x86-64 (also known as x64, x86_64 and AMD64) is the 64-bit version of the x86 instruction set. It supports vastly larger amounts (theoretically, 2⁶⁴ bytes or 16 exbibytes) of virtual memory and physical memory than is possible on its 32-bit predecessors, allowing programs to store larger amounts of data in memory. x86-64 also provides 64-bit general-purpose registers and numerous other enhancements. It is fully backward compatible with the 16-bit and 32-bit x86 code. Because the full x86 16-bit and 32-bit instruction sets remain implemented in hardware without any intervening emulation, existing x86 executables run with no compatibility or performance penalties, whereas existing applications that are recoded to take advantage of new features of the processor design may achieve performance improvements.