Front side bus
\'Front Side Bus (FSB') is the general term used to describe the CPU data bus. This bus carries all information that needs to be passed from the CPU to another device (such as RAM, a PCI expansion card, etc).
On older computers where the L2 cache was not integrated into the CPU, the back side bus goes to the L2 cache cache, and has always been faster than the front side bus.
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2 Current Usage 3 Overclocking / Related Bus Speeds 4 External links |
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+
History
The front side bus as it is traditionaly known is slowly 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.
EV6 Bus
HyperTransport
Current Usage
Most 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.
Most motherboards offer the ability for the user to manually set the clock multiplier and FSB settings by changing jumpers or BIOS settings. Many CPU manufacturers now usually "lock" an unchangeable preset multiplier setting into the chip, meaning manually-set multiplier settings are ignored in favour of the preset multiplier. It is possible to unlock some locked CPUs (namely those from AMD) through the process of connecting electrical contacts across points on the CPU's surface.
For some processors, the FSB speed can be increased to boost processing speed (called overclocking). This overclocking can take different forms, such as overclocking the front side bus higher than the motherboard was designed to go, or overclocking the front side bus for the purposes of overclocking a (usually, locked) CPU.
When a person overclocks or just increases the front side bus speed away from one of the normal speed increments, the PCI bus (and AGP bus) will therefore be overclocked too. For example, if Joe increases the front side bus from 166 MHz to 170 MHz, the PCI bus will also increase up to 34 MHz and the AGP at 68 MHz. If the motherboard were to change dividers to 1/6, the PCI and AGP speeds would be far lower than spec at 28 MHz and 56 MHz. If the FSB is increased all the way to 200 MHz, the board should switch to the 1/6 divider to bring the PCI and AGP speeds back to 33 MHz and 66 MHz.
It is technically possible for the PCI and AGP buses to run at speeds not derived from the front side bus's clock. In the past, few motherboards have ever supported asynchronous PCI. Examples included some Socket 7 motherboards which supported an 83 MHz FSB for clock-doubled 166 MHz Cyrix 6x86s, but wished to be able to run the PCI at a safe speed; 41 MHz was, and still is, beyond the ability of many PCI components to cope with. Nowadays, many motherboards oriented toward power users include the ability to lock the AGP and PCI bus speeds to a value within specifications, while still being able to increase the clock rate of the FSB. Some PCI devices (such as sound cards) won't handle the PCI bus going too far above the default speed. Sometimes hard drive controllers will behave incorrectly in such environments as well.
Similar to the PCI and AGP buses, the Memory bus can sometimes also be ran asynchronously from the front side bus. In Pentium 4 systems, it is common to see memory dividers of ÃÂÃÂ5:4ÃÂÃÂ 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 faster than DRAM manufacturers can keep up, it is sometimes necessary to run the RAM slower than the system bus. This incurs a performance penalty as well, but allows slower RAM to be used with the fast bus speeds of today.
In a typical image processing application where the data set (acquired images) 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.
Overclocking / Related Bus Speeds
CPU
The speed your processor (CPU) runs at is determined by applying a clock multiplier to the front side bus speed. For example, a processor running at 550 MHz might be using a 100 MHz FSB; this means there is a clock multiplier setting of 5.5, thus the CPU is set to run at 5.5 times the MHz speed of the front side bus: basically equating to 100 MHz x 5.5 = 550 MHz. By varying either the FSB or multiplier, several different speeds of CPUs can be made.
PCI / AGP
The PCI and AGP buses, which usually run much slower than the front side bus, use dividers to reduce the clock speed. Typically the PCI bus runs at 33.3 MHz, and the AGP bus runs at twice the PCI bus's speed (66.6 MHz). These dividers (such as ÃÂü, 1/5, 1/6, etc) set the fraction of the FSB speed the PCI bus will run. For example, a 166 MHz FSB will very likely use a 1/5 divider to bring the clock down to 33.2 MHz. A motherboard usually automatically sets these dividers to keep the PCI bus as close to 33 MHz as possible, though some allow the user to manually change them for greater performance.
Memory
When choosing a FSB speed for the CPU you chose, be aware that you'll need to purchase memory capable of this faster speed. Pushing the front-side bus to 170 MHz means you are also pushing your memory, be it PC2700 (or higher, i.e. PC3200), to 170 MHz.
External links