The AGP Express slot technically works like an ordinary PCI slot, but is powered by the electrical lines of two of them.
Without doubt, the real highlight of the 915P-A is the AGP-compatible slot. As we mentioned before, Intel decided to drop AGP support with the 900 chipset series. As a result, users are normally forced to buy PCI Express hardware. People that recently spent up to several hundred dollars on a new AGP graphics card are unlikely to be willing to spend that much money again for PCI Express hardware that doesn’t really provide any further benefit.
ECS obviously has been thinking about this issue for a long time, and came to the conclusion that users might be able to live with a "weakened" AGP port for a limited period of time. To this end, the manufacturer decided to provide an AGP-compatible slot based on PCI, by merging the voltage feeds of two slots into the AGP connector. This slot, dubbed AGP Express , is not an enhancement to AGP in any way, but rather a PCI slot physically adapted to look like AGP.
Though it is not quite AGP compliant, AGP Express will work with the majority of AGP cards available. Obviously, AGP Express is not capable of DIME (Direct Memory Execute), the AGP feature responsible for sourcing textures out into main memory. Also, a chipset function named GART (Graphics Address Remapping Table) would be required to take care of graphics memory allocation within the system RAM.
However, the biggest disadvantage of the PCI approach is the fact that bandwidth within a bus is shared with all devices present. Even though the graphics board does most of the heavy lifting in the system, it will never be able to use the PCI bus exclusively.