What Does Serial Presence Detect Mean?
Serial presence detect (SPD) is information stored on an EEPROM chip when a computer is booted. It is located on an SDRAM memory module and communicates to the BIOS the module size, data width, speed and voltage, which are used to configure the module memory controller for maximum reliability and performance.
Techopedia Explains Serial Presence Detect
The manufacturer of the memory module will put the SPD information on the EEPROM chip. When a computer is turned on, if the BIOS is not provided with SPD, it will assume the memory module information, which presents no problem to some memory modules.
Parallel presence detect (PPD) data was used in earlier 72-pin SIMMs. However, the standard changed to SPD with the later model 168-pin DIMM. SPD encodes much more information.