DRAM freq is not integer multiple of FSB

Abit Nf7-s2 motherboard
June 23, 2011 at 14:46:54
Specs: Windows 7, Athlon XP 2400+ OC
Are there any implications when the DRAM frequency is not an INTEGER multiple (2x, 3x, ..) of FSB, as compared to integer multiples ?
For example, a core2duo T5870 800Mhz FSB (200x4) with a PC3-8500 533Mhz (133.33x4), so basically a 1.5x ratio between DRAM:FSB. Asked another way would a PC2-6400 or a PC3-6400 or PC3-12800 be a better choice since those are integer multiples of the FSB ?

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#1
June 23, 2011 at 15:02:45
The optimal CPU:DRAM frequency ratio for a Core 2 Duo system is either 1:1 or 1:2.

That means an 800MHz FSB CPU (200MHz frequency) should be paired up with DDR2-400 (200MHz frequency) or DDR2-800 (400MHz frequency).

"For example, a core2duo T5870 800Mhz FSB (200x4) with a PC3-8500 533Mhz (133.33x4)"

PC3-8500 (aka DDR3-1066) does NOT run at 133MHz x 4. It runs at 533MHz frequency. And if it's paired up with an 800MHz FSB CPU, the frequency ratio is 200:533, which is the same as 3:8. Obviously 3:8 is not 1:1 or 1:2, therefore it is not the optimal configuration. It would be best to underclock the RAM to 400MHz frequency.


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#2
June 23, 2011 at 18:33:57
But if the cpu is not OC'd i.e. its running at the rated fsb of 800Mhz(200x4) then wouldn't it automaticallybe running the memory at 400Mhz(ddr3 - 200x2) ?

So this would be a 1:2 DRAM:FSB ratio, right ?
What's throwing me off is that CPUID is reporting 1:2 ratio, but why does Everest Ultimate report the DRAM:FSB ratio as 12:6 then ? Can you please help me understand this ? Thanks!


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#3
June 24, 2011 at 08:59:04
You didn't list anything about the system you're asking about but I know it's not an "Abit Nf7-s2 motherboard" & "Athlon XP 2400+ OC". It would help to know exactly what you're referring to.

"But if the cpu is not OC'd i.e. its running at the rated fsb of 800Mhz(200x4) then wouldn't it automaticallybe running the memory at 400Mhz(ddr3 - 200x2) ?"

Generally, if the BIOS settings aren't manually configured, both the CPU & RAM will run at their default speeds. And to make things easier/clearer, please refer to actual frequencies, not the bullsh!t QDR & DDR ratings.

"So this would be a 1:2 DRAM:FSB ratio, right ?"

The ratio is CPU frequency:DRAM frequency, not the other way around. But in the example above (fsb of 800Mhz(200x4) / memory at 400Mhz(ddr3 - 200x2)), the ratio would be 1:1 because both the CPU & RAM are running at 200MHz frequency.

Are you sure what your wrote is correct? Could it be CPU at 800MHz FSB (200MHz frequency) & DDR3 at 800MHz (400MHz frequency)? Because that would be 200:400, in other words,1:2.

"What's throwing me off is that CPUID is reporting 1:2 ratio, but why does Everest Ultimate report the DRAM:FSB ratio as 12:6 then ? Can you please help me understand this ? Thanks!"

CPUID is reporting the ratio correctly - CPU:DRAM. It appears Everest has it backwards - DRAM:CPU. But if you reduce 12:6 to 2:1 (which is the same thing), then reverse the order, you have 1:2.


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#4
June 25, 2011 at 08:46:19
yeah its not about the system in my sig. This is a Thinkpad SL410 notebook. Here are some screen shots -
The first is from Everest, second from Sandra, and the rest are cpu-z -

http://i94.photobucket.com/albums/l...
http://i94.photobucket.com/albums/l...
http://s94.photobucket.com/albums/l...
http://s94.photobucket.com/albums/l...
http://s94.photobucket.com/albums/l...

still beats me why Everest would report 12:6.


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