RAM Parity - Error Correction

Error Correction

Simple go/no go parity checking requires that the memory have extra, redundant, bits beyond those needed to store the data; but if extra bits are available, they can be used to correct, as well as detect, errors. Earlier memory as used in, for example, the IBM PC/AT (FPM and EDO memory) were available in versions that supported either no checking or parity checking (in earlier computers that used individual RAM chips rather than DIMM or SIMM modules, extra chips were used to store parity bits); if the computer detected a parity error it would display a message to that effect and stop. The SDRAM and DDR modules that replaced the earlier types are usually available either without error-checking or with ECC (full correction, not just parity).

An example of a single-bit error that would be ignored by a system with no error-checking, would halt a machine with parity checking, or would be invisibly corrected by ECC: a single bit is stuck at 1 due to a faulty chip, or becomes changed to 1 due to background or cosmic radiation; a spreadsheet storing numbers in ASCII format is loaded, and the number "8" is stored in the byte which contains the stuck bit as its eighth bit; then another change is made to the spreadsheet and it is stored. However, the "8" (00111000 binary) has become a "9" (00111001).

If the stored parity is different from the parity computed from the stored data, at least one bit must have been changed due to data corruption. Undetected memory errors can have results ranging from undetectable and without consequence, to permanent corruption of stored data or machine crash. In the case of the home PC where data integrity is often perceived to be of little importance—certainly true for, say games and web browsing, less so for Internet banking and home finances—non-parity memory is an affordable option. However, if data integrity is required, parity memory will halt the computer and prevent the corrupt data from affecting results or stored data, although losing intermediate unstored data and preventing use until any faulty RAM is replaced. For the expense of some computational overhead, of negligible impact with modern fast computers, detected errors can be corrected—this is increasingly important on networked machines serving many users.