AGP Interface

The AGP interface is in many ways still quite similar to PCI. The slot itself is similar physically in shape and size, but is offset further from the edge of the motherboard than PCI slots are. The AGP specification is in fact based on the PCI 2.1 specification, which includes a high-bandwidth 66 MHz speed that was never implemented on the PC. AGP motherboards have a single expansion card slot for the AGP video card, and usually one less PCI slot, and are otherwise quite similar to PCI motherboards.

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 133 MB/s as with PCI, AGP in its lowest speed mode has a bandwidth of 266 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(100 MHz), 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--normally information is only sent once per clock cycle, but with this 2X mode information can be sent twice. The result is that the performance doubles again, to 533 MB/s theoretical bandwidth. There is also a plan to implement a 4X mode, which will perform four transfers per clock cycle: a whopping 1.066 GB/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 533 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 800 MB/s, a definite step in the right direction, but still not enough to make 4X transfers feasible.

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