DDR-DRAM is basically SDRAM which has been modified so as to make it more advanced, and ultimately faster. Double Data Rate Synchronous Dynamic Random Access Memory is the complete wording of the product, but DDR-DRAM is much easier to deal with. To dive you an idea of why it is called this, one must consider the design of SDRAM. On SDRAM, data, along with commands and addresses, are transferred on the rising portion of the clock cycle. With DDR, there is some special circuitry at the data pins that allows it to transfer this information on both the rise and fall of the clock cycle. This gives the memory a doubled speed when compared to SDRAM, this the term “Double Data Rate”.
Data rate is doubled by adding some extra circuitry between the DRAM itself and the data pins. This extra logic produces a data output strobe which synchs the data flow to the external clock speed, and allows the memory to transfer data on both sides of the clock cycle.
DDR-SDRAM and Rambus DRAM have been the two competitors for the memory of the future. Rambus DRAM was seen by most, myself included, as the future. This was caused by the continuing trend that when Intel speaks, everybody listens. Intel said that RDRAM was the future, and as such it was accepted as inevitable. However, RDRAM was very expensive. It was to the point of people needing an RDRAM slush fund, right next to their Roth IRA, in case one needed to upgrade. A number of RDRAM chipsets were produced, primarily for high end servers and whatnot, due to the effectiveness of Rambus. Then we forgot about it. Rambus is still used on many Pentium IV systems, but DDR-DRAM is by far more popular. This is caused by a number of fiscally interesting factors. Primarily, unlike RDRAM, this stuff doesn’t force you to sell your grandmother’s organs to buy some. Its price is comparable to that of the run-of-the-mill SDRAM available at your neighborhood tech kid’s garage computer chop shop.
The speeds of SDRAM were typically rated by the bus speed at which they are capable of operating, so PC100 and PC133 SDRAM sticks could operate on 100 and 133 MHz frontside busses respectively. DDR-DRAM, though, is rated a bit differently. Common ratings are PC 1600, PC2100, and PC3200. The rating systems are different, and are based on the theoretical maximum bandwidth of the memory units in megabytes per second. How is this calculated? Well, as I stated already, DDR-DRAM manages this by working with data both on the rising and falling clock ticks. This allows the 100MHz version to effectively run at 200MHz. Math time: 64 bit architecture, running at 2 times the normal rate, on a 100MHz bus. Divide by 8 bits to the byte, and we get a transfer rate of 1600MB/s. In my mind, I would say that that is more than acceptable. Do the same to the 133MHz bus and you get 2133MB/s. Hence, we get PC1600 and PC2100.
This rating system was also used for matketing reasons, no doubt. Rambus is sometimes called PC800, because of the rate at which it operates. Manufacturers of DDR-DRAM can’t go out and use small little numbers like PC133. It’ll give RDRAM the marketing advantage.
When motherboard manufacturers whiched from standard SDRAM to DDR-DRAM, ti gave the market a nice performance boost. But, users craved more, of course. PC1600 and PC2100 were nice SDRAM replacements, but did not give enough of a performance boost at first to make the techie drool on himself. So, newer and faster speeds were released: PC2400 and PC2700. Some of this memory was true PC2400/2700 memory, some sticks were actually overclocked. But, beyond that, there is now PC3000 DDR and some faster than that. Some of these faster units some with copper heat spreaders on them to aid in the dissipation of heat.
One reason that DDR-SDRAM needs a modified motherboard is because of its different package. It comes in 184-pin instead of 168-pin. It also only has one notch in the unit, as opposed to two on more conventional SDRAM.
DDR-DRAM is now the new standard for consumer memory. The new renditions of the technology compete with RDRAM quite well, and it is sure to increase performance on Athlon-based systems as well.