need DEFINITIVE answer on memory speed confusion

jjd228 · 04-01-2007, 02:53 PM

posted this a while back and got a few answers but now im being told i was given bad advice...

mobo: Intel D975Xbx2
proc: Intel Core 2 Duo 2.6 w/ 1066 fsb

Now, the board supports memory 533/667/800 MHz. I was told that when running the memory in dual channel mode (equal DIMM's in each channel) that using memory above 533MHz is useless because 533x2=1066 (the fsb speed). So if I put 2 DIMM's in @ 800MHz (1600 combined) that it would be throttled back to 1066 anyway... if this is so, WHY does memory come at 800MHz for my board??? why wouldnt each channel simply run at 800??

At any rate, I bought 2GB ram (2x1GB DIMM's) @ 533MHz. Is the above true for a FACT? Or would I benefit from using the 800MHz ram?

If someone has a definitive answer and can explain, I would appreciate it.

Big B · 04-01-2007, 08:00 PM

There seems to be some confusion here. Memory channels do not add up to a faster memory speed. They add the bit width: each channel is 64-bits wide, and you get 128-bits in dual-channel mode. It does not add each 800MHz or other speed grade to give 1600MHz. This operates like a highway speed limit. If the limit on a 4 lane highway is 75MPH, that limit doesn't go to 300MPH, it stays at 75.

Now, for the FSB, there's another thing that needs to be clarified. Core 2, like it's predecessor, the Pentium 4, operates with a quad-pumped FSB. There's a base frequency that the memory and CPU anchor on. It's this number that the CPU speed is determined in conjunction with the multiplier.

Here's an example of the math:

FSB=(Base Freq) x 4

Assuming that the FSB is 1066 you would do the following:

1066/4=266 (266.5, to be precise, but that's not an option we work with here).

The third piece of the puzzle is in regards to DDR/DDR2 speeds. I put a table together here.

Quote:

Originally Posted by me

DDR=Double Data Rate. Specifically, this is DDR SDRAM (Synchronous Dynamic Random Access Memory), but it's easier and shorter to just use DDR. 'SDRAM' is typically used to to refer to SDR (Single Data Rate) SDRAM.

DDR transmits data on the rise and fall of the clock. DDR at 100MHz is equal to SDR at 200MHz. This is why PC1600 is also known as DDR200. DDR (and DDR2) are 64-bits wide, or 8-bytes wide (8 bits to a byte). 8B x 200MHz = 1.6GB/s peak bandwith.

Keep in mind that the calculations are maximum speeds guaranteed for that RAM. They will function fine at slower speeds, but only produce the bandwith at the given speed rating.

Unless you're overclocking, then DDR2-533 is exactly what you need. Now, if you wanted to overclock, you'd want to have PC6400/DDR2-800.

jjd228 · 04-01-2007, 08:04 PM

thank you!

Dean me · 08-10-2007, 09:42 PM

sorry guys im a bit slow on this.
I have basically the same problem as JJD228 had.

I just need to know which two frequency values we are trying to match, to prevent either the CPU or the RAM slowing by the bottleneck effect?

I've been looking for this answer for a a while and any help would be appreciated, thanks.

04-01-2007, 02:53 PM	#1 (permalink) Top
jjd228 Geek Trainee Join Date: Sep 2006 Posts: 23 Status: Offline	need DEFINITIVE answer on memory speed confusion posted this a while back and got a few answers but now im being told i was given bad advice... mobo: Intel D975Xbx2 proc: Intel Core 2 Duo 2.6 w/ 1066 fsb Now, the board supports memory 533/667/800 MHz. I was told that when running the memory in dual channel mode (equal DIMM's in each channel) that using memory above 533MHz is useless because 533x2=1066 (the fsb speed). So if I put 2 DIMM's in @ 800MHz (1600 combined) that it would be throttled back to 1066 anyway... if this is so, WHY does memory come at 800MHz for my board??? why wouldnt each channel simply run at 800?? At any rate, I bought 2GB ram (2x1GB DIMM's) @ 533MHz. Is the above true for a FACT? Or would I benefit from using the 800MHz ram? If someone has a definitive answer and can explain, I would appreciate it.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Memory Confusion...	Tru Profit	CPU, Motherboards and Memory	7	08-01-2006 07:59 AM
The Definitive BIOS Optimization Guide Revision 8.3	syngod	Overclocking & Cooling	0	09-09-2005 10:06 PM
video card-core and memory speed question	efx2003	Video Cards, Displays and TV Tuners	7	30-08-2005 02:06 AM
Memory Timings/Speed	Matt555	CPU, Motherboards and Memory	23	20-08-2005 09:51 AM
Flash Memory to Speed Up Hard Drives	syngod	News and Article Comments	0	01-08-2005 08:33 PM

04-01-2007, 08:04 PM	#3 (permalink) Top
jjd228 Geek Trainee Join Date: Sep 2006 Posts: 23 Status: Offline	thank you!

08-10-2007, 09:42 PM	#4 (permalink) Top
Dean me Geek Trainee Join Date: Oct 2007 Posts: 6 Status: Offline	sorry guys im a bit slow on this. I have basically the same problem as JJD228 had. I just need to know which two frequency values we are trying to match, to prevent either the CPU or the RAM slowing by the bottleneck effect? I've been looking for this answer for a a while and any help would be appreciated, thanks.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)