Did you notice how the preceding diagrams (Figure 7.9) and, in and Figure 7.12, at the extreme left edge (albeit very small!), a single page at the very beginning of the user space, named the null trap page? What is it? That's easy: virtual page 0 is given no permissions (at the hardware MMU/PTE level). Thus, any access to this page, be it r, w, or x (read/write/execute), will result in the MMU raising what is called a fault or exception. This will have the processor jump to an OS handler routine (the fault handler). It runs, killing the culprit trying to access a memory region with no permissions!
It's very interesting indeed: the OS handler mentioned previously runs in process context, and guess what current is: why, it's the process (or thread) that initiated this bad NULL pointer lookup! Within the fault handler code, the SIGSEGV signal is delivered to the faulting process (current), causing it to...