The North Bridge
Contents
The north bridge chip, also called MCH (Memory Controller Hub), is connect directly to the CPU and has basically the following functions:
- Memory controller (if available)
- PCI Express controller (if available)
- AGP bus controller (if available)
- Interface for data transfer with the south bridge chip
Current Intel CPUs have an integrated memory controller and an integrated PCI Express controller, meaning that these CPUs have an integrated north bridge chip; therefore, they don’t require this chip on the motherboard. See Figure 3. CPUs from AMD have an integrated memory controller but don’t have an integrated PCI Express controller. Because of that, CPUs from AMD still require an external north bridge chip with this component. AMD says its processors have an “integrated north bridge,” but what the company really means is that the CPUs have an integrated memory controller. This creates much confusion. Several users don’t understand why motherboards for AMD processors have a north bridge chip if the CPU manufacturer says the CPU has an integrated north bridge chip.
Figure 3: Chipset configuration with current Intel CPUs
Figure 4: Chipset configuration with current AMD CPUs
With older CPUs that don’t have an integrated memory controller, the system will follow the diagram presented in Figure 5. With older CPUs, since the memory controller is located inside the external north bridge chip, this chip plays an important role in the computer’s performance. One chipset may have a better memory controller and present higher performance. Nowadays, however, since the memory controller is embedded in the CPU, there is almost no performance difference between different chipsets.
Figure 5: North bridge with CPUs without an integrated memory controller
The PCI Express controller embedded in the north bridge chip or in the CPU may provide several lanes. The most common configuration is for it to provide 16 lanes, allowing the motherboard to have one PCI Express x16 slot or two PCI Express x16 slots, each working at x8. Additional PCI Express lanes required to connect the other slots and devices available on the motherboard are provided by the south bridge chip. High-end PCI Express controllers usually provide more than 16 lanes, allowing the motherboard manufacturer to either provide more PCI Express x16 slots for video cards or allow the connection of other slots and devices directly to the north bridge chip or CPU.
The connection between the north bridge and the south bridge is accomplished through a bus. Initially, the PCI bus was used, but later it was replaced by a dedicated bus. We will explain more about this later.