[LargoLasskhyfv]

On X86/AMD64 there is also ACPI and SMM (System Management Mode), which overlap in parts. ACPI calls into code provided by whichever bootloader, running post-boot above to operating system, or behind its back, so to speak. It's complicated.

[1] https://en.wikipedia.org/wiki/System_Management_Mode

[2] https://en.wikipedia.org/wiki/Advanced_Configuration_and_Pow...

Mostly for dynamic powermanagement and voltage regulation, monitoring temperatures, hotplugging, etc. That's just the way it is (now).

I remember when this first made the rounds over a decade ago with some mainboards from CENSORED Apple supplier which were pretty usable, except when running under something else than Microsoft Windows. IIRC it wasn't malice, only implemented in a way which made this hard to interface with from Linux, or the likes. Of course undocumented. Later, but about the same timeframe the first so called open source/security 'zealots' damning ACPI in general for those reasons.

It may be called differently on other platforms, but similar/equivalent mechanisms exist on almost everthing which is not in a museeum or landfill.