VIA’s KT133A Socket-A Chipset: AMD 760 Performance without DDR SDRAM

With 25% less memory bandwidth, the Pentium 4’s performance here drops approximately 17%, indicating that it is heavily dependent on memory bandwidth.  So what does this have to do with the Athlon?

In spite of the fact that the Athlon on the AMD 760 has 2.1GB/s of available memory bandwidth, the line representing it on the above two Linpack charts doesn’t even approach that of the Pentium 4 with PC600 RDRAM.  The Pentium 4’s clock speed advantage isn’t the reason for its lead here, since once you get out of L2 cache clock the Linpack test is clock speed independent.  So why is it that the Athlon gains so little from having so much memory bandwidth?

If you look at the numbers closely, it actually gains quite a bit, 50% to be exact over the KT133 + PC133 SDRAM configuration.  And we already know that the Athlon is noticeably faster than the Pentium 4 in almost all of today’s benchmarks, so why the performance difference here?

It turns out that the Athlon, surprisingly enough, doesn’t follow the same design theories that were implemented with the Pentium 4.  While the Athlon was built around the assumption that it would always have a high bandwidth FSB to feed the processor, it wasn’t designed around the idea of having a large amount of memory bandwidth that it could stream from.  This is much like the design of the Pentium III, which although doesn’t have the same benefit of a high bandwidth FSB, it wasn’t designed around the idea of having a large amount of memory bandwidth which is why RDRAM resulted in no real performance benefit on the Pentium III.  Chances are that RDRAM would have a similar effect on the Athlon as well. 

The Pentium 4 on the other hand would suffer horrendously if it weren’t for its high bandwidth memory subsystem.  The latency issues we noticed with RDRAM on the Pentium III platform don’t come into play too much with the Pentium 4 since the NetBurst architecture was designed around a serialized concept that RDRAM fits into perfectly.  The Pentium 4’s trace cache, low latency L1, and 100MHz quad pumped FSB were all designed to interact with a high bandwidth memory subsystem, which the i850’s dual RDRAM channels are perfect for. 

Judging by this information, it wouldn't be too surprising if quite a bit of the 50% performance jump in Linpack that the AMD 760 offered over the KT133 were due to its 133MHz DDR FSB and not DDR SDRAM.

Assuming that this is true, the KT133A could be an instant hit.  Imagine having the performance of the AMD 760 chipset, without having to invest in DDR SDRAM.  Before we get too caught up in ourselves, let’s have a look at the rest of the KT133A chipset as well as the board we tested on.