bochs offers you that to some extent.
Bochs not only has a built in debugger, but it has a mechanism to define new CPU instrumentations (via bochs source code, recompile required) that you can enable and disable from the debugger. Very cool feature if you need to investigate some code or some performance issue. Hoewever bochs is quite slow and supports some old ia32 system. Tim Newsham | www.thenewsh.com/~newsham | thenewsh.blogspot.com
