AGP interfaces and signaling voltages
There are three versions of the AGP specification, as follows:
AGP1.0 specifies an interface that supports 1X and 2X speeds with 3.3V signaling and a keyed 3.3V connector. AGP1.0 supports DIME, pipelined transactions, source-synchronous clocking, texturing, and sidebanding.
AGP2.0 specifies an interface that supports 1X, 2X, and 4X speeds with 1.5V signaling. AGP2.0-compliant interfaces and adapters may use either a 1.5V keyed connector, which accepts only 1.5V devices, or a universal connector, which accepts 1.5V or 3.3V devices interchangeably. AGP2.0 adds Fast Writes to the protocols supported by AGP1.0.
AGP3.0 specifies an interface that supports 4X and 8X speeds with 0.8V signaling. AGP3.0 uses the same 1.5V keyed connector or universal connector used by AGP2.0. AGP3.0 adds several protocol elements to those supported by AGP2.0, and removes support for some AGP2.0 features.
There are six types of AGP interfaces, and six corresponding types of AGP adapters, as follows:
AGP3.3V interfaces and adapters support 1X or 2X operation at 3.3V only, using a 3.3V connector. An AGP3.3V adapter can be installed in an AGP3.3V, UAGP, or UAGP3.0 interface. An AGP3.3V interface accepts an AGP3.3V, UAGP, or UAGP3.0 adapter.
AGP1.5V interfaces and adapters support 1X, 2X, or 4X operation at 1.5V only, using a 1.5V connector. An AGP1.5V adapter can be installed in an AGP1.5V, UAGP (described next), U1.5VAGP3.0, or ...