CPU FSB and RAM speed

[heyimsu]

Hi, a question about CPU FSB and RAM speed.

Say for example I have a Q6600. Wiki says the FSB is 1066MHz. Therefore should I be buying RAM with a speed of 1066Mhz as well?

Cheers.

[bubblegoose]

Its not necceserry, but faster ram, will give better performance.
most 800mhz ram should be able to run at 1066mz anyway.

[LostBenji]

Yes, the CPU is designed to run with a 1066FSB with the host freq set at 266 with a multiplier max of 9.

Most will Overclock the board to 400x8 with the RAM set to 1:1 where DDRII800 is a good all round solution. End result is 3.2Ghz @ 1600FSB and RAM driven at 400Mhz. Nice and efficient.

[Tubby2]

Originally Posted by heyimsu

Hi, a question about CPU FSB and RAM speed.

Say for example I have a Q6600. Wiki says the FSB is 1066MHz. Therefore should I be buying RAM with a speed of 1066Mhz as well?

Cheers.

From my limited knowledge trying to wrap my head around this for ages. It takes a bit of thought each time.

Technically speaking I think the FSB for the Q6600 is 266. These CPU's are quad pumped, so 266 x 4 equals the 1066mhz stated in wiki (or near enough).

So if the std FSB is 266 and the multiplier for the cpu is 9 then 266 x 9 =2394 or 2.4Mhz which is the speed of the Q6600 but most people can easily get them to 3.0mhz and over, but I digress...

DDR2 RAM (Double Data Rate) runs, as the name suggests, at double the FSB so at a 1:1 RAM divider the normal RAM speed will be 532mhz (266 x 2). So if you ran your system at stock speeds all you would need is DDR2 600mhz and it would be fine.
If you wanted to o/clock your machine, every time you increase the FSB, you are increasing the speed at which the RAM has to run.
For eg, if you wanted to try and o/clock a q6600 from 2.4 to 3mhz (a 600mhz o'clock) you would need to increase the FSB by 150 (600mhz/4) so you would have a FSB of 416 (266+150).
To run your system at an FSB of 416 you would need RAM that could run at 832 (416 x 2), So you would need DDR21066 but in reality DDR2800 would most probably run at the speed also.

So there you have it. If I'm totally wrong, I'm sorry and I'm sure I'll be told in no uncertain terms very quickly, but thats the way it was explained to me.


Oh and to answer your question if you didnt catch it, You dont need to buy 1066mhz RAM, Without o/clocking 600mhz RAM would be fine, but with the price of RAM now you may as well go and get some 800mhz RAM which will see you thru

[heyimsu]

OK, thanks for your explainations. Though I still don't understand where the 266 comes into it...

So the Q6600 runs at 1066FSB. Since there are 4 cores, that means 1066/4 = 266 ??

Would that mean a E6600 which also has a 1066FBS be 1066/2=533 ?

[animate]

@ Tubby2
You did a pretty good job. also typo 2.4mhz?? lol Ghz ;D
you cannot get 600mhz ram afaik. Doubel data rate is correct, the speeds are

266.5 x 2 = 533
333.5 x 2 = 667
400 x 2 = 800
533 x 2 = 1066
(i am pretty sure of this)

Getting 1066 ram would be pretty sweet if you actually plan to adjust the bios so you get a ratio of 1:1. otherwise its kind of pointless as the system will be running at a much higher ratio, resulting in performance loss. (I would assume so anyway)

So basically what it comes down to is. Do you want to overclock and optimise your sytem for best performance or are you planning on just buy it build it and leave it?
I would definately get the 1066 as it gives your more headroom if you plan to overclock anyway.

@ bubblegoose: SOME 800mhz ram will clock to 1066. not most lol. just gotta get a lucky stick/s

just my thoughts.

peace

[animate]

Originally Posted by heyimsu

OK, thanks for your explainations. Though I still don't understand where the 266 comes into it...

