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Reading a fascinating online book about the history of computing, I came across this passage on http://ds-wordpress.haverford.edu/bitbybit/bit-by-bit-contents/chapter-seven/7-5-assembly-language-programming/

The original FORTRAN compiler ran only on the 704, but programmers went on to develop FORTRAN compilers for other IBM computers, and competing manufacturers, licensed by IBM, adapted FORTRAN to their own machines.

Wait a minute. Competing manufacturers, licensed by IBM? In those days, you couldn't patent software. You couldn't even copyright it. Even nowadays, thanks to the recent Supreme Court decision in Oracle versus Google, it is legal to write your own implementation of someone else's programming language.

So why would other manufacturers need a license? Why would there be any legal problem with them just going ahead and unilaterally writing their own Fortran compilers?

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    Are you sure you couldn't copyright the text? Assuming you could, someone who "adapted FORTRAN to their own machines" who started with the assembler code for the compiler (instead of starting totally from scratch) might license from IBM to avoid copyright problems. Apr 13 at 14:21
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    To put a data point, per the wikipedia: in 1974, the Commission on New Technological Uses of Copyrighted Works (CONTU) was established. CONTU decided that "computer programs, to the extent that they embody an author's original creation, are proper subject matter of copyright."[15][14] In 1980, the United States Congress added the definition of "computer program" to 17 U.S.C. § 101 and amended 17 U.S.C. § 117 to allow the owner of the program to make another copy or adaptation for use on a computer. [ en.wikipedia.org/wiki/Software_copyright#History ]
    – Foon
    Apr 13 at 14:23
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    Also any documentation would have been copyrighted by IBM. If (and I don't know if this is the case or not) the canonical FORTRAN programmer's guide of the time was copyrighted by IBM, writing another compiler to the specs defined by that document would not have been a strict copyright violation but working out licensing with IBM so that documentation could be reused would make such a compiler more useful for potential customers. Apr 13 at 15:32
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    Let me join the guessing. Beside the fact, that the term licencing might be used rather loose, it does make sense to licence the source code to speed up development, saving time and avoiding pitfalls. In addition incompatibility between the COBOL predecessors about I/O was already a well known problem, so Time to Market and Compatibility might have been the reason for some - others just at down and wrote their own.
    – Raffzahn
    Apr 13 at 16:11
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    Don't forget the economics. It may have been cheaper to license IBM documentation and code than reinvent the wheel. When Fortran was invented, compiler-writing wasn't something you taught in a first-year computer science course, it was "guru-level" technology. As the reference says, it took Backus and his team 18 months of preliminary work before they even started writing the code and another 18 months to complete the project.
    – alephzero
    Apr 13 at 19:08
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The documentation, possibly including a language's keywords and their descriptions, could be copyrighted. This is why Zilog Z80 assembly language is not compatible with or a strict superset of Intel 8080 assembly language; because, IIRC, Intel registered a copyright.

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