How does reverse debugging work?

Question

GDB has a new version out that supports reverse debug (see http://www.gnu.org/software/gdb/news/reversible.html). I got to wondering how that works.

To get reverse

Answer 1

Reverse debugging means you can run the program backwards, which is very useful to track down the cause of a problem.

You don't need to store the complete machine state for each step, only the changes. It is probably still quite expensive.