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Matching Ram with Bus speed?

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November 1, 2004 4:53:47 AM

I tried searching the forums, but couldn't find anything regarding this. I bought a new LGA 775 925x Motherboard. It is an ABIT AA8-DuraMAX. I plan on getting a P4-3.2LGA775 Processor for it. It says it suports DDR2 533/400. Does this mean I can't use a 675mhz DDR2 set on it? My Main question is, do I have to match the RAM with the buspeed of the motherboard? If so, what speed of ram should I get? I always think the faster the better, but is this not the case? Do I have to use a 400mhz ram because of the multiplying factor? Sorry for all the questions, I just don't want to buy the 675 or 533 ram if it makes my computer slower then just buying the 400mhz modules. Thank you for reading this.

More about : matching ram bus speed

November 1, 2004 11:26:14 AM

You can use 675Mhz RAm on that board, but it will be under-clocked to 533. The chipset is not Guaranteed to go over 533Mhz DDR2, but overclocking will allow you go increase the speed. I'm not sure what speed the LGA775 boards top-out at.


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November 1, 2004 1:06:45 PM

I didnt think 400Mhz DDR2 existed?
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November 1, 2004 4:59:37 PM

There is an article on this motherboard you can find at:
http://www.anandtech.com/mb/showdoc.aspx?i=2162&p=2

Yes DDR2-400 (PC2-3200) certainly exists, it is just not that popular because for a better price DDR-400 (PC-3200) performs better.

You do not necessarily have to match BUS SPEED with memory speed. If you use a higher speed DDRII module, your chipset will just run it at 533MHz. You can always try overclocking your board, the highest I believe is 260FSB.

But yeah.. check out that article. Be sure to check out the section on 'Memory Stress Test Results'
November 1, 2004 6:34:07 PM

Thank's for all of the help. I am learning a little more about how DDR speed and latency works. Say if I were to get 2 675mhz 512mb sticks, and if I had to overclock to get the most out of the ram, what would I run it at? Basically since the RAM is only 25 dollars more, I'd rather get the better stuf now then have to upgrade the ram again later.
November 1, 2004 8:42:55 PM

Since the LGA 775 CPUs are multiplier locked, the only way to overclock the CPU is to increase the FSB. 260 FSB is the maximum I've heard for this board.

The ANANDTECH article says that your board is completely stable at latency at 3-3-3-10 in dual channel, with crucial/Micron DIMMS that support that latency. If you have 4 DIMMS, then CL 4-4-4-10 is best.

Unless you need to push the latency down, it's generally not a good idea to do so unless you understand what you are doing. Some motherboards will allow you to adjust these settings yourself (overiding the programmed DIMM latency), but if you adjust them too low your system will suffer from data corruption.
November 1, 2004 10:08:42 PM

I appreciate the answer, but, let's say I get the 667 ram, what would the FSB speed be? Or, how far would I have to underclock it to run on my MB with a max of 260 FSB? Here is the link to the RAM I want to purchase.

http://shop4.outpost.com/product/4158473?site=sr:SEARCH...

I basically dont care to much about the latency, I just want to know if it is benificial to me to get the 667 ram so I can overclock the FSB on my motherboard to it's Maximum potential. Or if I can do this with just the 533 ram. I still don't understand how the RAM equates the FSB speed, and how the multiplyer works. Thanks in advance.
November 2, 2004 12:05:19 AM

Maybe I should talk a little about FSB and memory speed. They are actually not directly related. FSB is a property of your processor. We always hear FSB expressed as 533FSB, 800FSB or 1066FSB. But these are effective frequencies. Since most motherboards are quad pumped bus (QPB = x4), the core frequency is 133, 200, and 266 respectfully. The processor frequency is a product of this core frequency and a multiplier:
processor speed = (core FSB) x multiplier

For a P4 3.6GHz prescott processor you usually see it rated at 800FSB, with a multiplier of 18 (default for p4). For a quad pumped bus this is a core frequency of 200MHz. Its no surprise that 200MHz x 18 = 3.6 GHz.

Your memory speed is determined by the motherboard chipset and memory modules you use. The FSB connects to the MEMORY BUS through the MCH (memory controller hub). The FSB speed is usually much higher than the MEMORY BUS speed. Because of this, the MEMORY BUS often has two channels (dual-channel) allowing for twice the bandwidth.

