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An '''execution model''' specifies how work takes place. Every programming language has an execution model, which is specified as part of the language specification, and is implemented as part of the language implementation. Execution models can also exist independently from programming languages, an example of which would be the Posix Thread library. The details in the specification of an execution model cover things such as what is an indivisible unit of work and what are the constraints on the order in which those units of work take place. For example, the addition operation is an indivisible unit of work in many languages, and in sequential languages such units of work are constrained to take place one after the other.
In particular, the [[C (programming language)|C programming language]], defines statements, which are terminated by a ";". This can be viewed as defining one level of indivisible unit of work. The language spec goes on to say that statements proceed in-order. This can be viewed as defining constraints on the order of performing those units of work. It also specifies the order of precedence, which can be viewed as defining another level of indivisible work unit, while
An implementation of an execution model controls the order in which work takes place during execution. This order may be chosen ahead of time, in some situations, or it can be dynamically determined as the execution proceeds. Most execution models allow varying degrees of both. For example, the C language fixes the order of work within a statement and it fixes the order of all statements, except ones that involve an IF statement or a form of loop statement. Hence, most of the order of execution may be chosen statically, before execution begins, but a small portion must be chosen dynamically, as execution proceeds.
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