[Wizzard~Of~Ozz]

DDR uses the leading and falling edge of a clock cycle, A clock is a square wave, So it goes

+voltage 0 +voltage 0......

SDR transfers on the + voltage, DDR transfers when the voltage goes up to +voltage and when it goes to 0, allowing double the transfers per clock cycle.

I'm not going to go into Rambus Memory since it's a completely different technology, And SGRam hasn't been used in a long time, but I believe it's Synchronus Graphics Memory, This was a more expensive faster memory then the current 60ns an 50ns memory That was availible at the time (Memory used to run much slower, But back then it was EDO or Fastpage but those haven't been used since 486/Pentiums. some modules ran at 100ns)

So DDR is not twice the speed, however it is as effective as double the speed. Marketing departments are the ones that do a lot of this

330MhzDDR (660Mhz)

Which means they suck at electronics and don't understand the technology, but in the Marketing department it's all about selling product, even if you have to mutilate a technology.

Also, SDram and Dram are different modules, But from what I remember the difference is mainly voltage, Dram was at 5v, SDram is at 3.3. Dram was also Fastpagemode and Did NOT usually run at the same clock speed as the processorFSB (P166MMX @ 66Mhz FSB with 25Mhz Memory.)

Dram was common in Pentium systems since it meant you could put 1 module in in place of 2 72 pin Simms (Single In-line Memory Module)

SDR SDRam chokes an Athlon System, The processor can send information to the northbridge at 133Mhz DDR, from there if it's memory intensive it can send the data at 133Mhz DDR to memory modules, however if you are only running SDR, then the processor sits idle 1/2 the time while waiting for memory to be written (since data transfer is 133DDR (266Mbits/S) to NB, then NB -> memory is only 133Mhz (133Mbits/sec)

SDR is very horrible for Athlon Systems. it strangles the processor.