Northbridge (computing)

A northbridge or host bridge is one of the two chips in the core logic chipset architecture on a PC motherboard, the other being the southbridge. Unlike the southbridge, northbridge is connected directly to the CPU via the front-side bus (FSB) and is thus responsible for tasks that require the highest performance. The northbridge is usually paired with a southbridge, also known as I/O controller hub. In systems where they are included, these two chips manage communications between the CPU and other parts of the motherboard, and constitute the core logic chipset of the PC motherboard.

On older Intel based PCs, the northbridge was also named external memory controller hub (MCH) or graphics and memory controller hub (GMCH) if equipped with integrated graphics. Increasingly these functions became integrated into the CPU chip itself, beginning with memory and graphics controllers. For Intel Sandy Bridge and AMD Accelerated Processing Unit processors introduced in 2011, all of the functions of the northbridge reside on the CPU, while AMD FX CPUs still require external northbridge and southbridge chips.

Separating the different functions into the CPU, northbridge, and southbridge chips was due to the difficulty of integrating all components onto a single chip. In some instances, the northbridge and southbridge functions have been combined onto one die when design complexity and fabrication processes permitted it; for example, the Nvidia GeForce 320M in the 2010 MacBook Air is a northbridge/southbridge/GPU combo chip.

As CPU speeds increased over time, a bottleneck eventually emerged between the processor and the motherboard, due to limitations caused by data transmission between the CPU and its support chipset. Accordingly, starting with the AMD Athlon64 series CPUs (based on the Opteron), a new architecture was used where some functions of the north- and southbridge chips were moved to the CPU. Modern Intel Core processors have the northbridge integrated on the CPU die, where it is known as the uncore or system agent.

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