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[Table of Contents]

[4. Tool encapsulation architecture]

[6. Language processing]

5. Design information management

5.1 Overview

In this section we describe the design information management (DIM) service of an EDA framework. A conceptual schema is presented that fulfils the requirements on design structure management summarized in Section 1.6(1). It can be used by tightly integrated and encapsulated design tools to access design objects and by designers to query the state of their design. The conceptual schema is associated with an application programming interface used by design tools and framework services such as the encapsulation service described in Chapter 4. The application programming interface in turn is implemented by invoking functions from an underlying data manager to access design objects, their structure and representation. In Section 5.4 we explain different approaches for an implementation of a DIM service and illustrate these approaches with two case studies conducted on the ObjectStore OODBMS and on the Nelsis CAD Framework. While the implementation on an OODBMS is easier, implementing the new service on an existing CAD framework has two advantages, assuming the framework already is the basis for a complete design environment with design tools:

  1. Design tools encapsulated using the new service can inter-operate with tools already integrated by other means.
  2. Designers can stick with a proven and familiar design system and need not be trained to use a completely new system.

5.2 Conceptual schema

The conceptual schema described in this section allows to configure a DIM service to manage structural information about design objects (R1). We use the Xplain semantic data modelling technique [terBekke 92] introduced in Section 3.1. The schema will not be presented as a whole but rather developed incrementally. The diagrams will sometimes not show all attributes of the depicted types. The textual type definitions, however, are complete. In each step, we focus on a specific aspect of structural design information, identify relevant concepts and formalize them using the Xplain notions of types, aggregation, and specialization. As new types are defined in terms of other types, previously defined types may be redefined to avoid redundancies.

5.2.1 Interfaces, implementations, and configurations


Figure 12.
Interfaces, implementations, and configurations.

As an electronic design project proceeds, the specification of a designed module is refined stepwise; its interface evolves according to the levels of detail, each having a number of implementations in different domains. In our schema, a module represents an electronic system and aggregates a set of design objects that all describe the same electronic design. It forms the root of an inheritance tree of refined interfaces on various levels of detail which in turn have implementations in different domains. Interfaces are arranged in an inheritance tree where interfaces at low levels of detail are situated near the root and refined interfaces at high levels of detail can be found near the leaves. No dedicated relationship is reserved for the representation of this inheritance hierarchy. Rather, we use equivalence relationships between the interfaces in which the more abstract design objects assume the role of sources and the more refined design objects assume the role of targets (Figure 12).

type Date: INTEGER.
type Designer: STRING.
type Name: STRING.
type DesignObject = Date, Designer, Name.
According to our definition in the Introduction, a design object describes an electronic design. However, the type DesignObject defined here does not have an attribute to store a design description. The capability to be associated with design representation information is not represented explicitly in our schema. It is instead hard-coded into the framework module that provides design information management services to design tools through its programming interface.

type Class: STRING.
type Tool: STRING.
type Equivalence = source_DesignObject, target_DesignObject,
     Class, Tool.
Equivalence relationships are very versatile. Here, they are used to arrange interfaces in an inheritance tree.(2) It is not trivial and generally depends on the nature of the design domain to check if two design objects are equivalent with respect to a certain property (or, phrased differently: if the design objects belong to the same Equivalence Class). This somewhat vague notion of equivalence makes us introduce explicit equivalence relationships and not equivalence sets as proposed in [Katz 86]. Equivalence relationships are thought to be established by tools and not automatically by the DIM service. Equivalence transitivity is therefore introduced on a case-by-case basis by establishing explicit equivalence relationships between design objects and is not answered in advance by some global property of equivalence sets. Our use of equivalence relationships follows their use in the Nelsis schema as motivated in [vanderWolf 93], p. 88.

type LevelOfDetail: 
     { `architecture', `algorithm', `functional_block', `logic', `circuit' }.
type Domain: { `functional', `structural', `physical' }.
type Module = Name.
type Interface = [DesignObject], LevelOfDetail, Module.
type Implementation = [DesignObject], Domain, Interface.
type Configuration = [DesignObject], Implementation.
Interfaces, implementations, and configurations are special design objects. As such, they inherit the capability to be associated with design representation information. They are arranged in a "static" (i.e., fixed by the schema) access hierarchy. Relatability ensures that whenever we want to create a configuration for a module, a corresponding implementation and interface have been defined first.

With this schema in mind, we regard a module as a set of design objects all describing the same electronic system from different view-points. The set of design objects is partitioned into three disjoint subsets according to the role the elements of each set play in the design description of an electronic system. Objects in the interface subset describe the appearance of an electronic system viewed from the outside. Each interface is related to a set of implementations that describe different approaches as to how this outside view is accomplished technically. An implementation itself may use components specified by another module. Which design description of either component exactly is used is described by design objects in the configuration subset. Again, there may be different alternative configurations for each implementation. This schema satisfies requirements R2, R3, and R5.

