A common 64-bit architecture provides 32-bit backward compatibility. Minor incompatibilities that exist can be addressed by compiler and operating-system implementations. Advanced Micro Devices (AMD) took the lead with 64 bits and x86 compatibility in its AMD64 architecture utilized in the Opteron and Athon 64 processor line. AMD's use of HyperTransport was interesting as well, but it does not have an impact on the 64-bit architecture. The approach taken by AMD was to utilize code prefix bytes that were unused, allowing access to the 64-bit features and registers.
Intel's 64-bit Itanium architecture deviated from the x86 architecture, although emulation provided support for x86 processors. Very large caches put the Itanium beyond reach for many users. New applications were needed to take advantage of the 64-bit architecture.
Recognizing the demand of x86-compatible processors with 64-bit support, Intel has created the EM64T (Extended Memory 64 Technology) for the Pentium 4 and Xeon processors. It implements an identical register architecture as AMD64 and the same instruction set enhancements, except for some minor variations. Systems require a EM64T-enabled BIOS. Operating-system recognition of AMD64 and EM64T is different but easily handled by the operating system. The support is then transparent to applications.
Key operating systems like Linux and Windows already handle AMD64 with EM64T support in the wings. Linux has the lead in 64-bit applications for these platforms due to its early availability. As usual, the x86 architecture is pounding on the competition.