introduction to virtual memory layout, chaotic style:

On so-called 64-bit platforms, unlike on 32-bit platforms, by convention the kernel is placed in virtual memory directly before userspace memory. Negative addresses refer to kernel memory, positive addresses refer to userspace memory. For example, on x86-64 Linux without KASLR, the kernel text mapping is at -0x80000000. When hardware capabilities expand to accomodate larger virtual addresses, the address space grows in both directions.