Help with DDR-2 and FSB...

[johnjkr1]

I'm new to the DDR-2 world and am a bit confused how DDR-2 speeds and FSB speeds tie together.

For instance, let's say I have one of the new 2.4 Core Duo chips with a 1066 FSB. Do I need DDR2-800? DDR-2 667? DDR-2 1066? How does FSB = DDR2 speeds?

Also, what about the upcoming 1333 FSB on newer intel chips? What speed DDR-2 will I need for that?

Appreciate any help.

Thanks!

 

[Gusty987]

Are you gonna be overclocking? If not, DDR2 533 is all you will need. If OCing, you can get faster memory and then either increase the FSB or the RAM multiplier to take advantage of it. (the memory speed is FSB*multiplier).

 

[johnjkr1]

Gusty,

Thanks for the reply. I will not be overclocking, just standard speeds.

Can you give me an example of FSB * Multiplier with a core duo? I'm assuming the 4x (quad pumped) bus has something to do with this.

Thanks!

 

[Roguestar]

This thread might help, check my second post down.
http://forums.anandtech.com/messageview.aspx?catid=27&threadid=2036150

 

[johnjkr1]

Thanks for the link....lots of good info in there, still trying to understand the link though.

 

[bamacre]

Well, that is true, all you need is 533mhz DDR2 but it needs to be ran in dual channel.

2 x 533 = 1066

The "2" above is from running the memory in dual channel. The "533" is the speed of the ram. The "1066" of course is both the FSB of the cpu and the relative speed of the memory.

 

[yh125d]

to figure the ram speed you need for intel cpus, do this

take the rated fsb of the cpu, which for 6xxx C2D's is 1066, and divide it by two.

1066/2=533

That is the ram speed you need to run the cpu at stock speed. If you're overclocking, you'll want some ram with a higher speed, like 667 or 800

The upcoming penryn's will have a 1333 fsb, so

1333/2=667

you will need ddr2-667 to run the cpu/ram at stock speed, more to o/c

 

[johnjkr1]

Thank you all very much, that is exactly what I was looking for. So, DDR2-800 = safe for a long time, including 1333 bus speeds.

Thanks!

 

[Roguestar]

Originally posted by: bamacre
Well, that is true, all you need is 533mhz DDR2 but it needs to be ran in dual channel.

2 x 533 = 1066

The "2" above is from running the memory in dual channel. The "533" is the speed of the ram. The "1066" of course is both the FSB of the cpu and the relative speed of the memory.

No, dual channel doubles the lines available for the memory to transfer along, doubling the memory bandwidth. It doesn't double the speed of the memory as such. You can have a single stick of DDR2-533 running in single-channel and still in a "1:1" ratio with the FSB on a standard Core 2 Duo.

 

[hans007]

the 2 in ddr2 is not dual channel.


its because its the second gen of ddr memory.


the intel gtl bus is 64bits wide. a 240 pin dimm is a 64bit module. dual channel is obviously 128bits, so a module at half speed with twice the bandwidth is the same as the bus @ full speed.


2 x 64bits * 1/2 speed = 64bits * 1.0 speed.


that said, it is possible to get more performance without overclocking if you use ddr2-800.


ddr2-533 is typically cl4. that is 4 cycles of 533mhz clocked latency. a 533mhz clock is 1.87ns. so if you bought ddr2-800 at cl4 it is 4 cycles of 800mhz latency, which is 1.25ns.


so the latency is 1.87/1.25 more on ddr2-533 at cl4.


so if you are buying new memory now, since cl5 ddr2-800 is pretty cheap and only a little more than ddr2-533 i would suggest you buy that. since 5 x 1.25 is still less than 4 x 1.87. additionally you will probably want the ddr2-800 if intel phases out ddr2-533 support in future chipsets like the g35 , x38 ones coming in the fall.