How can parity modules be used for ECC? Doesn't ECC require more than 36 bits?

• An 'ECC' 72-pin module usually refers to a 39-bit module.
• A 39-bit module would be required if the memory controller was using a 32-bit word because 7 bits are required for single-bit error correction of a 32-bit word.
• Due to the nature of ECC, only 8 bits are required for single bit error correction of a 64-bit word.
• Two 72-pin SIMMs that are addressed together create a 64-bit wide data path allowing the computer to read 64-bit words.
• Two 72-pin parity SIMMs have a combined 8 check-bits that could be used for parity or single bit error correction, depending on the memory controller.
• Compaq was among the first to utilize this efficiency in its 'TriFlex' memory bus.
• Many motherboard manufacturers now provide the option of using pairs of standard 36-bit parity modules as either ECC or parity.
• Using parity SIMMs in matched pairs is the most effective use of a 64-bit wide memory bus and the check-bits present on parity SIMMs. This yields error detection and correction as capable and reliable as any other form of ECC commonly available.
• This is not 'fake' or 'emulated' ECC. With rare exception, ECC is a function that takes place on the motherboard.

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