Real FSB speed 4

[nagornyi]

I have Abit BL7 motherboard with 400 MHz FSB. However, it never runs on more then 100 MHz because of the chipset restrictions on the RAM that can be only PC100.
Now I am thinking about upgrading and looks like these days 800 MHz FSB mobo is the way to go. But again the same issue: does it make sence to put bucks into 800 MHz FSB (as compared to 533 or 400) that would never work higher then 400 because of the max DDR400 RAM?

 

[cdogg]

I think you are confusing actual speed with theoretical speed. Intel P4's use a quad-pumped FSB. But what exactly does "quad-pumped" mean? Well, back when PC100 and PC133 topped the market, frontside busses ran in synch with memory. So if you had a Pentium III with a 133MHz FSB, then you also had PC133 SDRAM.

But the need for a faster FSB became essential to overall performance. Because of the length associated, it was near impossible to just simply crank the bus up to 400MHz or beyond - at these speeds the system becomes unstable. Instead, Intel borrowed part of the idea behind DDR SDRAM which doubles the amount of data sent on each clock cycle. It can do this by sending data on the rising and falling edge of each cycle. They took it one step further allowing for twice as much data on the rising and twice as much on the falling edge to be sent. So "theoretically", you've quadrupled your output. Hence the term "quad-pumped".

OK, that was a mouthful!

So the next thing to realize is that a 400MHz quad-pumped bus is really only running at 100MHz. That's why you see what you see in the BIOS. Contrary to how things were back in the older days, newer systems don't have to run the memory in synch with the FSB. The setting for each is separate. You could have an 800MHz FSB running with a 400MHz memory bus no problem. Of course, that's why Intel backed Dual-DDR boards a couple years ago to help double that theoretical number to 800MHz to match its FSB.

Now we won't get into the efficiency here, but just know that a quad-pumped 400MHz bus isn't nearly as good as having a true 400MHz bus. The same thing is true about 533MHz (133MHz x 4) and 800MHz (200MHz x 4). Just don't get all caught up in the hype and multiplier games...


~cdogg
[tab]"All paid jobs absorb and degrade the mind";
[tab][tab]- Aristotle
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[nagornyi]

Thank you. This explains a lot. I am a bit counfused about the last paragraph though. So, is there much of a difference between 800FSB/DDR400 versus 400FSB/DDR400, if the same CPU is used?

 

[SkipCox]

I'll try to get this right.

"So, is there much of a difference between 800FSB/DDR400 versus 400FSB/DDR400"

Yes, all the difference in the world if we're talking about an intel rig. FSB is performance and a 200Mhz (200x4=800) fsb will trounce a 100Mhz (100x4=400) fsb.

"if the same CPU is used?"

Yes, on the same motherboard if it will support both chips.
The 100Mhz x 4 chips were a couple generations before the P4C and P4E chips that run on motherboards that do 200Mhz x 4. In between were the 133 x 4 chips that cdogg mentioned above so we're really comparing apples to oranges here.

This is a crappy explanation and I'm sure someone will be along soon to clear the muddy water.

Skip

http://home.comcast.net/~skip9/index.html

http://scc.i8.com

 

[nagornyi]

OK.
So now for my 400FSB/PC100 mobo using 1.6 GHz P4 CPU I see in the BIOS that CPU is set for 100MHz x 16.
If I had 400FSB/DDR400 I would see 200MHz x 8.
If I had 800FSB/DDR400 I would still see 200MHz x 8 ..?
What am I missing here?

 

[cdogg]

Skip covered it pretty well. In regards to your question about whether "[blue]there much of a difference between 800FSB/DDR400 versus 400FSB/DDR400, if the same CPU is used?[/blue]", I'd have to say no.

The frontside bus speed is an internal setting within the CPU itself. Upgrading your motherboard to one that supports an 800MHz FSB will not help a CPU that was built for 400MHz. When running the two together, the FSB will still run at 400MHz (100 x 4).

Also in regards to memory... For a 400MHz P4, DDR400 is overkill, since the memory will only run at 200MHz. Now you can tweak the FSB/Memory ratio in the BIOS to increase that speed to 300MHz+, but that's not going to help a whole lot since there will be a slight bottleneck from the two running at different speeds (the FSB and the Memory Bus). Usually you want both running in synch with each other for maximum efficiency. Upgrade to a CPU that supports a 200MHz FSB to get the most out of DDR400.


