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AGP Bus Width, Speed and Bandwidth

The AGP bus is 32 bits wide, just the same as PCI is, but instead of running at half of the system (memory) bus speed the way PCI does, it runs at full bus speed. This means that on a standard Pentium II motherboard AGP runs at 66 MHz instead of the PCI bus's 33 MHz. This of course immediately doubles the bandwidth of the port; instead of the limit of 127.2 MB/s as with PCI, AGP in its lowest speed mode has a bandwidth of 254.3 MB/s. Plus of course the benefits of not having to share bandwidth with other PCI devices.

In addition to doubling the speed of the bus, AGP has defined a 2X mode, which uses special signaling to allow twice as much data to be sent over the port at the same clock speed. What the hardware does is to send information on both the rising and falling edges of the clock signal. Each cycle, the clock signal transitions from "0", to "1" ("rising edge"), and back to "0" ("falling edge"). While PCI for example only transfers data on one of these transitions each cycle, AGP transfers data on both. The result is that the performance doubles again, to 508.6 MB/s theoretical bandwidth. There is also a plan to implement a 4X mode, which will perform four transfers per clock cycle: a whopping 1,017 MB/s of bandwidth!

This is certainly very exciting, but we must temper this excitement somewhat (and not just because AGP is new and we don't have much that is practical to evaluate yet). It's great fun to talk about 1 GB/s bandwidth for the video card, but there's only one problem: this is more than the bandwidth of the entire system bus of a modern PC! If you recall, the data bus of a Pentium class or later PC is 64 bits wide and runs at 66 MHz. This gives a total of 508.6 MB/s bandwidth, so the 1 GB/s maximum isn't going to do much good until we get the data bus running much faster than 66 MHz. Future motherboard chipsets will take the system bus to 100 MHz, which will increase total memory bandwidth to 763 MB/s, a definite step in the right direction, but still not enough to make 4X transfers feasible.

Also worth remembering is that the CPU also needs to have access to the system memory, not just the video subsystem. If all 508.6 MB/s of system bandwidth is taken up by video over AGP, what is the processor going to do? Again here, going to 100 MHz system speed will help immensely. In practical terms, the jury is still out on AGP and will be for a while, though there can be no denying its tremendous promise.

Next: AGP Video Pipelining

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