CPU FSB Vs Memory speeds

Stonedofmoo · 11 Dec 2007 at 13:01

I'm looking to buy a budget Intel rig, but with a top motherboard so I can upgrade in future.

My plan is get a good motherboard now with a cheaper core duo chip and then upgrade to the Yorkfield quad chips when they come out next year.

The best looking chip for me is the Core Duo 2 E6750 at £115.
The best memory is Geil 6400 memory at £35 for 2gig

The CPU runs at 1333fsb, the memory at 800fsb?
How much of a hinderance would this be. The next upgrade in memory speed seems a lot more expensive for some reason.

Any thoughts?

semi-pro waster · 11 Dec 2007 at 13:04

Long story short, the Core2Duo/Quads use a quad-pumped FSB so to get 1333mhz you need 4x333mhz (i.e. PC5300 speeds) so PC6400 (400mhz actual) is no hinderance whatsoever.

I'd personally probably go with the E2180 since you want to overclock anyway, it is around half the price but with decent cooling and Ram should easily be pushing on to over 3ghz and quite a few people have got 3.6ghz+ out of them.

Stonedofmoo · 11 Dec 2007 at 13:09

Oh really? That's great news and makes OcUK's Geil bargain look all the more attractive.

Thanks for the answer, I'm off to hunt for a suitable motherboard now

Corasik · 11 Dec 2007 at 13:54

The 'full' explanation....

Intel CPU's have a 64bit wide quad pumped interface running at 333mhz (assuming 1333FSB). The northbridge has a DDR2 interface with a 128 bit wide 'dual channel' interface running at 333mhz (when running 1:1). So PC5300 is a 'perfect' match for a 1333 FSB CPU.. However faster memory give you overclocking overheads.

Simplistic example, based on 1333 Hz bus rather than Mhz

64 x 1333 = 85312 bps
128 x 667 = 85367 bps.

Whatever FSB your going to overclock to, divide by two, and pick the DDR memory that matches.
So if you overclock to 400mhz (1600FSB) your going to need PC6400 which is 800@400mhz DDR ram. While it is possible to run memory 'faster' than the bus, the gains are generally poor. In theory it can improve latency a tiny bit, but in practice it most 'fast' memory chips need slacker (slower) timings to compensate for the increased clock speed anyway.

In reality neither 1333, or 667 are the actual speeds, 333 x 4 = 1332, and 333x2 = 666... But I guess someone in marketing decided against calling it 666 Mhz

Dual channel does increase latency slightly, but the overall bandwidth is a 1:1 match.

Apparently the 'enthusiasts' version of Nehelem may even come with a tripple channel 192 bit DDR3 interface, and as you probably know graphics cards are already making use of 256, 320 and even 384bit wide memory controllers.

Stonedofmoo · 11 Dec 2007 at 13:56

I see

So whats the point in this really fast and expensive DDR3 memory? It runs much faster than the chip and has higher latencies?

Corasik · 11 Dec 2007 at 14:05

Stonedofmoo said:
I see

So whats the point in this really fast and expensive DDR3 memory? It runs much faster than the chip and has higher latencies?

Mhz sells? Its all about marketting isnt it. It does offer advantages when overclocking to the extreme, Take a LN2 cooled system with a 2664FSB, you would need 1333 DDR3 to keep up in dual channel mode.

In the 'real world' its not unheard of for people to get their motherboards up to 450mhz and even 500 isnt unheard of, but that is pretty extreme. But 500=2000FSB, and needs dual channel DDR memory at 1000 (actually 500mhz) to keep the cpu filled with data.

So the point of fast memory.. Overclocking, and marketing.

Far more cost effective would be if intel switched to a Quad channel 256bit interface and stuck with cheaper memory.

semi-pro waster · 11 Dec 2007 at 21:24

Cheers for that explanation Corasik, I didn't know it in that much detail, I knew the basics and that is enough to explain most of what is necessary but that is interesting.