A northbridge or host bridge is one of the two chips in the core logic chipset
architecture on a PC motherboard. Unlike the southbridge, northbridge is
connected directly to the CPU via the frontside bus and 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 or integrated
memory controller hub if equipped with an integrated VGA memory controller hub.
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 some high-performance
CPUs still require northbridge and southbridge chips externally.
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 northbridgeGPU combo chip. As CPU speeds increased, 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, 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.
Overview The northbridge typically handles
communications among the CPU, in some cases RAM, and PCI Express video cards,
and the southbridge. Some northbridges also contain integrated video
controllers, also known as a Graphics and Memory Controller Hub in Intel
systems. Because different processors and RAM require different signaling, a
given 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. For example, the northbridge from the Nvidia nForce2
chipset will only work with Socket A 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 Celeron, but it can use DDR or DDR2 memory.
Etymology The name is derived from drawing the
architecture in the fashion of a map. The CPU would be at the top of the map
comparable to due north on most general purpose geographical maps. The CPU would
be connected to the chipset via a fast bridge located north of other system
devices as drawn. The northbridge would then be connected to the rest of the
chipset via a slow bridge located south of other system devices as drawn.
Overclocking The northbridge plays an important part
in how far a computer can be overclocked, as its frequency is
commonly used as a baseline for the CPU to establish its own operating
frequency. This chip typically gets hotter as processor speed becomes
faster, requiring more cooling. There is a limit to CPU overclocking, as digital
circuits are limited by physical factors such as propagation delay which
increases with operating temperature; consequently most overclocking
applications have software-imposed limits on the multiplier and external
clock setting. Evolution
The overall trend in processor design has been to integrate more functions
onto fewer components, which decreases overall motherboard cost and improves
performance. The memory controller, which handles communication between the
CPU and RAM, was moved onto the processor die by AMD beginning with
their AMD64 processors and by Intel with their Nehalem processors. One of the
advantages of having the memory controller integrated on the CPU die is
to reduce latency from the CPU to memory.
Another example of this kind of change is Nvidia's nForce3 for AMD64 systems.
It combines all of the features of a normal southbridge with an Accelerated
Graphics Port port and connects directly to the CPU. On nForce4 boards it was
marketed as a media communications processor.
AMD Accelerated Processing Unit processors feature full integration of
northbridge functions onto the CPU chip, along with processor cores, memory
controller and graphics processing unit. This was an evolution of the AMD64,
since the memory controller was integrated on the CPU die in the AMD64.
The northbridge was replaced by the system agent introduced by the Sandy
Bridge microarchitecture in 2011, which essentially handles all previous
Northbridge functions. Intel’s “Sandy Bridge” processors feature full
integration of northbridge functions onto the CPU chip, along with processor
cores, memory controller and graphics processing unit. This was a further
evolution of the Westmere architecture, which also featured a CPU and GPU in the
same package. See also
Southbridge Chipset
References