1.9 Program execution [intro.execution]
1 The semantic descriptions in this International Standard define a parameterized nondeterministic abstract machine. This International Standard places no requirement on the structure of conforming implementations. In particular, they need not copy or emulate the structure of the abstract machine. Rather, conforming implementations are required to emulate (only) the observable behavior of the abstract machine as explained below.
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6 The observable behavior of the abstract machine is its sequence of reads and writes to volatile data and calls to library I/O functions.