RDRAM

RIMM redirects here. For the psychologist, see Sylvia Rimm.

Direct Rambus DRAM or DRDRAM (sometimes just called Rambus DRAM or RDRAM) is a type of synchronous dynamic RAM, created by the Rambus Corporation.

It has been used in the Nintendo 64, the Sony PlayStation 2, and a line of Cray supercomputers. Intel originally advocated the use of RDRAM in desktop PCs. Due to fears that Intel was trying to control the memory market through Rambus Corp., most of the main memory manufacturers sought to replace RDRAM with an alternative standard, DDR (Double Data Rate) DRAM. Better manufacturability lead to DDR prevailing over RDRAM in the cost-conscious PC market. Rambus's successor to RDRAM is XDR DRAM, used by IBM’s "Cell" processor technology and the PlayStation 3.

The Direct RDRAM, or RDRAM, features an architecture and a protocol that were designed to achieve high bandwidth. The Rambus channel architecture has a single-device upgrade granularity, offering engineers the ability to balance performance requirements against system capacity and component count. The narrow, high-performance channel also offers performance and capacity scalability through the use of multiple channels in parallel. In addition, the validation program created by Intel and Rambus promotes system stability by ensuring that devices and modules conform to published specifications. Although RDRAMs have a low pin count, a single device is capable of providing up to 1.6 GB/s bandwidth. Memory systems that use RIMMs (Rambus inline memory modules), also known as RDRAM modules, employ a narrow, uniform-impedance transmission line, the Rambus Channel, to connect the memory controller to a set of RIMMs. Low pin count and uniform interconnection topology allow easy routing and reduction of pin count on the memory controller. While a single channel is capable of supplying 1.6 GB/s of bandwidth, multiple channels can be used in parallel to increase this number. Systems that use, for example, the Intel 840 chipset have two parallel Rambus channels, and are able to handle up to 3.2 GB/s.

Providing high bandwidth from a single device also allows memory systems to be constructed from small numbers of RDRAMs. The Sony PlayStation 2 uses two RDRAM channels, each with a single RDRAM, to achieve a total of 3.2 GB/s memory bandwidth.

In order to ensure stability of RDRAM memory systems, design guidelines and a validation program have been put in place that surpass requirements set for previous memory technologies. Intel and Rambus have defined system specs to ensure robustness of RDRAMs and of the channel to the memory controller. In addition, they have created a rigorous validation programs for certification of RDRAMs and RIMM modules.

Due to the way RDRAM is designed, early implementations suffered from much higher memory latency when compared to contemporary SDR SDRAM and DDR SDRAM designs. While RDRAM's very high bandwidth allowed this effect to be masked to a certain degree, randomly accessed memory cells would suffer large performance drops with RDRAM, making SDR SDRAM or DDR SDRAM a better choice. Latency became worse as additional RIMM's were added to the system, further hurting performance.

Later designs mitigated the latency issue to the point where it was mostly competitive with DDR SDRAM, but by then RDRAM's parent company Rambus had largely alienated hardware manufacturers due to its litigious nature. Consumer sentiment turned against Rambus in the same manner and also because of RDRAM's higher price compared to other memory types. As of 2004, Intel has essentially abandoned RDRAM entirely, with all new products using DDR SDRAM or DDR-2 SDRAM.de:RIMM he:RDRAM nl:RIMM