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A front-side bus (FSB) is a computer communication interface (
bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...
) that was often used in
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...
-chip-based computers during the 1990s and 2000s. The EV6 bus served the same function for competing AMD CPUs. Both typically carry data between the
central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, an ...
(CPU) and a memory controller hub, known as the northbridge. Depending on the implementation, some computers may also have a
back-side bus In personal computer microprocessor architecture, a back-side bus (BSB), or backside bus, was a computer bus used on early Intel platforms to connect the CPU to CPU cache memory, usually off-die L2. If a design utilizes it along with a front-sid ...
that connects the CPU to the
cache Cache, caching, or caché may refer to: Places United States * Cache, Idaho, an unincorporated community * Cache, Illinois, an unincorporated community * Cache, Oklahoma, a city in Comanche County * Cache, Utah, Cache County, Utah * Cache County ...
. This bus and the cache connected to it are faster than accessing the system memory (or RAM) via the front-side bus. The speed of the front side bus is often used as an important measure of the performance of a computer. The original front-side bus architecture has been replaced by
HyperTransport HyperTransport (HT), formerly known as Lightning Data Transport, is a technology for interconnection of computer processors. It is a bidirectional serial/ parallel high-bandwidth, low- latency point-to-point link that was introduced on Apri ...
,
Intel QuickPath Interconnect The Intel QuickPath Interconnect (QPI) is a point-to-point microprocessor, processor electrical connection, interconnect developed by Intel which replaced the front-side bus (FSB) in Xeon, Itanium, and certain desktop platforms starting in 2008. I ...
or
Direct Media Interface In computing, Direct Media Interface (DMI) is Intel's proprietary link between the northbridge and southbridge on a computer motherboard. It was first used between the 9xx chipsets and the ICH6, released in 2004. Previous Intel chipsets had us ...
in modern volume CPUs.


History

The term came into use by Intel Corporation about the time the
Pentium Pro The Pentium Pro is a sixth-generation x86 microprocessor developed and manufactured by Intel and introduced on November 1, 1995. It introduced the P6 microarchitecture (sometimes termed i686) and was originally intended to replace the original P ...
and
Pentium II The Pentium II brand refers to Intel's sixth-generation microarchitecture (" P6") and x86-compatible microprocessors introduced on May 7, 1997. Containing 7.5 million transistors (27.4 million in the case of the mobile Dixon with 256  KB ...
products were announced, in the 1990s. "Front side" refers to the external interface from the processor to the rest of the computer system, as opposed to the back side, where the
back-side bus In personal computer microprocessor architecture, a back-side bus (BSB), or backside bus, was a computer bus used on early Intel platforms to connect the CPU to CPU cache memory, usually off-die L2. If a design utilizes it along with a front-sid ...
connects the cache (and potentially other CPUs). A front-side bus (FSB) is mostly used on PC-related
motherboard A motherboard (also called mainboard, main circuit board, mb, mboard, backplane board, base board, system board, logic board (only in Apple computers) or mobo) is the main printed circuit board (PCB) in general-purpose computers and other expand ...
s (including personal computers and servers). They are seldom used in
embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...
s or similar small computers. The FSB design was a performance improvement over the single
system bus A system bus is a single computer bus that connects the major components of a computer system, combining the functions of a data bus to carry information, an address bus to determine where it should be sent or read from, and a control bus to dete ...
designs of the previous decades, but these front-side buses are sometimes referred to as the "system bus". Front-side buses usually connect the CPU and the rest of the hardware via a
chipset In a computer system, a chipset is a set of electronic components An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are ...
, which Intel implemented as a northbridge and a southbridge. Other buses like the
Peripheral Component Interconnect Peripheral Component Interconnect (PCI) is a local computer bus for attaching hardware devices in a computer and is part of the PCI Local Bus standard. The PCI bus supports the functions found on a processor bus but in a standardized format th ...
(PCI),
Accelerated Graphics Port Accelerated Graphics Port (AGP) is a parallel expansion card standard, designed for attaching a video card to a computer system to assist in the acceleration of 3D computer graphics. It was originally designed as a successor to PCI-type connect ...
(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 synchronized to it. In response to
AMD Advanced Micro Devices, Inc. (AMD) is an American multinational semiconductor company based in Santa Clara, California, that develops computer processors and related technologies for business and consumer markets. While it initially manufactur ...
's
Torrenza Torrenza was an initiative announced by Advanced Micro Devices (AMD) in 2006 to improve support for the integration of specialized coprocessors in systems based on AMD Opteron microprocessors. Torrenza does not refer to a specific product or specif ...
initiative, Intel opened its FSB CPU socket to third party devices. Prior to this announcement, made in Spring 2007 at
Intel Developer Forum The Intel Developer Forum (IDF) was a biannual gathering of technologists to discuss Intel products and products based on Intel products. The first IDF was held in 1997. To emphasize the importance of China, the Spring 2007 IDF was held in Beiji ...
in
Beijing } Beijing ( ; ; ), alternatively romanized as Peking ( ), is the capital of the People's Republic of China. It is the center of power and development of the country. Beijing is the world's most populous national capital city, with over 21 ...
, Intel had very closely guarded who had access to the FSB, only allowing Intel processors in the CPU socket. The first example was
field-programmable gate array A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturinghence the term '' field-programmable''. The FPGA configuration is generally specified using a hardware d ...
(FPGA) co-processors, a result of collaboration between Intel-
Xilinx Xilinx, Inc. ( ) was an American technology and semiconductor company that primarily supplied programmable logic devices. The company was known for inventing the first commercially viable field-programmable gate array (FPGA) and creating the fi ...
-
Nallatech Nallatech is a computer hardware and software firm based in Camarillo, California, United States. The company specializes in field-programmable gate array (FPGA) integrated circuit technology applied in computing. As of 2007 the company's prim ...
and Intel-
Altera Altera Corporation was a manufacturer of programmable logic devices (PLDs) headquartered in San Jose, California. It was founded in 1983 and acquired by Intel in 2015. The main product lines from Altera were the flagship Stratix series, mid-rang ...
-XtremeData (which shipped in 2008).


