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The Northbridge/Southbridge architecture.
Motherboard chipsets include a Northbridge chip and a Southbridge chip, referring to their geographic location on the board. Communications between the CPU, memory, and graphics controller are handled by the Northbridge part of the chipset while the Southbridge chip controls the expansion bus and I/O functions such as IDE, SATA, USB, etc. Because the speed of data flowing between Northbridge and the CPU is critical to system performance, the two chips are usually located in close proximity to each other.
The Northbridge controls the Southbridge which is generally further away because its slower speeds make latency less of a performance issue. The CPU communicates with the motherboard Northbridge via the front side bus (FSB), a design that has worked well in the past, but has largely become viewed as a limitation in light of the increasing clock speeds of modern CPUs. Still, both Intel based and AMD based motherboards utilize the Northbridge/Southbridge architecture and new chipsets such as the recent AMD 890GX Northbridge and the AMD SB850 continue to be released.
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Because of the clock rates handled by Northbridge, the chips get very hot, so they are typically equipped with passive heat sinks from the manufacturer. Because overclocking tends to cause the Northbridge to get hotter, users who choose to push their system to higher speeds purchase third party Northbridge heat sinks, some of which are equipped with fans.
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Computer power users often increase the FSB speed in an effort to get more performance from a Northbridge chip. Based on the FSB speed, different “straps" or latencies come into play within the Northbridge. Some motherboards give users more control over the FSB speeds and chipset straps than others, so if you’re interested in this, make sure you buy a motherboard that features a chipset recommended by overclockers.
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Limitations of the Motherboard Northbridge
As noted earlier, the speed limitations of the FSB are beginning to become a limiting factor in the wake of faster CPUs. The Northbridge architecture is slow because as data travels from the CPU across the FSB into the Northbridge, it follows a circular pattern like a traffic circle with exits for Southbridge, DMI, graphics, and memory. The CPU has to wait for an entire cycle to complete prior to issuing new instructions to the Northbridge. Similarly, data bottlenecks at the Northbridge waiting to get back onto the FSB on its way to the CPU.
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Is Northbridge on the Way Out?
Some CPUs from Intel (such as Nehalem) and AMD (such as the AMD 64) have eliminated Northbridge in favor of memory controllers that are built into the CPU die, eliminating bottlenecks between graphics, memory, and I/O subsystems. These processors offer significant performance increases over the traditional chipset architecture.
Some in the industry see the Northbridge/Southbridge architecture being phased out over time as new technologies make it easier (and cheaper) to put more chipset functions onboard the CPU.