storage 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).