Related component speeds


CPU

The
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
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 The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
might be using a 400 MHz FSB. This means there is an internal
clock multiplier In computing, the clock multiplier (or CPU multiplier or bus/core ratio) sets the ratio of an internal CPU clock rate to the externally supplied clock. A CPU with a 10x multiplier will thus see 10 internal cycles (produced by PLL-based frequency ...
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. Different CPU speeds are achieved by varying either the FSB frequency or the CPU multiplier, this is referred to as
Overclocking In computing, overclocking is the practice of increasing the clock rate of a computer to exceed that certified by the manufacturer. Commonly, operating voltage is also increased to maintain a component's operational stability at accelerated spe ...
or
Underclocking Underclocking, also known as downclocking, is modifying a computer or electronic circuit's timing settings to run at a lower clock rate than is specified. Underclocking is used to reduce a computer's power consumption, increase battery life, redu ...
.


Memory

Setting an 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. Due to differences in CPU and system architecture, overall system performance can vary in unexpected ways with different FSB-to-memory ratios. In
image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
,
audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound *Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum *Digital audio, representation of sound ...
,
video Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syste ...
,
gaming Gaming may refer to: Games and sports The act of playing games, as in: * Legalized gambling, playing games of chance for money, often referred to in law as "gaming" * Playing a role-playing game, in which players assume fictional roles * Playin ...
,
FPGA A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturinghence the term '' field-programmable''. The FPGA configuration is generally specified using a hardware de ...
synthesis and scientific applications that perform a small amount of work on each element of a large
data set A data set (or dataset) is a collection of data. In the case of tabular data, a data set corresponds to one or more database tables, where every column of a table represents a particular variable, and each row corresponds to a given record of the ...
, 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 computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
. In newer systems, the PCI, AGP, and
PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common ...
peripheral buses often receive their own
clock signal In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits. A clock sign ...
s, which eliminates their dependence on the front-side bus for timing.


Overclocking

Overclocking In computing, overclocking is the practice of increasing the clock rate of a computer to exceed that certified by the manufacturer. Commonly, operating voltage is also increased to maintain a component's operational stability at accelerated spe ...
is the practice of making computer components operate beyond their stock performance levels by manipulating the frequencies at which the component is set to run, and, when necessary, modifying the voltage sent to the component to allow it to operate at these higher frequencies with more stability. Many motherboards allow the user to manually set the clock multiplier and FSB settings by changing
jumper Jumper or Jumpers may refer to: Clothing *Jumper (sweater), a long-sleeve article of clothing; also called a top, pullover, or sweater **A waist-length top garment of dense wool, part of the Royal Navy uniform and the uniform of the United State ...
s 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 AMD
Athlon Athlon is the brand name applied to a series of x86-compatible microprocessors designed and manufactured by Advanced Micro Devices (AMD). The original Athlon (now called Athlon Classic) was the first seventh-generation x86 processor and the fi ...
processors can be unlocked by connecting
electrical contact An electrical contact is an electrical circuit component found in electrical switches, relays, connectors and circuit breakers. Each contact is a piece of electrically conductive material, typically metal. When a pair of contacts touch, they ca ...
s across points on the CPU's surface. Some other processors from AMD and Intel are unlocked from the factory and labeled as an "enthusiast-grade" processors by end users and retailers because of this feature. 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 The Hewlett-Packard Company, commonly shortened to Hewlett-Packard ( ) or HP, was an American multinational information technology company headquartered in Palo Alto, California. HP developed and provided a wide variety of hardware components ...
or
Dell Dell is an American based technology company. It develops, sells, repairs, and supports computers and related products and services. Dell is owned by its parent company, Dell Technologies. Dell sells personal computers (PCs), servers, data ...
, 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.


