Serial Presence Detect
A means of identifying a memory chip to the memory controller logic. The original scheme, Parallel Presence Detect (PPD) used a separate pin for each bit of information. The limited number of pins available only gave the density and the speed of the chips. To pass more information, the Serial Presence Detect (SPD) scheme was introduced, storing the information in serial EEPROM with one pin to enable it and one for data.