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On the other hand, systems applications require the ability to stay rather close to the machine, and not only for reasons of efficiency. For example, defining a hardware descriptor must be done in terms of the physical properties, the bit positions, and so on. A mapping different from that prescribed by the hardware would not merely be inefficient - it would be incorrect and would not work at all. To produce a correct program in such cases we are forced to abandon the abstract view and to work in terms of the physical representation. This contradiction cannot be avoided. The language must deal with objects at two different levels, the logical and the representation level [Wo 72].
Clearly, dealing with physical representation is inherently dangerous. However, some control can still be achieved if the language enforces a clear separation of the logical properties from their physical representation.
This separation principle is discussed below, along with the problem of changing representation and an analysis of the issues raised by the different forms of representation clause available in the language. This chapter also covers the ways to specify the parameters of a given configuration, and conversely how to ascertain them by environment enquiries. Finally we present the means available for interfacing with other languages.
