The Mill

stateA

read·

step0

ones (Σ)0

statusready

head

speed 14/s

A real Turing machine — the abstract computer Alan Turing imagined in 1936. A single head shuttles over an unbounded tape, reading and writing one symbol at a time; a tiny table of rules — the program — says what to do for each (state, symbol). Step it, or set it running, and watch a computation unfold cell by cell.

Program

Input

Transition table — the program (click a cell to edit)

Each rule is write·move·next: the symbol to write, the move (L/R/N), and the next state. A blank rule means halt (stuck). Editing rebuilds the machine and resets it.

Skin

The tape is unbounded — blanks stretch forever both ways; only the visited cells are stored. The busy beavers are the rigorous centerpiece: from a blank tape, BB(4) halts after exactly 107 steps leaving 13 ones — the proven 4-state record. Try editing one of its rules and watch the proof break. For a halting question no one can answer, see The Collatz Bench — run n→n/2 | 3n+1 as a program and every tested input halts at 1, but no machine can promise it always will.