Overclocking and FSB Strap

Our open testbed used for overclocking experiments was built with the following components:

• Mainboard: MSI P35 Platinum Combo (MS-7338), ver. 1.1, P.0B3 BIOS;
• Graphics card: NVIDIA GeForce 8800 GTS 320MB;
• HDD: Seagate Barracuda 7200.10 (ST3320620AS), 7200 RPM, 16MB, SATA 320GB;
• CPU cooler: Zalman CNPS9700 LED;
• PSU: Antec NeoPower HE 550 (550W).

First we ran the tests with DDR3 SDRAM: 2x1024MB OCZ PC3-14400 Platinum Series (OCZ3P18002GK). With the voltage increased to 1.95V, these modules could work at 1800MHz frequency. We know very well what Intel Core 2 Duo E6300 processor (1.86GHz, 266MGz FSB, 2MB, Conroe-2M, rev. B2) is capable of: with the Vcore increased to 1.45V it can overclock to 490MHz FSB. However, the combination of 490MHz FSB and 1568MHz memory frequency didn’t work: the mainboard wouldn’t even start. When we dropped the memory frequency to 1470MHz, things didn’t get too much better: the board would start but would then hang on boot-up.

We managed to boot Windows with the 470/1410MHz combination of FSB and memory frequency respectively. In this case, the board set memory timings to 7-6-6-15 in Auto mode, however, the Performance Level was different for both channels: 7 for one and 8 for another. We assumed the system didn’t work because the memory timings were too low, so we set them to 8-8-8-24 in the BIOS, however, it didn’t help.  The maximum memory divider that allowed setting the frequency a little over 1500MHz turned out non-operational. The board didn’t even start with this divider. 490MHz FSB also remained unattainable for us, and the best we could get the system to boot at was 480/1440MHz.

I have to admit that it is pretty weak result for both: the CPU and the memory. However, when we replaced DDR3 modules with DDR2, everything got clear. We installed 2x1024MB Corsair Dominator TWIN2X2048-9136C5D and the system booted in no time, loaded Windows OS and then passed the stability test at 490/980MHz FSB/memory frequencies.

Unfortunately for overclocking fans, it turns out that the ability of MSI P35 Platinum Combo mainboard to work with two memory types is fiction. DDR3 memory support hasn’t been implemented in a satisfactory way. Moreover, with DDR3 SDRAM the mainboard doesn’t allow setting its frequency to the maximum, and high frequencies are actually the only really exciting advance of DDR3 over DDR2. Besides, the mainboard loses some of its CPU overclocking potential. 

It is sad news, but still let’s continue our experiments with DDR2 SDRAM, which doesn’t set any limitations. In Auto mode MSI P35 Platinum Combo mainboard set the “correct” timings recorded in the modules’ SPD, although the Performance Level again turned out different for two channels: 8 and 9 respectively.

To estimate the performance of an overclocked system we launched Everest Cache & Memory Benchmark and were shocked to see dramatically low read, write and copy speeds (first line). They were about three times as low as they were supposed to be for these settings!

However, we did expect results like that, although it was still frustrating. MSI P35 Platinum and MSI P35 Platinum Combo mainboards have a pair of jumpers between the PCI Express slots that set the starting FSB frequency to 200, 266 or 333MHz. There is nothing bad about having these jumpers onboard. For example, DFI LANParty LT X38-T2R mainboard we have recently reviewed also allows changing the stating FSB frequency on the hardware level. However, it is an optional feature on DFI mainboard and we can still overclock the system using only BIOS settings. MSI, however, reminded us about FSB Strap phenomenon.

Here we have to remind you that the notion of FSB Strap was introduced to overclocking reality when Intel P965 Express based mainboards appeared in the market. It turned out that at some point the performance of an overclocked system came crashing down, and they called this phenomenon FSB Strap. For example, overclocked ASUS mainboards demonstrated pretty expected performance increase up to 400MHz FSB. However, once the frequency was set at least at 401MHz, the performance dropped down significantly, and could only be regained by further overclocking to 430-450MHz. All mainboards on newer Intel chipsets that came out afterwards were free from this drawbacks already, except MSI solutions.

To figure out what’s going on, you can take a look at our earlier MSI P35 Platinum Mainboard Review, namely the discussion of FSB Strap influence on the system performance. As we discovered, for CPU with 200MHz bus the performance drops when we switch from 381 to 382MHz FSB, and for CPUs with 266MHz bus the same performance drop occurs when we move from 447MHz to 448MHz FSB. It is an unpleasant effect, but there is nothing dramatic about it: just move the jumper from 200 to 266MHz, or from 266 to 333MHz to shift the FSB Strap threshold another 66MHz further. As a rule, this is enough to successfully overclock the system without losing any speed.

MSI P35 Platinum and MSI P35 Platinum Combo mainboards are very similar that is why it is quite logical to assume they may have similar problems, too. We overclocked Intel Core 2 Duo E6300 processor with the default FSB frequency of 266MHz to 490MHz FSB, which is much higher than 448MHz where the performance should drop. No wonder that the results of Everest Cache & Memory Benchmark turned out so low. Let’s try and set the jumper to 333MHz and retest the system without changing anything else.

Compare these results with those on the previous screenshot to see that the performance numbers got back to normal. However, this doesn’t end our discussion. Switching the jumper doesn’t solve the problem, but just moves it aside. The difference between 200, 266 and 333MHz is 66.6MHz, this is the exact increment for FSB Strap threshold on MSI mainboards (447-381=66). So, we can assume that CPUs with 333MHz nominal FSB frequency and well-overclockable CPUs with 266MHz nominal FSB frequency will inevitably stumble upon unacceptable performance drop when moving from 513MHz to 514MHz (447+66-513). Intel Core 2 Duo E6300 processor cannot go past 490MHz FSB, however, we have seen before that Intel Core 2 Duo E8400 processor (3.0GHz, 333MHz FSB, 6MB, Wolfdale, rev.C0) can actually overclock at least to 530MHz FSB with the frequency multiplier reduced to x8.