Execution model

In computing, a programming language consists of a syntax plus an execution model. The execution model specifies the behavior of elements of the language. By applying the execution model, one can derive the behavior of a program that was written in terms of that programming language. For example, when a programmer "reads" code, in their mind, they walk through what each line of code does. In effect they simulate the behavior inside their mind. What the programmer is doing is applying the execution model to the code, which results in the behavior of the code. Each and every programming language has an execution model, which determines the manner in which the units of work (that are indicated by program syntax) are scheduled for execution. Detailed examples of the specification of execution models of a few popular languages include those of Python, the execution model of the Unified Parallel C (UPC) programming language, a discussion of various classes of execution model such as for imperative versus functional languages, and an article discussing execution models for real-time embedded languages. Operational Semantics is one method of specifying a language's execution model. The observed behavior of a running program must match the behavior derived from the operational semantics (which define the execution model of the language). An execution model covers things such as what is an indivisible unit of work, and what are the constraints on the order in which those units of work may 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. To illustrate this, consider the C programming language, as described in the book by Kernighan and Richie. C has a concept called a statement. The language specification defines a statement as a chunk of syntax that is terminated by a ";". The language spec then says that "execution of the program proceeds one statement after the other, in sequence".