FSB & Overclocking Question

[OrbitzXT]

I'd like to look into overclocking my processor as I've heard the Core 2 Duo chips are quite good at overclocking. I bought the E6400 with a Gigabyte DS3-965G and have 2 GB PC2-6400 RAM. I looked at CPU-Z and I have a stock core speed of 2133 MHz, Multiplier is 8, Bus Speed 266.7 MHz, Rated FSB 1066.7 MHz. While looking in the memory tab it shows my Timings as:

Frequency 400.0 MHz
FSB:DRAM 2:3
CAS# Latency 5.0 clocks
RAS# to CAS# Latency 5 clocks
RAS# Precharge 5 clocks
Cycle Time (Tras) 18 clocks

Can anyone give me a quick review of what this means for my overclocking potential or redirect me to a good site for noobs? My CPU is idling at 32C right now and I haven't seen it hit the 40's yet under load, I have a Zalman 9500 on it. Thanks in advance!

[azzey]

I'll do the 'generic' warning... basically, if you overclock you are putting your system on the line. It is very easy to ruin expensive pieces of equipment, especially if you don't know what you're doing. My rule is to never overclock anything you don't want to replace.

If you still wish to proceed, then someone with a little more experience will be able to help you (I've only had limited experience with overclocking).

Basically, each CPU is different, so while some schmuck somewhere may be able to get an extra 500mHz, you may be only able to get 50. It really depends. I wish you the best of luck :)

[OrbitzXT]

I've been doing some reading, based on the timings of my memory I posted above, does this mean I could either increase my FSB to overclock my processor? If I chose not to overclock the processor, can I lower the timings of my memory for increased performance? The FSB:DRAM ratio is 2:3 and I read the following:

"Running the FSB and RAM with a ratio causes gaps in between the time that the FSB can communicate with the RAM. This causes slowdowns that wouldn't be there if the RAM and the FSB were running at the same speed. If you want the most speed out of your system, using an FSB:RAM ratio wouldn't be the best solution."

These timings were like this after putting my computer together, I haven't changed anything. So am I losing performance right now with a 2:3 ratio? If I raised the FSB to 400 (Not right away of course), this would give me a 1:1 ratio I believe? And also give my C2D a clock speed of 3200 MHz since I have a 8x multiplier.

So when raising FSB what are the things you have to look out for? I'm not certain but it seems my memory is ready for an overclock, is temperature the only thing I have to worry about? With such a large overclock I read raising voltage may be necesary. I'm not certain I want to do that though as I've heard it shortens the life span of your parts. I'm just trying to think out loud here, can anyone tell me if my thoughts are near correct or am I severely wrong?

[schusterjo]

Overclocking is my thing here you go.

significant complaint about DDR2 when it came out--much higher latencies. On the other hand, DDR2 is capable of much much higher speeds--if you shell out you can pick up DDR2 RAM up to DDR2-1111mhz! As a comparison the fastest DDR1 RAM I've seen is 600mhz. Twice the latency, but twice the speed.

As usual, to go faster, often you must either sacrifice latency or use very high voltages.


If you are confused about the mathematics of the whole thing, here is a cut'n'paste from an overclocking guide, but explains the math well:

CPU speed = CPU multiplier x Front Side Bus

Example:

Core 2 Duo E6600 2.4ghz = 9 * 267mhz

However, even this simple equation has become complicated. For starters, Intel likes to “quad” pump their FSB so they will report it as 267*4=1066mhz, front side bus. I think its best to reduce the math to the lowest denominator and call the front side bus 267mhz. Intel also makes cpus with front side buses of 200mhz and 133mhz (or 800mhz/533mhz quad pumped).

To further complicate things, the latest processors from AMD have a completely different architecture, specifically, the memory controller is built right onto the cpu die. So technically, there is no front side bus, if by front side bus you mean the speed at which the core communicates with the memory controller. AMD chips do have a number which they retain in order to maintain the equation.

