Limitations of Practical Processors
In principle, a 64-bit microprocessor can address 16 exabytes of memory. In practice, it is less than that.
For example, the AMD64 architecture as of 2011 allows 52 bits for physical memory and 48 bits for virtual memory. These limits allow memory sizes of 4 PiB (4 × 10245 bytes) and 256 TiB (256 × 10244 bytes), respectively. A PC cannot contain 4 petabytes of memory (due to the physical size of the memory chips, if nothing else) but AMD envisioned large servers, shared memory clusters, and other uses of physical address space that might approach this in the foreseeable future, and the 52-bit physical address provides ample room for expansion while not incurring the cost of implementing 64-bit physical addresses. Similarly, the 48-bit virtual address space was designed to provide more than 65,000 times the 32-bit limit of 4 GiB (4 × 10243 bytes), allowing room for later expansion without incurring the overhead of translating full 64-bit addresses.
Read more about this topic: 64-bit Computing
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