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What Is a Processor Bus?

By Jessica Reed
Updated May 21, 2024
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A processor bus, also known as the front-side bus, refers to a specific electrical connection within a computer that connects a computer's processor to a chip known as the north bridge. For a computer to properly function, the processor, also known as the central processing unit (CPU), must send out orders and submit pieces of information to the computer’s memory. The processor bus brings messages back and forth between the processor and the north bridge, which in turn sends messages to the computer's memory and other parts of the computer. This allows the computer to perform multiple tasks at once at fast speeds.

The front-side or processor bus is not the only bus found within a computer. The motherboard chipset consists of two main chips, the north bridge and the south bridge. The processor bus transmits messages from the north bridge, also known as the memory controller hub, to the CPU. This area deals primarily with memory and graphics, and messages travel faster on the processor bus. The south bridge deals with processes which do not need to be acted on as quickly as those in the north bridge and sends signals at a slower rate using the PCI bus.

Imagining a motherboard in a computer as a highway and the buses as actual buses carrying passengers is one way to understand computer buses. Each passenger represents an electrical signal that the processor needs to send to the memory to store until it's ready to use it. The data gets on the bus and the bus transports the data to the north bridge, which then sends it on to the memory. When the process needs to use it, the data "rides" the bus back to the processor to wait for further instructions.

Other buses found on the motherboard which help connect parts to the main north and south bridge include the internal bus, the memory bus, and the AGP bus. The internal bus connects the north bridge to the south bridge so they can communicate with each other and other parts of the computer. The memory bus connects the north bridge to the memory. When the CPU needs to save something to the computer's temporary memory, it sends it to the north bridge which passes it on to the computer's memory. The AGP bus, short for accelerated graphics port, works with information relating to graphics display but is becoming less common and is often replaced by an expansion card known as the PCI-E or PCI Express.

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Discussion Comments

By anon285329 — On Aug 15, 2012

What happens if I install a CPU whose front side bus speed is faster than the motherboard front side bus (i.e., 1333 vs 800)?

By NathanG — On Dec 14, 2011

@miriam98 - I disagree. I don’t think consumers need to be concerned with all the minutiae of different bus speeds. Not all buses need fast speeds.

As the article points out the south bus is not that speed intensive, so why would you care how fast the bus speed is? And seriously, do you really need to know how fast the bus speed for your DVD-ROM is?

I think it’s easy to get carried away with these specs, which marketers sometimes use to hype their systems. Basically, processor speed, memory and frontside bus speed are all you really need to know, in my opinion.

By miriam98 — On Dec 14, 2011

@everetra - There is actually more than one bus operating inside your computer. A bus is simply a path between components, and of course the typical computer will have more than one component inside.

But to make things simple, you basically have two categories of buses: internal and external. The internal (which is covered in the article) is more generally called the system bus and deals with CPU, memory and things like that.

External bus refers to those pathways that are used to connect the components to devices like the CD-ROM drive, hard disk and so forth. In all of these cases, you want the maximum speed possible, in my opinion.

So I second your advice about going for the fastest bus, but I would extend it to all buses – to the extent that information is available to the consumer.

By David09 — On Dec 13, 2011

@everetra - I used to have AGP bus on my computer but my latest system has the PCI-Express card that the article talks about. It’s blazing fast and has its own processor and bus to move data back and forth very quickly.

When you’re doing high definition graphics and intense gaming like I do, you want to offload as much of that processing from the CPU bus onto another processor, if at all possible. The PCI-Express card does that, and it allows for fast screen refresh rates so that I can play games without any noticeable flicker.

By everetra — On Dec 12, 2011

As should be obvious, the processor bus is important in facilitating the fast movement of data between the computer’s processor and its memory. It follows then that the faster your processor bus speed, the faster this transfer will take place.

However, I’ve noticed that when people buy computers they rarely think of frontside bus speeds (in most cases because they don’t know about them, from what I can tell).

Typically users are concerned about things like RAM and CPU speeds alone, not processor bus. Well, you don’t have to be an expert, but I think that you should compare processor bus speeds on the different systems that you buy. The faster the bus, the better.

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