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On the subject of memory errors, there are basically three categories: ‘hard’ errors, ‘timing’ errors, and ‘soft’ errors. These are further broken down into ‘data’ errors and ‘addressing’ errors.
‘Hard’ Errors – These would be any permanent errors that occur without fail, and thus are easy to reproduce and diagnose. These are usually caused by shorted or open connections in the RAM chips or modules. They include:
‘Timing’ Errors – This type of error is harder to spot. They occur infrequently, more often during a period of heavy RAM access. The errors can be either incorrect data or an incorrect address, and each may involve a different address or data than the previous error. One cause of timing errors is ‘over-clocking’, by setting the RAM clock higher than the spec, or by putting in DIMM modules with a lower speed rating, which amounts to the same thing. You can also get errors if the clock is too slow, because the memory cells must be ‘refreshed’ periodically, and the refresh timing signal is derived from the clock. Rounding out the picture, dirty contacts on the DIMM or an uneven clock signal can give symptoms that are indistinguishable from over or under clocking.
‘Soft’ Errors – This is a catch-all term for errors that do not fit the above categories. It includes errors that only happen once, or ones that are intermittent like timing errors but don’t have anything to do with timing. They can be caused by faulty RAM chips, dirty or intermittent contacts, or by external factors such as a nearby noisy power line or device. An often overlooked source of one-time errors is cosmic rays. There is no point trying to track down these or other one-time errors, but tests designed for timing errors should also catch intermittent soft errors.
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