This is a visualiser and evaluator for register machines (the theoretical kind, not a specific CPU). The model is the Lambek/Minsky register machine from 1961 with increment and decrement-with-test instructions.

The example is the same as on slide 30 of the Cambridge Computer Science course, Computation Theory lecture notes.

Type an expression into the following text area (using the fn x => body syntax), click parse, then click on applications to evaluate them. Also have a look at the examples section below, where you can click on an application to reduce it (e.g. click on pow 2 3 to get 3 2, then fn x => 2 (2 (2 x))).

This is ported from my original one at my alma mater webpage, somewhat updated with better diagrams and more modern code.