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Rambus may come back onto the market of mainstream hardware for personal computers with its XDR DRAM memory technology on the forthcoming generations of graphics cards. Sources close to the company said there have been negotiations between graphics chip companies and Rambus amid some memory makers’ hopes on the ramp of XDR for graphics applications.

XDR for Next Breeds of Graphics Processors?

XDR memory’s novel system topology allows point-to-point differential data interconnects to scale to multi-gigahertz speeds, while the bussed address and command signals allow a scalable range of memory system capacity supporting from 1 to 36 DRAM devices.

XDR DRAMs will be available in multiple speed bins, device densities, and device widths. With densities ranging from 256Mb to 8Gb, and device widths ranging from x1 to x32, XDR DRAM satisfies the needs of both high-bandwidth and high-capacity systems, the company indicated.

Toshiba has been sampling 512Mb XDR DRAM 3.20GHz devices since late 2003 and is on track to deliver the actual products in 2005 or 2006. XDR roadmaps currently include devices with up to 6.40GHz core-clock.

Sony’s PlayStation 3 is expected to use XDR DRAM and is currently the most well-known XDR-based product.

XDR Inevitable for Graphics, Consumer Applications?

“Graphics seems to be one of the important initial targets for XDR, as graphics applications today have nearly unlimited need for bandwidth out of a single DRAM. Networking is another important market as networking cards need high bandwidth but low capacity,” a source close to Rambus and GPU makers told X-bit labs.

Representatives for Toshiba recently said that small amount of XDR memory for graphics applications will be available already next year. In 2006 equal volumes of XDR will be sold for consumer applications, including Sony PlayStation 3, and graphics devices. According to Toshiba, networking devices will use XDR memory in 2006, but it is unlikely that XDR will be in extremely high demand from networking companies: graphics and consumer electronics XDR markets will be much larger.

Since the data that is stored in a networking card packet buffer does not stay around long – it is sent out relatively quickly – there is no need for high capacity. High bandwidth from a single DRAM is an advantage for networking, as it provides high bandwidth, but remains relatively cost-efficient because of low capacity. Consumer devices are much like networking cards, with the additional constraint that consumer devices are much more cost-sensitive. For instance, Sony’s PlayStation 2, which uses RDRAM, needs high bandwidth from a small memory capacity to reduce device count, mainboard area, controller pin-count (which reduces packaging costs and routing complexity), and thus total system cost.

High-end graphics cards, in contrast, require both speed and capacity. Modern graphics cards, such as ATI RADEON X800 XT and NVIDIA GeForce 6800 Ultra, carry 256MB onboard, the amount of memory that mainstream personal computers had in 2001-2002.

“Rambus has been in discussion with many different graphics processor manufacturers about XDR memory. There are not many choices for high-speed memory for GPU manufacturers, so it is natural that they would like to know about XDR and what it offers for their products,” the source noted.

XDR Compatible with DDR, DDR2, GDDR2, GDDR3

Even though Rambus’ memory devices have always been faster compared to available SDR or DDR solutions, customers in PC segment have been giving preference to cheaper mainstream memory products, especially keeping in mind that the difference in performance between RDRAM and competing solutions was not high. With graphics applications the things may change – in case visual processing units (VPUs) require extreme bandwidth, at some point XDR may become the only choice, especially if graphics processors’ developers may obtain compatibility between various memory types by using recently unveiled Rambus’ guidelines for memory controller.

Memory controller from Rambus provides support for mainstream DDR and DDR2 at speeds of up to 800MHz and graphics DDR, including GDDR1, GDDR2, and GDDR3 at up to 1600MHz data rates, Rambus DDR memory controller interface cells are full-featured drop-in physical layer (PHY) cells. The interface cells use proven technology that allow customers to improve time-to-market, minimize design risk and avoid potential re-spin costs. Rambus also offers system engineering services to further accelerate time-to-market, and ensure the interface operates at high frequency in the system environment. Rambus DDR interface solutions are ideal for a broad range of applications, from consumer multimedia and graphics systems to mainstream PCs and servers, the company said in late June, 2004.

Challenges on Rambus’ Path

Current GDDR3 that is used for high-end graphics cards nowadays may be clocked at up to 2.00GHz, providing bandwidth of 64GB/s for an application with 256-bit memory bus. The next incarnation of GDDR memory will be GDDR4 that is likely to be out next year and is expected to allow memory frequencies of up to 2.80GHz and effective bandwidth of about 89GB/s.

Today’s graphics processors require computing power more than memory bandwidth and developers of graphics chips will consider whether possibly more expensive XDR will deliver tangibly higher performance compared to GDDR4.

It is also not clear whether XDR supports 256-bit operation, but if it does, and with no substantially higher pricing, graphics processing units developers may bid on astonishing 102.4GB/s or higher bandwidth. In case it does not, 3.20GHz XDR devices are unlikely to become serious rivals for GDDR4 DRAMs able to work in 256-bit mode, a crucial feature for modern memory for graphics cards.

Officials for Rambus did not comment on the news-story.


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