AMD's Athlon XP 3000+: Barton cuts it close

What about the 400MHz FSB?

After Comdex, the word on the street was that AMD would be moving Barton to a 400MHz FSB in the near future but that the CPU would debut with a 333MHz FSB. As you can tell by today's release, we are still dealing with 333MHz FSB CPUs, but what is there to be said about the potential impact of a 400MHz FSB?

A larger L2 cache means that Barton has to go to main memory much less often (assuming that our applications do abide by the principles of spatial and temporal locality), which means that it has to send requests and receive data across the FSB much less frequently compared to an identically clocked Thoroughbred.

Since Barton is being launched at speeds slower than the fastest Thoroughbred, the immediate need for a 400MHz FSB isn't apparent - remember, FSB traffic should be reduced by the larger L2 cache. However, as Barton ramps up in clock speed, the move to a 400MHz FSB may become more appetizing as higher clocked Athlon XPs will require data at a faster rate to keep their pipelines filled.

So today, Barton would benefit less from a 400MHz FSB than the Thoroughbred core, which isn't much at this point either. Remember that the main benefit of the 333MHz FSB was latency reduction because of the fact that the FSB and memory bus were finally operating at the same clock speed once again, and not because of the increase in FSB bandwidth.

Headroom with Barton?

More transistors and a die-size that is on the edge of AMD's manufacturing sweet-spot means that Barton isn't going to be any sort of overclocking monster, but just for kicks, we decided to see how far we could push the core.

Using conventional cooling, we were able to hit 2.324GHz at stock voltage, an overclock of 7%, but maintaining stability was another question. We're sure that with little effort the 2.2 - 2.3GHz range is attainable, but anything above that will require a bit more. With AMD working on an Athlon XP 3200+ to go up against Intel's forthcoming Pentium 4 3.2GHz, we can at least see that the current Barton cores shouldn't have much of a problem taking AMD there.