RAM parity
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RAM parity determines whether a random access memory unit stores a parity bit for error detection purposes. Non-parity RAM does not include a parity bit, and parity RAM does. Parity RAM requires one additional bit to be stored with each byte of information, decreasing the information storage capacity per chip compared to non-parity RAM manufactured with the same technology. Parity RAM may[citation needed] also have worse memory access performance than non-parity RAM.
The fundamental reason to have parity RAM is to detect the fact that a memory error (bit dropout) has occurred. Undetected memory errors can have varying results; from simply annoying to catastrophic. In the case of the home PC where data integrity may be of little importance, non-parity memory is an affordable option. However, if any sort of data integrity is required, parity memory would be the minimum level of protection. Further levels of protection such as detection and correction codes may also be warranted in the case of an application with a requirement for high availability.
Logic parity RAM is non-parity RAM that can be used in computers that require parity RAM. Logic parity RAM recalculates the parity bit each time a byte is read from the memory, instead of storing the parity bit when the memory is written to. Therefore logic parity RAM has no error detection capability.
RAM with ECC or Error Correction Code can detect and correct errors. As with parity RAM, additional information needs to be stored and more processing needs to be done, making ECC RAM more expensive and sometimes slower than non-parity and parity RAM. This type of ECC memory is especially useful for any application where uptime is a concern. Failing bits in a memory word are detected and corrected on the fly with no impact to the application. The occurrence of the error is typically logged by the operating system for analysis by a technical resource. In the case where the error is persistent, server downtime can be scheduled to replace the failing memory unit. This mechanism of detection and correction is known as EEC or Extended Error Correction.
