3.2 Next-generation 64-bit Instruction Sets
Two new instruction sets can claim to be the 64-bit successor to IA-32. One of them builds on top of IA-32 but has a different name, while the other one discards IA-32 completely but has a similar name.
3.2.1 IA-64
Intel's IA-64 architecture is not directly compatible with the IA-32 instruction set. It completely discards all IA-32 instructions, and starts from scratch with a completely different instruction set. It can run IA-32 instructions through an instruction emulator -- basically a piece of software that translates IA-32 instructions into IA-64 instructions on the fly. However, since it was designed by Intel, the original creator of the IA-32 instruction set, it gets to keep the "IA" prefix despite the lack of any real familial connections.
It is the instruction set that is used inside their Itanium line of processors.
3.2.2 AMD64
AMD's AMD64 instruction set, aka x86-64, is largely built on top of IA-32, and thus maintains the x86 family heritage. While extending the instruction set, AMD took the opportunity to clean up some of the odd behaviour of this instruction set that has existed (plagued?) since its earliest 16-bit days, while the processor is operating in 64-bit mode. They also doubled the number of general purpose registers from 8 to 16; and the general purpose registers are now much more truly general-purpose registers. They also doubled the number of SSE registers from 8 to 16 as well. They have also deprecated most of the functionality of the segment registers, since their usage has steadily declined even during the IA-32 days.
3.2.2.1 EM64T
As of February 2004, Intel has implicitly acknowledged the logic of the AMD64 instruction set, and it will begin using it itself in its own products soon. However, Intel does not want to directly acknowledge its rival AMD, so it will not call it AMD64, it will call it EM64T instead. Intel has also prevented itself from ever using the term IA-64 for this architecture, since it has obviously already given that name to the architecture behind its Itanium processors.
4 See also
5 External links
Computer terminology
Computer architecture
Microprocessors
Intel 386
