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Old 08-07-2003, 07:32 PM   #1
Yes, Yellowbeard
specmike's Avatar
Join Date: Oct 2002
Location: Metro Atlanta, GA.
Posts: 2,647
Post A Noob's Guide to Selecting RAM and a CPU written by a Noob

(I typed this aimed at a buyer that knows about the same or less than I did when I first started coming in here, virtually nothing. Please, veterans and gurus, offer up any corrections or additional advice.)

On most if not all of the new Canterwood/Springdale chipset boards, or 875/865 if you prefer, the ratios offered are 1:1, 5:4, and 3:2. The reason for this is that Intel P4 CPUs have a locked multiplier. The multiplier is a number assigned to that CPU by Intel

The current P4s run at an internal clock speed of 200mhz. Intel "quad pumps" that thru the CPU which is where we get the 800mhz FSB, 200 X 4. The multiplier is the factor that determines the external speed of the CPU. For example, on my P4 2.4c, my multiplier is 12. 12 x 200mhz = 2.4ghz. A 2.6 CPU has a multiplier of 13, 2.8 has 14 and so on. On an equivalent Athlon CPU, the multiplier is not locked. To overclock, you can manually change this number to get higher clock speeds and/or higher RAM frequencies.

Intel does not like or allow this so that's where the RAM ratios come into play. Since you can't change the multiplier, each step change of the CPU FSB results in a given linear change of the RAM freq. The MOBO makers have made it where the OCing user can select a ratio that will allow higher OCs. Thus, the choices of 1:1 which is default, and 5:4 and 3:2. The 5:4 and 3:2 ratios make each step increase of the FSB effect the RAM less. This allows higher FSB with existing RAM.

The ratios are basic math. Obviously a 1:1 ratio will yield a 200FSB and a RAM frequency of 200, 205 = 205 etc. However, since Dual Data Rate RAM transfers data on the rise and fall of each cycle (each half of the sine wave for those more technically inclined) you actually get data X 2 or DDR400 at FSB200.

A 5:4 ratio is figured like this. Example: FSB 250 divided by the first number of the ratio, the CPU factor of 5.....250/5 = 50. You then multiply the memory factor of 4 by 50.....4 X 50 = 200 or DDR400. A 3:2 is for example FSB200/3 = 66.6 X 2 = 133.2 or DDR266.

This is important since right now our CPU speeds are sort of "outrunning" our RAM. What I mean by this is that we have P4s out there but there that are approaching a FSB of 300 when OCd! That is DDR600 People that are running their FSB that high are using a 5:4 or a 3:2 ratio. 300/5 = 60 X 4 = 240 or DDR480. 3:2...300/3 = 100 X 2 = 200 or DDR400.

All you have to do is find a CPU that will do it and risk smoking the little fella. There is no RAM out there that will run FSB300 @ a 1:1 ratio or PC4800. I'm sure Corsair, OCZ, Geil, Kingston, et al are working on it though.( DDR2 will be out soon and it will start at DDR533 or PC4300. )

I have not seen bios screen shots on other MOBOs but the current Asus boards are using a bios from American Megatrends Inc. In their bios, you choose a ratio by either selecting DDR400, DDR333, or DDR266. That is what freq. the RAM will run at if you choose a FSB of 200. Each speed actually represents a ratio, DDR400/1:1, DDR333/5:4, and DDR266/3:2. When you change the FSB it changes the RAM frequency a given amount depending on which option you have chosen.

What confuses many, (including me initially) is that the bios screen does not change the DDR freq. you see. It stays at either 266, 333, or 400. You have to do a little math like above to figure your OC. It will get to the point that you will know them automatically if you tweak much. 1:1 is the easiest, it's basically FSB X 2. So FSB200 = DDR400, FSB217 = DDR433(434 actually when you round it) FSB233 = DDR466, FSB250 = DDR500 and on and on. We'll need to learn more math as the technology increases.

So here is my take on buying a new Canterwood/Springdale based board. Part of this works on the assumptions that a. the 1:1 ratio is the best to use on these boards and b. that users have a Springdale based board that will allow the activation of PAT.

This may get a bit lengthy or boring:ZZZ: but I did not force you to click this link

With the release of the PC3700 and PC4000 DIMMs, many are asking about proper combinations of RAM+CPU. Normally you would think that the fastest RAM going and the fastest CPU going would be the BEST combination, cost not withstanding.

But, it seems that this is not the case. To fully utilize PC3700 RAM on current P4c CPUs and on the current Intel 875/865 chipsets, you must run the RAM at a 1:1 ratio according to the benchmarks I have seen. Some debate the validity of synthetic benchmarks and say they prove nothing. If that is your belief, then don't buy any RAM above DDR400. The 5:4 ratio is not as good as the 1:1 as far as overall memory bandwidth which is why we buy good fast RAM.

