Abacus machines, sometimes called register machines, are abstract machines that form a mathematical model of computation. They are equivalent to Turing machines, so any possible Turing machine algorithm can be implemented on an abacus machine and vice-versa. An interesting alternative to Turing machines, to be sure, but only really used in academia.
Here's an abacus machine that multiplies the values in  and , storing the result in .
While in Brown, I was introduced to these machines in a class taught by Prof. Richard Heck. For class credit, I designed a language to describe abacus machines and wrote a compiler, runtime, and debugging tools for it. It includes features like:
- Built-in testing, with
whereblocks at the end of each function definition. Tests are automatically run on each compilation, and tests are marked as passed or failed.
- Static analysis is performed, detecting some types of infinite loops, unexitable functions, and unreachable lines.
- Runtime analysis detects recursion (which is not permitted by the abacus machine model), and loops without exit conditions.
- A debugger with the ability to:
- step into, through, over, and out of functions,
- halt execution at user-set breakpoints, and —
- inspect (and change!) the current stack frame.
- A compiler that can take a set of functions, each its own abacus machine, and compiles it down to a single abacus machine with no dependencies. The ability to reduce any program without recursion into a single program with a finite number of states is an important theoretical property, and this compiler provides constructive proof of that property.
- Graphviz output.
The compiler can be run in a browser and comes with an implementation of division from first principles. Take a look!