The x86-64 specification was designed by Advanced Micro Devices (AMD), that have since renamed it AMD64. The first family of processors to support this architecture, which AMD calls AMD64, was the AMD K8 family of processors. This was the first time any company other than Intel made significant additions to the IA-32 architecture. Intel was forced to follow suit, introducing modified NetBurst family processors, initially referred to as "IA-32e" or "EM64T" and now called Intel 64 and almost identical to AMD64. x86-64 is backwards compatible with 32-bit code without any performance loss. For example, The Developers Manuals available from Intel on the IA-32 architecture refer to IA-32 and IA-32e in tandem. [1] AMD licensed its x86-64 design to Intel, where it is marketed under the name Intel 64 (formerly EM64T). AMD's design replaced earlier attempts by Intel to design its own x86-64 extensions which had been referred to as IA-32e. As Intel licenses AMD the right to use the original x86 architecture (upon which AMD's x86-64 is based), these rival companies now rely on each other for 64-bit processor development. This has led to a case of mutually assured destruction should either company revoke its respective license.[2] Should such a scenario take place, AMD would no longer be authorized to produce any x86 processors, and Intel would no longer be authorized to produce x86-64 processors, forcing them back to the now-obsolete 32-bit x86 architecture. The last processors Intel manufactured which did not use AMD's x86-64 design were early versions of the Pentium 4 "Prescott", introduced in February 2004.
