[marcosdumay]

The kernel can write almost anywhere. (Well, actually, nothing can write on most addresses in a 64 bits machine, but if it's usable for something, the kernel can use it directly.)

And yes, some addresses are special. (AFAIK, on all current mainstream architectures.) This is the expected way to set those signal handlers, output (and input) data, configure devices, etc.

That said, there are some gotchas on using specific addresses in C. AFAIK none apply to x86, but it's something you usually do in assembly.