Intel64 and IA64 are two very different instruction set and architecture used in the Intel processor.
Intel 64 (formerly known as EM64T or IA32e) is the 64-bit extended instruction set based on x86 processor architecture. Modern processor that incorporated Intel64 technology could also run 32-bit application on a 32-bit operating system without performance losses although the 32-bit operating system would not be able to utilize any features in the Intel64 technology. The reason for such design is for backward compatibility which is the key to the success of Intel x86 processor.
IA64 however is based on an entirely different architecture. Only Intel Itanium processor employs IA64 architecture. It does not provide any backward compatibility with the IA32 software. the emulation of such 32-bit application is very slow. Originally Intel incorporated hardware emulation to the 32-bit application but since then Intel has been relying on software emulation.
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