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.^[1] 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

Sequential models include:

• Finite state machines
• Post machines (Post–Turing machines and tag machines).
• Pushdown automata
• Register machines
  • Random-access machines
• Turing machines
• Decision tree model

Functional models

Functional models include:

• Abstract rewriting systems
• Combinatory logic
• General recursive functions
• Lambda calculus

Concurrent models

Concurrent models include:

• Actor model
• Cellular automaton
• Interaction nets
• Kahn process networks
• Logic gates and digital circuits
• Petri nets
• Synchronous Data Flow

Some of these models have both deterministic and nondeterministic variants. Nondeterministic models are not useful for practical computation;^{[citation needed]} they are used in the study of computational complexity of algorithms.

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.

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.

See also

• Stack machine (0-operand machine)
• Accumulator machine (1-operand machine)
• Register machine (2,3,... operand machine)
• Random-access machine
• Abstract machine
• Cell-probe model
• Robertson–Webb query model
• Chomsky hierarchy
• Turing completeness

References

Further reading

• Fernández, Maribel (2009). Models of Computation: An Introduction to Computability Theory. Undergraduate Topics in Computer Science. Springer. ISBN 978-1-84882-433-1. 
• Savage, John E. (1998). Models Of Computation: Exploring the Power of Computing. Addison-Wesley. ISBN 978-0201895391. 

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