[Out_of_Characte]

"each process has 4 gigabytes of virtual address space all to itself"

A process does indeed have all 4GB of VIRTUAL adress space to itself. unless I'm misunderstanding you.

[dezgeg]

Yes, VIRTUAL memory. Most operating systems (Windows, Linux) leave 2 or 3 GB for the user process and reserve the of the address space for themselves.

That way userspace-to-kernel switch does not require changing active page table (and also avoids switcharoo each time kernel needs to access userspace memory).