and serial communications. In 1990s and early 2000s, the interface between a northbridge and southbridge was the PCI bus.

Before 2003, any interaction between a CPU and main memory or an expansion device such as a graphics card(s) -- whether AGP, PCI or integrated into the motherboard -- was directly controlled by the northbridge IC on behalf of the processor. This made processor performance highly dependent on the system chipset, especially the northbridge's memory performance