# Model of computation

https://en.wikipedia.org/wiki/Model_of_computation

In computer science, and more specifically in computability theory and computational complexity theory, a model of computation is a model which describes how an output of a mathematical function is computed given an input. A model describes how units of computations, memories, and communications are organized.  The computational complexity of an algorithm can be measured given a model of computation. Using a model allows studying the performance of algorithms independently of the variations that are specific to particular implementations and specific technology.

## Models

Models of computation can be classified into three categories: sequential models, functional models, and concurrent models.

Sequential models include:

Functional models include:

Concurrent models include:

Models differ in their expressive power; for example, each function that can be computed by a Finite state machine can also be computed by a Turing machine, but not vice versa.

A nondeterministic model of computation is associated with some of these models of computation. Nondeterministic models are not useful for practical computation; they are used in the study of computational complexity of algorithms.

## Uses

In the field of runtime analysis of algorithms, it is common to specify a computational model in terms of primitive operations allowed which have unit cost, or simply unit-cost operations. A commonly used example is the random-access machine, which has unit cost for read and write access to all of its memory cells. In this respect, it differs from the above-mentioned Turing machine model.

## Categories

There are many models of computation, differing in the set of admissible operations and their computations cost. They fall into the following broad categories: