[SOLVED] How does motherboard knows I re-inserted CPU?

[bratan]

I'm super curious how did my motherboard's BIOS knew I temporary took out CPU while it was completely powered off (PSU was turned off with a rocker switch)?
Some details: I have AMD Ryzen 9 3900X and TUF Gaming X570-PLUS Mobo. I turned off switch at PSU, removed CPU and temporary installed it into another motherboard that needed to have BIOS updated (I had trouble with it not recognizing AMD 5000 series CPU). After I finished with it, I put CPU back, and powered up my system. BIOS told me "New CPU has been detected"... How did it know? I know motherboard has 3V backup battery, so maybe it powers some kind of logic circutry other than clock. Or actual AMD CPU has non-volatile flash and it records which motherboard it's inserted into?

 

[drea.drechsler]

.... Or actual AMD CPU has non-volatile flash and it records which motherboard it's inserted into?

I've wondered the same thing...have suspected it's related to one of two things, or both.

One is memory training, where the CPU's IMC 'learns' how to initialize the memory on the board. It only goes through training at an initial power-on, that's when it might reboot three times or more. But every one thereafter goes much faster so something's stored to facilitate it. I've never been certain it's in CMOS, especially since CPU changes exhibit the behaviour you've noted.

The other possibility is the CPU's fTPM, the security device provided by the CPU that can be used to hold credentials and keys for logging into various services such as Bitlocker secured drives. Exactly how that works I don't know but it sort of makes sense to zero it out upon detecting it's been removed to a new motherboard for security reasons.

 

[bratan]

The other possibility is the CPU's fTPM, the security device provided by the CPU that can be used to hold credentials and keys for logging into various services such as Bitlocker secured drives. Exactly how that works I don't know but it sort of makes sense to zero it out upon detecting it's been removed to a new motherboard for security reasons.

Yeah that sounds more plausible, thank you for the insight! I didn't know about fTPM... That has to be it