ECC RAM

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Error correcting code (ECC) RAM is used for workstations and servers that require a high level of reliability.

• For each transfer:
  • ECC RAM performs a hash calculation on the data value and stores it as an 8-bit checksum
  • This checksum requires an extra processor chip on the module and a 72-bit data bus rather than the regular 64 bits
  • the memory controller performs the same calculation and should derive the same checksum
  • This system can detect and correct single-bit errors and allow the PC to continue functioning normally
    • ECC can also detect errors of 2, 3, or 4 bits but cannot correct them
    • Instead, it will generate an error message and halt the system
• Most types of ECC are supplied as registered DIMMs (RDIMMs)
  • A registered DIMM uses an extra component to reduce electrical load on the memory controller
  • has a slight performance penalty, but makes the system more reliable,
    • especially if large amounts of memory are installed
• Most types of non-ECC memory are unbuffered DIMMs (UDIMMs)
  • Some types of ECC RAM are packaged in UDIMMs, though this is rarer

Factors to consider when selecting memory for a system:

• Both the motherboard and CPU must support ECC operation for it to be enabled
• Most motherboards support either UDIMMs or RDIMMs, but not both
  • If a motherboard does support both, UDIMM and RDIMM modules cannot be mixed on the same motherboard
    • system will not boot if there are different types
• Mixing non-ECC UDIMMs and ECC UDIMMs is unlikely to work