Master control program OS
If you are an avid reader who is interested in exotic stuff and computing history, better check the mcp article on wiki.
As it costs millions of dollars potentially, like mainframe articles, it wasn't vandalized by "advertisement" claims.
I thought plan 9 (which is the future) was interesting until I read about mcp.
Its "initial" release being 1961 with the code of summer intern student Knuth should be a good teaser.