Some useful models of computation are deterministic. But modern computer systems are rarely deterministic as a whole (even if some of their components are). Fo

Some useful models of computation are deterministic. But modern computer systems are rarely deterministic as a whole (even if some of their components are). For example, CPUs can change their clock rate in response to temperature. This can make it impossible to trace exact behavior, especially if there's any timing dependent code or complex multi-threading going on.