http://www.amd.com/us-en/assets/content_ty...64_overview.pdf Virtual Addressing and Paging
In long mode, a virtual address is a uniform 64-bit offset into the virtual address space. The mechanisms that translate virtual addresses into physical addresses are changed to support the larger virtual-address size.
Virtual-Address and Physical-Address Size
The long-mode architecture provides for 64 bits of virtual address space and 52 bits of physical-address space. The maximum supported virtual-address space is 264 bytes (16 exabytes) while the maximum supported physical-address space is 252 bytes (4 petabytes).
Implementation Specifics. Implementations can support smaller virtual-address and physical-address spaces than the maximums defined by the long-mode architecture.
The first implementation of the Hammer family of processors supports 48 bits of virtual address and 40 bits of physical address. The CPUID instruction returns the number of virtual address
and physical-address bits supported by the implementation.