Disassembly counts as reverse engineering, just fyi, and it holds legal grounds in courts, at least in NA as far as I know. So Nintendo can't claim nothing because RE is legal.
This is the same reason why Nintendo hasn't done jackshit to the already widely available Pokemon disassemblies, which vary from Gen 1 to Gen 3 as of this time of writing, and they've been available for years in GitHub of all places.
I don't know in what world a sane human would try to "reverse engineer" something just from observation and tried to recreate it from scratch. That's completely stupid. It's like trying to waste 1 year of your life to recreate Mario's jump from observation 1:1, while you can very much do it by debugging the ROM and see how the game engine does it with disassembly, alongside what routines are tied to it and the exact timing ticks, gravity variables, etc.
Disassembly is the very first step to any kind of RE, and most of the time they go hand by hand.
From here, people adept enough in the MIPS assembly language can work wonders.
And someone adept enough in both MIPS and C++ can start recreating the whole game in C++ from the base MIPS disassembly.
And remember, all of this is LEGAL.
So Nintendo can go fuck themselves with their bullshit DMCAs already and their IP copyright asshole attitude towards everything related to them.
Glad this was done.
Disassembly, and reverse engineering as a larger umbrella, might be legal for purposes of interoperability, accessibility, defeating anti piracy that is dead and preventing a work from being used, and all the other "substantial non infringing uses" that allow emulator projects to happen out in the open over decades but firing your decompiler or disassembler over a piece of code and releasing the result (even tarted up a bit) is hard to justify as some kind of fair use any more than me taking bitmaps of every frame of a film and releasing those as a slideshow is legal.
I am not sure why the pokemon stuff has remained so unbothered, however everything I have seen from Nintendo other than the emulator which bundled ROMs has been more from a trademark angle from what I can see, and I don't think the disassembly terribly bothers that.
I agree it would have been a nightmare, one you know you can end trivially with existing tools, to recreate a game even as old as this from observation -- such methods are traditionally saved for things like database formats, document formats, low level protocols and things like that to allow people to operate with them or convert between them. The method described before, generally known as clean room reverse engineering, is the traditional way you generate free and clear info for your projects that you want to interact with someone else's standard, code or similar if the have not provided the code in a manner you care to use.
I am also happy to see such things made, and happy to skirt the edges of laws to have cool things like this. Does not change the technical reality of the law though.
how were those sophisticated romhacks, with new features, physics etc possible if the source hadnt been altered?
Source makes things potentially a lot easier, however it is not impossible without it.
I am not sure how much depth is required for a reply like this but in an emulator you can watch for certain things to happen (buttons, standard cheat searches will yield character location, direction and speed, from there you can either watch things watching for the buttons or do something like that and figure out what jump routines are like) and from there figure out what code is fiddling with what. At that point you can do some fairly radical changes. Rinse and repeat for the various hats, walls/obstacles, enemies and the like.
Prior to the N64 (and PS1) most things were written in the assembly language you will likely be getting spat back out at you from the emulator and 16 bit era games still managed a fair bit of complexity. Or if you prefer you can have a read of
https://stuff.pypt.lt/ggt80x86a/asm1.htm (it is for the PC but you can see how quickly you find yourself doing useful operations).
Source
