Taken from whatis.comWhen a computer is booted (started), serial presence detect (SPD) is information stored in an electrically erasable programmable read-only memory (EEPROM) chip on a synchronous dynamic random access memory (SDRAM) memory module that tells the basic input/output system (BIOS) the module's size, data width, speed, and voltage. The BIOS uses this information to configure the memory properly for maximum reliability and performance. If a memory module does not have SPD, the BIOS assumes the memory module's information. With some memory, this does not cause problems. But SDRAM memory has to have SPD or the computer may not boot at all. If it does boot, the assumed information may cause fatal exception errors
Before SPD, memory chips were identified with parallel presence detect (PPD). PPD used a separate PIN for each bit of information, which meant that only the speed and density of the memory module could be stored because of the limited space for pins. The EEPROM chip on an SPD memory module only requires two pins so more information can be stored.
