[vbezhenar]

CPU is pre-configured by OS as well. So IMO it's the same as memory. May be it would be more appropriate to say that userspace program accesses directly some parts of CPU and RAM.

With modern computers, AFAIK even OS does not have absolutely full control over CPU.

[sophacles]

But they don't directly access memory. They access an address that may be translatable by the mmu to a physical location in ram. They may also write to an address that the kernel hasn't allocated a page for yet, but that the kernel has agreed to map into the process' memory. In this case the kernel handles the trap and maps a page of actual ram (etc) and then the process continues forward progress.

[vbezhenar]

That's just the way CPU works. It has nothing to do with kernel or userspace. Kernel code will behave identically.

[justsomehnguy]

> With modern computers, AFAIK even OS does not have absolutely full control over CPU.

It's even more amusing with Type 2 hypervisors.