FSB Help

[Garet Jax]

Hello,

I am trying to start getting back into hardware and I am having a tough time finding a site that explains the concept of Front Side Bus speed and how it interacts with the RAM speed (PC100 and PC133) and the effect overclocking has on all of the above.

Does anyone have site recommendations or documents? PM, email (garet_jax23@hotmail.com), or response to this thread would be wonderful.

Thanks a lot.

 

[rkoenn]

I am hardly an expert, but think I can explain the basics. The FSB speed is the primary clock in the system and the industry standard settings are 66, 100, and 133. CPUs run on a multiple of these to reach their operating frequency by multiplying it internally. Most CPUs now have moved to either 100 or 133. AMDs CPUs are pretty much strictly 100 with the data moving into the CPU at a DDR rate of 200 MHz. They will be shortly releasing the 133 core CPUs. Memory nowadays is typically rated at 100 or 133 with newer DDR memory rated at twice those frequencies, being able to transfer data on both ends of the clock cycle. With some of the current motherboards where the chipset allows for asynchronous operation of the memory and CPU, you will find 100 MHz core CPUs with 133 MHz memory to the memory bus. Every little bit helps in the grand scheme, although you hardly get proportional increases in performance since so many other system components just can't keep up, i.e. video, HDD, etc. Finally, due to the PCI spec, the PCI slots in the system are clocked at 33 MHz so the cards will function properly.

Then when you get into overclocking, since everything depends on the FSB clock, as you increase the FSB clock all the other subsystems, i.e. memory bus and PCI bus increase in speed proportionally to the FSB increase. So your overclocking will fail at the point where the first component can't function at that speed, be it video card, memory, CPU or whatever. Hope this helps and does not just confuse you more.

 

[Garet Jax]

Robert,

Thanks for taking the time to respond.

I think you have cleared up a few things. Where is the FSB set? Is it in the BIOS? Is it a jumper on the MB? Elsewhere? Is increasing the FSB the only way someone can overclock their machines?

As I understand it, the FSB is the lowest common denominator between the MB, the CPU and the RAM(not quite sure how the PCI or AGP bus fits into the equation). As the FSB increases so does the communication speed between all these components.

Assuming the CPU is 800MHz with a core of 100MHz, can I deduce that the multiplier is 8? This means that running with an FSB of 100MHz I get 800MHz out of the CPU. If I increase the FSB to 133MHz, the CPU is running at 1064MHz right? How do I find out what the core of a CPU is?

Are there ways to determine before buying CPUs, RAM sticks and MB how overclockable they are?

Thanks again.

 

[rkoenn]

FSB is set within the BIOS for most motherboards nowadays. On my Asus (Award BIOS) motherboard, I get a choice of 133:133:33 which refers to CPU, memory, and PCI clock. I then have another item for CPU speed. For my system with an P3-866EB built around a 133 MHz core, I can increase it in small steps using this setting. The core multiplier for this CPU is 6.5 and cannot be changed. The CPU frequency setting jumps in about 3 or 4 MHz steps and as you increase it, you can see the previously mentioned FSB, memory, PCI setting also increasing propotionally. I have found with the latest BIOS that I can get up to at least 920 MHz for the CPU. Only thing I have found odd is my AGP (video card) setting drops from 4X to 2X above about 900 MHz. Really doesn't affect the video anyway as 4X AGP has almost no benefits over 2X. Other motherboards may adjust the clocks differently. Also, some boards have jumpers or switches on them to set these speeds or hard lock these speeds in. Some system builders don't like to allow the end user to play with the settings as you can fry your CPU at too high a clock setting. And the AMD athlon/duron CPUs can be modified by connecting traces on the CPU so that even the multiplier can be changed.

In your question about the 800 MHz CPU with the 100 MHz core, yes it would have an 8X multiplier and you could increase the FSB to 133 and it may run OK, and it may not. Finally, if you purchase RAM you should purchase the PC133 memory. Price is no different than PC100 and it will work fine at 100 MHz if that is what you use it at now. The newer DDR memory will not fit in the DIMM sockets for the standard SDRAM and is currently more than twice the price of the standard SDRAM. Motherboard chipset must support DDR also. And you can buy PC133 memory rated at different capabilities. There is a CAS rating that improves performance slightly and a CAS 2 module is better than a CAS 3. I have tried using the CAS2 setting for my memory, which is CAS 3, (another BIOS setting) and it won't work at that speed. The system locks up during the BIOS checks.

As for other components, you'll never know how fast they will OC until you try. Most PCI cards will run fine at higher speeds but I have heard of some AGP cards that sometimes have trouble when the speed gets real high. Usually the memory or processor will cause problems first.

Good luck with your experiments.