IOS/Kernel

The IOS kernel is responsible for dispatching interrupts to processes, handling syscalls, and running the IOSP threads. It is independent of the Wii's specific architecture, as Wii-specific functions such as high-level title launching are provided by ES.

Threads

IOS uses this struct to keep track of a thread. The first 0x40 bytes may belong to an OSContext-like struct.

struct IOS_Thread {
	u32 cpsr; // 0x0
	u32 r0; // 0x4
	u32 r1; // 0x8
	u32 r2; // 0xC
	u32 r3; // 0x10
	u32 r4; // 0x14
	u32 r5; // 0x18
	u32 r6; // 0x1C
	u32 r7; // 0x20
	u32 r8; // 0x24
	u32 r9; // 0x28
	u32 r10; // 0x2C
	u32 r11; // 0x30
	u32 r12; // 0x34
	void *sp; // 0x38
	void *lr; // 0x3C
	void *resumeAddr; // 0x40
	struct IOS_Thread *next; // 0x44
	u32 unknown; // 0x48
	s32 priority; // 0x4C
	u32 state; // 0x50 - 0 for a destroyed/uncreated thread, 1 for an active thread, 2 for the current thread, 3 for a thread that has not been started, 4 for a blocked thread
	u32 processId; // 0x54
	bool detached; // 0x58
	u32 result; // 0x5C
	IOS_ThreadQueue joinQueue; // 0x60
	IOS_ThreadQueue *queue; // 0x64
	u32 unknown2[0x42]; // 0x68
}

Note that IOS_ThreadQueue is just a pointer to an IOS_Thread.

Unlike in Revolution OS, where the initial thread is created over the current code with __OSThreadInit, the first IOS thread (IOSP) is created by the reset vector, launching new code with the standard IOS_CreateThread function.

Memory allocation

Memory allocation is similar to in the IPC library, although the IOS kernel supports 16 heaps instead of 8.

struct HeapBlockHeader {
	u16 magic; // 0xbabe
	u16 status; // 0 = free, 1 = allocated, 2 = aligned alias for header
	u32 size;
	struct HeapBlockHeader *prev; // depends on status; status 0 has the previous free block, status 1 has NULL, status 2 has the main block
	struct HeapBlockHeader *next; // NULL for anything besides status 0
}

struct Heap {
	void *base;
	u32 processId;
	u32 size;
	struct HeapBlockHeader *firstBlock;
}

When writing an aligned copy of a block, IOS does not check if it overlaps the existing copy; this could potentially be exploited.