Johnny is a simulator of a simple (virtual) von-Neumann Computer. Its principal application is education; therefore a number of simplifications are made: rather reduced instruction sets, simplified bus operations, etc.
It visualizes the function of Memory, Arithmetic Logic Unit and Control Unit. Machine Programs can be created and edited using a comfortable GUI and run on the virtual machine step by step.
Micro Code, too, is editable: Instructions can be added and modified using the GUI.
Thanks for Johnnysimulator, it's excellent!