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The single most important piece of hardware that you will buy is the motherboard - the very core of your PC. The processor plugs into it, drives connect to it with cables, expansion cards live in special slots and everything else, from the mouse to the printer, is ultimately connected to and controlled by the motherboard. If you buy a PC from a shop, chances are you'll never think about or even see the motherboard; but when you build a system from scratch, it must be your primary consideration. Everything else follows from here.

Form factor: This is the way of describing the motherboard's size and shape, important because it involves industry-wide standards and ties in with the computer case and power supply. Form factors have evolved through the years, culminating since 1995 in a popular and flexible standard known as ATX. Not just one ATX standard, of course: there are MiniATX, MicroATX and FlexATX motherboards out there, all progressively slimmed-down versions of full-size ATX. The upside of a smaller motherboard is that you can use a smaller case and reduce the overall dimensions of your computer; the downside is a corresponding reduction in expandability. A full-sized ATX motherboard can have up to seven expansion slots while a MicroATX motherboard is limited to four.

One technical benefit of ATX over the earlier BabyAT form factor from which it directly evolved is that full-length expansion cards can now be fitted in all slots; previously, the location of the processor and memory on the motherboard meant that some slots could only take stumpy (not a technical term) cards. Another is the use of a double-height input/output panel that lets motherboard manufacturers build-in more integrated features. All in all, it's a definite improvement.

But from your point of view, the main attraction has to be the guarantee that any ATX motherboard, including the smaller versions, will fit inside any ATX computer case. That's the beauty of standards.

Chipset: The real meat of a motherboard resides in its chipset: a collection of microchips that together control all the major functions. Without a chipset, a motherboard would be lifeless; with a duff chipset, it may be inadequate for your needs. Indeed, as one motherboard manufacturer explained it to us, the chipset is the motherboard: don't ask what this or that motherboard can do -ask instead what chipset it uses and there you'll find your answer.

So what does a chipset do, precisely? Well, at one level it controls the flow of data between motherboard components through a series of interfaces. Each interface, or channel, is called a bus. The most important buses are:

FSB (Front Side Bus) The interface between the Northbridge component of the chipset and the processor.

Memory bus The interface between the chipset and RAM.

AGP (Accelerated Graphics Port) The interface between the chipset and the AGP port. This is gradually disappearing from motherboards as more and more video cards are designed for the PCI Express slot.

PCI (Peripheral Component Interconnect) bus The interface between the chipset and PCI expansion slots. Pretty much any expansion card can be installed here, including sound cards, network cards and TV tuners. The exception is a video card, as these are, or were, designed for the higher bandwidth AGP interface. Like AGP, PCI is gradually giving way to PCI Express.

PCI Express bus The interface between the chipset and PCI Express expansion slots. There may be two separate buses determined by the bandwidth of the slots. For instance, the motherboard may have a 16-speed PCI Express slot for the video card and one or more slower slots for standard expansion cards.

IDE (Integrated Drive Electronics) bus The interface between the chipset and hard & CD/DVD drives.

SATA (Serial Advanced Technology Attachment) bus An alternative interface between the chipset and hard & CD?DVD drives which will eventually completely replace the IDE bus.

And then there are buses controlling the floppy disk drive, parallel and serial ports, USB and FireWire, integrated audio and more.

Bus bandwidths

Not all buses are equal. Far from it, in fact: they operate at different speeds and have different 'widths'. For example, the basic single-speed (lx) AGP specification has a clock speed of 66.6MHz (usually expressed as 66MHz). This means that over 66 million units of data can pass between the video card and the chipset through the bus per second. However, the AGP bus transfers 32 bits of data (that's 32 individual I's and O's) with every clock cycle, so the true measure of the bus is not its speed alone but rather the overall rate at which data is transferred. This is known as the bandwidth of a bus. In this case, 32 bits pass through the bus 66 million times per second. This equates to a bandwidth of 266MB/sec.

Just to be clear, using round figures, here's the sum:

66,600,000 clock cycles x 32 bits = 2,131,200,000 bits/sec There are 8 bits in a byte (B), so this equals 266,400,000B/sec

There are 1,000 bytes in a kilobyte (KB), so this equals 266,400KB/sec There are 1,000 kilobytes in a megabyte (MB), so this equals 266MB/sec

Looked at another way, the AGP bus transfers sufficient data to fill a recordable CD every three seconds.

It's also possible to run the AGP bus up to eight times faster, which boosts the bandwidth to over 2 gigabytes per sec. This is the kind of speed you need for playing games. By contrast, the PCI bus runs at only 133MB/sec. This is fine for many purposes but not for three-dimensional video.

Chipset architecture We needn't linger on the physical design of chipsets except to comment briefly on the terminology you are likely to encounter:

Northbridge The primary chip in a chipset, it typically controls the processor, memory and video buses.

Southbridge A second chip that typically incorporates the PCI, IDE/SATA and USB buses.

Processor and memory support The two most important questions with any motherboard, and hence computer, are which processor family and what kind of memory does it work with?

For instance, if you decide that you want to build a Pentium 4-based system, you'll need a motherboard with either a Socket 478 or a Socket 775 to house it; and if you want an AthIon-based system, you'll be looking for a Socket 939, 940 or 974. So far, so confusing. It gets all the more so when you factor in the many possible permutations of memory support, including bus speed, number of slots on the motherboard and whether it offers single-channel or dual-channel performance. We'll cover all of this in due course.

Need a further complication? Intel makes its own chipsets, which means ifs easy to compare like for like, but AMD largely relies on third-party manufacturers to come up with compatible chipsets for its processors. There's nothing wrong with AMD's stance on this - and indeed it opens the market to chipset manufacturers which would otherwise be squeezed out by Intel's dominance - but it does make motherboard comparisons slightly trickier.