. <b>32</b> (2): <span class="nowrap">9–</span>16. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F2429574.2429576">10.1145/2429574.2429576</a><span class="reference-accessdate">. Retrieved <span class="nowrap">September 8,</span> 2024</span>.</cite></span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.well.com/~cwf/pro/Davidson%20and%20Fraser.%20The%20design%20and%20application%20of%20a%20retargetable%20peephole%20optimizer.pdf">"Davidson and Fraser; <i>The Design and Application of a Retargetable Peephole Optimizer</i>; ToPLaS v2(2) 191-202 (April 1980)"</a> <span class="cs1-format">(PDF)</span>.</cite></span>
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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><cite id="CITEREFMano1992" class="citation book cs1">Mano, Morris M. (1992). <i>Computer System Architecture</i> (3rd ed.). Prentice Hall. p. 94. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0131755633</bdi>.</cite></span>
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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>
<ul><li><a rel="nofollow" class="external text" href="https://gcc.gnu.org/onlinedocs/gccint/RTL.html">Register Transfer Language of GCC</a></li></ul></div><!--htdig_noindex--><div><div class="zim-footer">
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