Tutorial 1: Modelling Computing and Abstract Machines

Unit 2 · Section 1

Objectives

Model state, input, transition, and output.
Use an abstract machine to reason about execution.
Separate model assumptions from implementation.

An abstract machine is a simplified model with states, symbols or inputs, transitions, and outputs. A finite-state machine is useful for menus, protocols, validation, and device control.

state: Locked
input: validKey
transition: Locked -> Open
output: unlock signal

Exercises

Model a vending machine as states and transitions.
Identify invalid transitions.
Translate a state table into pseudocode.

Self-check

What is machine state?
What triggers a transition?
Why simplify a model?

Self-Check Quiz

1. What does a transition consume?

Answer

It responds to an input or condition and changes the current state, possibly producing output.

2. Is an abstract machine a physical CPU?

Answer

Not necessarily; it is a model used to reason about computation.

Homework

Build a state-machine model for an online checkout.
List states, events, outputs, and invalid actions.
Explain one implementation consequence.

Sample answer

States include Cart, Payment, Confirmed, and Failed. Payment success moves to Confirmed; invalid payment moves to Failed; a confirmed order cannot be paid again. The implementation needs explicit state and guarded transitions.