5.2.2 Projects

The initial schema introduced in the previous section does not yet meet the requirement that all design data must not be contained in a global pool. We introduce the concept of projects to structure the global object pool:

Definition:
A project is a local environment in which design activities may be performed. [vanderWolf 93], p. 71

Projects allow to structure the global object pool so that design activities can be performed locally without effecting other projects. A production design environment would have to go even further and allow projects to be hierarchically composed of other projects. Each level of hierarchy introduces an additional level of privacy in which projects near the root of the hierarchy represent published designs and completed design libraries. In such a scenario, a designer uses a local project as a workspace in which he performs experimental design steps. He can use locally created objects as well as ones imported from further up in the project hierarchy. Once a set of design objects is stable, it is checked into a parent workspace and thus made public for other designers.

We will not introduce a formal schema for projects as this concept is not directly related to the problem of design tool encapsulation. An implementation nevertheless will have to add the concept of projects. Both our implementation bases offer projects either directly (Nelsis, [Dimes 93a], p. 13) or in the form of a hierarchy of workspaces (ObjectStore, [ObjectDesign 94], p. 15).

5.2.3 Composition hierarchy

A useful means to reduce the complexity of an electronic design is to compose a module of smaller components, that in turn may again be composed of sub-components. Hierarchy relationships are established between a compound implementation and the components it uses. As each component may be instantiated more than once in an implementation, we need to keep track of these instances, too. Every instance is associated with a constructor that specifies the exact circumstances under which it is used in its parent.

Components do not reference the used design object directly but via component configurations. Component configurations bind a component to a child design object (either of interface, implementation, or configuration) for a particular purpose. A configuration collects component configurations and may be used as the representative of a design object for a specific purpose like "Release 2", "Fast-CPU", or "Simulate-ALU" (Figure 13).

type DesignObject = Date, Designer, Name.
type Interface = [DesignObject], Module, LevelOfDetail.
type Implementation = [DesignObject], Interface, Domain.
These are the same definitions for DesignObject and its subtypes as in the previous section.


Figure 13.
Composition hierarchy. The relationships between the subtypes of DesignObject are not shown.Interfaces, implementations, and configurations.
type Constructor: STRING.
type Component = parent_Implementation, Name.
type Instance = Constructor, Component, Name.
Components establish hierarchy relationships between a parent implementation and another design object. We explicitly distinguish between components and instances to accommodate different uses of the information that is manageable by this schema. Design tools generally need the exact instantiation information with details about each instance. On the other hand, the encapsulation service only needs to know which design objects are used in which implementation. This information is represented by components.

type Info: STRING.
type Configuration = [DesignObject], Implementation.
type ComponentConfiguration = 
     Component, child_DesignObject, Info, Configuration.
Component configurations establish the actual binding between a component and the design object that realizes it. This added level of indirection allows to manage a set of configurations as described above. With these definitions, we satisfy requirements R4 and R5.

Already more complex than a model for composition hierarchies without configurations, this model still does not allow to individually configure each occurrence of a sub-module in a composition hierarchy. If we wanted to model the configuration of individual occurrences as well, we would have to modify the type definitions as follows:

type Configurable = Name.
type ComponentConfiguration
     = Configurable, child_DesignObject,
     Info, Configuration.
type Component = [Configurable], parent_Implementation.
type Instance = [Configurable], Constructor, Component.
With this model, it is possible to individually configure each instance in an implementation. Although this amount of control may be desirable for purposes such as back-annotation, the introduction of additional components with different characteristics can still be applied when using the simpler model. In the sequel, we therefore choose the simpler model as depicted in Figure 13.

Figure 33 on page 176 depicts the composition hierarchy of the design description for the DP32 test bench circuit, including two configurations. The large diamonds in the figure represent configuration objects, the small diamonds represent component configurations. The links between configurations and component configurations are not shown. Rather, the two configurations and their respective component configurations are rendered in two different shades. As can be seen in this hierarchy graph, components never reference their child design objects directly but do so via component configurations.

A single configuration only references component configurations that configure components of a single parent implementation. Formally, the following assertion holds for all component configurations:

assert ComponentConfiguration its
  Configuration its Implementation
   == Component its Implementation
A single configuration may nevertheless configure components more than one level down the composition hierarchy. This is achieved by having component configurations specify a configuration, not an implementation or interface as child design object.

Consider, for example, the following excerpt from the configuration declaration for the DP32 test bench:

configuration dp32_rtl_test of dp32_test is
  for structure
    for cg: clock_gen
      use entity work.clock_gen (behaviour)
      generic map (Tpw => 8ns; Tps => 2ns);
    end for;
    for mem: memory
      use entity work.memory (behaviour);
    end for;
    for proc: dp32
      use entity work.dp32 (rtl);
      for rtl
        for all: reg_file_32_rrw
          use entity work.reg_file_32_rrw (behaviour);
        end for;
        for all: mux2
          use entity work.mux2 (behaviour);
        end for;
        -- more component configurations
      end for;
    end for;
   end for;
end dp32_rtl_test;
First, we note that in the configuration of dp32_test.structure the instances are configured individually. With our simplified model of component configurations we cannot in principle represent this fine "configuration granularity". As, however, each component is only instantiated once, we can safely replace the instance labels (cg, mem, proc) with all qualifiers without losing information. In more complex cases, the introduction of additional components would have been necessary.

