Processor Overclocking
First of all we decided to find out how high we could push the FSB frequency for ZOTAC nForce 790i-Supreme mainboard to remain operational and stable. Therefore, we increased the FSB voltage, memory bus voltage and NB voltage and lowered the Intel Core 2 Duo E8400 processor frequency multiplier to the minimal x6. The board started at 475MHz and 450MHz FSB, but failed to load the OS. It all ended with inevitable BSOD and “Machine Check Exception” error. Only at 425MHz FSB we could finally see the Windows desktop.
Well, here is the first disappointment. Despite the promises, this mainboard proved hardly fit for processor overclocking. Luckily, we noticed that at some point processor clock frequency multiplier in the CPU-Z window suddenly hanged to x9 and then returned back to x6. Basically it was not enough to set the CPU multiplier in the BIOS. We had to disable the EIST for the multiplier to stay where we set it. Of course, the CPU couldn’t work at 450MHz FSB and 475MHz FSB without the corresponding core voltage increase It was really surprising that it managed to boot Windows at 425MHz FSB in this case.
By the way, EVGA nForce 750i SLI FTW mainboard also based on Nvidia reference design showed the same result during our preliminary tests. Since these two mainboards have similar BIOS, it could be exactly the same problem that wouldn’t let us go any further. I guess we will have to double-check it.
Once we unveiled this peculiarity of the BIOS from the reference Nvidia boards, finding maximum operational FSB frequency went on much faster. ZOTAC nForce 790i-Supreme mainboard successfully passed 450, 475 and 500MHz FSB with the processor multiplier reduced to x6. Then the progressed stopped. At 515MHz FSB the system booted and worked for a while, however, we had to go down to 510MHz with the Intel Core 2 Duo E8400 overclocked to its maximum with x8 multiplier. Well, 510MHz is not record-breaking frequency, but it is nevertheless a very good result. At this point we considered ZOTAC nForce 790i-Supreme a good mainstream overclocker solution and decided to move on to experiments with quad-core processors. However, this is where the surprises started.
Of course, most mainboards overclock dual-core processors much better than quad-core processors. Overclocker mainboards usually stop at about 450-460MHz FSB, while non-overclocker boards may not hit even this frequency. So far the best result in dealing with our Intel Core 2 Quad Q9300 processor belonged to abit IP35 Pro mainboard: 475MHz FSB, and there wasn’t a single mainboard out there that could get even close to that. Therefore, we started with 475MHz FSB and intended to lower the frequency little by little in order to find what our ZOTAC nForce 790i-Supreme was capable of. However, the board passed the tests at 475MHz and 485MHz FSB and stalled only at extremely high frequency for quad-core CPUs of 495MHz! We couldn’t hit the beautiful 500MHz, however, it doesn’t make the performance of our ZOTAC nForce 790i-Supreme any less impressive.
To reach this extremely high frequency we had not only to adjust a few voltages (processor Vcore - to 1.475V, FSB voltage – to 1.4V and NB voltage to 1.5V), but also to play with the GTLVREF parameter. By the way, it was the first time in my experience when changing these parameters actually improved overclocking performance. Otherwise I would have had to stop at 490MHz.
No one knows what GTLVREF values the mainboards set. Only those know who have reference mainboards on Nvidia chipsets, because they do display these values. Their BIOS is indeed very informative. We noticed that when our Intel Core 2 Quad Q9300 processor was working in its nominal mode, the board set GTLVREF Lane 0 at -30mV, and GTLVREF Lane 1 at +10mV. When we overclocked to 450MHz FSB, these parameters change to -50 and -50mV respectively, and at 475MHz FSB and higher - to -80 and -100mV. By changing GTLVREF Lane 0 and GTLVREF Lane 1 to -50 and -50mV manually, we managed to achieve system stability at 495MHz FSB.
