Totally serious question: If I go back to 1990, can I distribute a current version of emacs? The toolchain to build the current source tree won't exist for a while.

GCC stage1 is deliberately written in K&R C so that you can do this kind of bootstrapping. You need a few unix tools (e.g. make - but again, a version from 1990 should work, find yourself a solaris machine or something); I don't remember exactly what emacs depends on, but it should be fine. Getting a modern environment on old unix boxes (or under windows SUA, whose unix stuff is old enough that it comes with X11R5) is fun and usually requires a bit of fiddling but nothing too serious.

Why not? Any computer program is just a very large number. Even thought the emacs build process that calculates the large number corresponding to the current version of emacs hadn't been performed in 1990, the number itself "existed" back then as the outcome of any number of other possible computations.

On the other hand, you might have problems with dll dependencies.

Or maybe Emacs can't run in a home machine of 1990

Remember? Eight Megs And Constantly Swapping

I assume @danielweber was asking whether it was possible in principle - as in, can information exist before it is created.

My remark about dlls was an attempt at humour, not a comment on practicalities.

Oh, youre running a binary that uses CPU extensions or dependent on a kernel that doesn't exist yet? Enjoy your segfaults.