Current PC Motherboard Technology

When discussing computers, people put much emphasis on the CPU, RAM, or graphics card, among other things. Many people, however, ignore what is perhaps the most important component of a PC system: the motherboard. The motherboard is the 'glue' that ties all of the other components together, and helps defines many of the PC's performance characteristics. Because the motherboard dictates that speed at which the computer components communicate with each other, a high quality motherboard is essential to PC performance. The most expensive CPU and RAM available won't do you any good without a motherboard that can support them.

One very important motherboard component is the front side bus. The front side bus, also known as the "system bus" is the bus that connects "connects the CPU with the main memory and is used to connect to other components within the computer. (Computer Hope)" Front side bus speed is very important to PC performance. Even with CPU speed reaching into the 3 GHz range, the CPU can only pull data out of RAM at the speed of the front side bus. Because of this, the front side bus can be a major bottleneck. currently, front side bus speeds range from 66MHz, to the more common 133MHz, and up to 533MHz.

When talking about modern motherboard solutions for the PC, it is important to understand two of the most important parts of a motherboard: the North Bridge and the South Bridge. The names refer to which components the two 'bridges' (also called hubs) interact with on the motherboard. The North Bridge deals with components that 'live' to the 'north' of the PCI bus, and the South Bridge deals with components that 'live' to the 'south' of the PCI bus. The North Bridge deals only with very fast components, such as the CPU, RAM, and the AGP bus (a very fast bus dedicated to graphics cards). The South Bridge "is intended as the place to integrate all the (slower) peripherals like IDE, ISA, USB, etc (Gardner)". Because of the increasing speeds of both bridges, the PCI bus now commonly reports to the South Bridge, although this was not always the case. The bridges both communicate with each other and directly to the CPU.

The main function of the North Bridge is memory arbitration. The North Bridge "must deal with a 4-way intersection where everyone wants easy access to the road that leads to DRAM. (Gardner)" Every device connected to the North Bridge needs fast access to the RAM. In order to supply fast, concurrent access to RAM, the North Bridge provides several "buffers," so that it can cache data that needs to be written to the RAM until the RAM is available to be written to.

The South Bridge's job is to coordinate the peripherals and report to the North Bridge. At one time, the North and the South Bridge communicated with each other over the PCI bus. However, this 33 MHz bus became a bottleneck, so motherboard vendors are now using various proprietary connections between the North Bridge and South Bridge on their motherboards. Some of these connection techniques, such as HyperTransport are "designed to operate at multiple clock speeds from 200MHz up to 800MHz. (HyperTransport Consortium)"

Another very important consideration when evaluating motherboards is the type of CPU connector used. There are currently six different common CPU connector type, all of them incompatible with each other! Then connector types include socket and slot designs, with the L2 cache both on and off the processor. The different CPU connectors do not differ much in terms of performance, but it is important to keep CPU connector type in consideration when purchasing a motherboard in order to insure your motherboard is compatible with your choice of CPU. As Arstechnica notes, "March forth fully aware that your CPU choice determines whether you buy Slot 1/PGA 370 on the Intel side, and Socket A/Slot A on the AMD side of the fence (Darwing & Caesar)"

Another consideration when purchasing a motherboard is the level of device integration it provides. Many motherboard manufacturers choose to integrate certain common components, such as a graphics adapter or LAN adapter, with the motherboard. This practice has both its advantages and disadvantages. This type of integration can be very convenient for the user. With no need to install a separate audio and video card, for example, the user can both save money and the hassle of trying to configure third-party components. However, if an integrated component breaks, it can not be easily replaced, as with an add-on card. Using a motherboard with integrated components also hurts upgradability and customizability. Many power users prefer to buy a motherboard with very few integrated components and purchase carefully chosen expansion cards from third-party vendors.

There are many important factors to consider when purchasing a motherboard. However, I believe that three important factors stand above the rest. First and foremost is a high speed front side bus. Without a fast front side bus, processor speed means almost nothing; what is a processor without data to process? After a fast front side bus, the North and South Bridge should be considered. These components tie together every other computer component. A low quality North Bridge or South Bridge will degrade the performance of the entire system. The next factor to consider is the level of component integration. It is important to decide whether you want the convenience of a highly integrated motherboard, or the customizability of a motherboard without many integrated components. The choice of motherboard should be made very carefully, after every factor is considered. The motherboard is, in many ways, the most important component in a PC, and therefore requires the most care in its choosing.

References cited:

"Front Side Bus," Computer Hope, <http://www.computerhope.com/jargon/f/fsb.htm> <http://www.computerhope.com/>

"PC Motherboard Technology," J. Scott Gardner, <http://www.extremetech.com/article2/0,3973,9962,00.asp> <http://www.extremetech.com/>

"HyperTransport Consortium FAQS," HyperTransport Consortium, <http://www.hypertransport.org/faqs.html> <http://www.hypertransport.org/>

"Motherboard Buying Guide," Darwing & Caesar, <http://arstechnica.com/guide/mobo/mobo-1.html> <http://arstechnica.com/>

Paper for cs312 (Computer Architecture)
Fall Quarter, 2002
node your homework

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