Wikipedia
RAM parity checking is the storing of a redundant parity bit representing the parity (odd or even) of a small amount of computer data (typically one byte) stored in random-access memory, and the subsequent comparison of the stored and the computed parity to detect whether a data error has occurred The parity bit was originally stored in additional individual memory chips with the introduction of plug-in DIMM, SIMM, etc modules, they became available in non-parity and parity (with an extra bit per byte, storing 9 bits for every 8 bits of actual data) versions
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