If you're a chip designer like AMD, how do you improve security when you know that there's all the chance in the world that someone's going to hack into the software your processor will be running and just simply take over. How would you stop that from ever happening right from the millisecond the hard code hits the CPU cache? By using a single core to prevent such software execution, that's how. IBM's Cell processor inside the Playstation 3 famously has eight Synergistic Processing Elements (Sony speak for, "we spent a truckload of money on it") cores but doesn't use all of them - six of those cores are available to developers while the seventh is used for the Playstation GUI and general usage. The eighth is reportedly disabled to improve chip yields, although I believe that may actually be used to prevent software execution that Sony doesn't like or want on its system.
While the reality is that the eighth core's purpose is still a mystery, AMD has recently teamed up with ARM to allow the use of ARM's TrustZone technology on their APUs and motherboards. Hit the jump for more info and some interesting possibilities.