Chewring is a Turing machine simulator for A-level Computer Science, designed for quick classroom demos with a clear interface, included student tasks, and no steep learning curve.
Examples and exercises are available from the menu.
Tape editing
Tap or click a cell to cycle symbols: 01#. Right-click a cell, or long-press on touch devices, to open a symbol picker. You can change the available symbols by selecting 'Alphabet' from the main menu.
Head position
Drag the tape with mouse or finger to move the read head to a new start position.
Rules
Use the + button in Delta Rules to add a rule row. Edit Current, Read, Write, Move and Next directly in the table. For Read, click the button and tap one or more symbols, then press Set: the rule applies when the head is over any of them. A red outline means two rules for the same state share a read symbol, and the first one in the table wins. For Write, click the button and tap a symbol, or "same as read" to leave the symbol unchanged: the choice applies immediately. For Move, click the button to open a picker: Stay applies immediately, while the arrow buttons build a sequence of moves (for example ←←↑) that is applied as a single step when you press Set. Use the red X to delete a rule.
States
Use the + button in Finite State Diagram to add a state. Click a state node to edit its name and role: Normal, Start, Accept, Reject, or Halt. In the diagram, a double ring means a halting state (Accept, Reject, or Halt). In that dialog, use Delete to remove the state.
Tape controls
Use the Tape controls button in Tape Workspace to open the controls popup. In this popup you can add or remove rows with + and -, adjust cell size, and set execution speed. With more than one row, machine rules can move Up (↑) and Down (↓) between rows.
Run controls
Run executes continuously, Pause stops, and Step executes one transition at a time when not halted.
