This is kinda great; I've thought about having something like it for "running" (simulating) embedded code on a PC, where the compiler just ignores (or asks the user for a value) when hitting anything that isn't available on the host pc (registers, etc).

What an amazing thing!

I wish I had this back in the day when I first learnt C and was doing all this by hand with pencil and paper. The VCR-style rewind is a nice touch.