Prior to the development of the VISTA data level some design constraints had to be taken into consideration. First of all, the principle of utmost portability had to be obeyed. This required to restrict the implementation to reliable standardized languages (ANSI C in this case [Kern88]), which are available on all platforms that are intended to be supported. Although it may have been favorable from a merely technical point of view to have advanced languages thus as C++ [Elli90] available for the implementation, or certain platform-specific extensions of a programming language (e.g. STRUCTs in DEC FORTRAN), this would have violated the overall concept of portability.
Second, the use of system functions has to be restricted to a minimal subset available on all supported operating systems, and if operating-system dependencies occur, these have to be isolated in a separate operating system interface, thus effectively creating a virtual operating system, which is interfaced to through a separate functional layer. This led to the development of the VOS (Vista Operating System), briefly described under Section 6.1.2.
Third, implementation had to obey strictly the VISTA coding and documentation conventions, having the effect that the resulting code fits homogeneously into the framework which in turn makes the code and its documentation immediately available for all other framework components. These conventions included usage of the VMAKE make utility (see Section 5.2), the TAC (see Section 5.4) and UNFUG (see Section 5.5).
Besides these merely implementational aspects, many technical design decisions had to be made, which were aimed at conceptual integrity (see Section 5.1 and [Hala94]) with the VISTA framework's design principles. In fact, these decisions were made sometimes simultaneously for both the data level and other framework components, and sometimes alternately, when a data level concept implied a certain design decision on another framework component or vice versa. These design decisions, targeted at conceptual integrity of the whole framework, are motivated in the following sections which describe the key components of the VISTA data level.