Every exercise so far handed you a function to fill in — the name, the return type, the arguments, all laid out:
s32 add2(s32 a, s32 b) {
// you write this part
}Those were training wheels. When you decompile for real, nobody gives you that line. A disassembly is a pile of instructions filed under whatever address the linker parked them at, and nothing else. How many arguments the function takes, what types they are, whether it returns anything at all — that is for you to work out, from the assembly, before you can write a line of C.
From here on, the editor starts empty and the names stop helping. Every exercise goes by its address: func_ followed by eight hex digits, exactly what a real decomp project calls a function nobody has figured out yet.
Those digits will always start 08, and that is not decoration. The GBA maps the game cartridge at address 0x08000000, so every byte of ROM code lives somewhere in 0x08000000–0x09FFFFFF. A function named func_0803f1a4 is simply the one that begins 0x3f1a4 bytes into the ROM. Open a real GBA decomp project and the undone functions are named exactly this way. (Code copied into fast internal RAM gets 03… names instead, but that is a detail for much later.)
The header hands you that address and nothing more. Everything else is yours.
The ABI — the calling convention — is a fixed set of rules about where arguments arrive and where results leave, and you already know the headline:
r0, r1, r2, r3, in order;r0.Learn to read those rules off a disassembly once and a signature falls out of the first few instructions almost every time.
We will start gently. For the next while every value is a plain 32-bit s32, so the return type is either s32 or nothing at all, and the only questions that change from function to function are:
Narrow types, pointers, floats and stranger returns all have tells of their own, and later chapters come back for them. Get counting down first — then you are not doing exercises any more, you are doing the real thing.