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The ARM9 SDK contains an OS library that handles various functions. It does not have many debugging strings, so most names listed on this page are taken from the Wii.


The scheduler only runs the thread with the highest priority, and does not switch between threads if two threads have the same priority. Because of this, rescheduling only happens when a thread-related function is called and the thread with the highest priority can no longer run.

struct OSMathContext {
	u64 REG_DIV_NUMER; // 0x0
	u64 REG_DIV_DENOM; // 0x8
	u64 SQRT_PARAM; // 0x10
	u16 REG_DIVCNT; // 0x18
	u16 REG_SQRTCNT; // 0x1a

struct OSContext {
	u32 psr; // 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 *pc; // 0x40
	void *kernelSp; // 0x44
	struct OSMathContext math; // 0x48

struct OSThread {
	struct OSContext ctx; // 0x0
	u32 state; // 0x64
	struct OSThread *nextRunning; // 0x68
	u32 threadId; // 0x6c
	u32 priority; // 0x70
	u32 unknown; // 0x74
	struct OSThreadQueue *queue; // 0x78
	struct OSThreadLink linkQueue; // 0x7c
	struct OSMutex *mutex; // 0x84 - set to the mutex the thread is currently trying to lock
	struct OSMutexQueue queueMutex; // 0x88
	void *stackLo; // 0x90
	void *stackHi; // 0x94
	u32 unknown2; // 0x98
	struct OSThreadQueue queueJoin; // 0x9c
	u32 unknown3[3]; // 0xa4
	struct OSAlarm *timedSleepAlarm; // 0xb0
	u32 unknown4; // 0xb4

struct OSThreadQueue {
	struct OSThread *head;
	struct OSThread *tail;

struct OSThreadLink {
	struct OSThread *prev;
	struct OSThread *next;

struct OSMutex {
	struct OSThreadQueue waitingQueue;
	struct OSThread *holder;
	u32 timesLocked;
	struct OSMutex *next;
	struct OSMutex *prev;

struct OSMutexQueue {
	struct OSMutex *head;
	struct OSMutex *tail;

Message queues

struct OSMessageQueue {
	struct OSThreadQueue waitForReceive;
	struct OSThreadQueue waitForSend;
	u32 *buf;
	u32 capacity;
	u32 rotation;
	u32 messagesEnqueued;

Memory allocation

3 types of heaps exist: EXPH (exponential heap), FRMH (frame heap), and UNTH (unit heap).


Time is kept with timers 2 and 3, while alarm interrupts are handled by timer 1; timer 0 is presumably reserved for the DMA sound channels. The OS orders the alarms by alert time, so that it only needs to keep track of the frontmost alarm.

struct OSAlarm {
	void (*handler)(void *userData);
	void *userData;
	u32 unknown;
	u64 alertTime;
	struct OSAlarm *prev;
	struct OSAlarm *next;
	u64 repeatInterval; // 0 for non-repeating alarms
	u64 repeatStart; // undefined value for non-repeating alarms

struct OSAlarmQueue {
	u32 unknown;
	struct OSAlarm *head;
	struct OSAlarm *tail;