For DDR 533, the bandwidth for a single channel is:
(533MHz) x (64bit data bus) / 8 (bits/byte) = 4264MB/s
DUAL CHANNEL: 4264MB/s x 2 = 8528MB/s

You want to match this bandwidth with your FSB for best performance:
8528MB/s = FSB x (4 quad pumped bus) x [(64bit data bus)/(8 bits/byte)]

So your FSB for max performance would be about 266MHz.

Hopefully this helps your understanding a bit. Or maybe you are more confused! :p 

In anycase, keep in mind these are theoretical maximums (for zero latency). The real world values are going to be lower because of the latencies.

ok, so that might explain the FSB.

Finally, your question about 667MHz DDR2. Actually, I would recommend sticking with 533MHz for now. Because most 533MHz DDR2 modules can be pushed up to 667MHz anyway. AND many retailers buy 533 DDR2 memory chips and sell them as DDR2 667MHz chips. Something to think about. If you want faster memory just buy a 533MHz module with better latency. If latency does not matter just stick with 533MHz DDR2 module for now.

For example OCZ 3700 GOLD uses DDR333 chips pushed up to DDR466
http://www.anandtech.com/memory/showdoc.aspx?i=1849&p=1...

If you read the chip number: K4H280838?-TCB3. It is a DDR 333 chip pushed up to DDR466.
a b } Memory
a b V Motherboard
November 2, 2004 3:59:11 AM

All computers run the RAM at a bus speed set by the chipset. Most enthusiast boards let you set the ratio compared to the bus speed of the CPU. The best ratio is 1:1.

P4 bus speed is Quad Data Rate. DDR/DDRII run Double Data Rate. Dual Channel makes up the bandwidth difference. So an 800 bus P4 has a bus clock of 200MHz (QDR, 200x4). That matches DDR400 (DDR, 200x2) with dual channel making up the QDR/DDR difference.

This applies across the board. DDRII 533 will perfectly match a P4 with 1066 bus. With the same processor, DDRII 667 would run at DDR 533 speed by default, a 1:1 ratio, which is the best setting.

So you have DDRII 533, with a clock speed of 266MHz. You have a P4 with 1066 bus, that has a clock speed of 266MHz. If you bought DDRII 667 and wanted to overclock the busses to reach that speed, you'd raise those busses from 266 to 333MHz.

The speed difference for the CPU when overclocking from 266 to 333MHz is 25%. That's enough to push a P4 3.46 to 4.3GHz! Now, your 3.2 runs a default bus clock of 200MHz. Pushing THAT to 333 bus would make it 5.3GHz. I doubt it would run that fast even in a freezer!

So say you're planning to overclock and know you'll never be able to take advantage of DDRII 667, you could always get DDRII 533. And at a 1:1 ratio, you could push the 3.2 to 4.26GHz before you reached the rated speed of the RAM.

Will you reach 4.26GHz? Maybe, but it's not likely. Still, DDRII 533 would be a better choice than DDRII 400 for overclocking.

But wait, there's more! DDRII performs worse than DDR, because it has higher latency. The reason DDRII was introduced was that it will allow Intel to further increase bus speeds. Since you're buying an 800 bus P4, you don't need to make that kind of sacrifice, you'd be best off getting a board that supports DDR400 instead of DDRII, and using DDR500 (aka PC4000, overclocking RAM) if you'd like to overclock.

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November 2, 2004 10:18:00 AM

Could he not just use a 4/5 FSB/Memory divider? That way the FSB would be 266Mhz and the RAM would be 333Mhz (effectivly 667Mhz).

I know on my 865 based board I cant run the RAM faster than the FSB, but im assuming on these 915/925 boards you can else dual channel 533Mhz DDR2 would be pointless for people with 800Mhz FSB CPU's.
a b } Memory
a b V Motherboard
November 2, 2004 3:49:48 PM

I can't see the point in running the RAM bus faster than the CPU bus in dual-channel mode. Perhaps you could force lower latencies instead.

