The SPD EEPROM is accessed using SMBus, a variant of the I²C protocol. This reduces the number of communication pins on the module to just two: a clock signal and a data signal. (There are three additional pins to provide each EEPROM with a unique device address.) The same SMBus is commonly used on motherboards for system health monitoring tasks such as reading power supply voltages, CPU temperatures, and fan speeds.
EPP is an enhancement of SPD and provides additional performance-specific information needed for overclocking applications. The EPP information is stored in the same EEPROM, but in bytes 99-127 which are unused by standard SPD.
Motherboard BIOSes read the SPD information to configure the memory controller. Other programs that read SPD information, with full source code available, includes:
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