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 also have worse memory access performance than non-parity RAM.

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.

EEC or extended error correction RAM can correct errors detected by parity checking. As with parity RAM, additional information needs to be stored and more processing needs to be done, making EEC RAM more expensive and sometimes slower than non-parity and parity RAM.

Motivation and uses

Memory error happens when cosmic rays or other interference causes an unintentional change to the information stored in a RAM chip. Modern semiconductor memory is very reliable, so this error usually amounts to a single bit changing from 0 to 1, or 1 to 0 (flipping). The effect of this change depends on where the change happens. A change in an unused memory region produces no effect, whereas a change to operating system or application software memory may lead to wide corruption of data.

Parity RAM and EEC RAM is almost exclusively seen in computers performing server tasks and scientific computing calculations. In these situations, ensuring correct and continued operation is often considered more important than minimizing cost.

See also

• Soft error

References

• SCL Cluster Cookbook on memory

Computer memory