So far we have only considered the construction of new programs, but of course there are very many programs which are already in existence and which we may need to use. Like much old electronics equipment they may or may not have been well-designed but in any case there is a great tendency to ‘leave well alone’ just in case they stop working.

When a program exists only in its object form (i.e. translated into machine code) little can be usefully done – decompilers, which translate a program back into a high-level language form, do exist but the resulting program is usually pretty unintelligible. Such (object only) programs should be candidates for rewriting as soon as time is available since in their current form they cannot be safely and quickly amended and this, in practice, is an ever-likely requirement.

Programs which exist in source form present a more approachable problem. Because of their age and the rapid development of programming methodology, it is likely that these programs will not have formal specifications from which their correctness could have been demonstrated. Also, in view of their preoccupation with making best use of slow and expensive hardware, it is very probable that the early programmers were not permitted the luxury of programming style. These economic factors served to encourage the production of unintelligible ‘spaghetti’ code which was very difficult to analyse and/or modify.