PC Architecture. Chapter 6. The CPU and the motherboard

Copyright Michael Karbo and ELI Aps., Denmark, Europe.

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    Chapter 6. The CPU and the motherboard

    The heart and soul of the PC’s data processing is the CPU. But the processor is not alone in the world, it communicates with the rest of the motherboard. There will be many new terms introduced in the following sections, so remember that you can find definitions for all the abbreviations in the back of the guide.

    Busses do the transfers

    Data packets (of 8, 16, 32, 64 or more bits at a time) are constantly being moved back and forth between the CPU and all the other components (RAM, hard disk, etc.). These transfers are all done using busses.

    The motherboard is designed around some vary powerful data channels (or pathways, as they are also called). It is these busses which connect all the components to each other.

    Fig.  41. The busses are the data channels which connect the PC’s components together. Some are designed for small transfers, others for large ones.

    Busses with varying capacities

    There is not just one bus on a motherboard; there are several. But they are all connected, so that data can run from one to another, and hence reach the farthest corners of the motherboard.

    We can say that a bus system is subdivided into several branches. Some of the PC components work with enormous amounts of data, while others manage with much less. For example, the keyboard only sends very few bytes per second, whereas the working storage (RAM) can send and receive several gigabytes per second. So you can’t attach RAM and the keyboard to the same bus.

    Two busses with different capacities (bandwidths) can be connected if we place a controller between them. Such a controller is often called a bridge, since it functions as a bridge between the two different traffic systems.

    Fig.  42. Bridges connect the various busses together.

    The entire bus system starts close to the CPU, where the load (traffic) is greatest. From here, the busses work outwards towards the other components. Closest to the CPU we find the working storage. RAM is the component which has the very greatest data traffic, and is therefore connected directly to the CPU by a particularly powerful bus. It is called the front side bus (FSB) or (in older systems) the system bus.

    Fig.  43. The PC’s most important bus looks after the “heavy” traffic between the CPU and RAM.

    The busses connecting the motherboard to the PC’s peripheral devices are called I/O busses. They are managed by the controllers.

    The chip set

    The motherboard’s busses are regulated by a number of controllers. These are small circuits which have been designed to look after a particular job, like moving data to and from EIDE devices (hard disks, etc.).

    A number of controllers are needed on a motherboard, as there are many different types of hardware devices which all need to be able to communicate with each other. Most of these controller functions are grouped together into a couple of large chips, which together comprise the chip set.

    Fig.  44. The two chips which make up the chipset, and which connect the motherboard’s busses.

    The most widespread chipset architecture consists of two chips, usually called the north and south bridges. This division applies to the most popular chipsets from VIA and Intel. The north bridge and south bridge are connected by a powerful bus, which sometimes is called a link channel:

    Fig.  45. The north bridge and south bridge share the work of managing the data traffic on the motherboard.

    The north bridge

    The north bridge is a controller which controls the flow of data between the CPU and RAM, and to the AGP port.

    In Fig. 46  you can see the north bridge, which has a large heat sink attached to it. It gets hot because of the often very large amounts of data traffic which pass through it. All around the north bridge you can see the devices it connects:

    Fig.  46. The north bridge and its immediate surroundings. A lot of traffic runs through the north bridge, hence the heat sink.

    The AGP is actually an I/O port. It is used for the video card. In contrast to the other I/O devices, the AGP port is connected directly to the north bridge, because it has to be as close to the RAM as possible. The same goes for the PCI Express x16 port, which is the replacement of AGP in new motherboards. But more on that later.

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