A notional machine is the idealized model of the computer implied by the constructs of the programming language.

The notional machine is an idealized, conceptual computer whose properties are implied by the constructs in the programming language employed. That is, the properties of the notional machine are language, rather than hardware, dependent.

The notional machine is a model of the machine that conceptually executes the program code. In principle, it answers the question: what does a machine need in order to understand, follow and execute the program code? As such it differs from the actual implementation on any level, and is therefore not bound to the physical computer, or a virtual machine.

A notional machine is not a physical computer but an abstraction of computer software and hardware; it is the set of capabilities that a particular programming language or environment affords to the programmer. Understanding a notional machine enables a programmer to answer questions such as: What can this programming system do for me? What are the things it can’t or won’t do? What is the division of labour between myself and the computer system (i.e. between the human instructor and the mechanistic instructee)? What changes in the system does each of my instructions bring about as my program is run? How do I reason about what my program does?

The notional machine is primarily determined through the programming language, libraries and frameworks used. An excellent treatment of the notional machine can be found in Sorva [23].

For a strategy based on a notional machine to be effective, the notional machine must conform to two important principles. First, the notional machine employed should be conceptually simple, and second, methods should be provided for the novice to observe certain of its workings in action.

Getting a good understanding of the notional machine is crucial for programming. When we regard programming as communication between man and the machine, with the programmer being the sender of the message (the program code), then the notional machine is the receiver of the message. It determines the effect of a given program code, and answers accordingly to the programmer. Of course, successful communication can only take place with a working model and understanding of the second party involved, i.e., the notional machine.