Reading a Global Through the Small Data Area

Globals, the SDA & Pools

globalssdasda21

Globals live a short hop from r13

Here's the problem: a GameCube address is 32 bits, and a single PowerPC instruction has nowhere to put all of them, so you can't lwz a global from its absolute address. The Small Data Area is MWCC's workaround. Boot code points register r13 at a fixed base, and the globals a game uses most are packed into a window that a signed 16-bit offset off r13 can cover. It's not a big window — 64 KB, ±32 KB. Spill past it and a global has to take the long way with @ha/@l, which shows up later. While it fits, one load does the job, and the linker bakes the offset in:

lwz   r3, g@sda21(r13)   # load global g, r13-relative
blr

R_PPC_EMB_SDA21   g

@sda21 is a relocation, not a real address. An unlinked object disassembles to lwz r3, 0(0) plus a dangling R_PPC_EMB_SDA21 g line, because the offset and base register stay unresolved until link time. That R_PPC_EMB_SDA21 line is your tell for a plain, non-array global, whether it's extern or defined right here.

Your task

Declare nothing yourself — extern int gFrameCount; is already provided. Write func_80021258 to match the target assembly above.

Hints

match func_80021258mwcceppc.exe -O4,p

src

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Hit “Compile & Check” to diff your code against the target.