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Overclock Failed...
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When any computer starts up it reads the SPD table (memory timing table) on the memory stick closest to the CPU and chooses the timings (from the table) that match the CPU speed that is set in the BIOS. It then applies those timings to all the memory sticks that are installed. If you mix memory (ie. don't buy all the memory as one matched set) the it's possible the the timings for one won't work with the other.

You can download CPU-Z and see part of the SPD tables on you memory.. Note that CPU-z and many other such programs don't show you all of the timings (there are over 20). RAMMon will show you more of them.

Then there's XMP memory profiles (in the BIOS) that can cause problems.

Background (simplified):

When any computer starts up the BIOS reads the SPD tables programmed into the memory. You can download and run CPU-Z to see the SPD tables in your memory. The BIOS then uses the memory speed and memory timings from the SPD tables that corresponds to the CPU speed that is set in the BIOS.

XMP profiles were developed to allow automatically setting not only memory speed and timings automatically, but also memory voltage. So, with the added ability to increase voltage, memory manufacturers could program SPD tables that run the memory even faster with the higher voltages available, and do that automatically too.

But the whole concept of XMP is faulted because the person writing the XMP SPD table for the memory has no idea what motherboard, CPU, what other peripherals are connected to your motherboard, nor even how many memory sticks you have installed. So, XMP profiles are just a guess at what memory overclock will work for the widest range of motherboards, CPUs, memory, and computer configurations. They have nothing to do with what your specific setup is. ... they're just a guess.

Do they work? Sometimes, sometimes not. And even if they do work it is unlikely they are the best settings for you particular setup.

Memory setting (with or without overclocking) is best done by manually setting the memory settings in the BIOS.
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