Reverse engineering Explorers of Sky requires multiple skills in varying degrees. If you're a complete beginner and just want a single resource to start with, I recommend working through AnonymousRandomPerson's Reverse Engineering a DS Game, which walks through the basics of reverse engineering using Explorers of Sky as an example.
If you're looking for more granular resources on specific topics, here's a breakdown of some of the individual skills involved in reverse engineering, with relevant links:
- How to read assembly, and enough low-level knowledge of computers to understand that assembly.
- For EoS, there are technically two forms of assembly you need to know, ARM and THUMB, although they're extremely similar, and for the most part you only need to know ARM. If you're just getting started, this guide is a great introduction to the concepts needed to read assembly, and this guide is a fantastic reference and introduction to the finer details of ARM assembly.
- Other systems-level knowledge of the Nintendo DS.
- Working with the EoS binaries requires you to have some specialized knowledge (but it depends on what you're doing). One of the biggest things to know about is probably the overlay system. If you're working with anything graphical, there are additional concepts you'll need to know, such as the OAM (this series of posts is a great introduction to these concepts for the Game Boy Advance, most of which are also relevant to the NDS).
- What reverse engineering tools are available, and how to use them properly.
- See General reverse engineering for some suggestions.
- With EoS, one of the biggest obstacles when starting out is figuring out how to even set up a project properly, because it's unfortunately not as simple as importing the whole ROM with one button.
This is a nonexhaustive list, and only contains subjectively "large" collections of information. Note that some information from the following docs have also been added into pmdsky-debug.
- Project Pokémon technical docs.
- SkyTemple and the associated Discord server (the
#tech-docschannel has some links not listed here). Also see this directory in skytemple-files, which has XML files with various collections of useful data, and this other directory with various graphics file format docs. - Other (older) tools for EoS include Sky Editor and PPMDU (which has been rebooted as PPMDU 2).
- Frostbyte's dungeon data doc and map generation doc.
- UsernameFodder's EoS RAM notes.
- UsernameFodder's enum collection. Most have been ported to the C headers in
pmdsky-debug, but this still has some obscure additional info. - ShinxHijinx's PMD Info Spreadsheet.
- PsyCommando's reverse engineering notes. Also see this Dropbox folder, which is a slightly older iteration, and the tools and research notes on Project Pokémon, which are even older.
- pmd-sky is an in-progress EoS decompilation project. It is among the most complex of the resources listed in this document, but if you know how to read it, the parts of the decompilation that are complete are also likely to contain the most precise specifications one could hope to find.
- dungeon-mystery is a readable reproduction of EoS's dungeon generation algorithm written in TypeScript, with a corresponding web application called Pokémon Dungeoneer. The source code is closely based on the documentation in
pmdsky-debug, so the function names, variable names, and general terminology are largely consistent with the debug info.- dungeon-eos is an older reproduction of EoS's dungeon generation algorithm written in Python. It's more a transliteration of the raw assembly, so
dungeon-mysteryis probably a better place to start.
- dungeon-eos is an older reproduction of EoS's dungeon generation algorithm written in Python. It's more a transliteration of the raw assembly, so
- damage-eos is a readable reproduction of EoS's damage and accuracy calculation algorithm written in C++, with a corresponding interactive damage calculator. The source code directly relies on
pmdsky-debug, so there's a high degree of consistency with the debug info. - eos-move-effects is a kind-of framework for coding move and item effects in EoS. It also has good documentation on relevant bits of the EoS asm.
- Some (not really organized) dumps of various bits of data from EoS on GameFAQs: Various in-game data, Explorers of Darkness Dungeon FAQ 0.5.
The above list is, of course, tailored to reverse engineering EoS. If you want to learn about general reverse engineering, you can Google around for various resources. If you want some concrete links, check out wtsxDev/reverse-engineering and r/ReverseEngineering.
Project Pokémon has a list of reverse engineering tools specifically geared towards NDS ROM hacking. GBATEK also has a full technical reference dump, which can be useful in very specific circumstances.
This is a list of tools that I (UsernameFodder) use for EoS research that might prove helpful to some people. Keep in mind that I usually only do a specific subset of things when it comes to reverse engineering, so this list won't be suited for all use cases.
- Ghidra (cross-platform) for the majority of asm research, since it's free and has one of the best decompilers out there. It has built-in tutorials (HTML preview of the beginner class). The other well known binary analysis tool is IDA, which you need to pay for (for ARM binaries). If you're a licensed IDA user, you probably don't need to be reading these docs anyway.
- DeSmuME on Windows for watching memory at runtime (unfortunately only the Windows version has these tools at the time of writing). I find its memory viewer/editor (Tools > View Memory) and RAM Search (Tools > RAM Search...) tools particularly useful.
- No$GBA (Windows only) for interactive debugging. However, I've found this emulator to be relatively unstable, and I try to avoid using it unless there's no other option.
- DSLazy (Windows only) for unpacking NDS ROMs, since it's the simplest thing with a GUI that works. It's a wrapper around
ndstool, which is cross-platform. See the Ghidra setup doc for detailed instructions. - SkyTemple (cross-platform), primarily for getting up to speed on things the community has already figured out.
- Hex Fiend (macOS only) and HxD (Windows only) on the rare occasion I need to do raw hex editing, but honestly any hex editor works.
There's a lot to learn if you're just starting out, especially if you don't already have a background in computer science. If you want just one thing to sink your teeth into initially, I recommend AnonymousRandomPerson's Reverse Engineering a DS Game. It'll teach you how to read assembly, guide you through setting up and using reverse engineering tools like Ghidra, and share basic reverse engineering strategies through practical examples.
After that, I recommend going through the Ghidra setup tutorial (the above article covers some of it, but stops short of actually importing debug info). By this point, you'll be well equipped to start exploring the EoS code on your own.