What Is a Front Side Bus?

On a computer’s motherboard, the front side bus is a communication interface that connects the central processing unit with the system memory and other components, such as peripherals, transferring data back and forth between the CPU and the other components.

The FSB enables communication between computer components via a chipset. In Intel-based computers, the chipset has a northbridge (the memory controller hub) and a southbridge (the I/O controller hub).

The northbridge typically connects to memory slots and also to the graphics card via a high-speed graphics bus (e.g. Accelerated Graphics Port). The southbridge handles connections with peripherals through a bus such as Peripheral Component Interconnect. The speed of the FSB generally determines the speeds of secondary system buses.

When it comes to an FSB’s effect 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’s speed. Speeds vary widely, from older models with a frequency of 66 MHz to more recent examples with speeds over 1 GHz.

The width (expressed in bits) of an FSB is another key specification. Most FSB models are 32 bits or 64 bits wide. The other important metric is the data transfer rate, which is generally expressed in transfers/cycle. Multiplying these factors together gives the bandwidth of the FSB, which is essentially the best possible throughput. For example, an FSB with a bus width of 32 bits and frequency of 100 MHz performing 4 transfers/cycle has a transfer rate of 3200 MB/sec.

It’s often more useful to express the FSB’s speed in relation to the CPU’s speed. For example, if your computer’s CPU has a speed of 2.0 GHz and the FSB runs 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 out, so the CPU must spend some time being idle.

A smaller ratio means there is less discrepancy between the capabilities of the CPU and the FSB. As such, a computer with a 3:1 CPU to FSB ratio would perform better than a machine with a 10:1 ratio. It’s usually possible to set the FSB speed using hardware on the motherboard: jumpers or the BIOS.

In computers with dual-bus architecture, 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 that the CPU accesses via the BSB is typically Layer 2 cache and/or Layer 3 cache. L2 cache is static RAM, and L3 cache is specialized memory that can feed the L2 cache.

In a dual-bus system, it’s faster to access system memory through the BSB, because it’s a dedicated connection that operates over a short distance. Faster access to memory improves the computer’s performance.

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 replaced by modern computer architecture.

Most computers nowadays don’t have an FSB or a 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 setup, the CPU is directly connected to the southbridge or I/O controller.

In computers without an FSB, the CPU has an integrated memory controller that accesses system memory independently. This setup significantly increases the efficiency of accessing memory and frees up the bandwidth of the fast connectors for other functions.

The front side bus is no longer a regular part of motherboard architecture, but it was ubiquitous in computers and servers at the beginning of the 21st century. The FSB was a significant improvement over the original single system bus, and it paved the way for the high-performance computers available today.