The shape of an update function
A trimmed spellstone_update: every frame the object maybe-spins, checks a game bit, and routes through an if/else of helper calls. This is the first capstone where you juggle a saved base pointer (r30) across multiple calls while the state pointer lives in r31.
typedef struct { u8 state; } SpellState;
typedef struct { int completeEvent; int activeEvent; } SpellDef;
typedef struct GameObject {
SpellState* state; SpellDef* def;
f32 worldPosX; f32 worldPosY; f32 worldPosZ;
s16 rotX; s16 rotY; s16 rotZ; s16 flags;
void* followTarget;
f32 posX; f32 posY; f32 posZ;
} GameObject;
extern int GameBit_Get(int e);
extern void GameBit_Set(int e, int v);
extern void Obj_RemoveFromUpdateList(GameObject* o);
extern void ObjHits_EnableObject(GameObject* o);
extern void ObjHits_DisableObject(GameObject* o);
The state field is a u8, so its compares are unsigned cmplwi — exactly the SFA rule that compare width tracks the operand type. Spinning is a += 0x100 on the s16 rotation field; the flag is set with ori 0x4000:
lbz r0, 0(r31)
cmplwi r0, 2 # state == 2 ? (unsigned, because u8)
bne- .nospin
...
lha r3, 20(r30)
addi r0, r3, 256 # rotX += 0x100
sth r0, 20(r30)
.nospin:
lwz r3, 0(r5)
bl GameBit_Get
cmpwi r3, 0
beq- .else
ori r0, r0, 16384 # flags |= 0x4000
bl Obj_RemoveFromUpdateList
b .end
.else:
lbz r0, 0(r31)
cmplwi r0, 0
bne- .enable
bl ObjHits_DisableObject
...
(The rotY = 0 and rotZ = 0 stores are elided from the excerpt above for brevity — only the rotX += 0x100 line is shown — but your solution still needs all three.)
The instructive detail: typing state as u8 is what makes those compares cmplwi instead of cmpwi. Get the field type wrong and the branch opcode mismatches even though the logic is identical.
Your task
With the structs above, write spellstone_step to match the assembly above. Pay attention to which rotation fields change and by how much, the direction of the GameBit_Get branch and which event it reads, and the (s16) on the flag write.