64 bit CPUs are computers that utilize 64 bit general registers for computation, indexing, etc. This provides the capability to directly manipulate very large memories, etc. It is perfectly possible to manipulate smaller units 8, 16, or 32 bits in 64 bit CPUs. Floating point already uses 64 bit values. Vector arithmetic operates on 128 bit entities. The addressing limit of 32 bit arithmetic is around 4300 million = 4.3GB of memory. 64 bits makes 18 million terabytes addressable. Early "64 bit" CPUs probably will not really make the full address space physically available as no one is going to be able to fill out a large fraction of the 18 million terabyte address space with actual memory for many years.

As of 2004, several PC based 64bit CPUs have been introduced without spectacular success.

There are numerous major differences in the design of the various 64 bit CPUs and their interfaces to their environment. Their instruction sets are all different although all can execute 32 bit x86 code somehow. Curiously AMD's 64 bit offering is to be an extension of the ubiquitous 32 bit x86 instruction set as native instructions, whereas Intel which developed the x86 line is selling the Very Long Instruction Word Itanium which executes the short and somewhat bizarre x86 instruction set in a special mode. The Itanium is subject to substantial delays when transitioning between x86 and native IA64 mode and thus runs rather slowly when executing legacy 32 bit code. It appears that Intel will supplement its Itanium offerings with a 64 bit Pentium that closely resembles the AMD instruction and register layout.

Another curiosity is that the best absolute performance of the 64 bit CPUs is generally felt to have been obtained with the Alpha up to the time that Alpha development was halted in 2003.

64 bit versions of Linux and Windows (Itanium only) are available.

Acceptance of 64bit CPUs has been mixed. They appear to work well enough, but few current applications actually need them or show much improvement in performance when run on one. Neither are there a large number of technologies that have been put on hold waiting for 64 bit computing. Intel shipped around 10,000 times as many 32 bit CPUs in 2004 as it did 64 bit CPUs.

Intel has announced that it does not intend to ship a 64 bit "desktop" Pentium CPU until the "Longhorn" Release of Windows -- probably in the 2005-2006 timeframe. This is somewhat confusing in that Intel has also announced some availability of a 64 bit Pentium-IV that is to be compatible with Microsoft's 64 bit version of Windows XP. Beta versions of the latter -- for the AMD Opteron only -- were released in February 2004.


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