DDR2 RAM Speeds

[[GSV]Trig]

How does the DDR2/PC2.... speed compair to the FSB of the CPU.
Reason I ask is theres some pretty fast ram out there but what's actually needed and whats overkill...
Looking at RAM for an Intel 6400

[htid]

I'll copy what i posted to someone else yesterday

From my limited knowledge of overclocking, your ram clock speed needs to be double your fsb speed (hope that's right). Since PC5300 is 667mhz half that is 333.5, and 333.5 x 9 (the multiplier on an E6600...not sure about the E6300) is around 3ghz so theoretically i would assume you can get 3ghz with that RAM, but you should definitely be able to overclock, just depends how high

(I may be talking crap here, so if I am can somebody please tell me or i'll never learn )

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Dunno what the multi on an 6400 is so you'll have to find that out yourself but the premise is the same.

Hope that helps!

[htid]

double post

[shaithis]

If you need to figure out the maximum rated speed for the RAM, just divide the PCxxxxx number by 8.

That gives you the max speed but because its double data rate, you then divide by 2 again to get the 'FSB value'...

[Hottentot]

Multiplier on the 6400 is 8.
Default bus speed is 266 which is DDR2 4200 (533MHz)
8 x 266 = 2.13 GHz

If you put DDR2 6400 (800MHz) in and ran 1:1
8 x 400 = 3.2GHz

Intel boards don't let you run the memory slower than the FSB so you need fast memory. The new Nvidia and ATI boards will let you run the memory slower so fast memory is not important.

[Hidden Hippo]

The AB9 series boards do support a 4:5 and 2:3 divider on the memory, so you are able to run 800MHz memory on the stock FSB. I myself am running my E6600 at 267 with a 2:3 ration for 800MHz.

[[GSV]Trig]

Multiplier on the 6400 is 8.
Default bus speed is 266 which is DDR2 4200 (533MHz)
8 x 266 = 2.13 GHz

If you put DDR2 6400 (800MHz) in and ran 1:1
8 x 400 = 3.2GHz

Intel boards don't let you run the memory slower than the FSB so you need fast memory. The new Nvidia and ATI boards will let you run the memory slower so fast memory is not important.

The 6400 tho states 1066Mhz FSB or is that just the quad pumped intel tosh

[Hottentot]

Yes that is the quad pumped tosh. It was easier back in the old days when memory was described by its speed which matched up with the motherboards FSB.
Its a choice now between an expensive mobo (£200) which will let you run with asynch slower memory or buying expensive fast memory. Tricky

[No.72]

way hay what part of poole?

[Hottentot]

Parkstone

[Mr Pants]

Intel boards don't let you run the memory slower than the FSB

Im sure that it can run slower or faster with dividers otherwise you would be stuck if you had slower ram

[No.72]

O nice I am from parkstone! where about in parkstone?

[[GSV]Trig]

So for the new Intel 1066Mhz chips you need PC2-8500+ then...

Bet thats not cheap then...

[BUFF]

So for the new Intel 1066Mhz chips you need PC2-8500+ then...

no, you don't.
DDR2-8500 is 533fsb but Intel's 1066 is really 266 quad pumped so for stock operation even DDR2-4200 is fine (there are actually chipsets that will run 1066MHz chips using DDR400).

[kalniel]

Some notes on speeds:

Core 2 Duo motherboards all have to support 1066 fsb. This is not the real fsb (front side bus) speed, but the speed of the connection to the CPU. In intels case they quad pump the real fsb for the connection to the CPU. 1066 is 4x the real fsb of 266.

The CPU speed works in multiples of the real fsb, so a 2.66ghz chip is using a 10x multiplier.

Ram also normally works at this same speed of 266mhz, but in the case of DDR (double data rate) RAM manufacturer's quote speeds at double the real speed, because they are transfering twice as much information per cycle.

Therefore we can conclude that at 'normal' settings, the RAM has to be 533mhz, and the motherboard has to be 1066mhz, to support Core 2 Duo.

Things get interesting when you either overclock or change the ram:fsb speed ratio. Overclocking can be done one of two ways - increasing the multiplier (ie go for 11x266 instead of 10x) or changing the fsb (going for 10x300 instead of 10x266). Extreme processors allow multipliers to be changed, but it is fixed for non-extreme models, leaving fsb as the only way to improve things.

Most motherboards these days allow a lot of head room for increasing fsb. They often do this by clamping the speed that other components talk to the fsb - so your graphics card still communicate at the normal speed for example. However if you are still running the ram at 1:1 with the fsb you have to make sure your ram is rated at higher speeds. DDR2 800 (real speed = 400mhz) gives a ridiculous amount of headroom for increased fsb, infact your motherboard will likely fail before you reach this speed. So how do you take advantage of this extra speed?

You run the RAM at a different speed to the FSB.

In earlier CPUs running the ram asynchronously gave a performance penalty, but with the new generation of chips this is no longer the case. That means you can run your FSB:RAM at 4:5 ratio for example, letting you run 266 fsb but ram running at 333 mhz. Then you can overclock the fsb on top, but as the rams running faster you don't need to overclock as much to get the most out of the ram potential.

[shaithis]

Most boards now let you set multipliers BELOW the CPUs default.