64-bit computing Limits of practical processors

https://en.wikipedia.org/wiki/64-bit_computing

In principle, a 64-bit microprocessor can address 16 EiBs (16 × 10246 = 264 = 18,446,744,073,709,551,616 bytes, or about 18.1 exabytes) of memory. In actual practice, it is orders of magnitudes less than that.

For example, the AMD64 architecture (as of 2011) allows 52 bits for physical memory and 48 bits for virtual memory.[8] These limits allow memory sizes of 4 PiB (4 × 10245 bytes) and 256 TiB (256 × 10244 bytes), respectively. A PC cannot currently contain 4 pebibytes of memory (due to the physical size of the memory chips), but AMD envisioned large servers, shared memory clusters, and other uses of physical address space that might approach this in the foreseeable future. Thus the 52-bit physical address provides ample room for expansion while not incurring the cost of implementing full 64-bit physical addresses. Similarly, the 48-bit virtual address space was designed to provide more than 65,000 (216) times the 32-bit limit of 4 GiB (4 × 10243 bytes), allowing room for later expansion and incurring no overhead of translating full 64-bit addresses.