The northbridge, also known as the Memory Controller Hub (MCH), is traditionally one of the two chips in the core logic chipset on a PC motherboard, the other being the Southbridge. Rarely, these two chips have been combined onto one die when design complexity and fabrication processes permit it. In general, however, core logic chipsets are divided into these two main parts.

The northbridge typically handles communications between the CPU, RAM, AGP port or PCI Express, and the southbridge. Some northbridges also contain integrated video controllers, which are also known as a Graphics and Memory Controller Hub (GMCH). Because different processors and RAM require different signalling, a northbridge will typically work with only one or two classes of CPUs and generally only one type of RAM. There are a few chipsets that support two types of RAM (Generally these are available when there is a shift to a new standard). For example, the northbridge from the NVIDIA nForce2 chipset will only work with Duron, Athlon, and Athlon XP processors combined with DDR SDRAM, the Intel i875 chipset will only work with systems using Pentium 4 processors or Celeron processors that have a clock speed greater than 1.3 GHz and utilize DDR SDRAM, and the Intel i915g chipset only works with the Intel Pentium 4 and the Intel Celeron, but it can use DDR or DDR2 memory.

The name is derived from drawing the architecture in the fashion of a map. The CPU would be at the top of the map at due north. The CPU would be connected to the chipset via a fast bridge (the northbridge) located north of other system devices as drawn. The northbridge would then be connected to the rest of the chipset via a slow bridge (the southbridge) located south of other system devices as drawn.

Importance

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The northbridge on a particular system's motherboard is the most prominent factor in dictating the number, speed, and type of CPU(s) and the amount, speed, and type of RAM that can be used. Other factors such as voltage regulation and available number of connectors also play a role. Virtually all consumer-level chipsets support only one processor, with the maximum amount of RAM varying by processor type and motherboard design. Pentium-era machines often had a limitation of 128 MB, while Pentium 4 machines have a limit of 4 GB. Since the Pentium Pro, the Intel architecture can accommodate physical addresses larger than 32 bits, typically 36 bits, which give up to 64 GB of addressing.

A northbridge typically will only work with one or two different southbridge ASICs; in this respect, it affects some of the other features that a given system can have by limiting which technologies are available on its southbridge partner.

Recent developments

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The traditional northbridge may be dying. The memory controller, which handles communication between the CPU and RAM, has been moved onto the processor die in AMD64 processors. Other CPU designers have considered this change for their own product lines, such as Intel and IBM, neither of which has committed.

In addition, with the development of the PCI Express bus, AGP will become obsolete. This will remove one of the traditional functions of the northbridge for all consumer-level systems once its adoption is widespread.

An example of this change is NVIDIA's nForce3 chipset for AMD64 systems that is a single chip. It combines all of the features of a normal southbridge with an AGP port and connects directly to the CPU. On nForce4 boards they consider this to be a MCP (Media Communications Processor)

Northbridge and overclocking

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The Northbridge plays an important part in how far a computer can be overclocked, as its frequency is used as a baseline for the CPU to establish its own operating frequency. In today's machines, the chip is becoming increasingly hotter as computers become faster. It is not unusual for the northbridge to now use some type of heatsink or active cooling.

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