When RDRAM made its debut on the desktop one of the biggest selling points of the Rambus technology was its very low pincount; at 16-bits per channel compared to the current 64-bit wide DDR buses, it's easy to see where the lower pin count comes from. As a serial interface (much like Serial ATA), lower pin count and the ability to run at very high speeds are major benefits of the technology; not to mention that it is much easier on motherboard manufacturers to design solutions with lower pin count memory interfaces.

Very few can argue that RDRAM did exhibit technical superiority to DDR; however there are issues such as cost, risk and availability of modules that kept the technology out of the mainstream market. Because of this, and the inability of DDR SDRAM to provide as much bandwidth as is necessary for CPUs like the Pentium 4, chipset manufacturers have had to start working on dual channel DDR solutions.

NVIDIA was the first with their 128-bit DDR interface on the original nForce later followed by Intel with their E7500 chipset. Both SiS and VIA will be joining the ranks of dual channel DDR chipset manufacturers but make no mistake, going to dual channel DDR (128-bit parallel memory interface) does not make the life of the motherboard manufacturer any easier.

It isn't impossible to implement a 128-bit memory interface, and you can actually do so on a 4-layer motherboard however that still doesn't mean that motherboard manufacturers wouldn't like to see a simpler solution. DDR-II may end up being that simpler solution, as a single 64-bit wide channel should be able to offer more bandwidth than present day dual channel DDR solutions; the downside is that DDR-II won't be hitting the mainstream market until 2004.

Intel's Prescott CPU will have a 667MHz FSB, demanding a memory interface capable of providing 5.3GB/s of memory bandwidth. While RDRAM definitely fits the bill, the market currently wants a DDR based solution to do the same and unfortunately it would take a dual channel DDR333 solution to provide that sort of bandwidth. It will be done as Springdale (Prescott's intro chipset) will be a dual channel DDR333 platform, but again, the motherboard manufacturers want something simpler.

Being very in touch with the motherboard manufacturers and desperately trying to drum up interest in their Pentium 4 chipsets, VIA announced an alternative memory architecture based on currently available DDR technology: Quad Band Memory (QBM).

The QBM Chipset Architecture

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