Originally Posted by respartan
why was changing the command rate from 2 to 1 such a issue... i have read in many places that the corsair vengeance i have should easily be able to do this...i actually havent been able to OC this ram at all....
@respartan, command rate is actually kind of a big issue, and usually the first thing people test in their RAM overclocks (which is why it's good you started with this!). You can think of it as the number of clock cycles the RAM controller has to identify the right block of RAM for an operation to read, write or copy data. Under your heat spreaders on your RAM is a bank of chips, right? Each little unit of that RAM is addressed by your CPU/memory controller so that they know where to find things. So each time your computer uses RAM, the CPU needs to figure out which block to use. At a 2T command rate, your computer has two clock cycles (at 1666 million cycles per second, or whatever your default RAM clock rate is, which means only 1.2 nanoseconds!) to find the right block for a given address. At 1T, it has only half as much time, or 0.6 ns! Whether your computer can correctly find the right block in one clock cycle or two depends on a bunch of factors. The biggest determinant is how many physical places it has to look, i.e., how many RAM chips it has to look in. So 1T command rates often only will work with smaller capacity sticks and only a single stick per channel (or just a single stick!), whereas 2T will work with a much wider range of setups. Other things that influence it are whether the RAM is within spec or overclocked, RAM voltage and its effect on signaling, signal-to-noise ratio on the circuit board and in the traces connecting the RAM to the CPU, etc.
So what happens if the CPU doesn't find the right memory when it needs it? It throws a BSOD error code or just crashes and restarts. But what happens if it "thinks" it has the right data and copies from or writes to an incorrect location? And the data it's copying is used for something critical like the OS kernel or a low-level system driver? And it gets copied back to a critical location on the boot disk? Boom. Splat. Bye-bye OS.
Don't stop overclocking just because of that, though! Just know that you may blow everything up and be ready to recover from it if you do. I'm overclocking some OCZ Platinum DDR2 on a Wolfdale platform right now, rated for 5-5-5-15 @ 1000 MHz, and the other night I had it up to 1112 MHz without loosening the timings and at boot, I got the "Cannot find Windows file X, missing or corrupt, etc." error and thought, oh snap, well that's ironic, looks like I've thrashed my OS install too, just like respartan. But I backed down to 1050 and it booted right back up again. Within the given 2T command rate at 1112 MHz, Windows couldn't find critical data in time and crashed; at 1050 MHz, it could. The only difference is in my case, Windows hadn't copied corrupt data back to the boot disk.
Sorry to be so wordy, hope it clears some things up for you. Don't give up on a good OC though!