Front side busFront Side Bus (FSB) is the term used to describe the CPU data bus. This bus carries all information that passes from the CPU to other devices within the system such as RAM, PCI expansion cards, hard disks, etc. On older computers where the L2 cache was not integrated into the CPU, the back side bus connects to the L2 cache, and using this was faster than accessing the RAM via the front side bus. With modern CPUs having L2 cache on the processor die itself, this bus no longer exists, as the CPU's cache is simply checked for the appropriate data before a read request is issued to main memory.
HistoryThe frontside bus has been a part of computer architecture since applications first started using more memory than a CPU (a very complex integrated circuit) could reasonably hold. 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 the use of more and more individual point-to-point buses. Three recent bus technologies are GTL+, EV6, and HyperTransport. Each bus is unique in how it moves data within the system between the CPU and devices. GTL+/AGTL+
EV6 Bus
HyperTransport
Current usageMost modern buses (both GTL+ and EV6) serve as a backbone between the CPU and a chipset. This chipset (usually a combination of northbridge and 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. In general, a faster front side bus means higher processing speeds and a faster computer for a number of reasons which are outlined below. Overclocking and related bus speedsCPUThe 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 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. Most 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, which causes the manually-set multiplier settings to be ignored in favour of the locked-in multiplier. 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, the FSB speed can be increased to boost processing speed (called overclocking). MemorySetting 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 like the frontside bus connects the CPU and northbridge. Often, these two buses must operate at the same frequency. Pushing the front-side bus to 170 MHz means pushing the memory to 170 MHz in most cases. Similar to the PCI and AGP buses, however, the memory bus can sometimes also be run asynchronously from the front side bus. In Pentium 4 systems, it is possible to see memory ratios of "5:4" and the like. The bus will run 5/4 faster then the memory in this situation, meaning a 200 MHz bus can run with the memory at only 160 MHz. With bus speeds increasing rapidly, it is sometimes necessary to run the RAM at a lower frequency than the system bus in order to stay within the limitations of the DRAM modules on the memory stick. This incurs a performance penalty, but it allows slower RAM to be used with the faster bus speeds that some processors were designed for. In a complex image, audio, video, scientific, and encryption 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. Some sample FSB frequencies and bandwidths
External links
Categories: Computer terminology | Computer buses |
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