<code>decode</code> function that takes an instruction and
returns the unpacked representation.
</p>

<p>
First, <code>decode</code> fetches the mode from the instruction. Its least significant bit
is 24 and it contains at most four bits.
</p>

<div class="org-src-container">

<pre class="src src-c" id="fetch_modes">/* Fetch the space modes from the instruction. */
uint8_t modes = (instr &gt;&gt; 24) &amp; 0xF;
</pre>
</div>

<p>
As the modes are stored in a bit vector, it&rsquo;s useful to have masks for e