ed from Pascal and has some features in common new one. Ada was the result. (Ada is a trademark of the US with Ada and some that are different. Department of Defence, Ada Joint Program Office.) CHILL has three mechanisms for communicating between pro- Ada incorporates almost all the features of Pascal, and much more cesses: besides: 1. Regions, which allow mutually exclusive access to informa- Information hiding tion. 2. Buffers, in which information can be left by one process and Ada introduced the notion of a package which was similar to that of retrieved by another. the module in the language Modula, an earlier descendant of Pascal. 3. Signals, which can be directly transmitted from one process to The package has two parts, its definition and its body. Only those another without buffers. entities defined in the definition part of the package can be made available for use in other packages. Those defined in the package Although CHILL is fairly well used in telecommunications, the body are local. This means that it is possible through the package total user population is not enough to attract tool makers from definition to provide details on how to use the package without companies not in the communications business. The tool support disclosing the implementation details. Information is hidden except therefore tends to be proprietary, and often limited. Other, more where it is explicitly made available. widely supported, languages are beginning to be used. 25/14 Software systems in telecommunications 25.6.2.4 specifications or designs becomes more formal, it will be possible to generate more formal tools. The language C was originally developed as the systems programm- ing language for the Unix operating system (Ritchie, 1978) but has been adopted widely as a programming language for software 25.7.1.3 Static analysis running not only on Unix but also on other operating systems. In the same way that new languages sprang from the Pascal route, C has Static analysers carry out a more detailed automated examination 