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November 2, 2004 4:14:54 PM

I went ahead and picked up the 667/675 RAM because it was the only one that Fry's had in stock, and after reading this article, it seems I can put it at the higher bus by running the ram at 686? Let me know if you guys have any thoughts on this.

http://www.anandtech.com/memory/showdoc.aspx?i=2112
November 2, 2004 9:16:08 PM

It might interest you that that article talks specifically about the board you want:

From: http://www.anandtech.com/memory/showdoc.aspx?i=2112&p=6

The following settings were tested with all eight DDR2 memories in the roundup:
800FSB/DDR2-533 - the highest stock speed supported on 925X/915 motherboards.
1000FSB/DDR2-667 - a standard speed rating that represents a future memory speed for DDR2 on the Intel platform
1032FSB/DDR2-686 - the highest setting currently usable on the test motherboard with PCIe Video Card and SATA Hard Drive

<b>Since the Abit AA8 is presently limited to a 258 setting for FSB, we were not able to test the highest memory speed with the DDR2 memory modules.</b> This is a current limitation of all 925X motherboards, since the Abit reaches the highest speeds that we have found so far on a 925X motherboard. The Asus P5AD2 is a very close second, with a highest FSB setting of 248. We will continue to monitor the development of overclocking capabilities on 925X motherboards, and we will update Highest Memory Speeds in an upcoming article as soon as we can find a BIOS or motherboard that can fully test the high-speed capabilities of DDR2.
November 5, 2004 7:14:12 PM

I have this board with Crucial ddr2 533. I'm waiting for a new CPU cooler, so I am running only a slight overclock at stock voltage. I found that I can run stable at 231 FSB and standard x17 multiplier (so 3.92 Ghz), with memory set at the 533/3:4 ratio and timings at 4-4-4-10. I played around a lot and found I could drop the timings to 3-3-3-8 and raise the FSB a little more if I boosted voltages. The temps were even ok at that mild voltage boost (63C under load by ABIT's sensors, so probably 53-56C actual), but I didn't see the need to stress the system for the slight performance gain.

I compared the mem bandwidth with basic benchmarks like AIDA and SANDRA, and there *is* a difference between latencies and ratio settings of course. Running the 3:4 ratio gives a slight increase over the 1:1, so I use that setting at 231 FSB. I tried DDR600 (2:3) but I couldn't get the system to boot even with voltage increases and 5-5-5-12 latencies. I can run the DDR600 setting at stock speeds (200/204) with timings at SPD, but there isn't any reason because the main performance gain is the FSB bump of course. I was sort of wondering the same thing as the original poster- would the 667/5300 rated memory allow a higher overclock? Somehow, I don't think my 533 memory is the problem. I think this board has some bugs due to being new tech that need to be ironed out in a new revision. We need some kind of PCI-e lock or fix, the AA8's SATA controller pretty much nixes good overclocking if you aren't using an IDE drive, and there are a lot of other little things bugging me that I can't quite nail down yet. One thing that really bothers me is how the PCI-e video drivers seem to crash the computer when you overclock- no sense in getting a good overclock if you can't run video apps.

Just speculating on the DDR2 thing, I think it needs a bit more oomph before it becomes useful. As in, tight timings like DDR1 developed. Certainly for AMD, that will be great when they finally adapt DDR2 (maybe that's what they're waiting for?). But even Intel will benefit eventually, since they are getting out of the long-pipeline/HT business, and will hopefully come up with something that can really handle memory aggressively.

<P ID="edit"><FONT SIZE=-1><EM>Edited by imp on 11/06/04 10:43 PM.</EM></FONT></P>
November 7, 2004 9:43:30 AM

The Duramax has multis unlocked from 14-17 at least on the 550 and 560 with the new bios. When I flashed the board and rebooted I also noticed that there is a new setting added to "PSB Strap"- 1066. Checking the documentation shows that they have indeed added support for the 1066 FSB processors.

Anyway, with the multiplier unlocked I was able to get the following overclock at base voltages:

FSB @ 260 with the divider, mem running at 346
CPU @ 3.65 Ghz
Latencies: 4-4-3-8
Temps by Uguru (so maybe a few degrees high): 45-47c idle, 60-63c full load

AIDA 32: 7218 Read, 2671 Write
Sandra Bandwidth test: 6439 Int, 6409 float
Everest latency score: 72.8 ns
Everest lists ram as operating at DDR692

You can push that Crucial Ballistix 533 pretty far then. I'm wondering if some of the manufacturers might be sneaky and just package the same product as two different things. I think for the most part I'll stick with the 231 FSB overclock that puts the CPU at 3.92Ghz. It just seems to be a bit quicker than the higher mem bandwidth settings when I'm doing my normal stuff.

I don't know how high you can actually push this rig, because I confined my tests to the lowest voltage setting. It seems like a good platform, but I think this particular board might have some bugs- certain things just nag at me when I'm using it. I'm going to RMA it, and I've ordered a Soltek mobo that has LGA w/DDR1 and a pair of 512 OCZ Platinum Rev. 2 memory. It should be interesting to compare the performance of the same processor when rigged with ddr1 vs ddr2. =)
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