FSB and RAM speed

[osuman]

I have the current setup for my CPU and RAM. The CPU is an AMD 800. It has a 100mhz clock speed with the multiplier locked at 8. The bios (Award) has a feature called FSB Plus with settings ranging from 0 to 28. I have set the FSB Plus setting to 12 and the chip clocks at 112MHz times 8 which gives me an internal speed of approximatly 896MHz. The RAM speed is set to "Host Clock" with two other options of Host+PCI or Host-PCI. So, my question is, if I have the FSB Plus setting at 12, is my RAM running at 112MHz or 100? I don't really have a physical understanding of what Front Side Bus is. Additionally, the motherboard manual (Abit KA7-100) says that it supports 200MHz Alpha EV6 for AMD Athlon Processor. With the 200MHz refering to FSB speed i think. However, in the Bios under the CPU setup nowhere does it have anything about 200 MHz speeds. As you can see, I am greatly confused by all this. Any info would be appreciated. Thanks Justin

 

[CitrixEngineer]

http://webopedia.internet.com/TERM/f/frontside_bus.html

FSB's currently run from 66-133Mhz, although some (like yours) are double-pumped (use a clock multiplier of 2), or, in the case of the P4, are quad-pumped.

The multiplier gives you an FSB rating of 200Mhz, although the main PCI bus runs at 100Mhz. The fastest a PCI bus can run is 133Mhz.

By upping the clock speed to 112Mhz, you will get a PCI -> CPU speed increase to 224Mhz, but the RAM, being the other side of the bridge, will run at 112Mhz. Most modern RAM can handle this.

In other words, the high numbers only apply to the data path between the FSB and the CPU. That's why you don't see that speed setting in your BIOS.

Hope this helps

 

[osuman]

Thanks for your response. However, I think you may have raised more questions than answered. I've never heard of this "double-pumped" thing before. If the FSB is actually at 200 mhz what is the 8x multipier in the chip multiplying? Or is it not even really multiplying anything by 8? If anybody knows of a good in depth article, please let me know. Or if you care to spend more of your time explaining it, I would be appreciative. My understanding of this doesn't really matter for anything, I'm just curious. Thanks again. Justin

 

[CitrixEngineer]

There are many multipliers - and many system clocks on modern PCs. Processors typically run between 66-200Mhz, but use multipliers to get the very marketable figures that we go and buy (1Ghz!!!!).

eg 200Mhz processor with 4x multiplier = 800Mhz processor.

100Mhz PCI bus will need a 2x multiplier on the FSB in order to communicate with the processor's bus (on the other side of the Bridge).

It's more likely that the PCI bus will be old skool, running at 33Mhz, with a 6x multiplier to get to 200Mhz (198 is close enough!).

This practice has taken place since around the time of the Intel 486DX/2 chips, which, although rated at 50 and 66Mhz, actually ran at 25 and 33Mhz respectively with a clock multiplier of 2. More confusingly, the DX/4 100 chips were generally 33Mhz chips with a multiplier of 3, not 25Mhz x 4 as implied.


Try these links for enlightenment:

http://arstechnica.com/cpu/2q99/clock-intel-1.html

http://www.bukkorosu.com/pages/athlonoverclocking.html

http://www.pcguide.com/ref/cpu/arch/ext_Clocks.htm

...Happy reading!