While hardware is expected to remain useful on timescales exceeding ten years (PDP 11s and IBM 470s are still functional machines today), software evolves on timescales less than two or three years. Long before the machines stop running, companies stop providing support for their operating systems, development tools and environments, networking upgrades and other necessary software. In the case of networking tools related to the Internet (like World Wide Web), change is currently taking place on scales nearer to months. In addition, the size and complexity of the programming environments continues to increase as developers build for larger and larger memory, discounting the continued use of small memory machines.
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