Intel 820 'Camino' Chipset

Rambus DRAM Support

The most controversial aspect of the 820 chipset is its native support for the RAMBUS DRAM or RDRAM specification. RDRAM is a new technology that Intel has been pushing in the industry for the past two years and they have just recently began talking about the standard more freely. If you look at the PC100 SDRAM that we’ve been using ever since the introduction of the i440BX chipset back in May of 1998 you can begin to realize Intel’s "need" for a higher bandwidth memory technology.

Operating at 100MHz, the PC100 SDRAM memory bus we have been used to allows for up to 800MB/s of bandwidth. While that may seem like a lot, add the fact that the AGP 4X specification is capable of transferring up to 1.06GB/s worth of data to and from the system memory over the AGP bus and you can quickly realize why a higher bandwidth memory technology is necessary.

VIA’s answer to this is the implementation of PC133 SDRAM which raises the memory bandwidth to 1.06GB/s and, in the future, their suggested PC266 DDR SDRAM should double that figure if it ever comes to market. Intel doesn’t like this option for various reasons, including the fact that they have spent quite a bit of time and money working on RDRAM and they’re not going to let that go to waste and support VIA’s standard…not yet at least.

RDRAM is a bit of a different approach to increasing available memory bandwidth. Instead of simply increasing the clock frequency by 33%, RDRAM makes use of very high speed memory, operating on a much narrower bus, resulting in a larger amount of available memory bandwidth. Let’s take a look at how we figure out the 800MB/s bandwidth provided by the memory bus at 100MHz:

Now let’s look at what happens with RDRAM:

Operating on a bus that’s 16-bits wide yet at a frequency of 400MHz and transferring on both the rising and falling edges of the clock (à la AGP 2X mode) and therefore doubling the effective transfer rate, RDRAM seems like the more intelligent option, right?

Not exactly. In exchange for this faster transfer rate, RDRAM sacrifices the effects of a higher latency than SDRAM, meaning that it takes longer to get the data you want from RDRAM than it does from SDRAM. At the same time, once the data is retrieved, it flows much quicker from RDRAM than it does from SDRAM courtesy of the increased transfer rates.

In some cases, RDRAM will offer a performance advantage over SDRAM, while in others it will be slower than SDRAM. In games that are doing a lot of transfers over the AGP bus (especially in 4X mode) the 1.6GB/s transfer rate of RDRAM will help get the data where it needs to go quickly. However in business applications, where there isn’t a lot of transferring of data and latency is king, then SDRAM should, in theory, reign over RDRAM.