Two-chip design

If you recall SiS's DDR chipset for the Socket A platform, SiS 735, one of the unique features SiS implemented in its design was its integration of the North Bridge and South Bridge into a single chip. In traditional chipset architectures, the North Bridge of the chipset is a discrete chip that handles communication between the processor, system memory, and AGP graphics.

The SiS 645 reference board

Two-chip design

The South Bridge of the chipset is another separate chip (located on the lower portion of the motherboard, hence the name South Bridge) which is responsible for system peripherals and other devices such as your hard drive and any USB accessories installed on your computer. Traditionally the North Bridge and South Bridge are joined together by the PCI bus. Offering up to 133MB/sec of bandwidth however the PCI bus was quickly becoming constrained by the components underneath it, USB printers and fast ATA hard drives can quickly sap its bandwidth, resulting in a glaring system bottleneck.

As a result, Intel moved to its "hub architecture" essentially renaming the North Bridge to the Memory Controller Hub, and the South Bridge to the I/O Controller Hub. Both are connected via a 266MB/sec pathway, twice that offered by the PCI bus. VIA offers the same amount of bandwidth between the North and South bridges with its V-Link technology.

In the SiS 645, 533MB/sec of bandwidth is available between the North and South Bridge of the chipset. Yes, you read that correctly, the SiS 645 departs from the SiS 735 in that the North Bridge and South Bridge are once again discrete chips in the SiS 645 chipset. SiS decided to go back to two separate chips to ease development costs.

In single-chip designs, adding a new feature (ATA/133 or Firewire for example) to one chip requires a redesign of the entire chipset. In contrast, with two-chip designs new features can be added to either bridge without affecting the design of the other. This will make it easier for SiS to add upcoming features such as ATA/133 or USB 2.0 to their upcoming products: rather than working on a new design, they only need to make a few modifications to the South Bridge. This new South Bridge then becomes a drop-in replacement for its predecessor.

As a result the end product is slightly less flexible for motherboard manufacturers, but as a result SiS can bring new products to market more quickly and at a lower development cost than if a single-chip design had been used.