Features

The memory controller

As we mentioned in the introduction, the SiS 645 chipset supports both SDR SDRAM and DDR SDRAM, just like the VIA P4X266 chipset. Unlike Intel's 850 chipset, the memory bus operates asynchronous of the system bus, i.e. both buses operate at independent speeds.

In the case of the system bus, the bus operates at a speed of 100MHz quad-pumped (effectively 400MHz), yielding up to 3.2GB/sec of bandwidth. Meanwhile, the memory bus supports speeds of 100MHz (for PC1600 DDR SDRAM or PC100 SDRAM), 133MHz (PC2100 DDR SDRAM, PC133 SDRAM), and finally, 166MHz (for PC2700 (also known as DDR333) memory). This latter feature in particular gives the SiS 645 chipset a unique advantage over VIA's P4X266 chipset and brings the memory bus more in line with the bandwidth offered by the system bus. ALi's DDR chipset for the Pentium 4 also supports PC2700 DDR SDRAM.

Maximum memory size has always been the biggest limitation of SiS's chipsets and this continues to hold true with SiS 645. In particular, the SiS 645 memory controller supports up to 3GB of memory, although up to three DIMM sockets are supported by the controller in PC2100 mode, and only two DIMM sockets in PC2700 mode. In comparison, VIA's P4X266 chipset supports up to 4GB of memory. While 3GB of memory is plenty for most users, keep in mind the DIMM limitations if you do end up building a system based around the SiS 645 chipset. We were able to run our SiS 645 test system with up to 512MB of memory in DDR333 mode (via two DIMMs) and 784MB in DDR266 mode (three DIMMs) with no problems.

The South Bridge

The South Bridge of the SiS 645 chipset is the SiS 961 chip. From a features standpoint, it builds largely from the SiS 735. For instance, up to six PCI bus masters are supported, just like SiS 735. Dual IDE channels are provided for ATA 33/66/100 support, as are dual USB 1.1 controllers for support of up to six USB devices. One new feature that has been added to SiS 645 over SiS 735 however is audio. The SiS 961 chip supports 5.1 channel audio, versus the 2.1 channel audio offered in SiS 735. For networking, 10/100MB Fast Ethernet support is provided as is 1/10MB HomePNA 2.0 for phoneline networking.