SiS755 Reference Board: Athlon64 from SiS

The launch of Athlon 64 has brought us 2 chipsets thus far: the nVidia nForce3 150 and VIA K8T800. As we have discussed in reviews of boards based on these chipsets, neither one really meets the specifications that we would like to see in Athlon 64 chipsets.

While VIA performs at the specified 800 Hyper Transport speed, it is hampered by lack of a real means to fix AGP/PCI lock. This severely limits the ability of VIA chipset boards to overclock, and is the most important complaint we have had with this chipset. VIA is also a 2-chip solution and the actual communication speed between the North and South Bridge chips is specified as 500MHz, which is much slower than the 1.6GHz to 3.2GHz that is used for internal communication of the other components on HyperTransport. While VIA has made a lot of splash about being the “only” chipset to implement 800 HT speed, they are really telling only part of the story, since the slower North/South bridge communications are certainly a bottleneck if VIA is correct about the impact of a slower HT speed on other chipsets. You simply cannot have it both ways.

nVidia, on the other hand, fully implements the ability to fix the PCI/AGP on nForce3-150 chipset boards. However, current 150 version chipsets cannot run with stability any faster than 600 HyperTransport. The nForce3 does offset this limitation partially with their one-chip implementation of nForce3 150. This assures the single chip is communicating with other HT components at the fastest speeds available, and is not hampered by a slower North/South bridge bus.

The practical reality is these differences in the nForce3-150 and VIA K8T800 have been impossible to measure at stock performance speeds. With the Athlon 64 memory controller on the chip, it appears the chipset has less impact on final performance, and our benchmarks with the VIA and nF3 chipsets have been virtually identical. Overclocking, on the other hand, has been quite different between the 2 chipsets. Using the same Athlon64 chip, we have been able to reach overclocks to the 230 (920FSB) range with nF3-150; VIA solutions, conversely, have never been able to reach much beyond the 214-215 range due to problems of the floating PCI/AGP speeds.

nVidia will have a new chipset in the next few months that will run at 800 HT called the nForce3-250. VIA is also rumored to have a new chipset with PCI/AGP lock in the works. Frankly, we are more skeptical as to whether or not the VIA solution will appear, since VIA has never had a chipset with PCI/AGP lock. We also believe that VIA would have implemented PCI/AGP lock if it had been doable with their current core logic in the year we all waited for Athlon64 to appear. While there is promise of better solutions from both nVidia and VIA, the reality today is neither chipset does exactly what we want.

SiS is now releasing their Athlon64 solution, which we are evaluating in this look at the SiS755 chipset. SiS does run at 800 HyperTransport, and SiS also has implemented the ability to run the PCI/AGP speed asynchronously, so it is possible to fix PCI/AGP speed. On paper, at least, SiS appears to be a better Athlon64 solution than either nForce3-150 or VIA K8T800. While it is also a 2-chip solution like VIA, the SiS755 chipset provides North/South communication using the proprietary Mutiol bus. For the 755, SiS has increased Mutiol speed to 1GHz, a higher speed with less chance of saturating the bus than the VIA solution.