X64

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.

The original specification, created by AMD and released in 2000, has been implemented by AMD, Intel and VIA. Various names are used for the instruction set; prior to the launch, x86-64 and x86_64 were used, while upon the release AMD named it AMD64. Intel initially used the names IA-32e and EM64T before finally settling on "Intel 64" for its implementation. Some in the industry, including Apple, use x86-64 and x86_64, while others, notably Sun Microsystems (now Oracle Corporation) and Microsoft, use x64. The BSD family of OSs and several Linux distributions use AMD64, and so does Microsoft Windows internally.

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