And yes, theoretically a 64-bit architecture can address 16.8 million terabytes of memory, or 2^64 bytes. But I don't believe the current popular implementations fully support this; for example, current AMD64 hardware can only address up to 1 terabyte of memory (implementing 40-bit physical addresses), although virtual address-space is 256 TiB, and future CPUs can support up to 52-bit physical addresses (4096 TiB) without any ISA changes to the page-table format (Why in x86-64 the virtual address are 4 bits shorter than physical (48 bits vs. 52 long)?).