In this example, a configuration for the register-transfer level implementation of the DP32 microprocessor is embedded in the configuration for the test bench circuit. Within this nested configuration, components are configured, this time directly using all qualifiers. The nesting of configurations resembles the logical nesting of sub-modules. Without losing information, we can unwind ("flatten") the configuration nesting by introducing explicit configuration declarations which are then referenced as children by the component configurations:

configuration dp32_rtl_test of dp32_test is
  for structure
    for all: clock_gen
      use entity work.clock_gen (behaviour)
        generic map (Tpw => 8ns; Tps => 2ns);
    end for;
    for all: memory
      use entity work.memory (behaviour);
    end for;
    for all: dp32
      use configuration work.dp32_rtl;
    end for;
   end for;
end dp32_rtl_test;

configuration dp32_rtl of dp32 is
  for rtl
    for all: reg_file_32_rrw
      use entity work.reg_file_32_rrw (behaviour);
    end for;
    for all: mux2
      use entity work.mux2 (behaviour);
    end for;
    -- more component configurations
  end for;
end dp32_rtl;
We can directly map this flattened configuration into an object graph according to our conceptual schema (Figure 14).


Figure 14.
Object graph for the dp32_rtl_test configuration. Interface and info objects are left out for simplicity.

Although configuration dp32_rtl_test was our starting point, the configuration dp32_rtl originally nested within dp32_rtl_test now is a first class object of type Configuration and can be referenced from component configurations other than those contained in dp32_rtl_test. While simplifying the model, flattening nested configurations therefore simplifies design reuse.

5.2.4 Version derivation and design transactions


Figure 15.
Version derivation, design transactions, and equivalence.

The relationships introduced in this schema are crucial to design management. However, we do not have to develop something new here as e.g. the schema of the Nelsis CAD Framework fits our needs well. We therefore take the following type definitions directly from the Nelsis schema [vanderWolf 93], only slightly extending the definition of DesignObject.

type Name: STRING.
type Date: INTEGER.
type Designer: STRING.
type Language: STRING.
type VersionNumber: INTEGER.
type VersionState: { `backup', `derived', `working', `actual' }.
type DesignObject =
     Date, Designer, Name, VersionNumber, VersionState.
type VersionDerivation = original_DesignObject, derived_DesignObject.
type CompletionMode = { `running', completed', `aborted' }.
type Tool: STRING.
type Transaction = Tool, CompletionMode.
type Module = Name, LastVersion.
The definition of attributes for version number and version state emphasizes the central role DesignObject plays in our schema. Design objects are the only entities that can be versioned. This property, of course, is inherited by its subtypes Interface, Implementation, and Configuration.

version derivation adds a further structuring mechanism to the set of design objects by allowing to relate a design object with a set of derived design objects and a derived design object with one or more originals. Note that in the case of more than one original a merge must have occurred to weed out potential incompatibilities introduced on the two branches. Design objects are versioned relative to a module. To uniquely identify objects within the module (other than by their object identifier), they are automatically assigned a version number on creation. In addition, a design object is assigned a version status for version selection in cases in which configurations are not used.

Design objects are involved in potentially long design transactions. Information about completed transactions and associated design objects is not simply deleted on transaction commit but is stored to render a complete design history.

5.2.5 Contexts

Encapsulated design tools access design information through design files. The design information manager, however, manages design objects. A mapping between design files and design objects is therefore necessary. To support this mapping, the structural information is enhanced with "contexts", objects that reflect the nesting of design objects in design files.

Each design object is associated with a textual design representation in a selected design description language. In addition, it is also associated with the lexical context in the design file from which it originates and the offset in this context. In more complex situations, the design object itself may again play the role of a lexical context for some sub-ordinate design object (Figure 16). Figure 17 gives a schematic example; in Figure 18, lexical contexts are applied to a piece of VHDL description.


Figure 16.
Design objects and lexical contexts.

Figure 17.
Nested contexts. The sequentialized, textual representation of nested contexts is mapped to a tree in the DIM manager where a single context may embrace any number of design objects, nested to arbitrary depth.

Figure 18.
Using lexical contexts to represent parts of the behavioural architecture specification of the DP32 processor. The shaded areas are textual design representation, with the insertion points of children marked with a (>) sign. The two boxes denote the extents of the types LexicalContext and Implementation. Note that Implementation is-a DesignObject is-a LexicalContext, so every Implementation object is also a LexicalContext.

In more complex languages or if the design methodology dictates a more fine-grained management of design objects, a refinement of lexical contexts is called for. Lexical contexts only allow to manage the lexical structure of a hierarchy of contexts, basically by storing offsets into the context of a given design object. If the context itself is a DesignObject, it may be modified as such, invalidating the offsets of design objects lexically nested within it. An obvious solution to this problem is not to store a numeric offset with the child but rather to insert a special marker into the parent. This marker may be moved around freely, and, as long as it is not deleted, may be replaced by the text of the child on export. A disadvantage of this approach is that the parent context must be searched for markers to insert the text of children.

