ECC (either "error correction [or correcting] code" or "error checking and correcting") allows data that is being read or transmitted to be checked for errors and, when necessary, corrected on the fly. It differs from parity-checking in that errors are not only detected but also corrected. ECC is increasingly being designed into data storage and transmission hardware as data rates (and therefore error rates) increase.
Here's how it works for data storage:
When a unit of data (or "word") is stored in RAM or peripheral storage, a code that describes the bit sequence in the word is calculated and stored along with the unit of data. For each 64-bit word, an extra 7 bits are needed to store this code.
When the unit of data is requested for reading, a code for the stored and about-to-be-read word is again calculated using the original algorithm. The newly generated code is compared with the code generated when the word was stored.
If the codes match, the data is free of errors and is sent.
If the codes don't match, the missing or erroneous bits are determined through the code comparison and the bit or bits are supplied or corrected.
No attempt is made to correct the data that is still in storage. Eventually, it will be overlaid by new data and, assuming the errors were transient, the incorrect bits will "go away."
Any error that recurs at the same place in storage after the system has been turned off and on again indicate a permanent hardware error and a message is sent to a log or to a system administrator indicating the location with the recurrent errors.
At the 64-bit word level, parity-checking and ECC require the same number of extra bits. In general, ECC increases the reliability of any computing or telecommunications system (or part of a system) without adding much cost. Reed-Solomon codes are commonly implemented; they're able to detect and restore "erased" bits as well as incorrect bits.
Verder van de Crucial site:
How do I know if I need parity, non-parity, or ECC memory?
Description:
How do I know if I need parity, non-parity, or ECC memory?
Solution:
The general rule of thumb in deciding what type of memory you need is to look at what's already installed in your system. To find out if you have ECC, parity or non-parity memory, count the number of chips on the module. If you can evenly divide the number of chips by three or five, the module is ECC or parity, if not, then it is a non-parity module.
So what if your system does have ECC or parity memory (the chips are evenly divisible by three or five), how do you know which of the two you have? One way is to look at the part numbers on the chips of your module. If each chip has the same part number, you have ECC. If one chip is different, you have parity. An even easier way is use the Crucial Memory Selector. Just select your system make and model and our Memory Selector will tell you which type of memory works in your system.
If you are building a PC and deciding which type to use, the following guidelines should help. If you plan to use your system as a server or a similar mission critical type machine, it is to your advantage to use ECC. If you plan to use your PC for regular home, office, or gaming applications, you are better off with non-parity.
Using ECC decreases your PC's performance by about 2%. Current technology DRAM is very stable and memory errors are rare, so unless you have a need for ECC, you are better served with non-parity SDRAM.
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