RDRAM (Rambus DRAM) is a type of computer device active memory developed and licensed by Rambus Inc. RDRAM competed with synchronous dynamic RAM (SDRAM) during the late 1990s and early 2000s.
As an overall memory subsystem, RDRAM consisted of the following:
- Random access memory (RAM);
- A RAM controller; and
- A bus, or path, connecting RAM to the microprocessor and devices in the computer that used it.
In 1999, Direct Rambus DRAM (DRDRAM) was released, which provided a 16-bit bus, rather than SDRAM's initial 8-bit bus. DRDRAM was the innovator of double data rate transfers, in which data is moved at both the beginning and end of the clock cycle. DRDRAM also used pipelining to move data from RAM to cache memory levels located closer to the microprocessor or display.
RDRAM latency was initially higher, but more consistent than that of SDRAM. That consistent latency -- coupled with the speed advantage of the double data rate transfers and pipelining -- made the technology attractive to video game systems, such as the Nintendo 64 and Sony PlayStation 2.
RDRAM vs. DDR SDRAM
With the introduction of Intel Corp. chipsets in 2002 that supported double data rate SDRAM (DDR SDRAM) and RDRAM, the competing technologies both reached a data transfer bandwidth rate of 4,266 megabytes per second (MBps). RDRAM stayed competitive with DDR SDRAM through DDR 200 and DDR 300.
In 2003, Intel introduced a chipset that supported DDR 400 memory, which increased the data transfer bandwidth rate to 6,000 MBps. The terms DDR 200, DDR 300 and DDR 400 are clock-speed designations of the original DDR SDRAM memory technology (DDR1 SDRAM); the speed indicators were dropped with the introduction of DDR2 and beyond.
RDRAM effectively became obsolete with the launch of DDR 400, since it could no longer keep up in terms of data transfer speed or low latency rates. DDR SDRAM then became the de facto memory standard in most computing systems by the mid-2000s.
In 2003, Rambus introduced extreme data rate DRAM (XDR DRAM), technology based on RDRAM. That was followed by XDR2 DRAM in 2005. Sony chose XDR DRAM for use in the PlayStation 3.
XDR DRAM was initially competitive with the next version of DDR SDRAM, DDR2, which increased top clock speeds in the memory chips to 200 megahertz (MHz). DDR3 had the same clock speed as DDR2, but twice the prefetch buffer width, resulting in twice the overall data transfer rate. When DDR4 increased both the clock speed of the chips and the bus transfer rate, XDR DRAM could no longer match data transfer speeds.