SAN MATEO, Calif. Rambus Inc. and some of its partners are set to deploy a 32-bit Rambus in-line memory module (RIMM) populated with faster, 1,066-MHz Rambus DRAM memory chips, as part of a renewed campaign to woo more PC makers over to the RDRAM camp.
The 32-bit RIMMs, which will provide 4.2 Gbytes/second of bandwidth to and from Intel's dual-channel RDRAM controllers, was designed to solve some nagging system design issues facing RDRAM. Taiwan-based Asustek Computer will be one of the first adopters of the 32-bit RIMMs using the 1,066-MHz RDRAMs.
RDRAM systems that use existing 16-bit RIMMs require a minimum of four RIMM slots to accommodate the two RDRAM channels supported by Intel's chip sets, even though just two RIMM slots are needed to meet most system memory needs. If the remaining slots are not used, they must include unpopulated dummy modules known as continuity RIMMs.
Besides taking up more board space, the current configuration requires that users and OEMs add two RIMMs at a time one for each memory channel when upgrading the memory subsystem. That is considered overkill for most systems.
Rambus says it has fixed that problem by widening the RIMMs' bus width from 16 to 32 bits. With that approach, each module can take in both 16-bit-wide Rambus channels, thus requiring only two slots to complete the memory subsystem instead of four. To do so, Rambus loaded more pins into open positions on the RIMM's connector edge and added termination. Today's 16-bit RIMMs terminate on the motherboard.
Rambus expects memory and module makers to provide 32-bit RIMMs with 128, 256 and 512 Mbytes based on current, 256-Mbit RDRAM chips. With just two slots, the highest memory capacity would reach 1 Gbyte, or two 512-Mbyte modules. The lowest capacity would be 128 Mbytes using one module and one dummy RIMM.
By moving to the 32-bit RIMMs, PC motherboard makers will no longer have to upgrade memory modules in pairs and will be able to exploit the freed-up board space to add features, said Melissa Frank, product marketing manager at Rambus. "It didn't make a lot of sense for most [of them] to go to 2 Gbytes," she said.
Thus far, memory manufacturers Elpida Memory Inc. and Samsung Electronics Co. Ltd. and module maker Kingston Technology Co. Inc. have built prototypes of the 32-bit RIMMs. Samsung last week announced the first 32-bit RIMMs using the 1,066-MHz RDRAM devices. The RIMM delivers 4.2 Gbytes/s of bandwidth, twice as fast as competing DDR-266 modules, according to Samsung.
This is where the battle against double-data-rate (DDR) architectures takes a twist: By year's end, Pentium 4-compatible chip sets should arrive that support two separate 64-bit channels of DDR memory, observers said. Under that scenario, upgrades to the DDR systems have to be made in pairs rather than as single modules, as they are today.
That means minimum increments of 512 Mbytes (one 256-Mbyte module for each channel), based on today's 256-Mbit DRAM technology.
In effect, the forthcoming 128-bit DDR systems are about to inherit the monkey Rambus just pried off its back.
Whether DDR memory granularity becomes a problem for OEMs may depend on what kind of system they are building. "For the high end, this is not going to be a big issue," said Tom Quinn, vice president of marketing for Samsung Semiconductor (San Jose, Calif.). "But it will be interesting to see what effect that will have when it goes to the midrange."
In theory, chip makers can get over the granularity hump by manufacturing 256-Mbit DDR devices in a x16 configuration instead of the more common x4 or x8 widths. By doing so, they could populate the modules with fewer components and churn out lower-capacity 128-Mbyte DIMMs, so that the dual-channel DDR systems can upgrade memory in smaller chunks.
The problem is that the x16 parts are not a slam-dunk for chip makers. They require a separate design and take up more die area than their narrower x4 and x8 siblings, which translates into higher cost. "It's not that cost-effective to build a 128-Mbyte module. The most cost-effective one is the 256-Mbyte module," Samsung's Quinn said.
But it's uncertain whether looming DDR issues and the new RIMMs will be enough to prompt PC and chip set makers to abandon SDRAM and its DDR derivative, observers said. RDRAM-based systems have received plaudits for high benchmark scores and are being snapped up in high-end desktop systems, but that has not stemmed the DDR tidal wave building this year.
"This will be a bit of a boost [for Rambus], but I would be surprised if it gives them more than a 1 or 2 percent jump," said Scott Thirwell, a marketing manager at Taiwan-based motherboard maker DFI.
And although RDRAM prices have come down, they are still higher than competing SDRAM and DDR tags. Some Taiwanese system designers remain skeptical that RDRAM can be anything more than a niche memory product if the price gap doesn't close.
"This [32-bit RIMM] will offer more flexibility on the systems, but Samsung won't drop its price on the chips. The main reason the platform is more expensive is the memory," said an executive at Asustek. "Intel is still behind DDR."
Kingston Technology, where RIMM sales amount to about 5 percent of total sales, has seen some increase in demand since the introduction of the 1,066-MHz RDRAM speed. But the pickup has been limited to those building high-end game machines, according to Stephen Rodriguez, director of strategic marketing at Kingston (Fountain Valley, Calif.).
Kingston is building prototype 32-bit RIMMs for companies like Asustek but probably won't ramp production until 32-bit volumes reach 5 percent of total memory module shipments. "Right now we're doing 2 million modules a month. Doing at least 100,000 [32-bit RIMMs] would be nice to warrant the investment in people and support," Rodriguez said.
That may not happen until tier-one OEMs start to place big orders for RIMMs. But many have been skittish about doing so because Intel thus far has not been validating the 1,066-MHz RIMMs for its chip set, Rodriguez said. DFI's Thirwell agreed, but he noted that a few system integrators might sign on.
Rambus insists that Intel's refusal to validate the 1,066-MHz RDRAM modules is not standing in the way of demand.
"We rely on individual motherboard makers and OEMs to handle it, and we don't consider it an issue, frankly," said Steve Tobak, senior vice president of worldwide marketing at Rambus. "We're absolutely seeing tremendous demand for the 1,066-MHz parts and the 32-bit RIMMs."
Indeed, many say the 1,066-MHz RDRAM is a good match for the 533-MHz system bus supported by the Intel 850e Pentium 4 chip set, as evidenced by the enthusiastic response it has received from PC makers that cater to the gamer community.
"The bottom line is that the channel is up and running, with or without official confirmation from Intel," said Samsung's Quinn.