Back to the point, the FSB for PC3700 (DDR466) at a 1:1 ratio is FSB233 and FSB250 for PC4000 (DDR500). This means that you have to be able to overclock (OC) your CPU to run the RAM at its stock speed. To OC the RAM past stock, you REALLY have to OC your CPU. Prometheus, big toe, Eva2000, et al are getting some fantastic numbers with the newest RAM out, check out their posts for more details.

Well Hawk, a frequent poster here and Master of all that Matters over at the BleedinEdge has tested retail samples of all 5 issues of the P4c CPUs. He has found the following:
P4 CPU Speed Stock MAX FSB
2.4c 288FSB
2.6c 270FSB
2.8c 266FSB
3.0c 247FSB
3.2c 238FSB

His results are exceptional and he knows how to get speed out of a rig. One of his nic names is "Overclocking Junkie" and it fits. The reason I am posting all of this is to show that you can't necessarily utilize the DDR466 and DDR500 RAM with EVERY CPU available. So, for any of you that are considering a new rig, consider your CPU choices.

Hawk's results are not etched in stone however so buy whatever CPU floats yer boat. For example, the Goog @ Corsair HoH stated recently that he has run 2 3.0s. One would go over 250FSB, one would not so it is a hit and miss. It is always hit and miss with OCing but so far, the 2.4c CPUs are the most consistant at hitting high FSB.

On the other end of the spectrum, if you want the MAXIMUM performance at box stock speeds, I'd say go with the good PC3200 RAM that will do TIGHT timings + a 3.2ghz CPU. Then, you are running fantastically high and don't have to worry with any OCing. Some users want TIGHT timings, some want HIGH FSB, some want a good balance of both.

Now, as to timings, I can attest that with my results + posts I've seen by Hawk et al, the PC3700 and PC4000 stix do NOT need tight timings to have AWESOME results. Good stock PC3200 is far better on a dual channel MOBO than any previous single channel MOBO but the PC3700 and PC4000 with more loose timings are EVEN BETTER. IMO, FORGET about tight timings unless you are running an AMD CPU.

So here are my rules of thumb for buying an 875/865 based board and CPU+RAM. If you can't hit a 233FSB, you will be underclocking the PC3700 and PC4000. If you can't hit a 250FSB you will be underlocking the PC4000.

1. If you don't want to OC and want to run stock CPU speed with TIGHT RAM timings buy the good PC3200 or PC3500 and the FASTEST CPU you can afford.
2. If you want a MILD OC with TIGHT timings, buy the BEST PC3500 and the FASTEST CPU you can afford.
3. If you want a MEDIUM OC your BEST buy would be the PC3700 with the FASTEST CPU YOU BELIEVE WILL HIT A 233 FSB. I am on the fence at this point with a 2.8c, will they consistently hit a FSB233? Buy one and let me know
4. If you want a HOT OC and want to go all out, buy the PC4000 and either a 2.4c or 2.6c. My choice is the w2.4c. It's not as "slow" as it sounds and you can spend the money you save on the lovely PC4000 Anyways, a 2.4c at FSB 250 is 3.0ghz, how much faster do you wanna go?

It seems to me the cap on the P4c CPUs is about 3.6ghz due to architectural limitations. This is still RARE and again, hit and miss. Do you want to play hit and miss with a $700 3.2ghz CPU? A 3.2 at 3.6 is only a FSB222 which is hard to get on a 3.2c. I have seen several people post that they cannot hit 220 on a 3.2c.

Since 3.6 is about the average cap on the P4 Northwood architecture, OCing it up to or past 3.6 is tough. The headroom of a 2.4c is its strength. All P4c CPUs are from the same die, and are just speed binned. The "best" chips become 3.2s and the "worst" become 2.4s. I've had my 2.4c at 3.0 on stock voltage so that tells you how "bad" a 2.4c is. IMO, save the money on a CPU and buy better faster RAM!

I have based a great deal of this on what I have observed so I may be wrong on some points. Also, I have based this on observations of benchmarks. Some argue the validity of benchmarks and what they “really” mean. However, many buyers base their purchasing decisions on published benchmarks and subsequent satisfaction based on their own marks. I think the published benchmarks in this forum are valid for these reasons.
1. They are end user equipment bought retail, except as noted. I believe people here are honest and fess up when they are using beta or sample units. Why lie here?
2. They are valid because regardless of what they “really” mean to the end user, they are the same marks and can be used to compare relative performance. Will the consumer ever see the difference in Sandra 5978/5997 and 6011/6046? Not with the naked eye but when they run their own benches, they will get some measure of how they match up to similar equipment and hopefully get some satisfaction of getting their money’s worth. If not, they can always RMA. Well, there it is, fire away!

spec "I'm out of work and have nothing to do but hang out here" mike

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