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One of the great limitations in Fortran is its inability to dynamically allocate storage. In a large program which must deal with problems are highly variable sizes, it is extremely valuable to allocate storage based on the size of the problem. Thus, a given computer can process a mix of problems as long as the aggregate size is not too large.
Even though Fortran does not allow dynamic storage allocation, we can conveniently simulate it so that arrays can be established at run-time with whatever size is necessary. These storage allocation schemes are written entirely in Fortran and do not require any machine dependent features.
The storage schemes described here are ubiquitous, and it is very difficult to follow the code without a good feel for these storage schemes. However, once they are comprehended, modifications are straightforward.
There are numerous sources in the computer science literature on stacks, heaps, and data structures. An excellent starting point is Jeffrey D. Ullman, Fundamental Concepts of Programming Systems, Addison Wesley (1976). Chapters 3, 6, and 7 are most useful.