What does a 64 bit processor do that a 32 bit processor does not? One of the fundamental rules of current computer design is that there is nothing one processor can do that another cannot, given enough time. Where 64 bit processors have an advantage is in the amount of memory they can directly address - 32-bit processors stop at 4GB, while 64-bit processors have a theoretical limit of more than one trillion terabytes. A 64-bit processor can work on very large data sets very efficiently, at least in theory. However, modern 32 bit processors can now work on multiple 32-bit instructions simultaneously - or 32-bit instructions with large amounts of data - as well as address more than their basic 4GB memory limit so the architectural lines are blurred. Can 64 bit processors run 32 bit code? Yes, with reservations. Intel's Itanium processors were redesigned from the ground up so that their instruction set - IA-64 - is completely incompatible with the IA-32 set used from the 80386 to the Pentium 4. However, the Itanium family has a compatibility mode where the chip runs IA-32 code, but this is passed off to what is in effect an entirely separate - and by today's standard, dismally slow -- Pentium circuit on the same silicon. AMD's Hammer also has new 64-bit instructions, but these are more tightly integrated with IA-32 in what the company calls its x86-64 architecture. Performance is still not independently tested, but is expected to be substantially faster than any Itanium running 32 bit code. Who's winning -- Intel or AMD? Intel's Itanium processor, in development for around a decade, was finally launched two years ago. AMD's Hammer processor is due to ship early next year, with the Clawhammer desktop chip coming out in the first quarter and the Sledgehammer - proper name, Opteron - coming out in the first half of 2003. However, many computers based around the Itanium have now been withdrawn by their manufacturers. The Itanium 2 has been sort of launched and is listed in HP's catalogues, but as yet nobody has taken delivery of one off-the-shelf and no delivery dates are known. Meanwhile, AMD has announced some major wins for the Opteron, including a 10,000 processor supercomputer from Cray. The best way of describing the 64-bit mainstream processor market at the moment is that it barely exists Which is better, AMD or Intel? It's impossible to say, while the companies even disagree as to whether the processors are comparable. Intel is sticking to its story that the Itanium architecture is best suited for very powerful scientific or engineering requirements, or for very powerful servers, while AMD says that multi-processor Opteron-based systems will be more than a match for Itanium. Until both architectures are available for side-by-side comparisons, any claims for superiority remain just that - claims. What 64-bit software is available? Both AMD and Intel have considerable support for their architectures, with Microsoft committed to both IA-64 and x86-64 versions of Windows. Similarly, the major Unix-based operating systems either are or will be available in both flavours. Existing 32-bit software will run well on x86-64, with AMD claiming that one Opteron will do the job of two 32-bit Xeons, while converting software to use the 64-bittedness of that architecture will be easier than to IA-64. IBM recently said it took two days to convert DB-2 to x86-64. But at the moment, there is little to no mainstream customer demand for 64 bit software and effectively no platforms to run it, so software vendors are waiting to see what happens before announcing strategies or product. Will Intel produce an Opteron-style processor? Intel has repeatedly denied the existence of a project that would combine 32 and 64 bit instructions, much as in x86-64. But very persistent rumours - including some from people very close to Intel - say that these projects have existed and some are still under development. Intel has also licensed x86-64 from AMD. While Intel certainly has been working on such ideas - specifically with a chip codenamed Yamhill, and possibly with a future design called Tejas - its major problems aren't technical but market-led. It has consistently disparaged the idea, in part to talk up the IA-64 against AMD and in part to concentrate the minds of developers into investing time and effort into IA-64. If next year sees Opteron making significant inroads against Intel's high-end IA-32 chips and selling strongly in competition with the Itanium 2, Intel may change its mind. But it's a marketing, rather than an engineering, decision.