Execution model: Difference between revisions

Revision as of 09:17, 9 March 2021

Revision as of 13:47, 23 May 2021

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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!. Those words tells us that statements are indivisible units of work and that they proceed in the same order as their syntactic appearance in the code (except when a control statement such as IF or FOR modifies the order). By stating that "execution of the program proceeds one statement after the other, in sequence", the programming model has stated constraints on the order of performing units of work.

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 ==