TDF Specification, Issue 4.0

January 1998

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1. Introduction

TDF is a porting technology and, as a result, it is a central part of a shrink-wrapping, distribution and installation technology. TDF has been chosen by the Open Software Foundation as the basis of its Architecture Neutral Distribution Format. It was developed by the United Kingdom's Defence Research Agency (DRA). TDF is not UNIX specific, although most of the implementation has been done on UNIX.

Software vendors, when they port their programs to several platforms, usually wish to take advantage of the particular features of each platform. That is, they wish the versions of their programs on each platform to be functionally equivalent, but not necessarily algorithmically identical. TDF is intended for porting in this sense. It is designed so that a program in its TDF form can be systematically modified when it arrives at the target platform to achieve the intended functionality and to use the algorithms and data structures which are appropriate and efficient for the target machine. A fully efficient program, specialised to each target, is a necessity if independent software vendors are to take-up a porting technology.

These modifications are systematic because, on the source machine, programmers work with generalised declarations of the APIs they are using. The declarations express the requirements of the APIs without giving their implementation. The declarations are specified in terms of TDF's "tokens", and the TDF which is produced contains uses of these tokens. On each target machine the tokens are used as the basis for suitable substitutions and alterations.

Using TDF for porting places extra requirements on software vendors and API designers. Software vendors must write their programs scrupulously in terms of APIs and nothing more. API designers need to produce an interface which can be specialised to efficient data structures and constructions on all relevant machines.

TDF is neutral with respect to the set of languages which has been considered. The design of C, C++, Fortran and Pascal is quite conventional, in the sense that they are sufficiently similar for TDF constructions to be devised to represent them all. These TDF constructions can be chosen so that they are, in most cases, close to the language constructions. Other languages, such as Lisp, are likely to need a few extensions. To express novel language features TDF will probably have to be more seriously extended. But the time to do so is when the feature in question has achieved sufficient stability. Tokens can be used to express the constructs until the time is right. For example, there is a lack of consensus about the best constructions for parallel languages, so that at present TDF would either have to use low level constructions for parallelism or back what might turn out to be the wrong system. In other words it is not yet the time to make generalisations for parallelism as an intrinsic part of TDF.

TDF is neutral with respect to machine architectures. In designing TDF, the aim has been to retain the information which is needed to produce and optimise the machine code, while discarding identifier and syntactic information. So TDF has constructions which are closely related to typical language features and it has an abstract model of memory. We expect that programs expressed in the considered languages can be translated into code which is as efficient as that produced by native compilers for those languages.

Because of these porting features TDF supports shrink-wrapping, distribution and installation. Installation does not have to be left to the end-user; the production of executables can be done anywhere in the chain from software vendor, through dealer and network manager to the end-user.

This document provides English language specifications for each construct in the TDF format and some general notes on various aspects of TDF. It is intended for readers who are aware of the general background to TDF but require more detailed information.

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