Serial presence detect (SPD) refers to a standardized way to automatically access information about a computer memory module. Earlier 72-pin SIMMs included 5 pins which provided 5 bits of parallel presence detect (PPD) data, but the 168-pin DIMM standard changed to a serial presence detect to encode much more information.
When an ordinary modern computer is turned on, it starts by doing a power-on self-test (POST). Since about the mid-1990s, this process includes automatically configuring the hardware currently present. SPD is a memory hardware feature that makes it possible for the computer to know what memory is present, and what timings to use to access the memory.
Some computers adapt to hardware changes completely automatically. In most cases, there is a special optional procedure for accessing BIOS parameters, to view and potentially make changes in settings. It may be possible to control how the computer uses the memory SPD data—to choose which settings to use, to selectively modify the memory timings, or possibly to completely over-ride the SPD data (see overclocking).
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