The northbridge is a chip which sits on PC motherboards based on a large group of chipsets by Intel, VIA and AMD. Northbridges and their southern siblings made their appearance roughly with the PCI bus.
A Pentium-class processor has hundreds of pins (478 on the new Pentium 4), and to many of us it would seem that this is plenty of connections for accessing memory and regulating signals on a bus or two. But there are design considerations which speak against the concept of a PC on a chip, at least for powerful desktops. For one thing, driving signals on high-speed buses consumes plenty of power, and a CPU isn't big enough to dissipate all the resulting heat. So the concept of delegation has proven its worth in computer architecture as in real life.
The northbridge communicates with the CPU over the Front Side Bus (FSB) and acts as the controller for memory, AGP and PCI. The southbridge takes care of most basic forms of I/O, such as USB, serial ports, audio, IDE and more. The southbridge sits on the northbridge's PCI bus, which is usually a 32-bit, 33MHz bus capable of providing 133MBps of bandwidth. So both of these chips are essentially "bridges" between different kinds of buses.
More recent Intel chipsets (the 8xx series) replace the north and south bridges with two similar but evolved chips which together participate in something Intel calls the Intel Hub Architecture (IHA).
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References:
- http://www.sharkyextreme.com/hardware/guides/intel_i815_chipset/2.shtml
- Several perspective corrections from call