A language, BLISS, is described. This language is designed so as to be especially suitable for use in writing production software systems for a speci c machine (the PDP-10): compilers, operating systems, etc. Prime design goals of the design are the ability t o p roduce highly e cient object code, to allow access to all relevant hardware features of the host machine, and to provide a rational means by which to cope with the evolutionary nature of systems programs. A major feature which contributes to the realization of these goals is a mechanism permitting the de nition of the representation of all data structures in terms of the access algorithm for elements of the structure.
IntroductionWhile the c.ontents of databases can be easily changed, their organization is typically extremely rigid. Some databases relax the rigidity of database organization somewhat by supporting simpIe changes to individual schemas. As described in this paper, OTGen supports not only more ccmmplex schema changes, but also database reorganization.A database administrator uses a declarative notation to describe mappings between objects created with old versions of schemas and their corresponding representations using new versions. OTGen genemtes a transformer that applies the mappings to update tb: database to the new definitions, thus facilitating improvements in performance, functionality, and usability of the database.l Separation of interfaces from implementation is by now a generally accepted technique for localizing the effect of change. It gives the implementor the freedom of improving the code without affecting the client, while the latter can continue to use the specified interfaces and apply the provided operations as before. Only when client and implementor agree on a change in the specifications will it be necessary to modify the client's application.However, changes in either specification or implementation not only affect programs, but also the representation of persistent data objects. The problem of modifying software is not limited to changing code, but also affects existing data objects that were generated with that software prior to modification. It is thus necessary to address the problem of how to adapt existing objects to the new requirements imposed by the modified software.
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