2 Definitions

The following definitions are used in this document:

Attributes

An entity is defined in terms of a set of attributes, which represent the entity's essential traits, qualities, or properties. Attributes can contain "primitive" data type values such as strings, integers, reals, and enumerations, or an attribute of one entity can be one or more other entities. EXPRESS does not distinguish between an entity's attributes and its relationships as separate notions. Rather, each attribute is also a relationship to the type that represents its value; see [1] in Section 7, "References".

Entity (Type)

An entity represents some thing or concept that is important in a schema. An entity type provides expressions which define the entity. An entity instance is the realization of a specific case of the entity type. Thus while there is an entity type named "CELL," the cell named "Half-Adder" is an Entity Instance of the type CELL. The word entity used alone means entity type. An entity is defined in terms of its data and behavior. The data definition is given as a set of attributes. Behavior is defined in terms of constraints and operations [1].

Entity-Level Model

An Entity-level model is an EXPRESS-G model that represents the definitions and their relations, which comprise a single schema [1].

EXPRESS

An information modeling language defined by the EXPRESS Language Reference Manual [1]. This document also describes the graphical notation EXPRESS-G.

EXPRESS-G

The graphical notation for EXPRESS, which is described in Annex D of the EXPRESS Language Reference Manual [1].

Objects---Entity Instances

Specific (allocated) cases of entity types, such as a particular "CELL," e.g., "Half-Adder" or "Adder" or "My 64 Bit ALU."

Policy

Rules restricting how entities and their operations may be manipulated.

Programming Interface

A set of procedures or functions used by an application to (in the case of DR) access design data.

Relationships

Attributes of entities that model references to one or more other entities. EXPRESS does not distinguish between attributes and relationships explicitly.

Schema

A schema encompasses a set of entity and other declarations which have a related meaning and purpose. A schema can contain other schemas [1].

Schema-Level Model

A schema-level model is an EXPRESS-G model which shows the relations between single schemas. It is an abstraction of a model removing much of the detail. Schema-level models should only show schemas, no entities or types.

Session

A session is defined as the interval of time in a program which starts with the "Initiate PI" procedure and ends with the "Quit PI" procedure [2].

Subtype and Supertype

A subtype is a thing which is different from other things of the same general kind. The entity which represents all varieties of that same general kind is called the supertype entity. For any supertype entity A which has a subtype entity B, the following statement is true:

Every B is an A, but not every A is a B.

Type

A type is used to associate an attribute or variable with a data representation. In EXPRESS, the type is the kind of an entity or variable. Some types are basic, e.g., integer, real, or string. Other types are more specialized and are defined by the user. Every entity forms a type.

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