Description domain and instance domain

Tutorial on SDL-88
Belina, Hogrefe (edits Reed)

First of all, a clear distinction must be made between the instance domain and the description domain. In the instance domain there is a system instance, consisting of a number of block instances, forming a tree structure with the system instance as the root. Leaf-block instances contain process instances instead of subblock instances. A system instance contains also instances of-channels, signal routes, signals etc. An instance has a life of its own, it is created and it may die, it has dynamic properties. The explanation of the concepts (the semantics of the language) is generally more simple in the context of the instance domain than in the context of the description domain. (Note, however, that instances should be regarded as imaginary objects, which are not created or built in reality, only in imagination.)

The user of SDL can specify and create instances of a process, signal, procedure etc. However, SDL does not provide mechanism for the specification and creation of system instances, block instances, channel instances etc (therefore some people may question the use of the term instance for these concepts). That means that for each block description in a system description there is one single block instance (having the same name as the block description) in a system instance. The same hold for channels, signal routes etc.

In the description domain there is a system description, containing block descriptions, channel descriptions, signal descriptions etc. Generally, the description of a concept is done in terms of the description of other concepts, in accordance with the syntax rules. In contrast to the system instance, the system description has only static properties and can contain optional parts (not to be confused with the option shorthand), see §9.3. In addition, the system description can also contain concepts (shorthands). Which have no counterpart in the system instance.