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I have an Asus ROG series motherboard (it's a Maximus IV Extreme) and there's a toggle switch on the board called the "LN2 Mode Switch". I gather this is used to try and remedy cold boot bugs when using liquid nitrogen to cool the CPU. By default the board ships with this turned on.

The manual states: "With LN2 mode activated the ROG motherboard is optimised to remedy the cold boot bug during POST at an extremely low temperature and help the system boot successfully."

So, my query: What actually gets changed by this switch? Power distibution? Bus timings? I assume that since it's on by default it won't hurt to leave it that way but I'd like to know if there's anything to be gained by disabling it because my cooling setup is far more pedestrian than a bucket of LN2.

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These mainboards have some self-timed circuitry (also known as asynchronous circuitry). Circuitry speed, for both synchronous and asynchronous circuitry, is sensitive to supply voltage level, fabrication process, and temperature level. Normally, for mainboards whose operating temperature range is no wider than room temperature, this isn't a huge issue that requires extraordinary measures. But when the temperature variations involved are as large as 77K–320K or more, this temperature sensitivity becomes a significant factor.

Self-timed circuitry can be adapted to cope with such temperature variations. (Synchronous circuitry is constrained by the PVT (process-voltage-temperature) assumptions that went into calculating its clock speed.) What the LN2 mode does is modify the operation of the self-timed circuitry to try to cope with its behaviour at such low temperatures.

Note that, as the ASUS employee points out, this doesn't fix the so-called "cold-boot bug". That is a problem that encompasses circuitry within the microprocessor chip as well, which a mainboard's LN2 mode cannot modify, for obvious reasons. LN2 mode ameliorates the problem.

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