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The front side bus (FSB) connects the CPU with system memory and peripherals. It has a northbridge and southbridge, and its speed is measured in frequency, width, and data transfer rate. The FSB has been replaced by point-to-point connections for better performance and scalability. The CPU to FSB ratio affects computer performance, and dual-bus architecture has a front and back side bus. The FSB has been phased out and replaced by newer systems like Intel’s QuickPath Interconnect and AMD’s HyperTransport.
On a computer motherboard, the front side bus is a communication interface that connects the central processing unit with system memory and other components, such as peripherals, by transferring data back and forth between the CPU and other components.
What does the bus do on the front side?
The FSB allows computer components to communicate through a chipset. In Intel-based computers, the chipset has a northbridge (the hub of the memory controller) and a southbridge (the hub of the I/O controller).
The northbridge typically connects to the memory slots and also to the graphics card via a high-speed graphics bus (e.g. Accelerated Graphics Port). The Southbridge manages connections with peripherals through a bus as a Peripheral Component Interconnect. The speed of the FSB generally determines the speeds of the secondary system buses.
Fun fact
In a computer, a bus facilitates two-way data transfers between components.
Bus terminology differs between manufacturers. Intel makes FSB while AMD offers EV6 buses.
The terms system bus, memory bus, and processor bus can refer to an FSB.
In most modern computers, the FSB architecture has been replaced by point-to-point connections which support faster performance and better scalability.
The FSB architecture was essentially phased out by 2008.
How do you measure the speed of the bus on the front side?
When it comes to the effect of an FSB on a computer’s performance, there are three main factors: clock frequency, width, and data transfer rate. The frequency of the FSB is measured in megahertz (1,000,000 cycles/second) and is often referred to as the FSB speed. Speeds vary widely, from older models with a frequency of 66MHz to newer examples with speeds above 1GHz.
The width (expressed in bits) of an FSB is another key specification. Most FSB models are 32 or 64 bit wide. The other important metric is data transfer rate, which is usually expressed in transfers/cycle. Multiplying these factors together gives the FSB bandwidth, which is essentially the best possible throughput. For example, an FSB with a bus width of 32 bits and a frequency of 100 MHz that does 4 transfers/cycle has a transfer rate of 3200 MB/sec.
It is often more useful to express the speed of the FSB in relation to the speed of the CPU. For example, if your computer’s CPU has a speed of 2.0 GHz and the FSB is running at 200 MHz, the CPU to FSB ratio is 10:1. In this case, the FSB is essentially a data bottleneck; the CPU processes data much faster than the bus can send it, so the CPU has to spend some time idle.
A smaller ratio means there is less discrepancy between CPU and FSB capabilities. Therefore, a computer with a 3:1 CPU/FSB ratio would perform better than a 10:1 machine. You can usually set the FSB speed using hardware on the motherboard – jumpers or BIOS.
What is the difference between a front side bus and a back side bus?
In dual-bus architecture computers, there is a front side bus and a back side bus. The BSB creates a connection between the CPU and cache memory and generally operates at the same clock rate as the CPU. The memory accessed by the CPU through the BSB is typically level 2 cache and/or level 3 cache. L2 cache is static RAM and L3 cache is specialized memory that can feed L2 cache.
In a dual-bus system, it is faster to access system memory through the BSB because it is a dedicated connection that operates over a short distance. Faster access to memory improves computer performance.
Is a front bus still used?
Computer architecture based on an FSB has largely been phased out in favor of newer systems with better performance. The FSB was common in most computers from the 1990s to the early 2000s, but has since been superseded by modern computer architecture.
What technology has replaced the frontal bus?
Most computers nowadays don’t have an FSB or Northbridge. Instead, these machines use point-to-point connections, such as Intel’s QuickPath Interconnect, Intel’s Direct Media Interface, and AMD’s HyperTransport. In this configuration, the CPU is connected directly to the Southbridge or I/O controller.
In computers without FSB, the CPU has an integrated memory controller that accesses system memory independently. This configuration greatly increases memory access efficiency and frees up fast connector bandwidth for other functions.
The front side bus is no longer a regular part of motherboard architecture, but was ubiquitous in computers and servers in the early 21st century. The FSB was a significant improvement over the original single system bus and paved the way for the high-performance computers available today.
