Northbridge
From TPU Reference
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Also: MCH (Memory Controller Hub), NB
The Northbridge is an integral part of the PC core chipset logic. It is usually paired with a Southbridge that takes over less important functions. The Northbridge chip resides on the motherboard and is connected to the CPU via a high-speed bus. It handles communications between CPU, Memory, AGP or PCI-Express Graphics devices and the Southbridge.
Often the Northbridge / Southbridge combination is called Chipset, putting more focus on the selected Northbridge than the Southbridge. For example you hear people talking about "I have an i975X chipset on my motherboard". Northbridges can not work with all Southbridges. Depending on the manufacturer and platform, there are several Southbridges to choose from when using a certain Northbridge. For example the ATI RD580 Northbridge can be paired with the ATI SB400, ATI SB600, ULi M1575 and others.
In some cases Northbridge and Southbridge are combined into a single chip. This is done to reduce cost of lower end chipsets, taking away a bit of design flexibility.
A Northbridge works for a single CPU architecture. For example the ATI Radeon XPRESS 3200 (RD580) is designed to work with the Athlon 64 architecture. A Northbridge is not bound to a CPU socket, Athlon64 chipsets for example can work with both Socket 939 and Socket AM2 processors.
On the Athlon64 architecture the memory controller has been moved from the Northbridge into the CPU. If you compare the following schematics, you can see that on Intel i975X (and all other chipsets for Intel CPUs) the memory is connected to the Northbridge. On the ATI RD480 (and all other chipsets for AMD Athlon64 CPUs) the memory is connected directly to the CPU. This means that technically an Athlon64 Northbridge is not a MCH.
The term Northbridge comes from the arrangement of the devices. If you look at the schematics like you would look at a map, the Northbridge and CPU are in the northern part of the map, the southbridge in the southern part.
Depending on the platform, the Northbridge plays a more or less important role in limiting the maximum overclock. On Intel Pentium 4 systems for example, the maximum memory clock is often limited by the Northbridge. On Intel Core 2 systems with i975X chipset the Northbridge is often the limiting factor for the maximum FSB clock. When the Northbridge limits your maximum overclock, you can usually gain more overclocking out of it by increasing its supply voltage.
