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Instability running TwinX XMS2-6400C5 RAM at 800Mhz on board with FSB at 200Mhz.

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Hello you experty people you. :) I've signed up to ask for some insight in a little prob I have with a brand new PC project.


I was asked to put together a workstation by a friend on a tight university student budget. After looking into what would be possible, I found the infamous article about overclocking a pentium D 805 to extreme levels. I figured if I did this right I could get his station working and stable at 4Ghz. I consulted him first of course as its his money I was spending. ;)


I invested his dosh in a good value water cooling system, in a decent case, in a quality motherboard (P5N32-Sli Deluxe) and, yes, in quality DDR2 RAM from Corsair. I bought him 2x1GB Corsair DDR2 XMS2-6400C5 TwinX. I was aiming for the system to run at a frequency of 200Mhz (rated FSB 800Mhz) so wanted to run the RAM at 800Mhz for a good FSB:DRAM ratio, full speed potential, and minimal bottlenecks.


At first, on an unoverclocked system (frequency at 133Mhz, CPU at 2.66Ghz), all the defaults were on and the RAM was reportedly running at 800Mhz (frequency was 400Mhz, ratio reported as 2:6, timings were 5-5-5-18). There were no issues at all.


I'm happy to report the overclocking part of the project has gone very well, but I had some trouble working through a particular issue. When I cranked the system up to a rated FSB of 800Mhz (actual FSB 200Mhz) to achieve a processor clock of 4Ghz, I found that the system became unstable, with Prime95 able to run sometimes and other times the system would refuse to boot at all. This seemed odd behaviour for just a low VCore problem. No matter how much voltage I gave to the CPU, it wouldn't sort itself out. The problems I was having reminded me of some memory speed issues I was having before, so I changed the target RAM bus speed to 750Mhz from 800Mhz. I also reduced the vcore back to how it was when I had the system running stable at a frequency of 190Mhz.


Once I'd reworked the voltage for the CPU, the system ran perfectly. Prime95 reported no errors for either the stress test configured to target the CPU (running an instance per core) nor the tests targeted at the RAM.


So I've currently got the processor running comfortably at 4Ghz on a motherboard operating at a frequency of 200Mhz (rated FSB is 800Mhz), but the RAM is running at 750Mhz. I've examined the PSU (Hiper Type-R 580W) voltages and it doesn't seem to be buckling under the power requirement at all. All seems fine if I leave the RAM set to 750Mhz. If I change this to 800Mhz, it all falls apart. Sometimes I don't even get to POST. If I'm lucky enough to get to the OS, I get BSODs at a variety of times.


CPU-Z reports that the memory looks like this (when speed is set to 750Mhz):

Frequency: 375Mhz

FSB/DRAM ratio:16:30

Timings: 5-5-5-15



Apart from the ratio and Trc, I manually set the above to match the specification of the RAM. Currently the memory is running on 1.9v. The RAM specs state this memory runs at 800Mhz no probs at that voltage. When I tried to change this to 2.0v when I was having issues, Windows booted up with only 4 colours available in the display and that worried me enough to set it back straight away. So as you can see, I've tried a few things. :)


What am I not understanding? Where am I going wrong?


Any help will be appreciated, and thanks in advance. Once I get this cracked, that'll be it over with and I'll be a very happy man! :)

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