The main function of the Northbridge is to handle the memory, CPU, and AGP data-transfer duties and to make the most efficient use of available resources. These are mission-critical jobs, as control of the CPU and memory makes up a high percentage of overall system activity. The Northbridge component is the key performance area of any motherboard, and it's usually where benchmark-test races are won or lost.

Out of the three main Northbridge duties, controlling access to system memory is arguably the most important, as it involves playing traffic cop to myriad requests from the CPU and AGP buses as well as the data link to the Southbridge functions. Keeping all these components supplied with data is the core duty of the Northbridge, and its performance is a a factor of its timings, buffers, and memory clock speeds.

The last is the easiest distinction to make -- due to extra memory bandwidth, a 2.0GHz Pentium 4 running with DDR400 system memory will outperform one using DDR266. The actual memory timings of the Northbridge itself are a bit harder to grasp, but even slight enhancements can have a significant impact.

Intel's 845PE is a good example: This chipset upgrades the i845E to DDR333 support, so it's no surprise that it surpasses its predecessor in overall performance, especially with memory-intensive applications. What's unexpected, however, is that the i845PE is also noticeably faster than the older, DDR333-equipped i845G platform. We know Intel did some revision work with the i845PE and GE chipsets, and it seems likely that faster memory timings were among the results.

Controlling the CPU bus -- moving data to and from the processor and interacting with various levels of cache -- is another important function of the Northbridge. The faster the chipset can do this, the more efficient the CPU can be in terms of making the most of its clock speed or internal performance.

The Northbridge also determines the motherboard's level of CPU support. Early Intel 845-series chipsets, for example, supported only then-current Pentium 4 processors' 400MHz front-side bus, not the newer 533MHz models. In the vast majority of motherboard architectures, the front-side or system bus drives the other bus speeds, so running a 533MHz Pentium 4 on a nonsupported platform will raise other bus speeds by a corresponding amount -- probably well out of specifications. Even the Athlon XP, a relatively minor revision to the Athlon core, ran into problems with some older chipsets, due to the strict chipset compatibility guidelines that both AMD and Intel require.

Between the Bridges

As you can imagine, the Northbridge-to-Southbridge interface is also an extremely important facet of chipset design. Older chipsets used the PCI bus for this data link, but while the overall bandwidth (133MB/sec at 33MHz) was adequate in those days, the number of other components riding the PCI bus served to lower available bandwidth quite significantly. With hard drives, sound cards, network interface cards, and other peripherals sharing the PCI bus, the latter became a real system bottleneck as CPU, AGP, and memory speeds continued to ramp up.

The solution was to create a dedicated Northbridge-to-Southbridge link apart from any other system bus. This really started to become prevalent once high-bandwidth DDR memory came into vogue; VIA, for instance, instituted a dedicated 266MB/sec V-Link for its KT266 and P4X266 chipsets and upgraded it to 533MB/sec for the newer KT400 and P4X400.