When a computer is booted (started), serial presence detect (SPD) is information stored in anelectrically erasable programmable read-only memory (EEPROM) chip on a synchronous dynamic random access memory (SDRAM) memory module that tells thebasic input/output system (BIOS) the module's size, data width, speed, and voltage. The BIOS uses this information to configure thememory properly for maximum reliability and performance. If a memory module doesnot have SPD, the BIOS assumes the memory module's information. With somememory, this does not cause problems. But SDRAM memory has to have SPD or thecomputer may not boot at all. If it does boot, the assumed information may causefatal exception errors
Before SPD, memory chips were identified with parallelpresence 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 storedbecause of the limited space for pins. The EEPROM chip on an SPD memory moduleonly requires two pins so more information can be stored.