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for railways equipments. This approach is fully supported by an off the shelf tool, which already offers various
code and documentation generators, as well as reverse engineering features. Finally, this approach is closely
connected to the COTRE project, sponsored by the French department of research (RNTL), and which should
provide enhanced real-time software verification techniques for this solution.
using the AADL for mission critical software development page 2
2 A summary of the AADL
The following definitions have been extracted from the current draft of the Avionics Architecture Description
Language definition (AADL v 0.95). The reader is invited to refer to the final definition of the standard, when it
will be released.
The AADL standard provides semantical and syntactical definitions to formally describe a real-time architecture
in terms of interacting specialized components. An AADL specification of a system consists of a set of packages
declaring a list of abstract components (components types and components implementations) and a static
structure of component instanciations to describe the executable application and its allocation to the execution
platform.
The abstract declaration of an AADL component in a package consists of one component type, and one or
several component implementations. A component type describe the visible functional interface of the
component, including a provided interface (list of declarations of public ports and subprograms), and a required
interface (list of references to remote component provided interfaces). A component implementation contains all
the additional details to fully define the architectural structure of the component, including a list of
subcomponents and connections between these subcomponents and modes to describe the various operating
states of the system. Several implementations may be defined for a same component type. All the
implementations must strictly comply with the corresponding type declaration.
In order to provide an advanced support for real-time modeling, the AADL standard offers a set of predefined
components categories which semantics is formally specified. These categories have been grouped into three
sets:
• Data, threads and processes are the software components categories.
• Processors, memories, bus and devices are the execution platform categories.
• Systems represent composite sets of software and execution platform components.
Each component category can contain a controled set of features. Features represent connectors in the provided
interface of the component, through which control flows and dataflows will be propagated. The AADL specifies
three kinds of connectors:
• Ports are point to point connectors for individual data or events.
• Subprograms are composed sets of a control flow and dataflows (subprogram parameters).
• Subcomponent access represent access to remote data or bus components.
Finally, components and features descriptions are completed by a set of predefined specialized properties,
providing all the necessary lower level additional information. Properties can be used to specify various kinds of
data, such as links to source files or real time attributes. Properties are grouped into property sets, which provide
a powerful extension mecanism for the language.
Due to these rigorous and well focused definitions, the AADL is the appropriate language to describe real time
or other mission critical system and software architectures. However, in order to bring a maximal benefit to the
projects, the use of this language must be considered within the overall development lifecycle. In particular, the
issues of specifying a precise software design process, providing standardized graphical notations and advanced
verification techniques, must also be addressed. Our proposed solution to meet these requirements, is to use the
proven HOOD design process and the new UML2.0 graphical notations, to support the AADL modeling
activities. The issue of verifying AADL model is being directly addressed by the COTRE project which is also
presented during this conference. Please refer to the corresponding paper to know more about the COTRE
project.
using the AADL for mission critical software development page 3
3 The HOOD method
The HOOD method first appeared in 1987 to meet the requirements of the European Space Agency (ESA). The
early versions already included a set of notations and precise design rules to support advanced software
engineering concepts and Ada code generation rules. In 1992, the version 3.1 of the HOOD Reference Manual
(HRM) was published. In 1995, two concurrent major releases of the method were issued : HOOD4, conducted
by the French Space Agency (CNES) improved widely the support of true Object Orientation to enable Ada95
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