RIMM and DDR memory-informational

Memory technology. Which direction it takes seems to be a hot topic of discussion. Rambus®, PC133, and DDR. You've heard the names before, but just what technology powers these memory solutions? The common thread that binds them all together is that they are all advancing the future of tomorrows memory technology.

The memory industry seems to be at a crossroad. As chipset performance and processor speeds continue to increase, the memory needed to drive these platforms to the next level needs to advance at a pace slightly ahead of the hardware pack. Memory needs to overcome the bandwidth and latency issues which often caused it to be called the bottleneck that slowed down advancement of technology.

Front side bus speeds have increased to 133MHz. Memory needs to perform at a speed of no less than 133MHz to fully utilize the new processor and chipset technology. Rambus, PC133 and DDR all perform at that level, and beyond.

RAMBUS
Rambus is a revolutionary memory technology. By not being backward compatible, consumers cannot upgrade their existing computers to use Rambus technology or RIMMs. The definition of RIMM™ is not an acronym for Rambus In-line Memory Module. It is simply a term that is used to describe the Rambus memory module. Rambus works by transferring data on both the leading and trailing edge of the clock cycle. The clock runs at the high speed of 400MHz. With two operations per clock cycle, the data is transferred at a speed of 800MHz. The 16-bit wide Rambus channel (2 bytes wide) lets the data transfer at a rate of up to 1.6 GB per second. Rambus uses transmission line technology, which enables the signal to enter one side of the module, access the RDRAM, exit the other side and travel through the next memory connector. Because of this, each connector must be populated. If only one connector is populated with a module, a C-RIMM should populate the empty connector. A C-RIMM or Continuity RIMM does not have RDRAM on it. A C-RIMM allows the signal to pass through the connector to let the data flow uninterrupted. When the system is upgraded, the C-RIMM will be replaced with a module. With the release of the Intel 820 (single RIMM channel) and 840 (dual RIMM channel) chipsets and motherboards, Rambus technology has become a reality in the computing industry.

PC133
Higher memory bandwidths are expected in the near future. The higher bandwidths will require today's SDRAM technology to evolve to match the higher speeds and throughput. PC133 evolved almost accidentally from a die shrink that was done on existing PC100 SDRAM dies. These new PC133 modules run at 133MHz, and allow for a 1.1 GB/sec bandwidth. This speed increase delivers a performance gain of 33% over PC100. This technology allows DRAM manufacturers to offer a bridge between existing technology and new technologies like DDR and Rambus.

DDR
Double-Data-Rate (DDR) SDRAM, often simply called DDR, is another evolutionary SDRAM memory technology. DDR allows for two transactions in one clock cycle, compared to one for PC100 or PC133 SDRAMs. This allows data to be transferred on both the rising and falling edges of the clock signal. The initial DDR module specifications, expected to be finalized by JEDEC in 2000, define 100- and 133-MHz DDR DIMM modules, resulting in an effective data rate of 200- and 266- MHz respectively. The 200-MHz data rate DDR DIMM is expected to be introduced first, followed by the 266-MHz DDR DIMM later on. Faster DDR DIMMs are also under discussion. DDR is expected to be introduced first in the server market, and then in the desktop market.