Memory Presence Detection

When a computer system boots up, it must detect the configuration of the memory modules in order to run properly. For a number of years, Parallel Presence Detect (PPD) was the traditional method of relaying the required information by using a number of resistors. PPD used a separate pin for each bit of information and was the method used by SIMMs and some DIMMs use to identify themselves. However, the parallel type of presence-detect proved insufficiently flexible to support newer memory technologies. This led to JEDEC defining a new standard, serial-presence detect (SPD). SPD has been in use since the emergence of SDRAM technology.

The Serial Presence Detect function is implemented using an 8-pin serial EEPROM chip. This stores information about the memory module’s size, speed, voltage, drive strength, and number of row and column addresses, parameter read by the BIOS during POST. The SPD also contains manufacturer’s data such as date codes and part numbers.