As others here, I strongly recommend reading the IA manuals on this subject, as well as the equivalent AMD doco. Most of the processor part (but not the firmware part) of this subject is in the manufacturer doco.

And yes, one has to be careful about outdated information.

* https://superuser.com/a/347115/38062

* https://superuser.com/a/695716/38062

* https://superuser.com/a/345333/38062

* https://unix.stackexchange.com/a/461774/5132