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Parity, Non-Parity and ECC Memory
Most memory modules are available as either non-parity or parity. Some also are available as dedicated ECC-only modules. Non-parity is "regular" memory, including one bit of storage for each bit of data. Parity memory includes one extra bit of storage for every eight bits of data, used to store information about the data that the system can later use for error detection or correction. It can be used in parity or ECC mode. ECC modules are newer and also include extra bits of information, but can only be used in ECC mode. See this section for a full description of the differences between these memory forms.
Parity or ECC memory is generally more expensive than non-parity. Originally, this was because of the extra 12.5% worth of memory chips needed (one bit for every eight). Now, the increase in cost is more due to supply and demand issues, since non-parity memory is produced in much larger quantities. ECC is beginning to become popular again, and the price differential is shrinking as a result.
Parity memory will usually work in a non-parity system; the extra parity bits are ignored. However, non-parity memory will not work in a parity system (unless parity checking is turned off in the BIOS setup program, and some older systems don't even allow that). If you use non-parity memory in a parity system, a parity error will be generated as soon as the system boots up.