I'm trying to find software that will let me create - and more importantly - apply a binary patch (for an exe file) under DOS.

Everything I've found (of which xdelta was the "best", IMO) only works under modern Windows, even command line tools. I don't mind if I have to create the patch in Windows, but I would really like a tool that can apply a patch under DOS (i.e. either in DOSBox or under true DOS on an old PC). If I can make an exe from the patch so it doesn't need a program to apply it, even better!

Am I looking in vain for something like this? I remember "patches" for DOS floppy games in my childhood so I'm sure something like this *used* to exist, for companies to use "back in the day", even though patches were nowhere near as commonplace as they are for games post-release as they are today.