# FSB and system bus ?



## tarazan (Feb 27, 2005)

Hello, I just would like to know the difference between the Front Side Bus (FSB) and the System Bus. I have a 1.8 GHz pentium 4 with FSB=100Mhz, Multiplier=18.0 and System Bus=400MHz. Any comment on my CPU ?
Thanks.


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## crazijoe (Oct 19, 2004)

The actual system clock speed is 100MHz. Intel uses a technology called "Quad Pumping" to raise the FSB to 400Mz. The memory bus runs at 200MHz because the use of DDR memory and its ability to perform two data operations in one clock cycle. 
Examples: The Pentium4 2.20GHz processor has a FSB of 100MHz. Its system clock is 400 / 4 = 100. The multiplier is 2000 / 100 = 20. Memory bus is 2 x 100 = 200
Required memory DDR200 or PC1600

The Pentium4 3.06GHz processor has a FSB of 533MHz. Its system clock is 533 / 4 = 133. The multiplier is 3,060 / 133 = 23. Memory bus is 2 x 133 = 266
Required memory DDR266 or PC2100

The Pentium4 3.2GHz processor has a FSB of 800MHz. Its system clock is 800 / 4 = 200. The multiplier is 3200 / 200 = 16. Memory bus is 2 x 200 = 400
Required memory DDR400 or PC3200 memory.


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## tarazan (Feb 27, 2005)

Thanks crazijoe, but I would like to understand something, u said for the 2.20GHz processor with FSB of 100MHz Its system clock is 400 / 4 = 100, from where did the 400 come from ? On the other hand u used the formula system clock=FSB/4 for the other computers. and how did get that the system clock is less than the FSB despite that in my cpu i have FSB=100 and system clock=400.
Thanks.


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## crazijoe (Oct 19, 2004)

My mistake on that. The FSB should be 400MHz.
The actual system clock is 100MHz. 
The FSB of the system is 100MHz but the FSB of the processor is 400MHz because of the Quad Pumping.

To equate the speed of the processor you take the system clock speed times the multiplier.


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## tarazan (Feb 27, 2005)

but according to pc wizard 2005 i have the following:

Front Side Bus Information : 
FSB Frequency :	100 MHz 
Bus Speed :	400 MHz (QDR) 


So is it the FSB of the cpu that is multiplied by 4 or the other way around. and one more question, when they say "2.8GHz E 800MHz/1MB" is the 800 Mhz the Bus Speed or The FSB ?

Thanks again.


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## crazijoe (Oct 19, 2004)

FSB Frequency is the clock speed. 
Bus Speed is the FSB of the CPU.
The FSB of the CPU is the clock speed times four. This pertains to P4 based processors only. 
On a 2.8GHz E 800MHz/1MB 800MHz is the FSB of the processor. The System clock speed is 200MHz.
There are 3 different bus speeds,
1) FSB of the Processor (400MHz)
2) Memory Bus speed (200MHz)
3) PCI Bus speed (33MHz)

These values can change depending on the chipset and the processor.


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## tarazan (Feb 27, 2005)

if the maximum FSB of the cpu is 800MHz thereore the maximum memory speed would be 800/4 * 2=400MHz and u stated that the required memory is the DDR400. When can we use the DDR533 then ?


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## KaTaLy5t (Feb 16, 2005)

No this is not the case the memory is controlled seperate from the CPU so you set the frequency of the RAM seperate from the CPU. For DDR400 RAM the frequency would be 200MHz but because it's DDR (Double Data Rate) you get an operation on the up and down side of the cycle. This means you get 400MHz DDR but the true frequency is 200Mhz.

Ross


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## crazijoe (Oct 19, 2004)

KaTaLy5t said:


> No this is not the case the memory is controlled seperate from the CPU so you set the frequency of the RAM seperate from the CPU. For DDR400 RAM the frequency would be 200MHz but because it's DDR (Double Data Rate) you get an operation on the up and down side of the cycle. This means you get 400MHz DDR but the true frequency is 200Mhz.
> 
> Ross


Very true. 
It's just the minimun memory speed for a 800MHz FSB processor is DDR400. If you use anything less then the memory bus speed would be downgraded.


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## KaTaLy5t (Feb 16, 2005)

Oops, my bad!


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## tarazan (Feb 27, 2005)

so if i use DDR533 with the 800Mhz FSB will I benefit from the 533 or the memory bus can only work at 400 ?


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## crazijoe (Oct 19, 2004)

To benifit from the faster memory, you will have to adjust the settings in the BIOS. If your BIOS is capable of letting you do that. Most OEM systems you not let you adjust the timmings and speed settings in the BIOS. 
I don't usually overclock a system because my pockets aren't as deep as some peoples are.


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## crazijoe (Oct 19, 2004)

Here is more info:
http://www.tomshardware.com/howto/20030701/index.html


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