X-bit labs

SiS655 - New Dual-Channel Chipset. Gigabyte GA-8SQ800 Ultra – First SiS655 Based Mainboard (page 2)

Category: Chipsets

by Ilya Gavrichenkov

[ 03/21/2003 | 01:49 AM ]

Now let’s talk about the main innovation of SiS655: new memory subsystem supporting two DDR SDRAM channels. The chipset’s dual-channel memory subsystem officially supports DDR266 and DDR333 SDRAM thus providing up to 5.4GB/s bandwidth. It’s even more than modern Pentium 4 processors can use up. Moreover, SiS655 delivers the highest memory bandwidth among all currently available chipsets for Socket 478 processors.

Note also that although the official SiS655 specs tell only about DDR266 and DDR333 support, the real capabilities of the chipset are much wider. Of course, we have DDR400 SDRAM support here: it was unofficially implemented in SiS645DX already. Moreover, the numerous possible values of the memory bus devisor allow setting memory frequency with the same flexibility as in NVIDIA nForce2 based systems.

The chart shows there are two independent memory controllers in SiS655 with their own address, data buses and so on. In other words, SiS engineers used the same approach as NVIDIA did in its nForce2 solution for Athlon XP platforms. This allows flexible configuration of the memory subsystem and the use of different-type DIMM pairs. SiS went even farther: SiS655 can work in three memory access modes chosen according to the configuration and number of installed modules and according to user’s settings, of course. Let’s describe each mode.

The simplest work mode implies single-channel 64-bit memory access. This mode is activated when there is only one memory module in the system, or two modules are installed into slots referring to one and the same channel. In this case, only one controller of the two is in use, the other one remains idle. So, the chipset works as a traditional single-channel chipset in this mode.

The second mode provides dual-channel 64-bit access. It can be used when you install modules into DDR DIMM slots referring to both controllers. As the controllers of SiS655 are fully independent, the modules assigned to each channel may differ in capacity, quantity and organization.

The third, single-channel 128-bit mode “couples” the controllers to work with the memory as if there were a 128-bit bus. This mode requires that the memory modules in both channels are installed in pairs and are absolutely identical.

SiS claims the memory subsystem performs about the same in 128-bit and dual-channel 64-bit modes. Meanwhile, our tests show that 128-bit mode allows getting a bit higher effective bandwidth, while dual-channel 64-bit mode allows getting lower latency of the memory subsystem.

Overall, SiS655 is one of the most advanced chipsets today. Only the upcoming chipsets from Intel with 800MHz bus support will probably push the performance edge further ahead. But, they are to come only 2 months later…

SiS655 against Intel Chipsets: Specification Comparison

To confirm our words, we present a table listing specifications of SiS655 and contemporary chipsets from Intel. It shows that SiS655 offers richer capabilities than analogous Intel products.

Well, we think the advantages of SiS655 are evident. At the same time, SiS655 is no match for Intel E7205 in the server market. First of all, it can’t elbow its way into the High-End because of the “non-aristocratic” origin and secondly, it lacks ECC support.