There is no such thing as a single thread running on multiple cores simultaneously.

It doesn't mean, however, that instructions from one thread cannot be executed in parallel. There are mechanisms called instruction pipelining and out-of-order execution that allow it. Each core has a lot of redundant resources that are not utilized by simple instructions, so multiple such instructions can be run together (as long as the next one doesn't depend on the previous result).

Hyper-threading is kind of extreme variant of this idea, in which one core not only executes instructions from one thread in parallel, but mixes instructions from two different threads to optimize resource usage even further.

Related Wikipedia entries: Instruction pipelining, out-of-order execution.

There is no such thing as a single thread running on multiple cores simultaneously.

It doesn't mean, however, that instructions from one thread cannot be executed in parallel. There are mechanisms called instruction pipelining and out-of-order execution that allow it. Each core has a lot of redundant resources that are not utilized by simple instructions, so multiple such instructions can be run together (as long as the next one doesn't depend on the previous result). However, this still happens inside a single core.

Hyper-threading is kind of extreme variant of this idea, in which one core not only executes instructions from one thread in parallel, but mixes instructions from two different threads to optimize resource usage even further.

Related Wikipedia entries: Instruction pipelining, out-of-order execution.