Amd64
x86-64 is a 64-bit extension of the IA-32 32-bit version of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on IA-32, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other enhancements. The original specification was created by AMD, and has been implemented by AMD, Intel, VIA, and others. It is fully backwards compatible with 16-bit and 32-bit x86 code. Because the full x86 16-bit and 32-bit instruction sets remains 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.
AMD's method of extending Intel's 32-bit x86 instruction set to be a subset of its x86-64 instruction set is the same technique Intel employed to extend its 16-bit x86 instruction set to 32 bits.
Prior to launch, “x86-64” and “x86_64” were used to refer to the instruction set. Upon release, AMD named it AMD64. Intel initially used the names IA-32e and EM64T before finally settling on Intel 64 for their 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 while the BSD family of OSs and the Debian Linux distribution use AMD64.
The AMD K8 core was the first to implement the architecture; this was the first significant addition to the x86 architecture designed by a company other than Intel. Intel was forced to follow suit and introduced a modified NetBurst family which was fully software-compatible with AMD's design and specification. VIA Technologies introduced x86-64 in their VIA Isaiah architecture, with the VIA Nano.
The x86-64 specification is distinct from the Intel Itanium (formerly IA-64) architecture, which is not compatible on the native instruction set level with the x86 architecture.
Read more about Amd64: AMD64, Intel 64, VIA's X86-64 Implementation, Differences Between AMD64 and Intel 64, Industry Naming Conventions, Licensing Issues