What is a Front Bus?

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

A front side bus connects a computer’s memory and hard drive to the main central processing unit.

What does the Front Bus do?

The FSB allows communication between computer components through a chipset. On Intel-based computers, the chipset has a north bridge (the memory controller hub) and a south bridge (the I/O controller hub).

Motherboard with some slots shown.

The northbridge normally connects to the memory slots and also to the video card via a high-speed graphics bus (eg accelerated graphics port). The south bridge handles connections to peripherals via a bus, such as the Peripheral Component Interconnect. The FSB speed usually determines the speeds of the secondary system buses.

A BSB can provide a more efficient connection between the CPU and cache memory.

Curiosities

In a computer, a bus facilitates bidirectional data transfers between components.

Bus terminology differs between manufacturers. Intel manufactures FSBs while AMD offers EV6 buses.

The terms system bus, memory bus, and processor bus can refer to an FSB.

On most modern computers, the FSB architecture has been replaced by point-to-point connections that support faster performance and better scalability.

The FSB architecture was essentially phased out in 2008.

The FSB is located on the computer’s motherboard, where it connects the CPU to memory and peripherals.

How do you measure Front Bus speed?

When it comes to the effect of an FSB on a computer’s performance, there are three main factors: clock frequency, bandwidth, and data transfer rate. The FSB frequency is measured in megahertz (1,000,000 cycles/second) and is often called the FSB speed. Speeds vary widely, from older models with a frequency of 66 MHz to newer examples with speeds above 1 GHz.

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Most modern computers use internal point-to-point connections instead of the traditional bus architecture.

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

The FSB transfers data back and forth from the CPU to other components.

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.

What Is the Difference Between a Front Side Bus and a Back Side Bus?

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.

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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.

Is a Front Side 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 replaced by modern computer architecture.

What Technology Replaced the Front Side Bus?

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.

A slow FSB can affect the computer’s performance when it doesn’t keep up with the processing speed of the CPU.

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