and <i>w'</i> happens before <i>r</i>.
That is, each read must observe a value written by a preceding or concurrent write.
</p>

<p>
Additionally, observation of acausal and “out of thin air” writes is disallowed.
</p>

<p>
Reads of memory locations larger than a single machine word
are encouraged but not required to meet the same semantics
as word-sized memory locations,