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Old 12-07-2005, 06:25 PM   #1 (permalink)
Junior Techie
Join Date: Dec 2005
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Default front side bus / bus speed

Hello, I'm in the process of researching to upgrade a computer with a new motherboard and cpu.

what's the difference when talking about the "front side bus" on the motherboard, and the "bus speed" on the processor?

From what I've learned from, the front side bus is the bus between the processor and northbridge. Then what is the bus speed? They say: "Bus speed usually refers to the speed of the front side bus (FSB)". So what they're saying is that: bus speed = front side bus speed.

Howerver, I found another site about bus speeds:

They say that "The system clock is the actual speed of your FSB with out any enhancements....The system clock is also sometimes just called the bus speed." They later say: "The Pentium4 3.06GHz processor has a FSB of 533MHz. Its system clock is 533 / 4 = ~133. The multiplier is 3,060 / 133 = ~23.
The AMD Athlon XP2700+ has a main clock speed of 2.17GHz and a FSB of 333MHz. Its system clock is 333 / 2 = ~166MHz. The multiplier is 2,170 / 166 = ~13"
Now I'm getting confused....It sounds like the system clock (which according to them is the "bus speed") is always slower than the FSB.

Here's a mobo/cpu combo at

The FSB on the motherboard is 800mhz, but the bus speed on the processor is 1600mhz. I don't understand what's going on here. said that the bus speed is always slower than the FSB.

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Old 12-07-2005, 06:54 PM   #2 (permalink)
Monster Techie
Join Date: Oct 2005
Posts: 1,246

I find FSB and bus very confusing, as well. I've never really understood it. I think the only thing I've gotten a hold of is RAM speeds, since that's pretty straightforward...

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Old 12-07-2005, 07:48 PM   #3 (permalink)
Wizard Techie
Join Date: Dec 2004
Location: Canada
Posts: 3,790

There are many factors you have to consider when talking about the FSB speed and there are many different ways of expressing its effective speed

First, I would like to point out that any recent PC built in the last four years or so uses DDR memory, which sends data on both the rise and fall of the clock cycle. In other words, 1MHz processed using DDR memory does effectively twice the IPC of what 1MHz would normally do...therefore memory operating at 200MHz is effectively equivelent to a "normal" 400MHz clock frequency

As you probably already know the FSB link is directly connected to the memory speed therefore a 200MHz FSB is effectively running at 400MHz

Intel specifically uses a quad-pumped architecture in its memory controllers which effectively sends data down the FSB four times every clock cycle...therefore on a system running DDR400 the equation on a Pentium 4 would be:

200MHz DDR * 2 * 4 = 1600MHz effective bus

It's still only technically running at 200MHz, however it's performing more IPC therefore it can be expressed as being equal to a faster FSB...that is the difference between the FSB and the bus speed, the FSB is the literal speed and the bus speed is the effective speed

On AMD64s it gets even more confusing...the memory controller is built directly into the CPU therefore there is no long a physical FSB between the northbridge and CPU, and the FSB is the effective speed as the core itself...for example a 2.2GHz chip would have a 2.2GHz effective FSB speed

To communicate directly between the memory and the CPU, AMD has implimented what is known as the Hypertransport bus (previously known as Lightning Data Transfer)...the Hypertransport operates at the memory speed, or on AMD64s, 200MHz, and is multiplied by the LDT ratio which is 5x on socket 939 also have DDR memory performing operations on both the rise and fall of clock cycles therefore an AMD64 FSB speed can be expressed as:

200MHz DDR * 2 * 5 = 2000MHz effective bus
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Old 12-07-2005, 07:56 PM   #4 (permalink)
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Very good now could you add in info for DDR2 for intel please and the new AMDs that supposidly support DDR2.
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