Perhaps in response to this confusion, Abit has gone with calling the “Front Side Bus” the “External Clock”. Since we are talking about an Intel platform, I will from here on out use the words FSB and External Clock interchangeably because they both indicate the speed at which the core communicates with the memory controller. In this situation the memory controller and the northbridge are also one in the same, and so I will use them interchangeably as well.

The CPU multiplier is a property intrinsically set by Intel, as far as I know, hard set into the chip itself. C2D cpus are a bit of new breed, for the first time, Intel has left downward cpu multipliers unlocked. Meaning, for example, the E6600 has an intrinsic maximal multiplier of 9—but the bios will also unlock 6, 7, and 8. This is useful in certain circumstances, especially in testing of maximal FSB speed (lower multiplier will allow for higher FSB at the same overall CPU speed ie 2.4ghz = 9 x 267 = 8 x 300 = 7 x 343= etc.). The Extreme Edition CPUs retain their tradition of being both upwards and downwards unlocked.

The Memory Bus

For those of you coming from the older platforms based on DDR SDRAM, you will need to pause a moment and re-orient your thinking. Because of the nature of DDR2 RAM, much much higher speeds are easily achieved.

As a baseline, when running stock FSB speeds, (fastest C2Ds being FSB 267), if you have Ram rated to FSB 267 (DDR2-533 or PC2-4200), the bios will automatically select a cpu:RAM ratio of 1:1.

However, sticks of RAM are commonly available at speeds of FSB 333/DDR2-667/PC2-5300 or even FSB 400/DDR2-800/PC2-6400. Therefore, if you run you cpu at maximal stock FSB 267, but put in these sticks of RAM that are rated much faster, the bios will automatically select a ratio whereby the RAM will be running FASTER than the cpu bus. Currently, these are the selectable RAM ratios and the corresponding RAM speeds:

1:1
FSB 267 (times 1 divide by 1) = 267mhz = DDR2-533 = PC2-4200 Ram

4:5
FSB 267 (times 5 divide by 4) = 334mhz = DDR2-667 = PC2-5300 Ram

2:3
FSB 267 (times 3 divide by 2) = 400mhz = DDR2-800 = PC2-6400 Ram

Even faster RAM modules are out there—(fastest I’ve seen as of writing is FSB 556 = DDR2-1111 = PC2-8888) obvioiusly these are for overclocking as (currently) these are faster than the fastest cpu FSB (267) with the highest RAM multiplier (2:3).

On the one hand, running the Ram faster than the cpu doesn’t give trememdous performance boosts in most settings--on the other hand, if the RAM is rated to do it, that’s free performance.


Memory Latency (and secret settings)

There are many parameters that must be set for RAM to operate appropriately. The most commonly acknowledged settings are:

CAS# Latency
RAS# to CAS# Delay
RAS# Precharge
Cycle Time (Tras)

I list them in this order and as termed because this is how they appear in the very popular and useful program cpuz. When not otherwise specified, if you see 4 unlabeled numbers, this is the order they are in. Not all bioses list these in the same order.

Here is how they appear in the AW9D max bios:

Advanced Chipset Features

DRAM Timing Selectable = Manual
X - CAS Latency Time (tCL) = 4
X - RAS# to CAS# Delay (tRCD) = 4
X - RAS# Precharge (tRP) = 4
X - Precharge Delay (tRAS) = 15

Which is in the same order.

The smaller the latency numbers, the faster the RAM turns over data. However, if you tweak them too tight, data will become corrupted. The performance boost from tighter latencies again is not necessarily tremendous in most applications. As a very very rough reference, I’ve generally found that a 10mhz increase in the cpu speed overcomes the performance boost from going from the loosest of timings to the tightest.

There are “secret settings”, ie parameters the bios automatically sets without allowing you to adjust. Furthermore, its been shown that these settings can affect your overclock, case in point: By using 3rd party applications such as Memset 3.0, a number of overclockers found that loosening one parameter, Refresh Cycle Time (tRFC), could increase their maximal RAM speeds. As of writing of this guide, Abit has a beta bios, that adds adjustability of this parameter.

I’ve seen reports of increased speeds with adjustment of Read Delay (tRD), only available by Memset.

There are certainly other settings, and their effects yet to be fully known.