So the Q6600 runs at 1066FSB. Since there are 4 cores, that means 1066/4 = 266 ??

Would that mean a E6600 which also has a 1066FBS be 1066/2=533 ?

Basically no

unfourtunately i have no idea how to explain it. you just have to read alot of shit to understand what is going on

[heyimsu]

I'm beginning understand it now...

@ 1:1 ratio

CPU 266 x 9 = 2.4GHz (default); DDR2 RAM 266 x 2 = 533MHz (minimum speed of RAM needed)

Now if I buy DDR2-800 and make full use of it;

CPU 400 x 9 = 3.6GHz ; DDR2 RAM 400 x 2 = 800MHz (full use of RAM)

Also, this 9 x multiplier, is this 'lock' by the CPU manufacturers?

[animate]

yep your getting it now :P

im not sure with the quad but i think 9 is max and like 6 or 7 is the minimum so its kinda locked the unlocked versions have a X in the name E.G QX6750

[SLATYE]

Originally Posted by heyimsu

So the Q6600 runs at 1066FSB. Since there are 4 cores, that means 1066/4 = 266 ??

Quick explanation:

[EDIT: Fine, so it's not actually a 'quick' summary. Unfortunately I can't think of any way to make it much shorter]

All Intel CPUs since the first P4 have used a QDR FSB. What this means is that in each clock cycle of the FSB, data gets transferred four times. That's where the 266 comes from. The actual FSB frequency is only 266MHz. However, because it transfers data four times in each clock cycle, the effective data frequency is four times 266MHz (1066MHz, or close enough).

Naturally Intel advertise this number (1066MHz), because most consumers think that higher numbers are always better. I perfer to use the real FSB frequency (266MHz in this case).

The FSB transfers 64 bits (8 bytes) of data at a time, so the total bandwidth for the Q6600's FSB is:
[code]1066 (million transfers per second) * 8 (bytes per transfer) = 8533MB/s, or 8.533GB/s[code]

Now we'll have a look at RAM. Current RAM is DDR, which means that it transfers data twice in each clock cycle. Again, the manufacturers almost always quote the effective data frequency instead of the actual clock speed ("DDR2-800" is really 400MHz; "DDR2-1066" is really 533MHz).

A single RAM module can transfer 64 bits (8 bytes) of data at a time. However, almost all current mainboards can use dual-channel. This links two modules together to double the amount of data transferred to 128 bits (16 bytes). This does require two RAM sticks, which is why everyone goes for 2x 1GB sticks instead of one 2GB stick.

Note that it doesn't apply for more RAM - you can't transfer 256 bits (32 bytes) at a time with four RAM sticks unless you've got a very expensive server mainboard. Four RAM sticks hurt overclocking anyway, so just stick with two.

Getting back on topic, the bandwidth of a pair of DDR2-800 sticks in dual-channel mode is given by:

800 (million transfers per second) * 16 (bytes per transfer) = 12800MB/s or 12.8GB/s

As you can see, the RAM bandwidth is actually much higher than the FSB bandwidth. There's not much point having this extra bandwidth - the CPU can never access more than 8.533GB/s because all the data going to or from it has to pass through the FSB.

In fact, for a Q6600 at stock speed, you can saturate the FSB with DDR2-533 RAM (not that it's a good idea now, since faster RAM is so cheap).

The simple rule of thumb is that you need to keep the RAM frequency greater than or equal to the FSB frequency (266MHz in this case), or alternatively keep the RAM data frequency at half the FSB data frequency (so DDR2-533 and a "1066MHz" FSB will go together well).

For overclocking this changes, because you overclock by increasing the FSB. This creates more bandwidth, so you can use faster RAM to take advantage of that. Some (most) boards will also force the RAM to run at the FSB frequency or faster, so you really do need decent RAM.

For example, if you get your Q6600 up to 3.6GHz (9x multiplier, 400MHz FSB frequency, 1600MHz effective FSB data frequency) then you'll want to have DDR2-800 RAM (really 400MHz) to go with that.

