I don't know whether it's legal, but http://www.interact-sw.co.uk/altair/index2.html has an annotated disassembly of Altair BASIC. I haven't given it more than the briefest glance myself, but allegedly there's some very nice stuff there. (Of course it's from a different era and no one outside deeply-embedded-land writes that sort of code any more other than for a twisted kind of fun.)
He wrote Microsoft's original Basic, probably parts of other languages. DOS must have been changed a lot after Microsoft bought it. He may have written some of the new stuff.
