A means of identifying a memory chip to the memory controller logic. The original scheme, called Parallel Presence Detect (PPD) used a separate pin for each bit of information. As the number of pins can not be very large this only allowed to identify the density and the speed of the chips. To allow for additional information the Serial Presence Detect (SPD) scheme was introduced which uses a serial EEPROM to store the presence detect information and requires only two pins (one for enabling it and one for data).
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