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Aetias
77 lines
4.2 KiB
Markdown
77 lines
4.2 KiB
Markdown
# Decompiling
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This document describes how you can start decompiling code and contribute to the project. Feel free to ask for help if you get
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stuck or need assistance.
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- [Pick a source file](#pick-a-source-file)
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- [Decompiling a function](#decompiling-a-function)
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- [Decompiling `.init` functions](#decompiling-init-functions)
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- [The Ghidra project](#the-ghidra-project)
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## Pick a source file
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See the `decomp` tag in the [issue tracker](https://github.com/AetiasHax/ph/issues?q=is%3Aopen+is%3Aissue+label%3Adecomp) for
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a list of delinked source files that are ready to be decompiled. This list grows as more source files are delinked from the
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rest of the Assembly code.
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You can claim a source file by leaving a comment on its issue, so that GitHub allows us to assign you to it. This indicates
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that you are currently decompiling that source file.
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If you want to unclaim the file, leave another comment so we can be certain that the source file is available to be claimed
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again. Remember to make a pull request of any notable progress you made on the source file, which can include
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[non-matching functions](/CONTRIBUTING.md#non-matching-functions).
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## Decompiling a function
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Say you've found a function you want to decompile. Here are the steps we recommend for decompiling it:
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1. Visit [decomp.me](https://decomp.me/) and start decomping.
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1. Under the platforms, select "Nintendo DS".
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1. Select compiler version `2.0sp1p5`.
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1. Copy and paste the target assembly for your function, including the `func_start` and `func_end` macros, and the pool constants.
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For example:
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```arm
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.global func_ov09_0211bf48
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thumb_func_start func_ov09_0211bf48
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func_ov09_0211bf48: ; 0x0211bf48
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ldr r0, _0211bf50 ; =data_ov09_0211f59c
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ldrb r0, [r0]
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bx lr
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nop
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thumb_func_end func_ov09_0211bf48
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_0211bf50: .word data_ov09_0211f59c
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```
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6. Run `m2ctx.py include/MyHeader.hpp -c` to generate a context and put it in your clipboard.
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- If no suitable header file exists, make a new one and put any structs and types you need in there.
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1. Paste the context into decomp.me, and create the scratch.
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1. Copy the `CC_FLAGS` from `Makefile` into the arguments field in decomp.me.
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- Also copy `CXX_FLAGS` if your function comes from a `.cpp` file. Otherwise, copy `C_FLAGS` if it's a `.c` file.
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- Replace the `-d $(REGION)` flag with whichever region you intend to decompile for. You can also delete the flag entirely
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if the function contains no region differences.
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1. Decompile the function and try to get a 100% match.
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- There's no ARM decompiler in decomp.me yet, but Ghidra does the job quite well. See [the Ghidra section](#the-ghidra-project)
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for more info.
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- If you're unable to get a 100% match, share your decomp.me scratch with other contributors and they may assist you.
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- In the worst case, you can also contribute [non-matching functions](/CONTRIBUTING.md#non-matching-functions) to this
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project.
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## Decompiling `.init` functions
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> [!NOTE]
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> This section will be updated as we learn more about global objects. Feel free to contribute or provide us with more
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> information!
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Functions in the `.init` section are static initializers. Their purpose is to call C++ constructors on global objects, and to
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register destructors so the global objects can be destroyed when their overlay unloads.
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Static initializers are generated implicitly and do not require us to write any code ourselves. So, to generate one, you must
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define a global variable by using a constructor.
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If the static initializer calls `__register_global_object`, that means the global object has a destructor. This means you'll
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have to declare a destructor if it doesn't exist already.
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Another consequence of having a destructor is that a `DestructorChain` object will be added to the `.bss` section. This struct
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is 12 (`0xc`) bytes long and is also implicit, so we don't need to define it ourselves.
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> [!IMPORTANT]
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> An important thing to keep in mind is that a static initializer can construct multiple global objects.
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## The Ghidra project
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We use a shared Ghidra project to analyze the game and decompile functions. To gain access to the project, install
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[Ghidra version 10.2.3](https://github.com/NationalSecurityAgency/ghidra/releases/tag/Ghidra_10.2.3_build) and request access
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from @aetias on Discord.
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