DDR SDRAM - Variations

Variations

DDR (DDR1) was superseded by DDR2 SDRAM, which had modifications for higher clock frequency and again doubled throughput, but operates on the same principle as DDR. Competing with DDR2 was Rambus XDR DRAM. DDR2 dominated due to cost and support factors. DDR2 was in turn superseded by DDR3 SDRAM which offered higher performance for increased bus speeds and new features. DDR3 will likely be superseded by DDR4 SDRAM, which was first produced in 2011 and whose standards are still in flux (2012) with significant architectural changes.

DDR's prefetch buffer depth is 2(bits), while DDR2 uses 4. Although the effective clock rates of DDR2 are higher than DDR, the overall performance was no greater in the early implementations, primarily due to the high latencies of the first DDR2 modules. DDR2 started to be effective by the end of 2004, as modules with lower latencies became available.

Memory manufacturers stated that it was impractical to mass-produce DDR1 memory with effective transfer rates in excess of 400 MHz (i.e. 400MT/s and 200 MHz external clock) due to internal speed limitations. DDR2 picks up where DDR1 leaves off, utilizing internal clock rates similar to DDR1, but is available at effective transfer rates of 400 MHz and higher. DDR3 advances extended the ability to preserve internal clock rates while providing higher effective transfer rates by again doubling the prefetch depth.

RDRAM was a particularly expensive alternative to DDR SDRAM, and most manufacturers dropped its support from their chipsets. DDR1 memory's prices substantially increased since Q2 2008 while DDR2 prices declined. In January 2009, 1 GB DDR1 was 2–3 times more expensive than 1 GB DDR2. High density DDR RAM will suit about 10% of PC motherboards on the market while low density will suit almost all motherboards on the PC Desktop market.