The 800Mhz Bus

What will the new 800Mhz front side bus do for overall system performance? That’s the question most of us are asking. To understand this, we first must understand how the Pentium 4’s front side bus works. Just like the previous bus speeds, the new “800Mhz” is actually the quad-pumped number of the main bus. This means that although the new front side bus will run at a true speed of 200Mhz, the memory bus is able to send data four times per clock, thus somewhat equal to a true 800Mhz bus (200 X 4 = 800). That’s 6.4GB/s of “theoretical” bandwidth. This number is arrived at by multiplying 200 MHz(clock speed) X 64 bits (bus width) X 4 (how many times per clock data is transferred) and then divide by 8 (because there are 8 bits in a byte).

When the CPU needs to read or write data on the memory it must first “address” it. The CPU sends out the addresses of the data it needs to work with. The addresses travel through the front side bus. The Pentium 4’s front side bus actually only transfers addresses two times per clock, unlike the data that it can transfer four times per clock. This means that the new bus can only request data at 400Mhz. This shouldn’t effect performance much however because data is what uses up the majority of bandwidth, not addresses. You will never actually see 6.4GB/s of bandwidth being used because of this. Hopefully it will be somewhat close.

The front side bus is what connects the processor to the rest of the computer system. In the case of the Pentium 4, the front side bus is 64bits wide. To improve performance, Intel is increasing the speed at which data is transferred (frequency). When you increase the frequency, you shorten the time it takes to transfer data thus improving performance. It would be much more difficult to increase the width of the bus; that would essentially be a new platform. A wider front side bus would offer much greater performance but unfortunately that would be very difficult to implement due to manufacturing reasons. So essentially, what we have is the same old bus running at a higher clock frequency.

Looking at the performance improvements the 533Mhz bus offered over the 400Mhz bus can give us a very good idea of what will be improved with the 800Mhz bus. With the 533Mhz bus we basically saw a performance increase across the board. Having a faster front side bus will definitely help performance in areas where heavy memory bandwidth is needed. New games and 3D applications will probably benefit the greatest. DOOM III anyone?

There is really no point having a fast bus if the memory can’t keep up. When the memory or any other component is holding back system performance, this is called a “bottleneck”. An example of a memory bottleneck would be if you were running single channel DDR 333Mhz with this new 800Mhz bus. The memory would only be providing 2.7GB/s of bandwidth while the bus would be capable of saturating 6.4GB/s of bandwidth. A situation like this would kill overall system performance.

In order to utilize this new 800Mhz front side bus and prevent bottlenecks, Intel along other manufactures, will be releasing chipsets that support dual channel DDR 400Mhz. DDR 400Mhz alone offers 3.2GB/s of memory bandwidth. Dual channel chipsets allow you to combine the memory bandwidth by using DDR in pairs, somewhat like Rambus. Dual DDR 400Mhz will provide exactly 6.4GB/s of memory bandwidth perfectly matching what the new 800Mhz bus has to offer. Another benefit is that the front side bus and memory will be running at the same “true” clock speed. DDR 400Mhz, really is running at 200Mhz double data rate. When both the memory and bus are running at the same speed, latency is reduced. This is called “synchronous” operation. Running the bus and memory at different speeds is called “asynchronous”.