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I've been reading a lot about how BIOS works and how to avoid BIOS related attacks. I understand things like TPM, but what doesn't make sense to me is how do you actually set the processor in a mode that allows you to write to the BIOS?

I've been reading through the Intel datasheet for my processor. Page 480 talks about BIOS security for registers and page 474 talks about descriptor vs non-descriptor mode. From what I can tell, when running normally the processor runs in descriptor mode which has all sorts of security access restrictions.

How does a BIOS flash utility legitimately upgrade the BIOS? I would think those security protections would stop it from working, but clearly there is a mode which allows carte-blanche write access.

Edit: Does it have something to do with section 22.5.3 on page 478?

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