A lot of developers made back ups of their games, probably out of love to their work or because they saw profitability in keeping that material. Nintendo, Al Lowe, William Salvador/Rebecca Ann Heineman, John Calhoun, John Romero, Dave Ashley, Digital Eclipse, BioWare employees, Kevin Edwards, Climax Studios, Electronic Arts, David S. Maynard, Chris Shrigley, Williams Electronics/Bally Midway Games, Ed Fries, Telltale Games, Microsoft, and even Tose Software preserved a lot of material.
>don't know if I ever told you, but the Tose pres told me they have all of the source code for every game they've done
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