jaihsonk wrote:
What about RAM?
Can I take out the regular CPU from my desktop, plug in the Xeon Phi (or another PCI CPU) and have that running by itself?
Probably not. PCs are designed to bootstrap using the main processor.
Some platforms do something similar though. The RaspberryPi, for example, is bootstrapped by the GPU, which then starts the "main" CPU(s) to run your OS. But this isn't the mainstream way of doing things, certainly on anything like a PC or similar machine.
But then this depends on where you draw the line. Some Intel chipsets contain a complete embedded computer, running a private copy of Minix OS, to run the Management Engine, completely seperately to the main CPU. I'm not sure if this Minix gets control before your main CPU is booted and is involved in booting the CPU, in a manner similar to the RPi.
This video might answer that (I've not watched it yet.)
jaihsonk wrote:
What should an OS do when it comes across a PCI CPU?
Absent a specific example, it's hard to say, other than to ignore the PCI card if the OS doesn't have a driver for it.