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Front Side Bus

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February 5, 2003 11:53:58 PM

Is the front side bus the speed at which the CPU links to the ram and the back side bus the main speed of the board. For example if I have a 700MHZ PIII with the 133MHZ FSB is the whole mother board at 133MHZ or is it at 66 or something else? I was told the older Athalon CPUs had a 133 FSB but the MOBO was at 200MHZ. Please explain FSB. I have built several computer and set the multiplier but I never thought about what the FSB is.

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February 6, 2003 1:29:38 AM

Hope this helps:
FSB is the speed at which the CPU communicates w/ RAM.
Don't worry about BSB...was only an issue when L2 memory existed off the CPU.
PIII's came in either 100MHz or 133MHz FSB varieties ("B" processors were 133).
Athlons (prior to the XPs) came as 100 or 133 FSB but because of data rate doubling, these came to be known as FSBs of 200 and 266 respectively.
I currently use an AthlonXP2700+ which runs at 2.17GHz. This can be expressed in 2 ways:
System clock (167MHz) x 13
or System bus frequency (333MHz) x 6.5
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a b à CPUs
February 7, 2003 3:18:33 AM

FSB used to mean from the CPU to RAM, but now it's been split, since the chipset north bridge is in the middle, to mean only the part from the CPU to the chipset northbridge. The newer definition makes more since because of the chipset, and was redefined like this because chipsets are being made that allow the memory bus and CPU bus (FSB) to run asychronously.

Anyway, these big numbers that AMD made up (and Intel followed) usually say MHz, but aren't. MHz refers to a real number, the clock speed. 133MHz is a common bus speed, called 266MHz by AMD and 533MHz by Intel. The inflation comes from AMD using Double Data Rate techonology (2 transfers per clock cycle) and later Intel using Quad Data Rate (4 transfers per clock cycle). Instead of simply telling you its 133MHz and DDR, they came out with a "performance equivalent" "speed" of "266MHz".

Various parts of the board have their own speed. PCI is 33MHz, AGP is 66MHz. All of these bus speeds are divided as such at the chipset northbridge by the clock generator, or at least they were...

Because the chipset southbridge runs on the same bus that all PCI divices did, the link between the chipset northbridge and southbridge was also 33MHz. But now chipsets are using a high speed link, so you'll see a higher frequency between the chipset parts, and 33MHz on the other side of the southbridge.

A PIII 700 used a 100MHz bus.

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February 7, 2003 10:34:17 PM

Thanks folks!

I guess I had it right for the most part. It is odd. I heard the RDRAM or Rambus would shine when the CPU got above 2 GHZ but Intel is doing the opposite. They are now using DDR or SDRAM now when the CPU could use the Speed. I know the Rambus can only send 16 bits at a time but if it can clock out above one GHZ it should rock.
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February 7, 2003 11:55:06 PM

Rambus RDRAM uses a DDR bus just like DDR SDRAM does, meaning the PC1066 memory clock is 533MHz. Now, since it's paired, it's 32-bit. Which is half as wide as the CPU bus, but with DDR technolgy it makes up for it, ie "1066" on 32-bits matches Intel's "533" 64-bit CPU bus.

<font color=blue>There are no stupid questions, only stupid people doling out faulty information based upon rumors, myths, and poor logic!</font color=blue>
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