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{{short description|Behavioral rules for all elements of a programming language}}
{{Program execution}}
A programming language consists of a grammar/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 (programming languages)|syntax]]) are [[Scheduling (computing)|scheduled]] for [[Execution (computing)|execution]]. Detailed examples of the specification of execution models of a few popular languages include those of Python,<ref>▼
▲Each and every programming language has an execution model, which determines the manner in which the units of work (that are indicated by program
{{cite web
| url=https://docs.python.org/3/reference/executionmodel.html
| title=Python Documentation: Execution Model
}}</ref>
the execution model of the [[Unified Parallel C]] (UPC) programming language,
<ref>{{cite web
| url=http://upc.lbl.gov/lang-overview.shtml
| title=UPC Language Features
}}</ref>
a discussion of various classes of execution model such as for [[Imperative programming|imperative]] versus [[Functional programming|functional languages]],<ref>
{{cite
| url=https://books.google.com/books?id=4xwWNCiF9CgC&
| title=Programming Languages and Execution Models
| author1=Cardoso, J.M.P.
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| publisher=Springer US
}}</ref>
and an article discussing execution models for [[Real-time computing|real-time embedded]] languages.<ref>{{cite journal
| url=http://pertsserver.cs.uiuc.edu/~mcaccamo/papers/PREM_rtas11.pdf
| title=A Predictable Execution Model for COTS-based Embedded Systems
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| author6=CACCAMO, M.
| author7=KEGLEY, R
|
| publisher=IEEE
| journal=Real-Time and Embedded Technology and Applications Symposium
| access-date=2015-05-20
| archive-date=2017-08-12
}}</ref>▼
| archive-url=https://web.archive.org/web/20170812100818/http://pertsserver.cs.uiuc.edu/~mcaccamo/papers/PREM_rtas11.pdf
| url-status=dead
▲ }}</ref>
== Details of an execution model ==
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To illustrate this, consider the [[C (programming language)|C programming language]], as described in the book by Kernighan and Richie.<ref name="k&r1e">{{cite book | last=Kernighan | first=Brian W. | authorlink=Brian Kernighan | author2=Dennis M. Ritchie | title=The C Programming Language | edition=1st | publisher=[[Prentice Hall]] | date=February 1978 | ___location=[[Englewood Cliffs, NJ]] | isbn=0-13-110163-3 | authorlink2=Dennis M. Ritchie | url-access=registration | url=https://archive.org/details/cprogramminglang00kern }}</ref>
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". Those words: "execution of the program proceeds one statement after the other, in sequence" are one piece of the execution model of C
The C language actually has an additional level to its execution model, which is the order of precedence. Order of precedence states the rules for the order of operations within a single statement. The order of precedence can be viewed as stating the constraints on performing the units of work that are within a single statement. So, ";" and "IF" and "WHILE" cover constraints on the order of statements, while order of precedence covers constraints on work within a statement. Hence, these parts of the C language specification are also part of the execution model of the C language.
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However, an [[Interpreter_(computing)|interpreter]] may also be constructed for any language, in which case all decisions on order of execution are dynamic. An [[Interpreter_(computing)|interpreter]] can be viewed as being part translator, and part execution model implementation.
== Assembly language execution model versus implementation by micro-architectures ==
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"In the case that ownership of the lock goes from thread A to thread B, A-post-gain-lock statements come before B-post-gain-lock statements."
▲Seems simple, right? The complication comes from the fact that the execution model does not have any means for the execution of "give up ownership of the lock" to have any influence over which execution of "gain ownership of the lock" in some other timeline (thread) follows. Very often, only certain handoffs give valid results. Thus, the programmer must think of all possible combinations of one thread giving up a lock and another thread getting it next, and make sure their code only allows valid combinations.
== See also ==
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