Editing Abstract machine (section)

== Hierarchies ==
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[[File:A_hierarchy_of_abstract_machines.png|thumb|A hierarchy of abstract machines]]
Abstract machine hierarchies are often employed, in which each machine uses the functionality of the level immediately below and adds additional functionality of its own to meet the level immediately above. A [[Computer hardware|hardware computer]], constructed with physical electronic devices, can be added at the most basic level. Above this level, the abstract [[Microcode|microprogrammed machine level]] may be introduced. The abstract machine supplied by the [[operating system]], which is implemented by a program written in [[Machine code|machine language]], is located immediately above (or directly above the [[Computer hardware|hardware]] if the [[firmware]] level is not there). On the one hand, the operating system extends the capability of the physical machine by providing higher-level primitives that are not available on the physical machine (for example, primitives that act on files). The [[Hypervisor|host machine]] is formed by the abstract machine given by the operating system, on which a [[high-level programming language]] is implemented using an [[Virtual machine|intermediary machine]], such as the Java Virtual machine and its byte code language. The level given by the abstract machine for the [[High-level programming language|high-level language]] (for example, Java) is not usually the final level of hierarchy. At this point, one or more applications that deliver additional services together may be introduced. A "web machine" level, for example, can be added to implement the functionalities necessary to handle Web communications ([[Communication protocol|communications protocols]] or [[HTML|HTML code presentation]]). The "[[Web service|Web Service]]" level is located above this, and it provides the functionalities necessary to make web services communicate, both in terms of interaction protocols and the behaviour of the processes involved. At this level, entirely new languages that specify the behaviour of so-called "business processes" based on Web services may be developed (an example is the [[Business Process Execution Language]]). Finally, a specialised application can be found at the highest level (for example, [[E-commerce]]) which has very specific and limited functionality.<ref name=":3" />