Quick RAM question if anybody knows

[librarian]

Heiho,

DDR RAM is a subspecies of SD RAM, but if you see SD RAM stated without DDR it usually refers to the outdated SDR SD RAM standard.

http://en.wikipedia.org/wiki/SDRAM

SDR SDRAM

Originally simply known as "SDRAM", Single Data Rate SDRAM can accept one command and transfer one word of data per clock cycle. Typical clock frequencies are 100 and 133 MHz. Chips are made with a variety of data bus sizes (most commonly 4, 8 or 16 bits), but chips are generally assembled into 168-pin DIMMs that read or write 64 (non-ECC) or 72 (ECC) bits at a time.

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DDR SDRAM

While the access latency of DRAM is fundamentally limited by the DRAM array, DRAM has very high potential bandwidth because each internal read is actually a row of many thousands of bits. To make more of this bandwidth available to users, a Double Data Rate interface was developed. This uses the same commands, accepted once per cycle, but reads or writes two words of data per clock cycle. Some minor changes to the SDR interface timing were made in hindsight, and the supply voltage was reduced from 3.3 to 2.5 V. As a result, DDR SDRAM is not backwards compatible with SDR SDRAM.

DDR SDRAM (sometimes called "DDR1" for greater clarity) doubles the minimum read or write unit; every access refers to at least two consecutive words.