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<h1 id="armv8"><a class="header" href="#armv8">armv8</a></h1>
<p>keywords: aarch64, arm64, A64, aarch32, A32, T32, abi</p>
<p>The <code>armv8</code> architecture introduces support for 64-bit and defines two
<em>execution states</em> <code>aarch64</code> and <code>aarch32</code>.</p>
<p>Implementations are <strong>not</strong> required to implement all execution states for all
<em>exception levels (EL)</em>. For example the <a href="https://en.wikichip.org/wiki/arm_holdings/microarchitectures/cortex-a32">coretex-a32</a> only
implements <code>aarch32</code>, while the <a href="https://en.wikichip.org/wiki/arm_holdings/microarchitectures/cortex-a34">coretex-a34</a> only implements
<code>aarch64</code>.</p>
<p>The execution states support different instruction sets.</p>
<ul>
<li><code>aarch64</code> only supports the new <a href="https://developer.arm.com/documentation/ddi0602/latest"><code>A64</code></a> instruction set, where all
instructions have the fixed size of of 32 bits.</li>
<li><code>aarch32</code> supports the <a href="https://developer.arm.com/documentation/ddi0597/latest"><code>A32</code></a> and <a href="https://developer.arm.com/documentation/ddi0597/latest"><code>T32</code></a> instruction sets. These
are updated versions of the <code>armv7</code> instruction sets, kept backwards
compatible allowing <code>armv7</code> programs to run on <code>armv8</code>.
<blockquote>
<p>In <code>armv7</code> the instruction sets <code>A32</code> an <code>T32</code> were called <code>arm</code> and
<code>thumb</code> respectively.</p>
</blockquote>
</li>
</ul>
<p>A <em>program</em> always runs in either the <code>aarch64</code> or the <code>aarch32</code> execution
state, but never in a <em>mixture</em> of both. Transitions between execution states
only occur when raising or lowering the exception level.</p>
<ul>
<li><code>aarch64 -> aarch32</code> can only occur when switching from <em>higher EL</em> to <em>lower
EL</em>.</li>
<li><code>aarch32 -> aarch64</code> can only occur when switching from <em>lower EL</em> to <em>higher
EL</em>.</li>
</ul>
<p>The following figure depicts which execution state Transitions are allowed.</p>
<pre><code> (user) EL0 ^ |
(os) EL1 | 32->64
(hypervisor) EL2 64->32 |
(secure) EL3 | v
</code></pre>
<p>This means for example, an <em>os</em> running in <code>aarch32</code> can only support <code>aarch32</code>
user applications, while an <em>os</em> running in <code>aarch64</code> can support
<code>aarch32 / aarch64</code> user applications.</p>
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