SerialICE is a flexible, software based system software debugger. Based on the open source processor emulator Qemu, SerialICE allows you to log and intercept hardware accesses of your firmware and single step your firmware with the GNU debugger. After a couple of quiet years on the development front, it has now adopted new filtering subsystem.
more here............http://www.serialice.com/Main_Page
more here............http://www.serialice.com/Main_Page