AMD's Athlon XP 2100+ The Last of the Palominos

The Future: 166MHz FSB & DDR333?

At the end of our Pentium 4 2GHz review we looked at the benefits that could be realized simply by bumping the FSB frequency from 100MHz (400MHz quad-pumped) to 133MHz (533MHz quad-pumped). The results were anywhere between a 0 - 15% improvement in real-world performance.

Now armed with memory that will work at 166MHz frequencies (pseudo-DDR333 spec) we are able to perform a similar test on the Athlon. We took an Athlon XP 2000+ and ran it at its native 133.33333MHz x 12.5 frequency, and then at 166.6666MHz x 10 to achieve 1.67GHz using two different approaches. In the second case we kept the FSB at 166MHz and the memory bus at 166MHz as well, thus increasing overall system bandwidth by 25%. This is an additional 25% bandwidth between the CPU and the North Bridge as well as another 25% between the North Bridge and memory.

We were able to do this on our regular KT266A platform making it clear that you don't necessarily need a KT333 chipset in order to use DDR333. The only differences are the ability to run your memory bus at 166MHz while your FSB is clocked at 100 or 133MHz and electrical validation for DDR333 on the KT333 chipset. Do note that neither chipset officially supports the 166MHz FSB.

The biggest performance boosts here are in the memory bandwidth tests taken from SiSoft Sandra 2002 of 21 - 25%. But when we shift to the real world we see that the majority of the performance increases come between 0 and 6%. This is definitely not enough of a performance increase to justify the additional validation of platforms that would have to go into making DDR333 with a 166MHz FSB a standard for future Athlon XP processors.

Granted that this additional bandwidth would be better used once the processor hits higher clock speeds but we'd be willing to bet that AMD would rather focus on ramping up their Hammer based successors rather than worrying about helping the current Athlon XP scale any better with clock speed.

So there you have it, it wouldn't make much sense for Thoroughbred to have a 166MHz FSB or use DDR333 SDRAM not only from a cost standpoint but also when it comes down to performance. In fact, we'd be willing to say that all signs point to Thoroughbred being nothing more than a 0.13-micron version of the Athlon XP to enable higher clock speeds. AMD prides themselves on having very cheap to manufacture small footprint cores and throwing additional cache on the CPU would definitely conflict with that. At this point however it would make more sense for AMD to add another 128KB or 256KB of L2 cache rather than move to a faster FSB for the Athlon XP.