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Technology in Brief

Although it has been very poorly publicized, the first thing to know about RDRAM is that there are actually three different speed grades that are being used in PCs. Although PC800 is mentioned in the media and in Intel promotional material as if it is the only type of RDRAM, it is in fact the fastest, most expensive, and rarest grade of three different speed levels of RDRAM. At the bottom of the performance ladder is PC600, which is also, as you would expect, the cheapest. It is so slow that it will probably not be used extensively by major system manufacturers in any computer systems.

In the middle of this RDRAM pack is PC700, which is a standard that was only introduced after RDRAM producers pressured Intel to accept this compromise grade. The reason this happened was because many of those producers were finding it next to impossible to reach acceptable yields at PC800. These producers also feared that falling to PC600 would result in the memory being rejected as too slow by their customers. Even though this standard was adopted a year ago, memory manufacturers are still having trouble reaching the PC800 standard today. Because of this, PC800 RDRAM is almost unattainable and most of the current systems are quietly shipping out with the slower PC700 RIMMs.

Bandwidth over Clock-speed

The second important thing to understand about RDRAM is that clock-speed cannot always be trusted. Yes, the fastest grade of PC800 does run at 800 MHz, but the bus width (which determines the amount of data that can be transferred at any instant in time) is only 16 bits (8 bits = 1 byte so 16 bits = 2 bytes) wide. This means that PC800 RDRAM is capable of transferring 2 bytes x 800 MHz = 1.6 GB/sec. The number 1.6 GB/sec is called the "bandwidth" for PC800 and represents the maximum possible rate that data can be transferred using this technology.

By the way RDRAM's bus is double-pumped meaning that it transfers data on the rising and falling edges of clock pulses. Accordingly PC800 is fed by a 400 MHz clock. Less intuitively., as stated in a recent article, PC700 actually runs at 712 MHz double-pumped from a 356 MHz clock. To insure that there is no logic to RDRAM speed ratings, PC600 real speed is 532 MHz triggered by 266 MHz clock.

SDRAM has a much larger 64-bit (8 bytes) bus width. Currently the fastest DIMMs (sticks of SDRAM) run at 133 MHz. This gives PC133 (SDRAM running at 133 MHz) a bandwidth of 8 bytes x 133 MHz = 1.064 GB/sec. What should be apparent is that due to their differing bus widths, RDRAM must run four times faster than SDRAM to provide the same bandwidth. So when comparing these memory technologies, be sure to take this important fact into consideration.

Positioned by Micron and many other manufacturers to replace SDRAM, DDR SDRAM also has a 64-bit bus, but like RDRAM, DDR SDRAM transfers data at the rising and falling edges of clock signals, effectively doubling its bandwidth. Therefore DDR SDRAM feed by a 133 MHz clock has a bandwidth of 2 x PC133 bandwidth = 2 x 1.064 GB/sec = 2.128 GB/sec. According to Micron, DDR SDRAM should be priced similarly to current SDRAM making it greatly cheaper than RDRAM. Although DDR SDRAM is currently in certain Nvidia graphics cards, it has not been released yet for use as main memory. DDR SDRAM should make its widespread debut around July and should first appear in systems built around AMD's Athlon.

Following is a simple chart comparing the bandwidths of these competing memory technologies.

Designation Bus Width
RDRAM PC800 2 Bytes 400 MHz 800 MHz 1.6 GB/sec
RDRAM PC700 2 Bytes 356 MHz 712 MHz 1.424 GB/sec
RDRAM PC600 2 Bytes 266 MHz 532 MHz 1.064 GB/sec
SDRAM PC133 8 Bytes 133 MHz 133 MHz 1.064 GB/sec
SDRAM PC100 8 Bytes 100 MHz 100 MHz 0.8 GB/sec
DDR SDRAM PC266 or DDR133 8 Bytes 133 MHz 266 MHz 2.128 GB/sec
DDR SDRAM PC200 or DDR100 8 Bytes 100 MHz 200 MHz 1.6 GB/sec

For Intel's Coppermine based systems, there is one final important issue to consider in regard to bandwidth. Coppermine's data bus is 64-bits wide, but only operates at 133 MHz. This means that it has the same effective bandwidth as PC133 SDRAM of 1.064 GB/sec. Because of this bottleneck, much of the bandwidth advantage that RDRAM has over SDRAM is lost. There is still a theoretical advantage when DMA (Direct Memory Access) requests and AGP transfers are issued simultaneously with processor requests, but our tests show that this is not enough to help RDRAM above its other problems.

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