This is where Apple failed to provide good details and we had to do a bit of digging. After talking to five or so Apple employees, we got the scoop. The PowerMac G4s have a 100-MHz system bus supporting up to 400MBps data throughput on the lower end motherboard and up to 800-MBps data throughput on the higher end motherboard. We were immediately curious as to how the high-end motherboards could have faster memory busses when both models used PC100 SDRAM and 100MHz busses. It turns out, the higher end motherboards have a new memory controller that is much more efficient. According to Apple's specifications, twice as efficient. One example of efficiency given was that, with the older memory controller, if information was requested from memory and it was not immediately available, the CPU would have to wait until that initial piece was available before any other memory requests could be fulfilled. With the new design, other requests will be fulfilled even when the first request has to wait to be completed , so the CPU will spend less time waiting.

Currently, 100MHz busses on PCs have a theoretical maximum throughput of 800MBps. We have to wonder whether Apple has, up until this point, been using a sub par memory controller with their G3s, or whether the PC memory busses are not really as fast as they claim. Either way, the point may be moot because 133MHz PC memory busses are right around the corner. Part Five of our Memory Guide sheds a lot of light on memory performance and is worth reading if you want to know more.