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Ask HN: How do CPUs handle bad transistors?

I've read that current CPUs have more than a hundred million transistors per square millimeter. I can't imagine that every single one of those works perfectly and will stay working perfectly for the entire lifetime of the CPU.

How do we design CPUs that don't die or stop working properly when one out of a hundred million transistors fails?

6 hours ago

Created a post 4 points @brokenmachine

Ask HN: How do CPUs handle bad transistors?

@HarryHirsch 5 hours

Replying to @brokenmachine 🎙

The keyword is "mercurial core", isn't it?

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@retrac 5 hours

Modern designs sometimes have some duplication of functional units that allow the final chip to be configured in a workable manner even if a few of the units don't turn out right. But otherwise, getting all of those transistors working right is the big challenge of chip design. Yes, they all have to work. Yields on new processes are often very low, only a fraction of the devices made actually working.

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@phendrenad2 6 hours

Transistors are actually very reliable, especially on a silicon wafer where they're protected from the elements. Unless there's a voltage spike somehow, they're unlikely to go bad.

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@wmf 4 hours

Cache has extra capacity so bad parts can be mapped out during the testing process. If there's a fault in a core the entire core is disabled. A fault in the uncore will probably cause the entire chip to be scrapped.

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@mardiyah 6 hours

That's why called IC, integrated circuit

either not one at all or entirely outright break down

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