Well, "day 1" was an exaggeration, since the first program was arguably written in 1843. And it's not exactly clear when the first commercial computer was put on the market (it depends on how you count).
But, looking back on the earliest times that I was involved with computers -- which predates microcomputers entirely -- all of the software that came with the machines was copyrighted, as evidenced by the copious amount of copyright notices all over everything.
> Richard Stallman's story about software sharing, and him explicitly saying how software was initially free "by default", and how it was companies that invented software copyright, which is the spirit my original comment was intended in.
Taken literally, Richard Stallman is simply wrong here. But I doubt that he meant that all software was free of copyright (simply because it wasn't). In the US, before 1978(?), you had to register your copyright and it was legally possible to intentionally not copyright works. Lots of software did that, so there was certainly software that wasn't under copyright. I suspect that's what Stallman was talking about. The need to file to obtain copyright protection does mean that copyright didn't apply "by default" then. But that's not saying that there wasn't copyrighted software!
There's no such thing as "software copyright", by the way. It's just "copyright", and software doesn't have any sort of special status now, nor did it then.
As to hard info regarding the history of US copyright, there's tons of it available online. You could do worse than reading the various histories and issues the government has at www.copyright.gov.