PC6400 or PC5300/5400 with C2D?

[Mark]

Hi,

I am still undecided on what RAM to buy for my C2D system. I'll get a
gigabyte GA-965P-DS4 with a E6420. I can't decide whether to go for
PC5300 CL4 or PC6400 CL5 memory. They are about the same price and
I'm not sure whether one option would be faster than the other.

The only web articles I can find on this concentrate on older P4
systems or overclocking performance, neither of which I am interested
in.

Does anyone have a real idea of the performance difference (if any)?

TIA, M

 

[John Jordan]

Hi,

I am still undecided on what RAM to buy for my C2D system. I'll get a
gigabyte GA-965P-DS4 with a E6420. I can't decide whether to go for
PC5300 CL4 or PC6400 CL5 memory. They are about the same price and
I'm not sure whether one option would be faster than the other.

They're both bandwidth-limited by the FSB. Latencies for 667MHz 4-4-4-12
are fractionally better than 800MHz 5-5-5-15, but the difference may
not even be measureable. There used to be cases with DDR where some
sticks were much faster despite identical headline timings, but I'm not
sure if that's true for DDR2.

 

[Mark]

They're both bandwidth-limited by the FSB. Latencies for 667MHz 4-4-4-12
are fractionally better than 800MHz 5-5-5-15, but the difference may
not even be measureable. There used to be cases with DDR where some
sticks were much faster despite identical headline timings, but I'm not
sure if that's true for DDR2.

I thought the FSB was 1066MHz? Surely this is faster than the 667/800
Mhz of the memory?

M

 

[Paul]

I thought the FSB was 1066MHz? Surely this is faster than the 667/800
Mhz of the memory?

M

Remember that when two DIMMs are installed in dual channel mode, you
have a 128 bit wide memory interface. The processor interface continues
to be 64 bits wide.

At FSB1066, the processor manages 8528MB/sec on its 64 bit bus.
Two DIMMs running at PC2-6400, is 12800MB/sec. (All theoretical maxes of
course. The real world is different.)

The performance improvement can be seen here. There are probably better
articles than this around.

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

The bottom has dropped out of the DDR2 market, as Vista didn't pick
up the demand. (I think I read a blurb that Samsung is shifting some
production capacity, from DDR2 to flash memory.) Thus the price of ordinary
DDR2 is pretty reasonable right now. If you insist on some "uber" RAM
(>PC2-6400), then expect to be taken to the cleaners. In fact, you have
to shop carefully, because some of the smaller retailers might charge
$400 for a product that has dropped to $200 on the high volume retailer
web sites. It helps to read the reviews on a site like Newegg, for the
various memory products, as you can get a better idea of which products
arrive DOA, which have overclocking headroom and so on. The memory
industry is very efficient, and even the floor sweepings are soldered
onto DIMMs these days

Paul

 

[Mark]

Remember that when two DIMMs are installed in dual channel mode, you
have a 128 bit wide memory interface. The processor interface continues
to be 64 bits wide.

At FSB1066, the processor manages 8528MB/sec on its 64 bit bus.
Two DIMMs running at PC2-6400, is 12800MB/sec. (All theoretical maxes of
course. The real world is different.)

So it looks like there is no advantage in PC6400 unless you are going
to overclock? I guess I'll go for some good 667MHz RAM.

The performance improvement can be seen here. There are probably better
articles than this around.

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

I've seen this article. It was done using a Pentium EE CPU - I'd like
to know if there's any significant difference from the C2D?

Cheers, Mark