In personal computers, the front-side bus (FSB) is the bus that carries data between the CPU and the northbridge.
Depending on the processor used, some computers may also have a back-side bus that connects the CPU to the cache. This bus and the cache connected to it are faster than accessing the system memory (or RAM) via the front-side bus.
The bandwidth or maximum theoretical throughput of the front-side bus is determined by the product of the width of its data path, its clock frequency (cycles per second) and the number of data transfers it performs per clock cycle. For example, a 64-bit (8-byte) wide FSB operating at a frequency of 100 MHz that performs 4 transfers per cycle has a bandwidth of 3200 megabytes per second (MB/s).
The number of transfers per clock cycle is dependent on the technology used. For example, GTL+ performs 1 transfer/cycle, EV6 2 transfers/cycle, and AGTL+ 4 transfers/cycle. Intel calls the technique of four transfers per cycle Quad Pumping.
Many manufacturers publish the speed of the FSB in MHz, but often do not use the actual physical clock frequency but the theoretical effective data rate (which is commonly called megatransfers per second or MT/s). This is because the actual speed is determined by how many transfers can be performed by each clock cycle as well as by the clock frequency. For example, if a motherboard (or processor) has a FSB clocked at 200 MHz and performs 4 transfers per clock cycle, the FSB is rated at 800 MT/s.
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The front-side bus is an alternative name for the data and address buses of the CPU as defined by the manufacturer's datasheet. The term is mostly associated with the various CPU buses used on PC-related motherboards (including servers etc), seldom with the data and address buses used in embedded systems and similar small computers.
Front-side buses serve as a connection between the CPU and the rest of the hardware via a chipset. This chipset is usually divided in a northbridge and a southbridge part,and is the connection point for all other buses in the system. Buses like the PCI, AGP, and memory buses all connect to the chipset in order for data to flow between the connected devices. These secondary system buses usually run at speeds derived from the front-side bus clock, but are not necessarily synchronous to it.
In response to AMD's Torrenza initiative, Intel has opened its FSB CPU socket to third party devices [1][2]. Prior to this announcement, made in Spring 2007 at Intel Developer Forum in Beijing, Intel had very closely guarded who had access to the FSB, only allowing Intel processors in the CPU socket. This is now changing, the first example being FPGA co-processors, a result of collaboration between Intel-Xilinx-Nallatech [3] and Intel-Altera-XtremeData [4] [5].
The frequency at which a processor (CPU) operates is determined by applying a clock multiplier to the front-side bus (FSB) speed in some cases. For example, a processor running at 3200 MHz might be using a 400 MHz FSB. This means there is an internal clock multiplier setting (also called bus/core ratio) of 8. That is, the CPU is set to run at 8 times the frequency of the front-side bus: 400 MHz × 8 = 3200 MHz. By varying either the FSB or the multiplier, different CPU speeds can be achieved.
Setting a FSB speed is related directly to the speed grade of memory a system must use. The memory bus connects the northbridge and RAM, just as the front-side bus connects the CPU and northbridge. Often, these two buses must operate at the same frequency. Increasing the front-side bus to 450 MHz in most cases also means running the memory at 450 MHz.
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 400 MHz bus can run with the memory at 500 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 image, audio, video, gaming, FPGA synthesis and scientific applications that perform a small amount of work on each element of a large data set, 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. However, if the computations involving each element are more complex, the processor will spend longer performing these; therefore, the FSB will be able to keep pace because the rate at which the memory is accessed is reduced.
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 are operated at a set fraction of the front-side 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 front-side bus for timing.
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. Almost all 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 by reducing latency between CPU and the northbridge.
This practice pushes components beyond their specifications and may cause erratic behavior, overheating or premature failure. Even if the computer appears to run normally, problems may appear under a heavy load. Most PCs purchased from retailers or manufacturers, such as Hewlett-Packard or Dell, do not allow the user to change the multiplier or FSB 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.
Although the front-side bus architecture is an aging technology, it does have the advantage of high flexibility and low cost. There is no theoretical limit to the number of CPUs that can be placed on a FSB, though performance will not scale linearly across additional CPUs (due to the architecture's bandwidth bottleneck).
The front-side bus as it is traditionally known may be disappearing, but it's still being used in all of Intel's Atom, Celeron, Pentium, and Core 2 processor models. Originally, this bus was a central connecting point for all system devices and the CPU. In recent years, this has been breaking down with the increasing use of individual point-to-point connections like AMD's HyperTransport and Intel's QuickPath Interconnect. The front-side bus has been criticized by AMD as being an old and slow technology that bottlenecks today's computer systems. While a faster CPU can execute individual instructions faster, this is wasted if it cannot fetch instructions and data as fast as it can execute them; when this happens, the CPU must wait for one or more clock cycles until the memory returns its value. Furthermore, a fast CPU can be delayed when it must access other devices attached to the FSB. Thus, a slow FSB can become a bottleneck that slows down a fast CPU. FSB's fastest transfer speed is currently 1.6 GT/s, which provides only 80% of the theoretical bandwidth of a 16-bit HyperTransport 3.0 link as implemented on AM3 Phenom II CPUs, only half of the bandwidth of a 6.4 GT/s QuickPath Interconnect link, and only 25% of the bandwidth of a 32-bit HyperTransport 3.1 link. In addition, in an FSB-based architecture, the memory must be accessed via the FSB. In HT- and QPI-based systems, the memory is accessed independently by means of a memory controller on the CPU itself, freeing bandwidth on the HyperTransport or QPI link for other uses.
