Intel Pentium III 533B & 600B

Unfortunately, things aren’t that simple because you also happen to have a bandwidth hungry AGP 2X graphics card sitting in your system. This is where things get complicated. Let’s take a look at the bandwidth requirements for the AGP 2X bus:

(32-bit AGP bus x 66MHz AGP clock frequency x 2X transfer mode) / 8-bits in a byte = 528MB/s

Now with that figure, plus the 800MB/s from your CPU, if both your graphics card and your CPU request a great deal of data from memory you are going to begin to see the effects of a saturated memory bus. Going back to the road analogy again, this is the equivalent of having rush hour traffic going across your roads, and for anyone that’s ever been stuck in rush hour traffic, things don’t move as quickly as you would like them to.

However, you have to realize that these are all mathematical maximums and only in heavy usage conditions will you see these limitations met. If you’re just opening up a document in Word or you’re just surfing the web, chances are you’re not going to be experiencing the effects of a saturated system bus. But, remember the peripherals we left out from the above calculation? Your PCI peripherals can eat away at those bandwidth numbers as well., How much data are they capable of moving over the PCI bus?

(32-bit PCI bus x 33MHz PCI bus frequency) / 8-bits in a byte = 132MB/s

If you factor that into the numbers we just talked about, you can see how the limits of the current system/memory bus can be considered as a threat to performance. As CPUs get faster and faster, and as AGP cards get faster and faster this is going to be (in theory) more of a problem until it reaches the point where a user tries to open up a document in Word and immediately feels the effects of a saturated system or memory bus. That is an extreme example, but it gets the point across. But wait a minute, if all this is going on in your system right now then why don’t we see users complaining about not getting enough memory bandwidth? And if no one is complaining, then why change?

Well, let’s toss another wrench into these gears and eat up some more bandwidth, shall we? You’ve all heard the term AGP 4X used over and over again by Intel and graphics card manufacturers. Essentially, it is capable of moving twice as much data over the AGP bus as the AGP 2X transfer mode was capable of doing. And you better believe that it eats up about twice as much bandwidth too.

(32-bit AGP bus x 66MHz AGP clock frequency x 4X transfer mode) / 8-bits in a byte = 1056MB/s = ~1GB/s

That is quite a bit of bandwidth, and what we’ll start to see is that during applications, such as games, that are very graphic intensive (and thus AGP) as well as memory intensive, the bottlenecks of the system and memory buses are illustrated. So how did Intel solve this problem?

Simple: they upped the FSB frequency to 133MHz which yields a 33% increase in system and memory bus bandwidth. Theoretically, this should offer a hefty performance improvement, but, in reality, that isn’t exactly the case. Since the intended platform for the Pentium III 533/600B was the i820, we will conduct all tests on the i820 platform in this review, but in the coming weeks we will investigate i820 alternatives as well.

The i820 makes use of a new memory technology known as Direct Rambus DRAM or RDRAM for short. The bandwidth allowed for by RDRAM is as follows:

(16-bit RDRAM x 400MHz operating frequency x 2X data rate) / 8-bits in a byte = 1.6GB/s

Something should look funny to you in that equation -- the 16-bit data path of RDRAM versus the 64-bit width of the current SDRAM technology. At the same time, RDRAM operates at 4X the clock speed of SDRAM and transfers on both the rising and falling edges of the clock, so, in theory, a 400MHz RDRAM module offers 8X the operating frequency of a 100MHz SDRAM module. The result of this, using our "road" analogy, is that with RDRAM you have a very narrow road with a speed limit 8 times that of SDRAM.

Take a look at our i820 review to learn more about RDRAM and the advantages/disadvantages to the technology as well as the different flavors of RDRAM (PC800, PC700, PC600).