This page is a work in progress. Put everything related to multi-threading here, threads, synchronization, multi-core support, etc.
The Nintendo 3DS offers support for threading through use of SVC calls.
For Kernel implementation details, see KThread.
Though it is possible to run multi-threaded programs, running those on different cores is not possible "as-is". One core is always dedicated to the OS, hence you will never get 100% of both cores.
Using CloseHandle() with a KThread handle will terminate the specified thread only if the reference count reaches 0.
Lower priority values give the thread higher priority. For userland apps, priorities between 0x18 and 0x3F are allowed. The priority of the app's main thread seems to be 0x30.
The thread scheduler is cooperative, therefore if a thread takes up all the CPU time (for example if it enters an endless loop), all the other threads that run on the same CPU core won't get a chance to run. The main way of yielding another thread is using an address arbiter.
svc : 0x08
Result CreateThread(Handle* thread, func entrypoint, u32 arg, u32 stacktop, s32 threadpriority, s32 processorid);
R0=s32 threadpriority R1=func entrypoint R2=u32 arg R3=u32 stacktop R4=s32 processorid
Result result=R0 Handle* thread=R1
Details The processorid parameter specifies which processor the thread can run on. Non-negative values correspond to a specific CPU. (e.g. 0 for the Appcore and 1 for the Syscore on Old3DS) Special value -1 means all CPUs, and -2 means the default CPU for the process (Read from the Exheader, usually 0 for applications, 1 for system services). Games usually create threads using -2.
With the Old3DS kernel, the s32 processorid must be <=2(for the processorid validation check in the kernel).
With the New3DS kernel: processorid must be less than or equal to <total cores(MPCore "SCU Configuration Register" CPU number value + 1)>(for the processorid validation check in the kernel). When processorid==0x2 and the process is not an APPLICATION mem-region process, exheader kernel-flags bitmask 0x2000 must be set otherwise error 0xD9001BEA is returned. When processorid==0x3 and the process is not an APPLICATION mem-region process, error 0xD9001BEA is returned. These are the only restriction checks done by the kernel for processorid.
The thread priority value must be in the following range, otherwise error 0xE0E01BFD is returned: 0x0..0x3F.
The stacktop must be aligned to 0x8-bytes, otherwise when not aligned to 0x8-bytes the ARM11 kernel clears the low 3-bits of the stacktop address.
The input address used for Entrypoint_Param and StackTop are normally the same, however these can be arbitrary. For the main thread the Entrypoint_Param is value 0.
svc : 0x09
svc : 0x0A
void SleepThread(s64 nanoseconds);
svc : 0x0B
Result GetThreadPriority(s32* priority, Handle thread);
.global svcGetThreadPriority .type svcGetThreadPriority, %function svcGetThreadPriority: str r0, [sp, #-0x4]! svc 0x0B ldr r3, [sp], #4 str r1, [r3] bx lr
svc : 0x0C
Result SetThreadPriority(Handle thread, s32 priority);
svc : 0x34
Result OpenThread(Handle* thread, Handle process, u32 threadId);
svc : 0x36
Result GetProcessIdOfThread(u32* processId, Handle thread);
svc : 0x37
Result GetThreadId(u32* threadId, Handle thread);
Details It seems that only the thread itself or one of its parent can get the ID. Calling this on the handle of a sibling or parent seems to always yield the ID 0.
svc : 0x2C
Result GetThreadInfo(s64* out, Handle thread, ThreadInfoType type);
Details This requests always return an error when called, it only checks if the handle is a thread or not. Hence, it will return 0xD8E007ED (BAD_ENUM) if the Handle is a Thread Handle, 0xD8E007F7 (BAD_HANDLE) if it isn't.
svc : 0x3B
Result GetThreadContext(ThreadContext* context, Handle thread);
The cores are numbered from 0 to 1 for Old 3DS and 0 to 3 for the new 3DS.
svc : 0x0D
Result GetThreadAffinityMask(u8* affinitymask, Handle thread, s32 processorcount);
svc : 0x0E
Result SetThreadAffinityMask(Handle thread, u8* affinitymask, s32 processorcount);
svc : 0x0F
Result GetThreadIdealProcessor(s32* processorid, Handle thread);
svc : 0x10
You are not able to use the system core (core1) by default. You have to first assign the amount of time dedicated to the system. The value is in percent, the higher it is, the more the system will be available for your application.
For example if you set this value to 25%, it means that your application will be able to use 25% of the system core at most, even if you never issue system calls.
If you set the value to a non-zero value, you will not be able to set it back to 0%. Keep in mind that if your application is heavily dependant on the system, setting a high value for your application might yield poorer performance than if you had set a low value.
Most synchronization systems seem to have both a "normal" and "light-weight" version
For Kernel implementation details, see KMutex
/!\ It seems that the mutex will not be available once the thread that created it is destroyed
Ciritical Section (light-weight mutex)
Same thread ownership as a mutex ?
Light Semaphore ?
Does it exist ?