P4X Brief

Born to be DDR

Compared to the i845 SDR chipset from Intel, the P4X266 is in a much better position to offer raw bandwidth for enthusiasts and gamers. Because the P4 relies on a quad pumped 100MHz/64-bit bus, it needs a memory bus that’s able to provide approximately 3.2GB/sec of data to perform at its peak. Realistically, standard PC133 SDRAM modules are only able to provide about 1.0GB/sec of usable bandwidth.

If only RDRAM came as cheap as DDR RAM is, there would be many more P4 users out there today, and those users would have speedier systems too. Natively, the P4 fits best with PC800 RDRAM because that’s what Intel had in its drawing plans when designing the P4. At 800MHz, PC800 RDRAM is able to offer the P4 3.2GB/sec. of bandwidth – perfectly matching the P4’s bus to the memory bus. If AMD had its way, and if RDRAM modules were cheap, the Athlon platform might be using RDRAM too.

Alas, we all know this isn’t the case. Hindered by poor execution, high manufacturing costs, and legal troubles galore, Rambus was unable to provide cheap RAM en mass. Intel resorted to bundling RDRAM along with its P4 processors just to get the platform moving along, and at speeds uncomfortably slow to what Intel is normally use to. VIA, not one to sit around and play Frisbee with wafers, sought to bring an affordable, high-bandwidth solution to the market. Thus they created the P4X266 chipset.

Kind of like Gabriel vs. Yu-law

Back in our i845 coverage, we noted that while the i845 was released as a chipset supporting only PC100 and or PC133 SDRAM, it internally was already hardwired to support PC1600/2100 DDR RAM. The i845 enjoys native support for DDR RAM by having a write cache directly on the chipset die. This enables it to write information extremely fast into the P4. Along with a L3-like write cache, the i845 also had deep buffers which keep data flowing at peak performance and eliminates spikes and valleys in data flow. Data is kept flowing to the P4 smoothly, and frequently used data is moved to the P4 even faster.

All the above features are useful when high-bandwidth memory like PC2100 DDR RAM is used. Because PC2100 is fast, the memory bus needs to be just as fast to keep up with the RAM. The key is to eliminate as many bottlenecks as possible without creating new ones and the trick is to make all buses as fast as possible, close to the fastest bus in the chain, but not exceeding it. If any of the buses speeds past the fastest bus (in this case, the P4’s 3.2GB/sec data bus), then the fastest bus becomes the new bottleneck, and further optimizations will yield little benefit.

The P4X266 comes with similar enhancements that are seen in the i845 chipset, but natively supported DDR RAM when it was released. While not as fast as PC800 RDRAM, PC2100 does hold its own, especially when dealing with latency. At 266MHz, PC2100 is able to provide 2.1GB/sec. – short of what PC800 can do, but nevertheless, rarely will the P4 bus be saturated 100% of the time.