The North Bridge
The SiS 648 chipset is composed of the SiS 648 North Bridge (which connects the chipset to the CPU, graphics card, and system memory) and the SiS 963 South Bridge (which supports system peripherals such as the keyboard and mouse, and contains the hard drive and USB controllers among other items). If a motherboard manufacturer desires, the SiS 963 chip can be substituted for the SiS 961 chip from the SiS 645 family.
The memory controller
The heart of SiS 648’s North Bridge lies in its memory controller. Like previous SiS memory controllers, SiS 648 supports both synchronous and asynchronous bus speed operation. This means that the memory bus and system front-side bus can operate at the same speed or independent of each other. In particular, eight multiplier options are available for the memory bus, which can make things intimidating for the inexperienced computer user. With two system bus options to chose from (100MHz or 133MHz if you want to remain in spec) and four different memory types (DDR200, DDR266, DDR333, and DDR400) this adds for up to eight different combinations, ensuring support for every flavor of Pentium 4 system available.
Say for instance you wish to run your memory bus at 333MHz but your system bus at 400MHz. Simply select the “3:5” option for the CPU/RAM Freq Ratio setting, reboot the system, and you’re good to go. As we said earlier, this can be a bit confusing for those who are new to system building, so hopefully motherboard manufacturers will do a good job of explaining all the options available with SiS 648 in their user manuals.
Looking at the basic specs of the memory controller, you’d assume SiS left things virtually unchanged. Memory support is still limited to 3GB (1GB per DIMM, which is the same figure as SiS 645 and SiS 645DX); in comparison Intel 845/845E is also limited to 3GB while VIA P4X333 supports up to 4GB of memory. In addition, only two memory modules are supported in DDR400 and DDR333 modes, while up to three modules of DDR266 are officially approved. If it weren’t for the addition of DDR400 memory, the basic specs would be the same.
Despite this, SiS has assured us that the SiS 648 memory controller has been slightly improved. During our briefing, we were told SiS’ engineers had witnessed a two to three percent performance improvement with the SiS 648 memory controller over SiS 645DX. With this in mind, we were eager to see if this held up in our testing. You’ll have to read our performance results to see if this held true.
The other significant addition to the SiS 648 North Bridge is its AGP 3.0 interface. This officially brings AGP 8X (and its 2.1GB/sec of bandwidth) to the Pentium 4 platform.