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Memory modules come in types, Non-Parity and Parity.
Older RAM was not as stable a solution as it is today. Irregularities in the RAM could cause the data in memory to get corrupt or alter in many ways that often led to a computer crash or hard disk data damage. This problem was solved with Parity RAM.
A SIMM or DIMM usually has nine chips of memory on it and eight of it is used for the memory itself. The ninth memory chip is used by parity checking to hold checksum data on the contents of the other eight chips in that memory bank. If the predicted value of the checksum matches the actual value, then all is well. If it does not, then the contents of memory is corrupted and unreliable. In such a case the computer is instructed to shut down to avoid data corruption.