There's no harm in having faster RAM, except for the cost. If you can get DDR2-1333 RAM cheaply (or if you move straight to DDR3-1333) then that'll still be fine with a low FSB - you just won't be fully using the available bandwidth.

[LostBenji]

Originally Posted by heyimsu

I'm beginning understand it now...

@ 1:1 ratio

CPU 266 x 9 = 2.4GHz (default); DDR2 RAM 266 x 2 = 533MHz (minimum speed of RAM needed)

Now if I buy DDR2-800 and make full use of it;

CPU 400 x 9 = 3.6GHz ; DDR2 RAM 400 x 2 = 800MHz (full use of RAM)

Also, this 9 x multiplier, is this 'lock' by the CPU manufacturers?

Correct. The one thing that most will do drop the max multiplier to 8 so that the CPU runs at 3.2Ghz instead of pushing hard to get 3.6Ghz stable. Most Q6600's with a good board will hit 3.6Ghz but this means much higher voltages and stresses on everything. The other little less spoken thing called the "Point of Deminishing Gains" is fast approaching after 3.2Ghz so its not such good value for money after this level unless the CPU core itself is able to go higher without the extra stress.

Originally Posted by SLATYE

Quick explanation:

[EDIT: Fine, so it's not actually a 'quick' summary. Unfortunately I can't think of any way to make it much shorter]

All Intel CPUs since the first P4 have used a QDR FSB. What this means is that in each clock cycle of the FSB, data gets transferred four times. That's where the 266 comes from. The actual FSB frequency is only 266MHz. However, because it transfers data four times in each clock cycle, the effective data frequency is four times 266MHz (1066MHz, or close enough).

Naturally Intel advertise this number (1066MHz), because most consumers think that higher numbers are always better. I perfer to use the real FSB frequency (266MHz in this case).

The FSB transfers 64 bits (8 bytes) of data at a time, so the total bandwidth for the Q6600's FSB is:
[code]1066 (million transfers per second) * 8 (bytes per transfer) = 8533MB/s, or 8.533GB/s[code]

Now we'll have a look at RAM. Current RAM is DDR, which means that it transfers data twice in each clock cycle. Again, the manufacturers almost always quote the effective data frequency instead of the actual clock speed ("DDR2-800" is really 400MHz; "DDR2-1066" is really 533MHz).

A single RAM module can transfer 64 bits (8 bytes) of data at a time. However, almost all current mainboards can use dual-channel. This links two modules together to double the amount of data transferred to 128 bits (16 bytes). This does require two RAM sticks, which is why everyone goes for 2x 1GB sticks instead of one 2GB stick.

Note that it doesn't apply for more RAM - you can't transfer 256 bits (32 bytes) at a time with four RAM sticks unless you've got a very expensive server mainboard. Four RAM sticks hurt overclocking anyway, so just stick with two.

Getting back on topic, the bandwidth of a pair of DDR2-800 sticks in dual-channel mode is given by:

800 (million transfers per second) * 16 (bytes per transfer) = 12800MB/s or 12.8GB/s

As you can see, the RAM bandwidth is actually much higher than the FSB bandwidth. There's not much point having this extra bandwidth - the CPU can never access more than 8.533GB/s because all the data going to or from it has to pass through the FSB.

In fact, for a Q6600 at stock speed, you can saturate the FSB with DDR2-533 RAM (not that it's a good idea now, since faster RAM is so cheap).

The simple rule of thumb is that you need to keep the RAM frequency greater than or equal to the FSB frequency (266MHz in this case), or alternatively keep the RAM data frequency at half the FSB data frequency (so DDR2-533 and a "1066MHz" FSB will go together well).

For overclocking this changes, because you overclock by increasing the FSB. This creates more bandwidth, so you can use faster RAM to take advantage of that. Some (most) boards will also force the RAM to run at the FSB frequency or faster, so you really do need decent RAM.

