Northbridge (computing)

The Northbridge 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 permitted. In general, however, core logic chipsets are divided into two main parts.

Purpose

The northbridge typically handles communications between the CPU, RAM, AGP port (if applicable), and the southbridge. Some northbridges also contain integrated video controllers. Because different processors and RAM require different signalling, a northbridge will typically work with only one or two classes of CPUs and only one type of RAM. There are rare chipsets that support two types of RAM, although none of these allows for the use of different types at once. For example, the northbridge from the nVidia nForce2 chipset will only work with Duron, Athlon, and Athlon XP processors combined with DDR-SDRAM, and the Intel i875 chipset will only work with Pentium 4 processors and Celeron processors that are >1.3 GHz.

Importance

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

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

Recent developments

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 such as Intel and IBM have at least considered this change for their own product lines, although the likelihood of either company making such a change is rather low.

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.

Northbridge and overclocking

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.