North Bridge / South Bridge
Chipsets are generally divided up into two separate chips: the north bridge and the south bridge. This is the most common way of designing the chipset.
The north bridge is tasked with the responsibility of communicating directly with the processor over the system front-side bus. It also acts as the memory controller, interfacing the processor with the system memory. The north bridge also acts as the go-between between the processor and the PCI and AGP buses. In systems with integrated video, it is usually the north bridge that contains this circuitry. By containing the internal video circuitry, the sub-system can deal directly with the system memory (this is AGP).
The south bridge is tasked with the primary I/O responsibilities, including dealing with the IDE devices, serial ports, parallel ports, USB bus, any audio capability which might be integrated on the board, etc. The south bridge usually sits on the PCI bus of the chipset’s north bridge. It communicates with the north bridge at 133 MBps over the PCI bus.
Intel Hub Architecture
Like I said, the northbridge/southbridge layout is the most common way of putting a chipset together. Chipset manufacturers other than Intel use this design. Intel used to use this design as well but went to another method after the 440BX chipset. On all of their i8xx chipsets, they use what is called the Intel Hub Architecture (IHA). This architecture still consists of two parts, but they are the AGP Graphics Memory Controller Hub (GMCH) and the I/O Controller Hub.
The GMCH fulfills many of the same functions as the north bridge: communucation with the CPU over the front side bus, memory access, and AGP bus. It also contains integrated video in some cases. But, unlike the northbridge, the GMCH does not have a PCI bus. The PCI bus is handled by the IHA, along with all the typical functions of the south bridge. The ICH communicates with the GMCH at 233 MBps over its own bus. This new bus is optimized by Intel to be able to differentiate certain types of data like streaming video and act accordingly.
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