A further refinement is to actually retain the syntactical structure of the parent with respect to its children. An extension of the schema shown in Figure 16 accomplishes this (Figure 19):


Figure 19.
Design objects and generalized contexts.
type LexicalContext = Language, Date, Designer, Name.
type DesignObject = [LexicalContext], LexicalContext, Offset.
type ParseTreeConstructor: STRING.
type SyntacticalDesignObject = [DesignObject], ParseTreeConstructor.
This schema answers two questions. (1) How are design objects embedded in their lexical context, and (2) how does the design object lexically or syntactically contain other design objects? Every design object is embedded in a lexical context. This results from the fact that we extract design objects from design files. There, the file (stripped of the actual representation of the design object) is the lexical context. There are two ways in which the design representation of textually nested design objects can be managed by the design information manager. In the simpler approach, a design object itself plays the role of lexical context for a set of embedded design objects as described above. In more complex scenarios, not the lexical but the syntactical structure of a design object is managed by the design information manager. A specialized form of DesignObject carries a parse tree constructor. A parse tree constructor is a piece of Tcl code that describes how to construct a parse tree from the values of a design object's attributes and those of related objects. In Section 6.5 on page 114 we will explain in detail what a parse tree constructor looks like and how it can be constructed largely automatically from a language specification.

5.3 The design information programming interface

5.3.1 Overview

The programming interface for our design information management component has to support the following tasks:

  1. browse the design structure
  2. find a design object based on its name
  3. access the design representation associated with a design object
  4. access design hierarchy by using configurations
While tasks in (1) and (2) only imply a short read or write access to design structure, tasks in (3) and (4) usually involve an interactive editing session or a lengthy design tool run. According to this distinction, different kinds of operations have to be employed for these tasks:

In addition, tasks in (4) allow to access a single design hierarchy "filtered" by a configuration; components encountered in a design hierarchy are bound to the design objects determined by the configuration. For hierarchy access special operations are provided that perform this filtering.

5.3.2 Accessing design structure

The operations to access design structure can be directly derived from our conceptual schema. A principal design decision is whether to provide a procedural interface or a query language. The Xplain semantic data modelling technique defines a query language that could be used directly as an access method to a design information management service in a framework. A query language is a good choice as an interface paradigm when interactive access is desired. It is, however, more difficult to implement, especially when portability across various design data managers is an issue. It also does not blend naturally into the code of a design tool written in "C" of "C++". We therefore choose to define a procedural interface to our design information management service in the programming language "C++". A query interface can still be constructed on top of it.

Objects of aggregate types are accessed through object references. Such a reference may be the actual memory address of an object or a database identifier. Overloading in "C++" allows to handle both cases in a uniform and transparent way. Object references are only valid in the scope of a project. Before any other operation is invoked, a project handle has to be obtained by invoking the static member function

static dim_Project* dim_Project::open (string name, dim_OpenMode openMode);
OpenMode can either be read or update. For a given project, only a single project handle in update mode may be requested at any one time. Of course, multiple read handles may be requested. A project handle is released by a call to

void dim_Project::close (dim_CloseMode);
CloseMode may either be commit to request that all changes are to be made permanent or abort to discard all changes made with this project handle. The interface is schematically derived from our conceptual schema. Base types are mapped to the corresponding base types in "C++":


Table 9.
Mapping of Xplain base types to C++ types
Xplain base typemapped to C++ type
INTEGERint
REALdouble
STRINGchar*

For every Xplain definition

type BaseType: base-type.
there is a corresponding type definition in "C++":

typedef base-type BaseType;
The definition of an aggregate

type Aggregate = role_Component.
type Component = ... .
is mapped to two class definitions in "C++":

struct Aggregate {
  Component* role_component();
  void role_component (Component*);
};
struct Component {
  Set<Aggregate*> aggregates();
  ...
};
Please note that for each component there are two access functions, one to read a value and one to write a value. We do not map components directly to data members to have more control over the underlying implementation. For all practical purposes, a pair of access functions is equivalent to a data member. Access functions, however, are free to retrieve the actual data values from an underlying data manager or cache data values for greater efficiency.

Access functions automatically maintain consistency. In the above example, whenever a component is set for an aggregate, the aggregate is automatically inserted into the aggregate set in the component.

Access from a component to its aggregate is accomplished by set functions. The usual bunch of functions is provided to iterate through the elements in the set and to insert and delete members from the set. Only one function is needed to access the actual set.

A specialization in Xplain is mapped to class inheritance in "C++". For example, the following definition of a tree structure in Xplain

type Node = Value.
type InnerNode = [Node], parent_Node.
is mapped to the following "C++" classes:

struct InnerNode: public Node {
  Node* parent_node();
  void parent_node (Node*);
};
struct Node {
  Set<InnerNode*> inv_parent_nodes();
};
With these mappings it is possible to access all the objects managed by the design information manager. The only thing missing is a way to retrieve an initial object reference. This can be accomplished by accessing an object by its name with the following member functions of dim_Project:

Set<Module*> getModule (string moduleName);
Set<Interface*> getInterface (
  string moduleName, string interfaceName, string levelOfDetail);
Set<Implementation*> getImplementation (
  string moduleName, string interfaceName, 
  string implementationName, string domain);
Set<Configuration*> getConfiguration (
  string moduleName, string interfaceName, 
  string implementationName, string configurationName);
These functions return sets of design object references, because object names need not be unique within a project. The sets can be traversed to find the desired object.

