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Serial presence detect (SPD) is a standardized way to automatically access information about a computer memory module. Earlier 72-pin SIMMs included five pins that 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 settings, selectively modify memory timings, or possibly to completely over-ride the SPD data (see overclocking). ==Stored information== For a memory module to support SPD, the JEDEC standards require that certain parameters be in the lower 128 bytes of an EEPROM located on the memory module. These bytes contain timing parameters, manufacturer, serial number and other useful information about the module. This data allows a device utilizing the memory to automatically determine key parameters of the module. For example, the SPD data on an SDRAM module might provide information about the CAS latency so the system can set this correctly without user intervention. The SPD EEPROM is accessed using SMBus, a variant of the I²C protocol. This reduces the number of communication pins on the module to just two: a clock signal and a data signal. The EEPROM shares ground pins with the RAM, has its own power pin, and has three additional pins (SA0–2) to identify the slot, which are used to assign the EEPROM a unique address in the range 0x50–0x57. Not only can the communication lines be shared among 8 memory modules, the same SMBus is commonly used on motherboards for system health monitoring tasks such as reading power supply voltages, CPU temperatures, and fan speeds. (SPD EEPROMs also respond to I²C addresses 0x30–0x37 if they have not been write protected, and an extension uses addresses 0x18–0x1F to access an optional on-chip temperature sensor.〔(JEDEC Standard 21-C section 4.1.4 ) "Definition of the TSE2002av Serial Presence Detect (SPD) EEPROM with Temperature Sensor (TS) for Memory Module Applications"〕) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Serial presence detect」の詳細全文を読む スポンサード リンク
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