For example, if you get your Q6600 up to 3.6GHz (9x multiplier, 400MHz FSB frequency, 1600MHz effective FSB data frequency) then you'll want to have DDR2-800 RAM (really 400MHz) to go with that.

There's no harm in having faster RAM, except for the cost. If you can get DDR2-1333 RAM cheaply (or if you move straight to DDR3-1333) then that'll still be fine with a low FSB - you just won't be fully using the available bandwidth.

This is an awesome little peice of know-how that all should read and understand before trying grasp how a PC really works when area of OCing.
The only thing i can add to this is that if you move to DDRII 1066 or 1200 then you dont actually need to push it at those speeds. What you can do is to keeep it at say 400Mhz (800) and tighten up the RAM timmings to make it more responsive.

[SLATYE]

Originally Posted by heyimsu

Also, this 9 x multiplier, is this 'lock' by the CPU manufacturers?

It's the maximum value you can use. I'm pretty sure that on Core 2 CPUs the minimum is 7x. You can adjust it between those values, if you want a bit more RAM bandwidth without overclocking the CPU further (for example, if you want some more performance but you don't want to overclock the CPU at all, you can change the multiplier to 8x and set the FSB to 300MHz. Then the CPU will still be at the stock 2.4GHz, but it's got a bit more bandwidth).

Originally Posted by LostBenji

The only thing i can add to this is that if you move to DDRII 1066 or 1200 then you dont actually need to push it at those speeds. What you can do is to keeep it at say 400Mhz (800) and tighten up the RAM timmings to make it more responsive.

Good point.

More often than not you won't be able to run the RAM at its ideal speed anyway, if you're aiming for the maximum CPU overclock (because you'll find that the maximum CPU speed is something like 3436MHz. CPUs tend not to reach their limits at particularly nice numbers like 3.6GHz). That'll reduce the RAM speed a bit, and you can take advantage of the lower RAM speed by tightening the timings.

[Tubby2]

Originally Posted by animate

@ Tubby2
You did a pretty good job. also typo 2.4mhz?? lol Ghz ;D
you cannot get 600mhz ram afaik. Doubel data rate is correct, the speeds are
266.5 x 2 = 533
333.5 x 2 = 667
400 x 2 = 800
533 x 2 = 1066
(i am pretty sure of this)

Sorry brain fade. I was thinking too much about FSB/Clock speed/bla/bla/bla to think about the easy stuff.

Originally Posted by Tubby2

For eg, if you wanted to try and o/clock a q6600 from 2.4 to 3mhz (a 600mhz o'clock) you would need to increase the FSB by 150 (600mhz/4) so you would have a FSB of 416 (266+150).
To run your system at an FSB of 416 you would need RAM that could run at 832 (416 x 2), So you would need DDR21066 but in reality DDR2800 would most probably run at the speed also.

See I told you brain fade...I'm glad I picked up this little nugget first

I should have said increase FSB by 66 (600mhz/9)


Oh! and another thing. It doesnt matter how many times you've read and thought you've got your head around it, when it comes time to use what you've learned, it all goes out the window.

[animate]

Originally Posted by Tubby2

Oh! and another thing. It doesnt matter how many times you've read and thought you've got your head around it, when it comes time to use what you've learned, it all goes out the window.

Lol yup thats true xD
it doesnt really go out the window just becomes second nature and you no whats going on and u dont need to think about it that much :P
hehehe

[TheFreshMaker90]

I’ve been researching the topic of Overclocking for the past couple of days and it’s safe to say that I’ve managed to acquire more knowledge in this particular thread than the past 10 sites I’ve visited.

So cheers to Slatye, Lost Benji, Animate, Tubby2 & heyimsu, for starting this helpful enquiry.

P.S. I'm only new to this community but it’s easily the best & most informative forum/site focusing on overclocking. Now I’m going to spend the next few hours familiarizing myself with this forum and OCing.