An Unlimited Register Machine is a kind of very basic computer. (The ‘unlimited’ part refers to the number of registers that can be used; with a simulator like this, there will be a limit imposed by the computer it's running on). It only has four instructions, described on the evaluator page. By convention, the inputs and outputs start from the lowest-numbered register and work upwards.
The Open University course M381 – Number theory and mathematical logic taught me about URMs (along with Gödel, Escher, Bach). As with the Legendre symbol evaluator, I made this to help me understand the concept, and if it’s useful to others, that’s a bonus. Source is available on GitHub at smcgivern/urm-evaluator.