Finite State Machines — A-Level Computer Science Revision
Revise Finite State Machines for A-Level Computer Science. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to Regular & Context-Free LanguagesWhat is Finite State Machines?
A Finite State Machine (FSM) is a mathematical model of computation used to design both computer programs and sequential logic circuits. It is an abstract machine that can be in exactly one of a finite number of states at any given time, and can transition from one state to another in response to some external inputs.
Board notes: Covered by AQA, Edexcel, and OCR. AQA and OCR students are expected to be able to design and interpret FSMs from a description of a problem.
Step-by-step explanationWorked example
A vending machine can be modelled as an FSM. It has states for 'Idle', 'CoinInserted', and 'ItemDispensed'. An input of a coin transitions it from 'Idle' to 'CoinInserted'. Selecting an item transitions it to 'ItemDispensed' and then back to 'Idle'.
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Common mistakes
- 1Forgetting to include a start state or final (accepting) states.
- 2Creating non-deterministic FSMs when a deterministic one is required.
- 3Incorrectly drawing the state transition diagram, for example, by missing transitions for some inputs.
Finite State Machines exam questions
Exam-style questions for Finite State Machines with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Finite State Machines
Core concept
A Finite State Machine (FSM) is a mathematical model of computation used to design both computer programs and sequential logic circuits. It is an abstract machine that can be in exactly one of a finit…
Frequently asked questions
What is the difference between a Mealy machine and a Moore machine?
In a Moore machine, the output is determined only by the current state, while in a Mealy machine, the output is determined by both the current state and the current input.
Can a finite state machine have no final states?
Yes, an FSM can have zero or more final states. If it has no final states, it can never be in an accepting state.