page_title: Serial presence detect
section_title: SDR SDRAM
hierarchical_section_title: Serial presence detect / Stored information / SDR SDRAM
caption_attribution_description: Deutsch: SPD-Speicher auf SDRAM-Modul
context_page_description: In computing, serial presence detect is a standardized way to automatically access information about a memory module. Earlier 72-pin SIMMs included five pins that provided five bits of parallel presence detect 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. 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 settings, selectively modify memory timings, or possibly to completely over-ride the SPD data.
context_section_description: The first SPD specification was issued by JEDEC and tightened up by Intel as part of its PC100 memory specification. Most values specified are in binary-coded decimal form.  The most significant nibble can contain values from 10 to 15, and in some cases extends higher.  In such cases, the encodings for 1, 2 and 3 are instead used to encode 16, 17 and 18.  A most significant nibble of 0 is reserved to represent "undefined".
  The SPD ROM defines up to three DRAM timings, for three CAS latencies specified by set bits in byte 18.  First comes the highest CAS latency (fastest clock), then two lower CAS latencies with progressively lower clock speeds.
caption: Memory device on an SDRAM module, containing SPD data (red circled)
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