5.3.3 Accessing design representation

As mentioned above, access to design representation is of a different nature. The operations to access design representation need to be associated with a transaction that provides protection against concurrent write accesses and preserves consistency by maintaining duplicates for read accesses while a write transaction is in progress. The following functions are provided to manipulate design objects in the context of a design transaction. All functions work on design object handles, objects of class dim_DesignObject and its sub-classes dim_Interface, dim_Implementation, and dim_Configuration. The following member functions of dim_Project are used to create design objects. The attributes Designer, Date, VersionNumber, VersionState, Language, LexicalContext, and Offset of Design Object are filled in automatically either with context information or with default values by the function implementation:

dim_Interface* createInterface (
  string moduleName, string interfaceName,
  string language, string levelOfDetail);
dim_Implementation* createImplementation (
  string moduleName, string interfaceName, string implementationName, 
  string language, string domain);
dim_Configuration* createConfiguration (
  string moduleName, string interfaceName, 
  string implementationName, string configName);
If the named object does not yet exist, a new design object is created. If an object of this name already exists, a new version of it is created with version status working. In addition, lexical contexts are created with the function

dim_LexicalContext* createContext (
  string contextName, string language);
Normal contexts are not manipulated in an interactive design transaction. Rather, they are dynamically constructed from parse tree templates at the time of export (cf. Chapter 6 on page 103).

The following member functions of dim_Project open existing design objects:

dim_Interface* dim_Project::openInterface (Interface*, dim_OpenMode);
dim_Implementation* dim_Project::openImplementation (
  Implementation*, dim_OpenMode);
dim_Configuration* dim_Project::openConfiguration (
  Configuration*, dim_OpenMode);
These functions yield a handle for the corresponding object type, opened for read or update. After manipulations on any design object, the changes made have to be committed with the function

void dim_DesignObject::close (dim_CloseMode closeMode);
This call creates a new version that is derived from the original. The design object reference for a design object handle can be accessed by the function

DesignObject* dim_DesignObject::id();
The actual design data, in turn, are accessed through a stream-based interface:

dim_Stream* dim_DesignObject::openStream (
  string name, dim_OpenMode);
dim_Stream::read (int count);
dim_Stream::write (string buffer, int count);
void dimStream::close (dim_CloseMode);
Once a stream is opened, operations can be applied to get its size and to read and write its contents. To conclude, we have the following hierarchy of calls that reflects the transaction hierarchy needed to access the actual design data:

dim_Project::open();
    dim_Project::openDesignObject();
      dim_DesignObject::openStream();
        // stream access
      dim_Stream::close();
    dim_DesignObject::close();
dim_Project::close();

5.3.4 Hierarchy and configurations

A configuration collects all the component configurations for its implementation. The function

Set<dim_InstanceInfo*> dim_Configuration::getInstances
  (dim_BindingMode bindingMode);
returns a set of instance information structures. These are an abstraction of the instance type directly derived from the conceptual schema, conveniently hiding the component configurations from the programmer:

struct dim_InstanceInfo {
  string name;
  string constructor;
  DesignObject* child;
};
With binding mode static, for each instance, this structure references the child design object currently bound by the configuration. With binding mode dynamic, for each instance, an implementation object is selected:

To define component configurations in an opened configuration, the function

void dim_Configuration::setInstances (Set<dim_InstanceInfo*>)
is used. Here, the set of instance information structures has to be constructed first and is passed to the function as a parameter.

Whereas the operations presented in Section 5.3.2 represent a low-level interface, the operations of Sections 5.3.3 and 5.3.4 work on lower levels of detail. Whenever a suitable, more abstract function is available to accomplish a certain task in design information management, it should be used for maximum protection. All three levels are open to the programmer, however, to provide a maximum in openness and flexibility. We will add a further, even more abstract level in Chapter 6 on page 103 where we describe the processing of design files in the encapsulation service.


Figure 20.
Three approaches to realize a design information management system.

5.4 Case studies: Implementing design information management

5.4.1 Approach

The conceptual schema presented in this chapter provides only the logical organization of a design information management system. It relies on a design data manager (DDM) that implements the necessary database functionality. There are basically three choices to build such a design data manager:

  1. Implementing one from scratch, based on a low level storage mechanism like the UNIX file system or rudimentary database facilities offered by the operating system, like the DBM library. The advantage of this approach is the flexibility in choosing an appropriate and efficient implementation; disadvantages are high efforts for coding and maintenance, and the difficulty to share design structure information with existing DIM systems (Figure 20-A).
  2. Using a general purpose, domain neutral database management system (DBMS). This approach is attractive because a robust, powerful, and fast database management system can be chosen. The complete freedom to choose an appropriate mapping between our conceptual schema and the modelling primitives offered by the DBMS makes an efficient implementation possible. Depending on the models supported by the database system, our conceptual schema may be mappable to the database with little or no effort (Figure 20-B). Of course, the capabilities of the DBMS must match our application domain:

    These capabilities can be found to varying degrees in database management systems based on each of the common data models. Object-oriented systems have the richest data model in terms of modelling power. As they are tailored towards engineering applications, many of them already support versioning as part of their data model. Support for large objects no longer is a privilege of object-oriented systems. Many relational database management systems today provide a similar feature as an extension to the classical set of column types. Navigational access is inherently weak in the relational model, whereas it is the standard access method in hierarchical and network models, which, on the other hand, have weak query facilities. Finally, concurrency control is available in all database models, but only the rich type system of the object-oriented model provides the necessary flexibility in access granularity. With all this, an object-oriented database system seems to have the best combination of features for our purposes.

