Front side bus
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In computers, the front side bus (FSB) or system bus is a term for the physical bi-directional data bus that carries all electronic signal information between the central processing unit (CPU) and other devices within the system such as random access memory (RAM), AGP video cards, PCI expansion cards, hard disks, the memory containing the system BIOS, etc.
Some computers have an L2 or L3 memory cache external to the CPU connected via a back side bus. This bus and the cache memory connected to it is faster than accessing the system RAM via the front side bus.
The maximum theoretical bandwidth of the front side bus is determined by the product of its width, its frequency and the number of data transfers it performs per clock tick. For example, a 32-bit (4-byte) wide FSB with a frequency of 100 MHz that performs 4 transfers/tick has a maximum bandwidth of 1600 MB/second. The number of transfers per tick is dependent on the technology used, with (for example) GTL+ offering 2 transfers/tick, EV6 4 transfers/tick, and AGTL+ 8 transfers/tick.
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[edit] History and Current usage
The frontside bus has been a part of computer architecture since applications first started using more memory than a CPU could reasonably hold.
Most modern front side buses serve as a backbone between the CPU and a chipset. This chipset (usually a combination of a northbridge and a southbridge) is the connection point for all other buses in the system. The PCI, AGP, and memory buses all connect to the chipset to allow for data to flow between the connected devices.
These secondary system buses usually run at speeds derived from the front side bus speed. While a faster CPU can execute individual instructions faster, this is wasted if it can't fetch instructions and data as fast as it can execute it, when this happens the CPU must wait (do nothing for one or more clock cycles until the memory returns its value). Further, a fast CPU can be delayed when it must access other devices attached to the FSB. Thus, a slow FSB can become a bottleneck slowing down a fast CPU.
The front side bus as it is traditionally known may be disappearing. Originally, this bus was a central connecting point for all system devices and the CPU. However, in recent years this has been breaking down with increasing use of individual point-to-point buses.
[edit] Related Component Speeds
[edit] CPU
The frequency at which a processor (CPU) operates is determined by applying a clock multiplier to the front side bus (FSB) speed. For example, a processor running at 550 MHz might be using a 100 MHz FSB. This means there is an internal clock multiplier setting (also called bus/core ratio) of 5.5; the CPU is set to run at 5.5 times frequency of the front side bus: 100 MHz x 5.5 = 550 MHz. By varying either the FSB or the multiplier, different CPU speeds can be achieved.
[edit] Memory
Setting a FSB speed is related directly to the speed grade of memory that a system must use. The memory bus connects the northbridge and RAM, just as the frontside bus connects the CPU and northbridge. Often, these two buses must operate at the same frequency. Increasing the front-side bus to 170 MHz means also running the memory at 170 MHz in most cases.
In newer systems, it is possible to see memory ratios of "4:5" and the like. The memory will run 5/4 times as fast as the FSB in this situation, meaning a 133 MHz bus can run with the memory at 166 MHz. This is often referred to as an 'asynchronous' system. It is important to realize that, due to differences in CPU and system architecture, overall system performance can vary in unexpected ways with different FSB-to-memory ratios.
In complex image, audio, video, gaming, and scientific applications where the data set is large, FSB speed becomes a major performance issue. A slow FSB will cause the CPU to spend significant amounts of time waiting for data to arrive from system memory.
[edit] Peripheral Buses
Similar to the memory bus, the PCI and AGP buses can also be run asynchronously from the front side bus. In older systems, these buses operated at a set fraction of the frontside bus frequency. This fraction was set by the BIOS. In newer systems the PCI, AGP, and PCI Express peripheral buses often receive their own clock signals, which eliminates their dependence on the frontside bus for timing.
[edit] Overclocking
Overclocking is the practice of making computer components operate beyond their stock performance levels.
Many motherboards allow the user to manually set the clock multiplier and FSB settings by changing jumpers or BIOS settings. Many CPU manufacturers now "lock" a preset multiplier setting into the chip. It is possible to unlock some locked CPUs; for instance, some Athlons can be unlocked by connecting electrical contacts across points on the CPU's surface. For all processors, increasing the FSB speed can be done to boost processing speed.
This practice pushes components beyond their specifications and may cause erratic behaviour, overheating or premature failure. Even if the computer appears to run normally, under heavy load, problems may appear. For example, during Windows Setup, you may receive a file copy error or experience other problems [1]. Most PCs purchased from retailers or manufacturers, such as Hewlett-Packard or Dell, do not enable the user to change the multiplier or Front Side Bus settings due to the probability of erratic behavior or failure. Motherboards purchased separately to build a custom machine are more likely to allow the user to edit the multiplier and FSB settings in the PC's BIOS.
