Intel have a magic communciation channel between the system firmware and the graphics hardware. It's based on a region of shared memory and judicious use of interrupts, and it's documented here.
Nvidia have a magic communication channel between the system firmware and the graphcis hardware. It's based on WMI and bonghits, and it's not documented.
Why, yes, I have spent half the day trying to work out how the NVIF method works.