FSB / PSB / DDRxxx (Help)

[Xorcist]

Um... I'm confused as hell. It used to be your front side bus times a particular clock ratio would determine your processor speed, and that base number would then determine your memory speed. For example:

100 FSB x 4.5 CR = 450Mhz and your memory would run at 100Mhz

But now, with these new technologies I can't figure out what to buy, let alone how they'll all work together. Here is my dilema. I'm looking at getting a Shuttle AV49N motherboard, which has three FSB settings (Auto, 133, or 100). However the motherboard is rated for 400/533Mhz PSB and up to DDR400 (i.e. PC3200).

Now from what I understand DDR400 works at a 200Mhz x 2 (I don't know if that's supposed to be FSB or PSB?). The motherboard only has settings for 100 or 133Mhz? But somehow the processor runs at 400 or 533Mhz? I have no idea what numbers I'm supposed to be matching up!

I think it works like this:

Given a P4 2.4Ghz with a PSB of 400Mhz
100 FSB x 24.0 CR = 2.5GHz and the memory would run at 400Mhz

OR

Given a P4 2.4Ghz with a PSB of 533Mhz
133 FSB x 18.0 CR = 2.4GHz and the memory would run at 533Mhz

But there is no DDR533, so I'm assuming that with DDR400 installed it would only run at 400Mhz (i.e. 133 x 3 = 399). But if that is how it works then the DDR on a 133 FSB only gets read 3 times per cycle rather than 4 at 100Mhz (i.e. 100 x 4 = 400).

Now for starters is this at all correct? It would be great is someone could list some explainitory pages on this subject for me. And secondly, would it be better to sync the PSB with the DDR rating? or is faster better no matter what? I'm looking to build this machine for gaming only, and want the best possible performance. Any insight or suggestions would be appreciated.

[Xorcist]

A friend of mine just passed me these:
http://www.x86-secret.com/articles/r.../km400en-2.htm
http://www.cadimensions.com/requirements2.htm

and that kinda cleared things up a bit. So I guess what it comes down to is since the FSB speed for the Shuttle AV49N maxes out 133Mhz, getting anything greater than DDR266 is only worth while if I plan to overclock. Which I don't really intend to do, though I do believe the motherboard supports it up 165Mhz FSB overclocked.

Now the question becomes what part if any does the VIA P4X400 chipset play in the scheme of things? The specs for the motherboard state support for DDR400, yet this page states the VIA PX400 only supports up to DDR333? There just seems to be so much more involved in building a new machine these days than there used to be.

And are there Pentium based motherboards that allow for a greater FSB? will a 2.4Ghz P4 even run at such a high FSB (say 200Mhz x 12) or would I have to buy a newer 3.0Ghz processor?

[Xorcist]

Hours of research later... and it appears that even with the FSB set to 133, I can set the DRAM clock to 200Mhz. So indeed the DDR400 would be supported. My question now is, exactly how does this effect the system? Is this a normal practice, or would this be considered overclocking. I'm more concerned with stabability over full throttle speed.

[Xorcist]

Soooo..... absolutely no one knows how this stuff functions together?

[Smolder]

HI,

ok it sounds like you might still be a little confused on the subject so here goes.

The motherboard comes with the option of auto/100/133 FSB

If you use a Processor that is equipped with a 400Mhz fsb, then it multiplys the motherboards setting of either auto/100/133
times 4. So that could be 100x4 =400fsb or 133x4=533fsb
If you set the jumper to 133mhz with a processor that only handles 400mhz then you would be overclocking. so if the processor was of the 533mhz flavor the jumper on 133mhz fsb would be fine.

DDR 400 would be better to get then ddr333 or ddr266 because the ddr 400 would lessen the bottleneck more between the processor and ram

[Xorcist]

I understand how the FSB and PSB match up, being that the PSB is nothing more a multiple of the FSB for how many times the CPU can read each clock cycle.

What I am really confused about is that DDR400 is rated for a 200Mhz FSB, yet the maximum the motherboard can reach is 165Mhz (overclocked). Which to me says I'll never be running the RAM as fast as I should be to maximize performance. Thus I assume it would be cheaper to go with the DDR266, which would sync with the 133Mhz FSB. However it became apparent to me that the VIA P4X400 chipset on the motherboard is where the DDR400 support was coming from... but does this mean I can run the RAM asynchronously from the FSB, and if so what trade-offs are involved in this? I was always under the premise that the RAM had to run synchronously with FSB.

[cbuddha42]

memory clock and fsb can be set independantly, only instance of faster memory slowing down i can think of is some of the old nforce 2 boards ran faster with 333 than 400 but with ultra 400 chipset i think that no longr true - anyway you shouldn't have slowdown from faster memory