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[[Image:SDR SDRAM-1.jpg|thumb|Eight [[Hyundai Electronics|Hyundai]] SDRAM ICs on a PC100 [[DIMM]] package]]
The earliest DRAMs were often synchronized with the CPU clock (clocked) and were used with early microprocessors. In the mid-1970s, DRAMs moved to the asynchronous design, but in the 1990s returned to synchronous operation.<ref>{{cite book | author=P. Darche | title=Microprocessor: Prolegomenes
The first commercial SDRAM was the [[Samsung]] KM48SL2000 [[memory chip]], which had a capacity of 16{{nbsp}}Mbit.<ref name="electronic-design">{{cite journal|date=1993|title=Electronic Design|url=https://books.google.com/books?id=QmpJAQAAIAAJ|journal=[[Electronic Design]]|publisher=Hayden Publishing Company|volume=41|issue=15–21|quote=The first commercial synchronous DRAM, the Samsung 16-Mbit KM48SL2000, employs a single-bank architecture that lets system designers easily transition from asynchronous to synchronous systems.}}</ref> It was manufactured by [[Samsung Electronics]] using a [[CMOS]] (complementary [[metal–oxide–semiconductor]]) [[fabrication process]] in 1992,<ref name="KM48SL2000"/> and mass-produced in 1993.<ref name="electronic-design"/> By 2000, SDRAM had replaced virtually all other types of [[DRAM]] in modern computers, because of its greater performance.
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In operation, CAS latency is a specific number of clock cycles programmed into the SDRAM's mode register and expected by the DRAM controller. Any value may be programmed, but the SDRAM will not operate correctly if it is too low. At higher clock rates, the useful CAS latency in clock cycles naturally increases. 10–15 ns is 2–3 cycles (CL2–3) of the 200 MHz clock of DDR-400 SDRAM, CL4-6 for DDR2-800, and CL8-12 for DDR3-1600. Slower clock cycles will naturally allow lower numbers of CAS latency cycles.
SDRAM modules have their own timing specifications, which may be slower than those of the chips on the module. When 100 MHz SDRAM chips first appeared, some manufacturers sold "100 MHz" modules that could not reliably operate at that clock rate. In response, Intel published the PC100 standard, which outlines requirements and guidelines for producing a memory module that can operate reliably at 100 MHz. This standard was widely influential, and the term "PC100" quickly became a common identifier for 100 MHz SDRAM modules, and modules are now commonly designated with "PC"-prefixed numbers (PC66, PC100 or PC133
== Control signals ==
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* M8, M7: Operating mode. Reserved, and must be 00.
* M6, M5, M4: CAS latency. Generally only 010 (CL2) and 011 (CL3) are legal. Specifies the number of cycles between a read command and data output from the chip. The chip has a fundamental limit on this value in nanoseconds; during initialization, the memory controller must use its knowledge of the clock frequency to translate that limit into cycles.
* M3: Burst type. 0
* M2, M1, M0: Burst length. Values of 000, 001, 010 and 011 specify a burst size of 1, 2, 4 or 8 words, respectively. Each read (and write, if M9 is 0) will perform that many accesses, unless interrupted by a burst stop or other command. A value of 111 specifies a full-row burst. The burst will continue until interrupted. Full-row bursts are only permitted with the sequential burst type.
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{{Main|DDR4 SDRAM}}
DDR4 SDRAM is the successor to [[DDR3 SDRAM]]. It was revealed at the [[Intel Developer Forum]] in San Francisco in 2008, and was due to be released to market during 2011. The timing varied considerably during its development
The DDR4 chips run at 1.2 [[Volt|V]] or less,<ref>{{cite web|url=http://www.pcpro.co.uk/news/220257/idf-ddr3-wont-catch-up-with-ddr2-during-2009.html|title=IDF: "DDR3 won't catch up with DDR2 during 2009"|work=Alphr}}</ref><ref>{{cite web|url=http://www.heise-online.co.uk/news/IDF-DDR4-the-successor-to-DDR3-memory--/111367|title=heise online
DDR4 did ''not'' double the internal prefetch width again, but uses the same 8''n'' prefetch as DDR3.<ref name="jedec_ddr3_ddr4">{{cite press release |url=http://www.jedec.org/news/pressreleases/jedec-announces-key-attributes-upcoming-ddr4-standard |title=JEDEC Announces Key Attributes of Upcoming DDR4 Standard |publisher=[[JEDEC]] |date=2011-08-22 |access-date=2011-01-06}}</ref> Thus, it will be necessary to interleave reads from several banks to keep the data bus busy.
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* [[GDDR]] (graphics DDR) and its subtypes [[GDDR2]], [[GDDR3]], [[GDDR4]], [[GDDR5]], [[GDDR6]] and [[GDDR7]]
* [[List of device bandwidths]]
* [[Serial presence detect]]
* [http://taututorial.yolasite.com/ SDRAM Tutorial]
* A concise but thorough review of SDRAM architecture/terminology and command timing dependencies in [http://drum.lib.umd.edu/bitstream/1903/11269/1/Gross_umd_0117N_11844.pdf High-Performance DRAM System Design Constraints and Considerations], a master thesis from the University of Maryland.
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