| CPU | FSB Clock | Number of Cycles | Bus Width | Transfer Rate |
|---|---|---|---|---|
| Pentium | 50 MHz-66 MHz | 1 | 64-bit | 400 MT/s-528 MT/s |
| Pentium Overdrive | 25 MHz-66 MHz | 1 | 64-bit | 200 MT/s-528 MT/s |
| Pentium MMX | 60 MHz-66 MHz | 1 | 64-bit | 480 MT/s-528 MT/s |
| Pentium MMX Overdrive | 50 MHz-66 MHz | 1 | 64-bit | 400 MT/s-528 MT/s |
| Pentium II | 66 MHz-100 MHz | 1 | 64-bit | 528 MT/s-800 MT/s |
| Pentium II Overdrive | 60 MHz-66 MHz | 1 | 64-bit | 480 MT/s-528 MT/s |
| Pentium III | 100 MHz-133 MHz | 1 | 64-bit | 800 MT/s-1064 MT/s |
| Pentium III-M | 100 MHz-133 MHz | 1 | 64-bit | 800 MT/s-1064 MT/s |
| Pentium 4 | 100 MHz-133 MHz | 4 | 64-bit | 3200 MT/s-4256 MT/s |
| Pentium 4-M | 100 MHz | 4 | 64-bit | 3200 MT/s |
| Pentium 4 HT | 133 MHz-200 MHz | 4 | 64-bit | 4256 MT/s-6400 MT/s |
| Pentium D | 133 MHz-200 MHz | 4 | 64-bit | 4256 MT/s-6400 MT/s |
| Pentium Extreme Edition | 200 MHz-266 MHz | 4 | 64-bit | 6400 MT/s-8512 MT/s |
| Pentium M | 100 MHz-133 MHz | 4 | 64-bit | 3200 MT/s-4256 MT/s |
| Core Solo | 133 MHz-166 MHz | 4 | 64-bit | 4256 MT/s-5312 MT/s |
| Core Duo | 133 MHz-166 MHz | 4 | 64-bit | 4256 MT/s-5312 MT/s |
| Core 2 Solo | 133 MHz-200 MHz | 4 | 64-bit | 4256 MT/s-6400 MT/s |
| Core 2 Duo | 133 MHz-333 MHz | 4 | 64-bit | 4256 MT/s-10656 MT/s |
| Core 2 Quad | 266 MHz-333 MHz | 4 | 64-bit | 8512 MT/s-10656 MT/s |
| Core 2 Extreme | 266 MHz-400 MHz | 4 | 64-bit | 8512 MT/s-12800 MT/s |
| Atom | 133 MHz-166 MHz | 4 | 64-bit | 4256 MT/s-5312 MT/s |
| Celeron | 66 MHz-266 MHz | 1-4 | 64-bit | 528 MT/s-8152 MT/s |
| Celeron D | 133 MHz | 4 | 64-bit | 4256 MT/s |
| Celeron M | 100 MHz-166 MHz | 4 | 64-bit | 3200 MT/s-5312 MT/s |
| Celeron Dual-Core | 133 MHz-200 MHz | 4 | 64-bit | 4256 MT/s-6400 MT/s |
| Pentium Dual-Core | 133 MHz-266 MHz | 4 | 64-bit | 4256 MT/s-8152 MT/s |
| Pentium Pro | 60 MHz-66 MHz | 1 | 64-bit | 480 MT/s-528 MT/s |
| Pentium II Xeon | 100 MHz | 1 | 64-bit | 800 MT/s |
| Pentium III Xeon | 100 MHz-133 MHz | 1 | 64-bit | 800 MT/s-1064 MT/s |
| Xeon | 100 MHz-400 MHz | 4 | 64-bit | 3200 MT/s-12800 MT/s |
| Itanium | 100 MHz-133 MHz | 1 | 64-bit | 800 MT/s-1064 MT/s |
| Itanium 2 | 100 MHz-166 MHz | 4 | 64-bit | 3200 MT/s-5312 MT/s |
| CPU | FSB Clock | Number of Cycles | Bus Width | Transfer Rate |
|---|---|---|---|---|
| Athlon | 100 MHz-133 MHz | 2 | 64-bit | 1600 MT/s-2128 MT/s |
| Athlon XP | 100 MHz-200 MHz | 2 | 64-bit | 1600 MT/s-3200 MT/s |
| Mobile Athlon 4 | 100 MHz | 2 | 64-bit | 1600 MT/s |
| Athlon XP-M | 100 MHz-133 MHz | 2 | 64-bit | 1600 MT/s-2128 MT/s |
| Duron | 100 MHz-133 MHz | 2 | 64-bit | 1600 MT/s-2128 MT/s |
| Sempron | 166 MHz-200 MHz | 2 | 64-bit | 2656 MT/s-3200 MT/s |
| Athlon MP | 100 MHz-133 MHz | 2 | 64-bit | 1600 MT/s-2128 MT/s |
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