Front-side bus

A typical chipset layout

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 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 do often 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.

History and current usage

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 so-called 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].

Related component speeds

CPU

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.

Memory

See also: Memory divider

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.

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 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

Main article: 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. 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.

Pros and cons

Pros

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 an FSB, though performance will not scale linearly across additional CPUs (due to the architecture's bandwidth bottleneck).

Cons

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 half the bandwidth of a 6.4 GT/s QuickPath Interconnect link, and 80% of the bandwidth of the HyperTransport 3.0 link on AM3 Phenom II CPUs. In addition, with a 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 up the available bandwidth on the QPI or HyperTransport link for other uses (thus giving QPI and HyperTransport a larger advantage over the FSB based systems than it might appear from a bandwidth comparison alone).

Transfer rates

Transfer rate [MB/s]	Frequency [MHz]	CPU
46-153	12-40	Intel 80386
61-191	16-50	Intel 80486
381-503	50-66	Intel Pentium
503-6104	66-200	Intel Celeron
762-1014	100-133	Intel Pentium III
2029-3052	133-200	AMD Athlon XP
3051-8133	100-266.5	Intel Pentium 4
3052-4066	100-133.25	Intel Pentium M
3052-6103	100-200	VIA C7
4066-12207	133.25-400	Intel Core 2

Transfer rate [MB/s]	Bus width [bits]	Cycle	Frequency [MHz]	CPU
528 – 800	64	1	66 – 100	Intel Pentium II
800 – 1064	64	1	100 – 133	Intel Pentium III
3200 – 8528	64	4	100 – 266.5	Intel Pentium 4
4264 – 8528	64	4	133.25 – 266.5	Intel Pentium D
3200 – 4264	64	4	100 – 133.25	Intel Pentium M
4264 – 12800	64	4	133.25 – 400	Intel Core 2

See also

• HyperTransport
• Intel QuickPath Interconnect