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No matter how simple or elaborate the chipset in a modern computer, it has three chief functions. It must act as a system controller that holds together the entire computer, giving all the support the microprocessor needs to be a true computer system. As a memory controller, it links the microprocessor to the memory system, establishes the main memory and cache architectures, and ensures the reliability of the data stashed away in RAM chips. And to extend the reach of the microprocessor to other system components, it must act as a peripheral controller and operate input/output ports and disk interfaces.

How these functions get divided among silicon chips and the terminology describing the chips has itself evolved.

When computers first took their current form, incorporating the PCI system for linking components together, engineers lumped the system and memory functions together as a unit they called the north bridge. Intel sometimes uses the term host bridge to indicate the same function. The peripheral control functions make up the south bridge, sometimes called the I/O bridge. The circuits that controlled the expansion bus often were grouped together with the north bridge or as a separate chip termed the PCI bridge.

Intel has reworked its design and nomenclature to make the main liaison with the microprocessor into the memory controller hub. This chip includes the essential support functions of the north bridge. It provides a wide, fast data pathway to memory and to the system's video circuitry.

The I/O controller hub takes over all the other interface functions of the chipset, including control of the expansion bus, all disks, all ports, and sound and power management. The host controller provides a high-speed connection between the I/O controller hub and the microprocessor.

The memory control hub links to the microprocessor through its system bus, sometimes termed the front side bus, which provides the highest speed linkup available to peripheral circuits. The logic inside the memory control hub translates the microprocessor's address requests, pumped down the system bus as address cycles, into the form used by the memory in the system. Because memory modules differ in their bus connections, the memory hub controller circuitry determines what kind of memory can connect to the system. Some memory hub controllers, for example, link only to Rambus memory, whereas others may use double data-rate (DDR) modules.

Although the connection with graphics circuitry naturally is a part of the input/output system of a computer, the modern chipset design puts the link in the memory hub controller. Video displays involve moving around large blocks of memory, and the memory host controller provides the highest-speed connection with the microprocessor, speeding video access and screen updates. In addition, the design of the current video interconnection standard, accelerated graphics port (AGP), uses a memory aperture to transfer bytes between the microprocessor and the frame buffer (the memory that stores the image on the display screen). Accessing this aperture is easiest and most direct through the memory controller.

The I/O hub controller generates all the signals used for controlling the microprocessor and the operation of the computer. It doesn't tell the microprocessor what instructions to execute. Rather, it sets the speed of the system by governing the various clocks in the computer, it manages the microprocessor's power usage (as well as that of the rest of the computer), and it tells the microprocessor when to interrupt one task and switch to another or give a piece of hardware immediate attention. These are all system-control functions, once the province of a dedicated system controller chip.

In addition, the I/O hub controller provides both the internal and external links to the rest of the computer and its peripherals, its true input/output function. It controls the computer's expansion bus (which not only generates the signals for the expansion slots for add-in cards but also provides the high-speed hark disk interface), all the ports (including USB), and network connections. Audio circuitry is sometimes built in to the I/O hub controller, again linking through the expansion bus circuitry.

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