DRAM Memory Technologies

DRAM is available in several different technology types. At their core, each technology is quite similar to the one that it replaces or the one used on a parallel platform. The differences between the various acronyms of DRAM technologies are primarily a result of how the DRAM inside the module is connected, configured and/or addressed, in addition to any special enhancements added to the technology.

There are five well-known technologies:

Synchronous DRAM (SDRAM)

An older type of memory that quickly replaced earlier types and was able to synchronize with the speed of the system clock. SDRAM started out running at 66 MHz, faster than previous technologies and was able to scale to 133 MHz (PC133) officially and unofficially up to 180 MHz. As processors grew in speed and bandwidth capability, new generations of memory such as DDR and RDRAM were required to get proper performance.

Double Data Rate Synchronous DRAM (DDR SDRAM)

DDR SDRAM is a lot like regular SDRAM (Single Data Rate) but its main difference is its ability to effectively double the clock frequency without increasing the actual frequency, making it substantially faster than regular SDRAM. This is achieved by transferring data not only at the rising edge of the clock cycle but also at the falling edge. A clock cycle can be represented as a square wave, with the rising edge defined as the transition from ‘0’ to ‘1’, and the falling edge as ‘1’ to ‘0’. In SDRAM, only the rising edge of the wave is used, but DDR SDRAM references both, effectively doubling the rate of data transmission. For example, with DDR SDRAM, a 100 or 133 MHz memory bus clock rate yields an effective data rate of 200 MHz or 266 MHz, respectively. DDR modules utilize a 184-pin DIMM (Dual Inline Memory Module) packaging which, like SDRAM, allows for a 64 bit data path, allowing faster memory access with single modules over previous technologies. Although SDRAM and DDR share the same basic design, DDR is not backward compatible with older SDRAM motherboards and vice-versa.

It is important to understand that while DDR doubles the available bandwidth, it generally does not improve the latency of the memory as compared to an otherwise equivalent SDRAM design. In fact the latency is slightly degraded, as there is no free lunch in the world of electronics or mechanics. So while the performance advantage offered by DDR is substantial, it does not double memory performance, and for some latency-dependant tasks does not improve application performance at all. Most applications will benefit significantly, though

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