First off, it is worth noting that on 64-bit systems, all 64 bits are not used for addressing. On a standard or typical Linux OS configuration for the x86_64 with a (typical) 4 KB page size, we use (the Least Significant Bit (LSB)) 48 bits for addressing. Why not the full 64 bits? It's simply too much! No existing computer comes close to having even half of the full 264 = 18,446,744,073,709,551,616 bytes, which is equivalent to 16 EB (that's 16,384 petabytes) of RAM!
"Why," you might well wonder, "do we equate this with RAM?". Please read on – more material needs to be covered before this becomes clear. The Examining the kernel segment section is where you will understand this fully.
