A patent does not apply to a "code as such", but to the algorithm the code implements.
Well sort of. The only patent I remember related to software was the right mouse button drop down menu from the top of the screen (there are probably others). That isn't what I'd call an algorithm.
The "copyright" is the code "as is", the particular implementation of an "IP-right" as stated in a patent. And the copyright did run out. It usually extends 30 years after the death of its creator, or even 70 years now with the Mickey-Mouse law in place.
I don't believe death comes into it in this case. My understanding is that if If you write software and have it published then your death is important to the copyright expiring. If someone pays you to write software then when you die is irrelevant, because you never owned the copyright.
https://www.law.cornell.edu/uscode/text/17/302"In the case of an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication, or a term of 120 years from the year of its creation, whichever expires first."
In the case of movies, they usually put at the end of the credits who the author was at the time the movie was created. I don't believe there is any doubt that most of the "AmigaOS" code was created as a work for hire. You'd need to pay an IP lawyer to figure out when the copyright expires on:
1. ARexx as it was licensed from IBM
2. The BCPL source used in 1.3 and earlier
3. Any arp code that turned up in 2.0 and later.
You can certainly re-implement the same algorithms(!) AmigaOs is based on without violating the copyright.
.....
And no again, simly typing down the code from the stolen source does not make the resulting work legal, it would still be a derived work.
Copyright only protects source code with artistic merit, so as long as the only similarities your source code contains are obvious then you are ok. The original used functions, variables, loops etc, so can you. If you just copy and paste or retype by hand and rename variables, functions, change spacing, tab indenting etc then you've violated the copyright.
If you want to be perfectly safe, "don't use, don't look, create all yourself".
It's better not to look at the original because your brain will see that implementation as the obvious one and you will struggle to figure out a better way and then be deluded into thinking it's ok to copy it.
You could follow Compaqs lead with how they re-implemented the IBM-PC BIOS
http://mashable.com/2014/05/29/halt-and-catch-fire-amc-compaq/#Jue1WEcrdiqx
