Introduction

Pentium 4: Bandwidth king

Besides its radically new Netburst microarchitecture, one of the most popular features mentioned in Intel's Pentium 4 (P4) literature (whether it's the back of a box or a webpage) is its 400MHz front-side bus. While it's physically only running at 100MHz, the P4 bus is quad-pumped effectively transferring like a true 400MHz bus. Offering up to 3.2GB/second of bandwidth, the P4's bus offers over 40% more bandwidth than AMD's Athlon and over two and a half times the bandwidth of Pentium III's 133MHz bus. This figure matches the amount of bandwidth offered by Pentium 4's dual-channel 800MHz RDRAM interface. Up until recently, Rambus' RDRAM was the only memory type available with Pentium 4 systems.

While all this sounds impressive, plans are underway at both Intel and Rambus to improve this figure next year. After all, technology never stands still. In particular, the targeted clock rate for the front-side bus is 533MHz and 1066MHz in the case of RDRAM. This will mark the first bus improvement for Pentium 4 and if history is any indication, it should bring a noticeable performance improvement.

Earlier this year we witnessed the performance benefits of AMD's 266MHz Athlon bus; bringing roughly a 10% performance boost at 1.2GHz, AMD's 266MHz bus brought VIA's SDRAM-based KT133A chipset a lease on life -- DDR Athlon systems based on AMD's 760 chipset only performed slightly faster when KT133A was originally introduced. Keep in mind that in the Athlon's case the bus speed was improved by 25%, this is the same margin we'll be seeing next year for Pentium 4. In an attempt to get a sneak peek at what kind of performance improvement we can expect out of these upcoming P4 systems, we tested a specially configured Pentium 4 system with RDRAM that has been modified to operate successfully at 533MHz. In addition, Rambus provided us with RIMM modules that have been confirmed to operate at 1066MHz.

Building a 533MHz FSB/1066MHz RDRAM P4 system

We mentioned these uniquely configured Pentium 4 systems in our Platform Conference 2001 coverage from August, it just so happens that Rambus and ASUS were able to supply us with the parts we needed to build such an extreme P4 setup. If you recall, Rambus' engineers discovered that one of the culprits holding back successful P4 overclocks was the clock generator on the motherboard itself, rather than the RDRAM modules. Once the clock generator was swapped out for a unit validated for use at 533MHz, their overclocking success ratio increased. In the case of our motherboard, the standard clock generator of the ASUS P4T was swapped out for a part from Texas Instruments. However, Rambus has successfully demonstrated 533MHz systems on motherboards from more than just ASUS.

Once we combined this custom P4T motherboard with Rambus RDRAM and NVIDIA's GeForce3 GPU, we were curious to see what kind of results we'd get. Also keep in mind that the Pentium 4 we used for testing was unlocked, so we set the CPU for a clock speed that was available for the Pentium 4, and let her rip!