FSB - RAM Speed - CPU Speed compatibility and efficiency

[Robert Langlois]

Hello

I am building my first computer and am unsure about the speed of my
memory. I did a lot of research but this is still not clear to me.

My CPU is the Intel Quad Core 2 Q6600 2.4 GHz / 1066 MHz FSB.

Does that mean I should put 1066 MHz SDRAM in there ? That would be
the PC2-8500 (1066 MHz) DDR2. If I do that, am I wasting money? At
first, I thought I should do this to match the CPU's FSB speed with
the speed of the memory but some people say it will be useless.

I read I should buy 533 MHz DDR2. That would be the PC2-4200. They
say that 533 multiplied by two gives 1066 MHz so it matches the CPU's
FSB? Buying 1066 MHz would only be good if I overclocked the CPU?

This is confusing because I thought for sure that the CPU's FSB should
match the memory's speed but if buying 533 MHz is just as good then
that's what I'll do since it's a lot cheaper.

Thanks

 

[Paul]

Hello

I am building my first computer and am unsure about the speed of my
memory. I did a lot of research but this is still not clear to me.

My CPU is the Intel Quad Core 2 Q6600 2.4 GHz / 1066 MHz FSB.

Does that mean I should put 1066 MHz SDRAM in there ? That would be
the PC2-8500 (1066 MHz) DDR2. If I do that, am I wasting money? At
first, I thought I should do this to match the CPU's FSB speed with
the speed of the memory but some people say it will be useless.

I read I should buy 533 MHz DDR2. That would be the PC2-4200. They
say that 533 multiplied by two gives 1066 MHz so it matches the CPU's
FSB? Buying 1066 MHz would only be good if I overclocked the CPU?

This is confusing because I thought for sure that the CPU's FSB should
match the memory's speed but if buying 533 MHz is just as good then
that's what I'll do since it's a lot cheaper.

Thanks

There is a table here, of tests they did. The execution time of SuperPI
is dropping slightly, as the RAM speed goes up.

http://www.anandtech.com/memory/showdoc.aspx?i=2732&p=4

People typically buy DDR2-800. Above that, there could be a price
premium. Also, the motherboard chipset may not support speeds above
DDR2-800, without some kind of overclocking going on. For example, you
can drop the multiplier value, raise the FSB, raise the memory bus speed,
to emphasize the effects of the memory, and make use of the really
high speed memory. All while keeping the core at or near the nominal
speed.

In terms of balance, dual channel memory is 128 bits wide. The
processor FSB is 64 bits wide. That is why you can have a ratio of
FSB1066 to two sticks of DDR2-533. But the memory subsystem is decoupled
from the FSB speed, and the flow of data is rate adapted. That example
is purely coincidental and not indicative of some real requirement.

The data bus on the memory DIMMs, doesn't have data on it during every
memory clock cycle. There will be small gaps, where a new command is
being fed to the DIMMs, or a delay while waiting for precharge or whatever.
The memory bus is not 100% efficient. So, in fact, the actual data transfer
rate, versus the theoretical, is quite different.

Paul

 

[DaveW]

The CPU in this case is quad powered, meaning it runs at four times the FSB.
The RAM in use is DDR2, Double Data Rate 2, thus runs at twice the FSB. So,
you should be using the 533 MHz RAM, unless you wanted to overclock. (2 x
533 MHz = 1066 MHz.)