In Northbridge/Southbridge chipset architecture designs, the Southbridge is the chip that controls all of the computers I/O functions, such as USB, audio, serial, the system BIOS, the ISA bus, the interrupt controller and the IDE channels. In other words, all of the functions of a processor except memory, PCI and AGP.
The Southbridge chip is one of two chips that control the functions of the chipset. The other is the Northbridge. The Northbridge can consist of more than one discrete chips while the Southbridge is typically only one discrete chip that rests on the Northbridge��s PCI bus.