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What Is a Front Side Bus?

A front side bus links a computer's memory and hard disk to the main central processing unit.
A computer motherboard.
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  • Written By: R. Kayne
  • Edited By: O. Wallace
  • Last Modified Date: 19 December 2014
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A front side bus (FSB) is an electrical pathway on a computer’s motherboard, which connects the various hardware components to the main microprocessor, or central processing unit (CPU). If you think of the CPU as the brain of the computer, and the memory, hard disk and other components as organs, the front side bus would be akin to the main nervous system than links the organs to the brain.

This bus is bi-directional, meaning data can flow both ways, allowing components to send and receive data from the CPU. Since so much data passes through the front side bus, a computer’s overall performance will be dependent, in part, on its speed. Speed depends on how wide the bus is, its frequency, and the amount of data it can process per clock tick of the CPU.

The front side bus’ width is determined by bit-size. A 32-bit FSB is twice as wide as a 16-bit one. Frequency is indicated by the FSB’s megahertz (MHz). All else being equal, a 400 MHz front side bus will be faster than a 300 MHz FSB. Also considered is how many data transfers per tick it can deliver.

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The FSB communicates with components through the computer’s chipset. The chipset consists of two processors, known as the Northbridge and the Southbridge. These two chips collect data from specific components, channeling all information to the CPU through the front side bus.

The speed of the Northbridge and Southbridge buses is dependent on the speed of the front side bus. A fast CPU installed on a motherboard with a FSB that is not efficient enough for the CPU will result in data bottlenecks. The CPU will perform computations faster than the bus can process them. While this doesn’t hurt the computer, the CPU will often sit idle, waiting for new instructions. To realize the full performance benefits of a fast CPU, a fast front side bus is also required.

To further improve performance, modern motherboards feature a back side bus that directly connects the CPU to special cache (memory) reserves. These CPU cache reserves are commonly known as Level 2 (L2) and Level 3 (L3) cache. The CPU will store frequently accessed data here for quick retrieval.

In modern computers systems, Accelerated Graphics Ports (AGPs) and Peripheral Component Interconnect (PCI) slots, both Standard and Express, use their own buses for processing graphic data directly, relieving the front side bus of traffic. This helps boost benchmarks and also improves video processing for gaming and streaming multimedia applications like movies.

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Discuss this Article

anon149388
Post 11

I am a student. I want to know how many buses are on the motherboard? please help me.

anon87755
Post 10

I'm a compliance engineer and I'm finding the front side bus emits a bunch of unwanted RF noise. Are there any EMI countermeasures for this?

anon56524
Post 8

i would say both, but if i had to choose one, then i would say the processor.

anon26115
Post 4

how do you know if the FSB is fast enough for the CPU? hope you can give me an answer, thanks!

anon9661
Post 2

Just make sure the processor matches the front bus speed of the motherboard and you're fine.

anon6203
Post 1

what's more important to consider when buying a laptop, processor speed or front side bus speed? Is there a good equalibrium for the average laptop?

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