Evolution

The front-side bus had the advantage of high flexibility and low cost when it was first designed. Simple
symmetric multiprocessor Symmetric multiprocessing or shared-memory multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all ...
s place a number of CPUs on a shared FSB, though performance could not scale linearly due to bandwidth bottlenecks. The front-side bus was used in all
Intel Atom Intel Atom is the brand name for a line of IA-32 and x86-64 instruction set ultra-low-voltage processors by Intel Corporation designed to reduce electric consumption and power dissipation in comparison with ordinary processors of the Intel Cor ...
,
Celeron Celeron is Intel's brand name for low-end IA-32 and x86-64 computer microprocessor models targeted at low-cost personal computers. Celeron processors are compatible with IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called ...
,
Pentium Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The original Pentium processor from which the brand took its name was first released on March 22, 1993. After that, the Pentium II and Pe ...
,
Core 2 Intel Core 2 is the processor family encompassing a range of Intel's consumer 64-bit x86-64 single-, dual-, and quad-core microprocessors based on the Core microarchitecture. The single- and dual-core models are single- die, whereas the quad-cor ...
, and
Xeon Xeon ( ) is a brand of x86 microprocessors designed, manufactured, and marketed by Intel, targeted at the non-consumer workstation, server, and embedded system markets. It was introduced in June 1998. Xeon processors are based on the same arc ...
processor models through about 2008. Originally, this bus was a central connecting point for all system devices and the CPU. The potential of a faster CPU is wasted if it cannot fetch instructions and data as quickly as it can execute them. The CPU may spend significant time idle while waiting to read or write data in main memory, and high-performance processors therefore require high bandwidth and low latency access to memory. The front-side bus was criticized by
AMD Advanced Micro Devices, Inc. (AMD) is an American multinational semiconductor company based in Santa Clara, California, that develops computer processors and related technologies for business and consumer markets. While it initially manufactur ...
as being an old and slow technology that limits system performance. More modern designs use point-to-point and serial connections like AMD's
HyperTransport HyperTransport (HT), formerly known as Lightning Data Transport, is a technology for interconnection of computer processors. It is a bidirectional serial/ parallel high-bandwidth, low- latency point-to-point link that was introduced on Apri ...
and Intel's
DMI 2.0 In computing, Direct Media Interface (DMI) is Intel's proprietary link between the Northbridge (computing), northbridge and Southbridge (computing), southbridge on a computer motherboard. It was first used between the 9xx chipsets and the I/O Con ...
or QuickPath Interconnect (QPI). These implementations remove the traditional northbridge in favor of a direct link from the CPU to the
Platform Controller Hub The Platform Controller Hub (PCH) is a family of Intel's single-chip chipsets, first introduced in 2009. It is the successor to the Intel Hub Architecture, which used two chips - a Northbridge (computing), northbridge and Southbridge (computing), ...
, southbridge or I/O controller. In a traditional architecture, the front-side bus served as the immediate data link between the CPU and all other devices in the system, including main memory. In HyperTransport- and QPI-based systems, system memory is accessed independently by means of a
memory controller The memory controller is a digital circuit that manages the flow of data going to and from the computer's main memory. A memory controller can be a separate chip or integrated into another chip, such as being placed on the same die or as an int ...
integrated into the CPU, leaving the bandwidth on the HyperTransport or QPI link for other uses. This increases the complexity of the CPU design but offers greater throughput as well as superior scaling in multiprocessor systems.


Transfer rates

The
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
or maximum theoretical throughput of the front-side bus is determined by the product of the width of its data path, its
clock frequency In computing, the clock rate or clock speed typically refers to the frequency at which the clock generator of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the pr ...
(cycles per second) and the number of data transfers it performs per clock cycle. For example, a 64-
bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...
(8-
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
) wide FSB operating at a frequency of 100 MHz that performs 4 transfers per cycle has a bandwidth of 3200
megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...
s per second (MB/s): :8 bytes/transfer × 100 MHz × 4 transfers/cycle = 3200 MB/s The number of transfers per
clock cycle In electronics and especially synchronous digital circuits, a clock signal (historically also known as ''logic beat'') oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits. A clock sig ...
depends on the technology used. For example, GTL+ performs 1 transfer/cycle, EV6 2 transfers/cycle, and
AGTL+ Gunning transceiver logic (GTL) is a type of logic signaling used to drive electronic backplane buses. It has a voltage swing between 0.4 volts and 1.2 volts—much lower than that used in TTL and CMOS logic—and symmetrical parallel resistive t ...
4 transfers/cycle. Intel calls the technique of four transfers per cycle
Quad Pumping Quad as a word or prefix usually means 'four'. It may refer to: Government * Quadrilateral Security Dialogue, a strategic security dialogue between Australia, India, Japan, and the United States * Quadrilateral group, an informal group which in ...
. Many manufacturers publish the frequency of the front-side bus in MHz, but marketing materials often list the theoretical effective signaling rate (which is commonly called
megatransfer In computer technology, transfers per second and its more common secondary terms gigatransfers per second (abbreviated as GT/s) and megatransfers per second (MT/s) are informal language that refer to the number of operations transferring data that ...
s per second or MT/s). For example, if a motherboard (or processor) has its bus set at 200 MHz and performs 4 transfers per clock cycle, the FSB is rated at 800 MT/s. The specifications of several generations of popular processors are indicated below.


Intel processors


AMD processors


References

{{DEFAULTSORT:Front-Side Bus Computer buses Motherboard