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Southbridge is a reference to a chipset on a PC motherboard. It is a group of microchips designed for a single function and manufactured as a single unit. This chipset controls or manages input and output (I/O). Examples of I/O interface connections controlled by southbridge are USB, serial, IDE and ISA. These are the slower capabilities of the motherboard. It is located on the northbridge of the PCI bus and is not directly connected to the CPU, but connected to the CPU through the northbridge.
Southbridge is one of two chipsets commonly referred to as northbridge/southbridge. Northbridge is a chipset controlling the processor, memory, PCI bus, level 2 cache and AGP (accelerated graphics port) functions.
The name comes from the original 1991 Intel motherboard design. This design had the PCI local bus (the backbone) in the center and the CPU, memory/cache and other high performance-critical components located above or to the north. The less performance-critical components were located below, or to the south of, the PCI local bus. Bridges to these two sets of components from the backbone are often called southbridge and northbridge, even though current architecture has replaced the PCI bus backbone with faster I/O buses.
Motherboard diagrams may generally refer to the southbridge as the I/O controller hub and the northbridge as the memory controller hub.