  3. Using an existing DDM system, maybe as part of a general purpose CAD framework. This approach is attractive in that it promises the least coding effort when the schema of the DDM system is close to our schema yet open to necessary extensions. If the mapping is done carefully there is even a high probability that we will be able to integrate design management data native to the DDM system with data manipulated according with our conceptual schema (Figure 20-C).
With the availability of both efficient object-oriented databases and powerful CAD frameworks alternative (1) is clearly not to be favoured. Both alternatives (2) and (3) have their specific merit and we will describe an implementation of our DIM PI using either approach in the following sections.

5.4.2 DIM on the ObjectStore OODBMS

ObjectStore [ObjectDesign 94] is an object-oriented database management system that allows to make "C++" objects persistent in a nearly transparent way. This is achieved by exploiting the virtual memory mapping capabilities of modern operating systems. Whenever a program tries to access the virtual memory address of an object not mapped into core memory, the memory management hardware generates a page fault. The memory page containing the memory address is loaded into the page buffer maintained by the operating system and control is returned to the program that caused the page fault, just before the malicious instruction. The instruction is executed again and now finds the addressed object in place. Memory mapping architectures of (object-oriented) databases intercept page fault signals and retrieve memory pages from a background database. On client/server architectures, this memory page may even reside in a remote machine. Persistence in ObjectStore is not a feature of a certain class, but is associated with individual objects. An object is created in a particular database or cluster within a database by passing a database or cluster handle to an overloaded version of the new operator.

Besides normal "C++", the programmer needs only a handful of additional procedures to

Other than that, pointers into virtual memory are used as object references. ObjectStore provides many other features that make it an ideal basis for an engineering database, e.g.:

Due to the support for persistent "C++" objects, the DIM PI can be mapped directly to persistent "C++" classes (Figure 21). Set-valued attributes are implemented through ObjectStore's collection classes. Consistency is automatically maintained by ObjectStore by the declaration of inverse members so each time an object reference is set as the value of a data member of an object, ObjectStore ensures that this object gets inserted into the corresponding set-valued attribute in the referenced object.


Figure 21.
Using ObjectStore's SchemaDesigner to map the DIM schema to "C++" classes. Role names are sometimes obscured due to deficiencies in the graphical user interface of the tool.
Accessing design structure
In our implementation on ObjectStore, each project is stored in a separate database. A project database contains a global workspace where shared objects are deposited. A call to dim_Project::open creates a new workspace as child of this global workspace and starts a transaction. The workspace is associated with the project handle returned by dim_Project::open and is used for check-in and check-out of design objects. The transaction guarantees that all changes can be rolled back when the project is closed with mode DIM_ABORT or when the application program crashes.

The classes DesignObject and Module are defined as sub-class of the system-defined class os_configuration. A configuration in ObjectStore defines a set of objects that evolve together. We use configurations together with hierarchical workspaces to implement automatic versioning. When an os_configuration is checked out from the global workspace into the project workspace associated with a project handle, a new version of it is created in the project workspace that is only visible there. Only on check-in this version is frozen and made visible to other users of the same project. Access by name through the functions getModule, getInterface, getImplementation, getConfiguration is supported by maintaining the class extents of the respective classes as database roots.

Accessing design representation
Transactions on design objects are implemented by check-in and check-out of design objects into the private workspace associated with an open project. As long as design objects are manipulated in this workspace, they are not visible from the global workspace or by other of its children. The exact effect of invoking either of the functions openInterface, openImplementation, or openConfiguration depends on the type of design object passed as first parameter as described in Section 5.3.3.


Figure 22.
Mapping to Nelsis. The shaded boxes denote the added types. Type is used to store the actual type of a design object (interface, implementation, configuration, or context). DoName on DesignObject stores the name of a design object. Info stores miscellaneous information items about design objects.

5.4.3 DIM on the Nelsis CAD Framework

Implementing design information management on the Nelsis CAD Framework [Dimes 93b] is not as straightforward as on ObjectStore. This was to be expected, however, because the individual object types in our conceptual schema have to be carefully mapped to suitable types in the Nelsis schema so that as much Nelsis functionality as possible can be used. The reward of a successful implementation is that the design tools and framework tools already integrated with Nelsis can be used, making it easier to build a complete design environment. Where no suitable concept exists in Nelsis (e.g. component configurations), the implementation has to emulate these types by

The fundamental object type in the Nelsis schema is the DesignObject. The framework manages design representation at the granularity of this object type. A design object represents an element in a single-viewtype-version-set, called Module. A module describes a specific aspect of an (electronic) system, enumerating different ways to achieve this aspect. It is uniquely defined by the pair (name, view type).

