The north bridge chip, also called MCH (Memory Controller Hub) is connect directly to the CPU and has basically the following functions:

• Memory controller (*)
• AGP bus controller (if available)
• PCI Express x16 controller (if available)
• Interface for data transfer with south bridge

(*) Except for socket 754, socket 939 and socket 940 CPUs (CPUs from AMD like Athlon 64), because on these CPUs the memory controller is located in the CPU itself, not in the north bridge.

Some north bridge chips also controls PCI Express x1 lanes. On other PCI Express chipsets it is the south bridge that controls the PCI Express x1 lanes. In our explanations we will assume that the south bridge is the component in charge of controlling the PCI Express x1 lanes, but keep in mind that this can vary according to the chipset model.

In Figure 3, you can see a diagram explaining the role of the north bridge in the computer.

click to enlarge
Figure 3: North bridge.

As you can see, the CPU does not directly accesses the RAM memory or the video card, it is the north bridge that accesses these devices. Because of that, the north bridge chip has an ultimate role in the computer performance. If a north bridge chip has a better memory controller than another north bridge, the performance of the whole computer will be better. That’s one explanation why you can have two motherboards targeted to the same class of processors achieving different performances.

As we mentioned, on Athlon 64 CPUs the memory controller is embedded in the CPU and that’s why there is almost no performance difference among motherboards for this platform.

Since the memory controller is in the north bridge, is this chip that limits the types and maximum amount of memory you can have in our system (on Athlon 64 it is the CPU that sets these limits).

The connection between the north bridge and the south bridge is done through a bus. At first the PCI bus was used, but later it was replaced by a dedicated bus. We will explain more about this later, since the kind of bus used on this connection can affect the computer performance.