~cdogg
[tab]"All paid jobs absorb and degrade the mind";
[tab][tab]- Aristotle
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[cdogg]

nagornyi,
Just saw your last post...

A CPU that uses a 400MHz FSB will run at a true speed of 100MHz. That's why you see 100MHz x 16. The PC100 is just running in synch with that.

Now if you upgraded to 400FSB/DDR400, the FSB would still run at 100MHz. The memory bus though would jump from 100MHz to 200MHz, which is just an internal clock speed. That's how DDR works - doubles the throughput. It's the same concept as Intel's quad-pumped bus, only double-pumped instead.

If you had a motherboard that supported 800FSB/DDR400, then you would also need to buy a new CPU in order to get 800MHz FSB speeds. Otherwise, (as I mentioned in my last post) your old CPU would still use a 400MHz FSB (100 x 4) with a multiplier of 100 x 16.




~cdogg
[tab]"All paid jobs absorb and degrade the mind";
[tab][tab]- Aristotle
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[dakota81]

I even get confused with the explanations. We can all thank the marketting departments for this mess.

You have to convert EVERYTHING from their fsb numbers, ddr speed, quad-pumped, double data rate, blah blah blah blah, convert EVERYTHING back to the correct clock speeds that they run on. Only then can any of this make sense. Otherwise you're just going to run around in circles not understanding the marketting speak - mainly because the marketting speak MAKES NO SENSE straight from the beginning.

Intel:
800fsb - 200MHz clock
533fsb - 133MHz clock
400fsb - 100MHz clock

AMD:
400fsb - 200MHz clock
333fsb - 166MHz clock
266fsb - 133MHz clock
200fsb - 100MHz clock

DDR:
DDR400 - 200MHz clock
DDR333 - 166MHz clock
DDR266 - 133MHz clock
DDR200 - 100MHz clock

SDRAM:
PC133 - 133MHz clock
PC100 - 100MHz clock


Now all you have to do is match up the CORRECT CLOCK SPEEDS, and you will be all set!

(isn't this fsb crap a faq yet on this forum?)

 

[ceh4702]

DDR itself is misleading because Memory is accessed twice per a given clock cycle. The CPU on the other hand may be quad pumped and not be fully utilizing all of its pipelines.

Article on Processors:

http://www.pcmech.com/show/processors/715/

If you do not like my post feel free to point out your opinion or my errors.

 

[nagornyi]

Much more clarity now guys, much more. Thank you all.
So clock is clock, FSB is FSB. But then... what the FSB speed REALLY mean? Sorry, I feel I do go in circles, but approaching the target

 

[dakota81]

FSB stands for "front side bus" and is an indication of how much data can be transfered between the cpu & the motherboard chipset in one second.

However, the actual numbers refered to is very much so misleading thanks to the marketting gurus who try and sum up the entire architecture within a single number, and as such you absolutely cannot match cpus with memory based on those numbers, you must revert back to actual clock speeds to find matching components.

 

[cdogg]

Umm, yeah! What Dakota said!![tab]

I agree. A FAQ is long overdue and this forum definitely needs one.


nagornyi,
You probably already understand it, but just don't realize it yet! The true speeds bolded in Dakota's post are, well, TRUE speeds. The numbers to the left are speeds that are marketed by either AMD, Intel, or Memory specs. This "marketing" is deceiving, because it has nothing to do with speed.

Take Intel for example. Their FSB never reaches 800MHz. Instead, they've taken the same old bus from the old Pentium days and widened it. Think of a 2-lane highway being expanded into 8. More traffic can flow, but did the speed limit change? No. It's the same concept here. 4 times as much data can be sent during each clock cycle giving it the illusion that you're running 4 times as fast.

There are many times when the extra 6 lanes from my example aren't needed, as ceh4702 pointed out. When the CPU isn't sending data at a constant rate, then the speed limit of 200MHz is the limiting factor. Data still takes the same amount of time to reach its destination, so there are times where it's only running at 200MHz instead of the marketed 800MHz.


Time for someone to stop and write a FAQ. Any volunteers?



~cdogg
[tab]"All paid jobs absorb and degrade the mind";
[tab][tab]- Aristotle
[tab][navy]For general rules and guidelines to get better answers, click here:[/navy] faq219-2884

 

[sixstrings4]

What if your clock speeds don't match between the cpu and the memory. For example, what if your FSB is running at 333(166 mhz clock) and your memory is running at ddr 266(133 mhz clock)?