Every design object is uniquely defined by its version number and its module. Additional attributes of design objects hold its version status (either backup, actual, working, or derived), and its modification date. A working version represents work-in-progress and is overridden when a new version is checked in. Only one working version may exist in a module at any one time. When the designer is confident about the maturity of the working version, she can freeze and publish it by changing its version status to actual. Nelsis assumes an actual version to be frozen and published. This version is used automatically for constructing composition hierarchies if no other version is explicitly selected. It is protected against overriding by the system. When a new working version is designated as actual, the old actual version becomes a backup. Backup versions are retained to maintain a complete design history. In addition, versions may be deliberately designated as being derived from another design object to protect them against overwrite. There may be many backup and derived versions in a module.

Design objects may be related by the many-to-many relationships Hierarchy and Equivalence. Transactions record design transactions performed on a particular design object and thus allows to retrieve its history. Equivalences are used for many purposes, one not so obvious being a version derivation relationship. Other uses include relating design objects from different view types. Here it is anticipated that the actual validation of the equivalence is performed by some tool such as a design rule checker, a circuit extractor, or a synthesis tool. We only describe the core of the complete Nelsis schema as far as it directly relates to our implementation of design information management. The production schema contains many more types for trans-project references and design flow management. A more thorough discussion of all the features of the Nelsis schema can be found in [vanderWolf 93].

Accessing design structure
We now describe our mapping from DIM types to types in the Nelsis schema. The added types are shaded in Figure 22. The changes have been kept to a minimum, trying to stick closely to the notions inherent to Nelsis design management. The central problem in mapping the conceptual design information management schema to Nelsis is how to map the specializations of DesignObject. In Nelsis a design object is the only entity that is actually associated with design data.

The DIM type Module is mapped to a set of Nelsis Modules that all have the same name, but different values of the ViewType attribute. The function getModule

Set<Module*> getModule (string moduleName);
is implemented with the query

get Module where Name == `$moduleName'(3)
The DIM type Interface is mapped to the Nelsis type Module. As a module in Nelsis cannot be associated with design data directly, a single design object in each module is designated as the container for the interface description.

The function getInterface

Set<Interface*> getInterface (
  string moduleName, string interfaceName, string levelOfDetail);
is implemented with the query

get DesignObject where 
  Module its Name == `$moduleName' and 
  Name == `$interfaceName' and 
  Type == `interface'
The DIM type Implementation is mapped to the Nelsis type DesignObject. The function getImplementation

Set<Implementation*> getImplementation (
  string moduleName, string interfaceName, 
  string implementationName);
is implemented by retrieving an interface design object with the function getInterface, storing its module as module, and then issuing the query

get DesignObject where 
  Module == `$module' and 
  Name == `$implementationName' and 
  Type == `implementation'
The DIM type Configuration is also mapped to the Nelsis type DesignObject. The function getConfiguration

Set<Configuration*> getConfiguration (
  string moduleName, string interfaceName, 
  string implementationName, string configurationName);
is implemented similar to getImplementation by retrieving an interface design object with the function getInterface, storing its module as module, and then issuing the query

get DesignObject where 
  Module == `$module' and 
  Name == `$implementationName' and 
  Info == `$configurationName' and 
  Type == `configuration'
The DIM type LexicalContext has to be modelled as Nelsis type DesignObject because it is associated with design data. For each lexical context a new module and in it a single design object is created. The module has view type `$language-context'. Syntactical contexts are not mapped to Nelsis. Instead, they are dynamically constructed on export from a parse tree template and attribute values of the participating design objects.

The other types of the DIM schema are mapped as follows. Instances are not represented at all. For the export of design objects as files it is irrelevant how many and which exact instances are used by a particular implementation as long as the child design object to be exported is known. ComponentConfigurations are identified with the Hierarchy objects a design object that implements a configuration is involved in. VersionDerivation is handled automatically by Nelsis and is mapped to Nelsis Equivalence relationships. Transaction is also handled automatically.

Accessing design representation
The function dim_Project::createInterface

dim_Interface* createInterface (
  string moduleName, string interfaceName,
  string language, string levelOfDetail);
first tries to create a new module using the query

insert Module its 
  Name = `$moduleName',
  Designer = `$env(USER)',
  ViewType = `$language-$levelOfDetail'
The variable language here denotes the hardware description language used for the interface. The variable env is a global associative array in Tcl that gives access to environment variables. An error is signalled when such a module already exists. Then, a design object do in this module is opened for update. As Nelsis does not know about the additional attributes on DesignObject in the Nelsis schema, they have to be set manually:

update DesignObject `$do' its
  Name = `$interfaceName',
  Info = `$language',
  Type = `interface'
Note that with this mapping approach, interfaces cannot be versioned. Versioning on interfaces has to be simulated by creating them in separate DIM modules. The function dim_Project::createImplementation

dim_Implementation* createImplementation (
  string moduleName, string interfaceName, string implementationName,
  string language, string domain);
inserts a new implementation design object into the module that contains the associated interface object. Versioning is done automatically by Nelsis. The additional attributes again have to be updated manually:

update DesignObject `$do' its
  Name = `$implementationName',
  Info = `$language',
  Type = `implementation'
An implementation design object also serves as its own first configuration. This accounts for the fact that Nelsis does not support explicit configurations. The function dim_Project::createConfiguration

dim_Configuration* createConfiguration (
  string moduleName, string interfaceName, 
  string implementationName, string configName);
works similar to dim_Project::createImplementation. The additional attributes are updated as follows:

update DesignObject `$do' its
  Name = `$implementationName',
  Info = `configurationName',
  Type = `configuration'
The function dim_Configuration::getInstances

