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It's more to do with the underlying electronics: digital is an abstraction, everything is analog underneath, and logic gates and registers are actually just ver

It's more to do with the underlying electronics: digital is an abstraction, everything is analog underneath, and logic gates and registers are actually just very non-linear analog circuits. What this means is that it's always in principle possible for an asynchronous edge (i.e. one that is not aligned to the other edges in the system) to be arbitrarily non-determinate: never actually being decided as a 1 or a 0 as it propagates through the digital circuit. This isn't an entirely theoretical issue, either: it can and will cause very strange behaviour in a digital system because a marginal or metastable value can be interpreted differently by different parts of the circuit that should be seeing the same value. It's possible in practice to push the chance of this happening extremely low, but it comes at the cost of latency: you need to spend more time to allow the circuit to actually settle into one state or the other. Within a fully synchronous system, it's possible to avoid this, though. But most multi-processor systems are not synchronous.

londons_explore 2y ago
> But most multi-processor systems are not synchronous. Is this actually true? A modern AMD or Intel CPU doesn't have one quartz crystal per core, so therefore all per-core clocks have to be derived from one clock source, meaning the relationship between all the clock frequencies and phases is deterministic, even if not specced in the datasheet.
- YZF 2y ago
  There would be a single clock typically but OP is right in the sense that as the signal propagates through the system you can get nondeterministic effects: https://en.wikipedia.org/wiki/Metastability_(electronics) https://en.wikipedia.org/wiki/Metastability_(electronics) In practice the probability of that is just driven low enough that it's not an issue.
- rcxdude 2y ago
  Multi-core isn't the same as multi-processor, and in practice while they are derived from the same reference, dynamic per-core clock scaling makes it pretty asynchronous, to say nothing of the I/O paths between the many independently clocked parts of a modern computer.

londons_explore 2y ago

> But most multi-processor systems are not synchronous. Is this actually true? A modern AMD or Intel CPU doesn't have one quartz crystal per core, so therefore all per-core clocks have to be derived from one clock source, meaning the relationship between all the clock frequencies and phases is deterministic, even if not specced in the datasheet.

YZF 2y ago
There would be a single clock typically but OP is right in the sense that as the signal propagates through the system you can get nondeterministic effects: https://en.wikipedia.org/wiki/Metastability_(electronics) https://en.wikipedia.org/wiki/Metastability_(electronics) In practice the probability of that is just driven low enough that it's not an issue.
rcxdude 2y ago
Multi-core isn't the same as multi-processor, and in practice while they are derived from the same reference, dynamic per-core clock scaling makes it pretty asynchronous, to say nothing of the I/O paths between the many independently clocked parts of a modern computer.