VIA's P4X266: The Pentium 4 gets DDR

Let’s talk bandwidth

The name of the game with the Pentium 4 has always been bandwidth and until now we’ve never been able to challenge that assumption.  The Pentium 4 was introduced with Intel’s own 850 chipset platform which only supported a dual channel RDRAM memory interface.  If Intel had it their way, all of the Pentium 4 solutions offered would be RDRAM based from the entry level platforms to the highest end.  The reality of the market has prevented Intel from doing this and has forced them to consider SDRAM as a solution for the lower price points. 

There is good reason for Intel to want to stick with the dual channel RDRAM solution that the i850 chipset offers.  With PC800 RDRAM, the two 16-bit RDRAM channels offer a combined peak bandwidth of 3.2GB/s.  This is exactly the same amount of bandwidth that the Pentium 4’s 64-bit quad-pumped 100MHz FSB offers to the processor.  In theory, an equal match of FSB and memory bandwidth would eliminate any bandwidth bottlenecks that could arise thus allowing the CPU to run at its full potential.  Another part of this theory of how bandwidth effects CPU performance is that as CPU speed increases, its dependency on a higher bandwidth memory bus will increase since it will be processing/requesting more data at any given time. 

With that said, how much is enough?  Obviously any more than 3.2GB/s isn’t going to give your CPU much of a performance increase.  The V-Link connection to the South Bridge and the AGP 4X controller will eat up a bit of the 3.2GB/s of memory bandwidth but exactly how much is very difficult to determine.  From what we’ve seen, the amount that they consume at this point is very limited. 

On the SDRAM side of things, PC133 SDRAM is obviously not enough as it can only deliver 1.06GB/s of memory bandwidth.  Even for today’s Athlon platforms PC133 SDRAM can be limiting although not nearly as much as with the Pentium 4. 

Otherwise known as PC1600 DDR SDRAM, DDR200 SDRAM offers a bit more bandwidth at 1.6GB/s but that’s still only half of what a dual channel PC800 platform offers.

The obvious and most competitive solution to the dual channel RDRAM of Intel’s 850 chipset is DDR266/PC2100 DDR SDRAM.  A single DDR266 channel offers 2.1GB/s of bandwidth.  This is currently as close as you’re going to get to the 3.2GB/s that RDRAM has been able to give the Pentium 4.