C2D: matching FSB and memory

[Freelance]

hopefully a simple question: if was to put this (Intel Core 2 Duo E6850 "LGA775 Conroe" 3.00GHz (1333FSB) - Retail) processor in a new rig, does the "1333FSB" bit refer to just what the mobo has to support (G33/P35 chipset, etc.), or does it also mean the memory *has* to match it?

 

[helmutcheese]

No, Intel CPU's are Quad Pumped, example old CPU like AMD had 133FSB x 2 as DDR so 266 DDR FSB, Intel had 100FSB x4 as DDR so 400 DDR FSB.

Now the 1066FSB CPU only need 533DDR Memory (PC4200), 1333FSB CPU only need 667DDR Memory (PC5300), thats obv not inc Overclocking, stick in a nice affordable 2GB kit of 800mhz (PC6400).

 

[Freelance]

hmmm, i guess i'm asking which combinations will or will not work. obviously 1333cpu/fsb +1333 mem and 1066/1066 will work, but can you combine a 1333 cpu/fsb with slower memory? obviously it wouldn't be optimal, but does intel's chip support this config (it's been a while since i've looked at choosing a cpu, and i can't find anywhere that gives a definitive yes or no)

 

[helmutcheese]

I have just told you above, a 1333FSB CPU only needs 667mhz Memory to be 1:1 in Sync (PC5300), any high speed Memory will allow you to overclock the CPU FSB higher keeping the Memory at 1:1 in sync.

 

[Justintime]

Now the 1066FSB CPU only need 533DDR Memory (PC5200),

PC4200

 

[helmutcheese]

Yes TYPO's hard to go back and forth checking store lol.

 

[Justintime]

Yes TYPO's hard to go back and forth checking store lol.

Thought it was a mistake since you edited it twice from 5400 to 5200

 

[Corasik]

hmmm, i guess i'm asking which combinations will or will not work. obviously 1333cpu/fsb +1333 mem and 1066/1066 will work, but can you combine a 1333 cpu/fsb with slower memory? obviously it wouldn't be optimal, but does intel's chip support this config (it's been a while since i've looked at choosing a cpu, and i can't find anywhere that gives a definitive yes or no)

Intel systems for some time now have used 64bit wide CPU interfaces and 128bit wide memory interfaces. This allows full memory bandwidth with memory clocked at 50% of the CPU's FSB.

IE 1066x64 = 68224, and 533 x 128 = 68224.

There is a small price to pay in absolute latency, as the memory can be up to 1 cpu clock cycle out of sync at any given time, but thats a small price to pay considering the saving of being able to use cheaper memory.

Infact in real world terms, memory latency has always lagged behind the CPU anyway, so a matching the bandwidth between memory and CPU is far more important than the clock speed.

Of course in a perfect world, FSB and Memory would both be clocked at the exact same speed as the CPU, that would virtually eliminate latency.. (Would be like haveing main memory that was as fast as Cache).. However it would be so expensive that nobody could afford it. So Dual channel, with some latency is an acceptable comprimise

Anyway, in simple terms, no matter how far you overclock an intel based PC, the memory only needs to be as fast as 50% of the Frontside bus speed for a 'correct match'.

 

[errata]

I have just told you above, a 1333FSB CPU only needs 667mhz Memory to be 1:1 in Sync (PC5300), any high speed Memory will allow you to overclock the CPU FSB higher keeping the Memory at 1:1 in sync.

Your talking about "need" for 1 to 1 sync, the questions now about will slower memory work (sub omptimally) say would a 1333 fsb processor work at all with pc4200 (266 mhz memory).

I guess yes as sync does not have to be at 1 to 1 but anybody tried it?

 

[melbourne720]

Most motherboards won't allow you to run the memory slower than the FSB. So 800 DDR (PC6400) would give you a max FSB of 400MHz (1600 quad pumped).

 

[Freelance]

thanks for the replies, the penny dropped and i'm busy putting shopping lists together for a buy soon