decomp_me_basics.md

Decomp.me Basics

decomp.me is an online collaborative tool that allows reverse engineers to match functions against the original code and share their progress with others.

Creating A Scratch

A "scratch" refers to a WIP playground that you can use to iterate on a function or functions until you've reached a match.

In order to create a new scratch, follow the steps below:

1. Open decomp.me.
2. Login or register for a new account.
3. Click on New scratch in the top-right corner of the page.
4. Select Gamecube/Wii as the target platform.
5. Under the compiler section select from the "preset" dropdown either Animal Crossing (REL) or Animal Crossing (DOL) depending on where the function you are reverse engineering is located. For most game-specific code you will using the REL option.

6. Copy the assembly for the function you want to match from the assembly file you generated.
7. Copy the context for the function from the context file you generated for it.
8. Press the Create Scratch button.

Matching

After your scratch has been created you can begin reverse engineering and writing your own C code. You can use tools such as m2c or Ghidra to help assist you. As you make modifications to your scratch you can see how close you are to matching the function.

Some important notes when matching:

• Variable ordering matters. If you're finding that you're stuck on the last percent or two of a function, try re-arranging the declaration of your variables.
• What you write may affect the assembly the comes before or after it. You may find that as you fill out an earlier/later section that your function matching can increase.
• Only include what you need in your pasted in context. If you include other functions/data you may find that it adds to the "matched" code. In these cases you may need to only forward-declare the respective function(s) and data.