Set<dim_InstanceInfo*> dim_Configuration::getInstances 
  (dim_BindingMode bindingMode);
with binding mode dynamic simply issues the query

get Hierarchy its Name, Constructor, Son-DesignObject, 
  Son-DesignObject its Type
to create a set of instance information structures. For binding mode static, the function starts off with the same query. According to the type of the child design object, the appropriate implementation design objects have to be selected as described in Section 5.3.

Discussion
The approach to a mapping between the DIM conceptual schema and PI to the Nelsis schema and associated queries described above is certainly not the only viable solution. Another way to map interfaces and implementations to types in Nelsis is to combine an implementation with its associated interface and represent this pair as a DesignObject. This approach ensures that interface and implementation evolve together and can be kept consistent. Configurations are not represented explicitly in this approach, but are emulated by the install operation provided by Nelsis and a special flag BindingMode on Hierarchy. When a dynamic hierarchy relationship needs to be established, a design object with empty implementation part is created and used as child design object in the hierarchy. In addition, the hierarchy's binding mode is set to dynamic.

Whenever a design hierarchy is used by a tool that needs a static binding (e.g. a simulator), hierarchies with binding mode dynamic are treated separately. The install operation is used to replace the child design object with empty implementation part with a design object from the same module, i.e. one with a "compatible", possibly refined interface. The actual version is chosen if one exists, otherwise the design object with the highest version number will be used.

Using this approach, a module in Nelsis does not represent a single interface but rather a set of implementations with similar interfaces (Figure 23). Implementations with incompatible interfaces are collected in separate modules, related to each other by equivalence relationships. While this approach is quite valid on its own, it does not fit equally well into the Nelsis philosophy of identifying a module by its name and view type. Many dissimilar design objects are collected within a single module. It is also not possible to support an actual version for each interface as Nelsis only allows one actual design object per module.


Figure 23.
An alternative mapping solution: Implementations with similar interfaces are represented by a module. The box at the root of the tree has no implementation and is the target for dynamic hierarchy relationships. Arrows denote version derivation. The figures are consecutive version numbers.

5.5 Related work

The work presented in this chapter originated from the need to manage structural design information and its attached design representation according to the requirements stated in Section 1.6 on page 15. In addition, the anticipated solution had to be formally defined by an information model and should be both flexible and natural to the designer. It should deviate as little as possible from existing work in the area of design data management using CAD frameworks. To our knowledge, no system fulfilling all these requirements has been described in the literature.

A configuration management approach more powerful than the one found in e.g. the Nelsis CAD Framework was called for by the need to represent VHDL configurations in the DIM service. Also, Nelsis does not distinguish between interface and implementation, a prerequisite for top-down design.

In [CFI-FAR 93], CFI outlines the concept of references that are bound by configurations. Although this document does not describe an existing, working system, the concepts described form the basis for existing frameworks such as the JESSI Common Framework. We have transcribed the intuitive drawings from [CFI-FAR 93], "Data Management Information Model" and "Version Information Model", into the Xplain notation to be able to compare it to our conceptual schema (Figure 24). This transcription can only serve as a coarse approximation of the original intent of the authors of the CFI document. Although no cardinalities were attached to relationships in the informal CFI schema, the appearance of an Xplain schema is largely determined by the different cardinalities so cardinalities had to be guessed from the textual description. Also, type Binding was only described textually.


Figure 24.
Transcription of CFI's "Data Management" and "Version" information models into Xplain.

The first observation is that this schema does not distinguish interface and implementation of a design object from each other. This may be due to the fact that more domain specific schemas are intended to augment this schema with domain specific specializations of the types defined here. The overall structure of the types Configuration/Binding/Reference seems to resemble our types Configuration/ComponentConfiguration/Component as depicted in Figure 13 on page 72. However, CFI does not restrict their retargetable Reference to composition hierarchies. The binding policy we have implemented globally in the programming interface (see Section 5.3.4 on page 87) is bound to the type ConfigurationTemplate in the CFI schema so that in principle different binding strategies can be used. We have no information as to whether such a system has been implemented yet.


Footnotes

(1)
Bracketed marks R1..R7 refer to this list of requirements
(2)
A potential danger of this flexibility is that possibly object properties are not properly represented in the schema but rather are mapped to obscure equivalence relationships, making it hard for the designer to understand the disposition of his design.
(3)
The function parameters in this and the following queries denote variable references using a `$' imposed by using the Tcl language as extension language in our implementation.

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