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PC-2700 vs. PC-2100 @ 133MHz FSB

N

Nilonym

Member
I understand that most mbs come with a 4:3 memory to fsb clock ratio preset, so:

FSB 100 x 1.333 = 133 x 2(DDR) = 266MHz (PC2100), and
FSB 133 x 1.333 = 177 x 2(DDR) = 355MHz (PC2700).

But isn't it the case that there are some efficiencies in having a 1:1 fsb to memory clock ratio?
If I ran an FSB at 133 and used DDR266 memory with 1:1, wouldn't I have "synchronous" memory access at that point? Any chance that it is faster (or as fast as) PC2700?

Ultimately, I am using an Intel 845 with 1.8a, and am going to overclock to 2.4, which means that my FSB will be at 133MHz. I am just trying to figure out which type of RAM to buy.

Jeff
 
I just setup my rig (below) for 133 FSB and ran a couple tests

P4 1.6A 133 FSB 2.1 GHz DDR266 cas2

Q3A demo4
Normal: 255
10x7x32xSHQ: 235
16x12x32xSHQ: 154

3Dmark 2k1SE: 10159

P4 1.6A 133 FSB 2.1 GHz DDR355 cas2

Q3A demo4
Normal: 282
10x7x32xSHQ: 256
16x12x32xSHQ: 157

3Dmark 2k1SE: 10680

The higher mem speed is obviously better. Be aware that the 4:3 mem ratio is not available on any 845 board except the Epox 4BDA/(2+) @ 133 FSB and higher. If you go with the P4B266, run it @ 132 FSB so you can use the 4:3. Why 1.8A @ 2.4? I run a 1.6A @ 2.4, 150 FSB, DDR400. The higher FSB and DDR speed will outperfom a 1.8A @ 2.4 for less $$.
 
Thanks oldfart.

So is it your belief that I have as good of a shot of having a stable oc system with 1.6a@2.4(150fsb) as a 1.8a@2.4(133fsb), using retail HSF? It seems like I am pushing a lot more out of spec for the 1.6a (CPU +800MHz, FSB +50MHz) vs. the 1.8a (CPU +600MHz, FSB + 33MHz).

On the other hand, if I go 150FSB, then my 1:1 mem ratio means 300MHz to ram, which is better than 266MHz for sure, but wouldn't be taking full advantage of DDR333 ram.

My understanding, then, is that if I want to use DDR333 at 133MHz FSB or greater, I must:
- use the Epox board that allows 4:3 ratios
- use a SiS 645 based chipset, which is reportedly flaky going beyond 150FSB (although it does officially support 133FSB and DDR333, so we know it must provide a 4:3 ratio)
- wait for the i845e chipset mbs

Can you confirm my understanding of all this and correct me if I'm mistaken please?

Thanks,
Jeff
 
So is it your belief that I have as good of a shot of having a stable oc system with 1.6a@2.4(150fsb) as a 1.8a@2.4(133fsb), using retail HSF? It seems like I am pushing a lot more out of spec for the 1.6a (CPU +800MHz, FSB +50MHz) vs. the 1.8a (CPU +600MHz, FSB + 33MHz).
Click to expand...
You are pushing the CPU the same amount. 2.4 GHz = 2.4 GHz. Just depends on how good the yield is. You are pushing the ram more. The Samsung PC2700 is great. It can handle the DDR 350 - 400.
On the other hand, if I go 150FSB, then my 1:1 mem ratio means 300MHz to ram, which is better than 266MHz for sure, but wouldn't be taking full advantage of DDR333 ram.
Click to expand...
Nod
My understanding, then, is that if I want to use DDR333 at 133MHz FSB or greater, I must:
- use the Epox board that allows 4:3 ratios
Click to expand...
Yes
- use a SiS 645 based chipset, which is reportedly flaky going beyond 150FSB (although it does officially support 133FSB and DDR333, so we know it must provide a 4:3 ratio)
Click to expand...
I'm not big on "flaky" myself. 845 is solid. I've seen posts of problems above 140 FSB on the 645
- wait for the i845e chipset mbs
Click to expand...
Only difference is "official" support for 533 FSB. 845D does it unofficially. The northbridge is the same on both chipsets. the 845D could always do it. The mobo makers just had to tweak the bios to enable it.
 
Does the Samsung usually run DDR400 at CAS2? I would think DDR355 at CAS2 would be better than DDR400 at CAS3 especially with programs like SETI, though I don't know for sure.
 
Originally posted by: Nilonym
I understand that most mbs come with a 4:3 memory to fsb clock ratio preset, so:

FSB 100 x 1.333 = 133 x 2(DDR) = 266MHz (PC2100), and
FSB 133 x 1.333 = 177 x 2(DDR) = 355MHz (PC2700).

But isn't it the case that there are some efficiencies in having a 1:1 fsb to memory clock ratio?
If I ran an FSB at 133 and used DDR266 memory with 1:1, wouldn't I have "synchronous" memory access at that point? Any chance that it is faster (or as fast as) PC2700?

Ultimately, I am using an Intel 845 with 1.8a, and am going to overclock to 2.4, which means that my FSB will be at 133MHz. I am just trying to figure out which type of RAM to buy.

Jeff
Click to expand...


Note that PC2100 is DDR266 and PC2700 is DDR333 and not 355 MHz ... They way you run PC 2700 memory at its rated speed (on most modern motherboards) is at an FSB of 133 MHz and a 5/4 divider. This is the case on the SiS chipset P4 motherboards (P4B333 and P4B533, etc), as well as the KT-333 motherboards for Athlons.

The 3:4 divider is used to run PC 2100 memory on P4's at their default speeds (which is a 100 MHz quad pumped FSB, not 133 MHz). That's also why the 3:4 divider disappears after 133 MHz, because its way above PC2100 speeds and even PC2700 speeds.

Other ways to run at PC2700 speeds (333 MHz memory) is at a 166 MHz FSB, or at a 125 MHz FSB and using a 3:4 ratio divider (I'm currently doing that on an ASUS P4B266 with a 1.8A @ 2.25 GHz, 125 MHz FSB).
 
Great post jiffylube. Thanks for the clarification.

So the statement has always been that other than the Epox, 845 mbs do not support 4:3 beyond at 133MHz and beyond. Now I see that that is not really an issue. What I want to know is if they support 5:4 at 133MHz (133x1.25x2=DDR333)?

Could I use my GA-8IRXP 845 based mb, run it at 333FSB, and set a memory ratio of 5:4, and have my 1.8a@2.4GHz with full DDR333 memory?

Side question. Do mbs commonly support other ratios, like 10:9? So I could have 1.6a@2.4GHz and full DDR333 memory (16x150FSB=2.4GHz, 150FSBx1.1111x2=DDR333).

Thanks for all info.

Jeff
 
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