(Switch 2) Compatibility Mode

When a Switch 1 game is running on the Switch 2, it is loaded together with 3 new modules:

• nnCompatTrampoline
• nnCompatThin
• nnCompat

These binaries are compiled with PAC enabled.

Hooking

Backwards-compatibility is achieved by (ab)using dynamic linking to selectively hook API functions. Three compatibility libraries are mapped into the game process. They provide symbols which override the original sdk codebin implementation, wherein a translation layer for Switch 2 is provided.

Most of these end up doing small tweaks to the parameters, and then calling into the original function implementation.

Switch 2 is mostly backwards-compatible, so most sdk codebin symbols remain unchanged and execute their original code. The overrides are mostly related to Graphics (i.e. translating to the new GPU), but there are also plenty other function overrides to deal with various system differences.

Below is a breakdown what some of the hooks and what their purpose is.

CompatibilityParameter

The system puts an additional information block in the ProgramArgument parameter for the process. This block contains certain extra information and overrides, which allows customization of the compatibility hooks depending on the game.

To retrieve this pointer, nnCompatThin intercepts the nnosInitialize function and retrieves it from there. Normally, a game doesn't use the ProgramParameter so it doesn't care this parameter was passed.

Offset	Type	Name
0x00	ModuleSdkInfo[11]	Modules
0xB0	u64	ModulesCount
0xB8	u64	GraphicsControlData
0xC0	u64	MovieControlData
0xC8	u64	AudioControlData
0xD0	u64	#MiscControlData

MiscControlData

Bit	Name	Description
0	PreventShaderCacheOpenFile	Prevents game from opening file: "ROM:/data/scaleform_cache/GFxShaders.cache".
1	PreventShaderCacheGetEntryType	Prevents game from getting the entry type on 5 different hardcoded paths.
2	PreventShaderCacheOpenDirectory	Prevents game from opening directory: "host:/shaders/build/"
4	SerializeSavedataAccess	A global mutex is taken when a file starting with "savedata" is opened, and held until the file has been closed. Commit-action also takes the mutex, so the commit will not complete until the file has been closed.
6	PreventNegativeVibrationValue
10	NeedsWaitingLaunchLogo
13	ExtraSleepForOfflineWebApplet

ModuleSdkInfo

Offset	Type	Name
0x00	const char*	SdkNameStringPtr. Points to a string of the form "SDK MW+Nintendo+NintendoSdk_nnSdk-XX_Y_Z-Release".
0x08	u32	SdkNameStringLength

nnCompatThin

Hooked Symbol	Difference	Conditional
nnosInitialize	Captures the ProgramArgument parameter where the #CompatibilityParameter is stored.
nn::os::CreateThread	No change.
nn::os::SetThreadCoreMask	No change.
nn::os::GetThreadAvailableCoreMask, nn::os::GetThreadAvailableCoreMask	Returns actual core mask masked with 0x7, effectively pretending that there are only 3 CPU cores available.
nn::audio::GetAudioInName	If the primary audio in name is "XUac", it returns the secondary device name instead. Presumably, the "XUac" device has been removed in Switch 2.
nn::audio::GetReleasedWaveBuffer	Now returns NULL under certain circumstances.	AudioControlData bit0
nn::web::OfflineWebSession::Appear	Added sleep 50000000 before calling the original function.	MiscControlData bit13
nn::ssl::Connection::SetSocketDescriptor	Calls nn::socket::Fnctl(..., 3); nn::socket::Fnctl(..., 4); on the socket fd.	MiscControlData bit15
nn::swkbd::InlineKeyboard::GetImage, nn::swkbd::InlineKeyboard::Calc	Takes a shared mutex before calling original functions. No other changes. Probably prevents a race condition that only manifests on Switch 2.
nn::socket::Socket	If nn::socket::IsInitialized() is 0, sleep for 1 second, and see it reaches 1 before calling the original function. Probably prevents a race condition that only manifests on Switch 2.	MiscControlData bit9
nn::os::TimedAcquireSemaphore	Force arg1=true.	MiscControlData bit8.
nn::os::QueryMemoryInfo,nn::os::QueryMemoryInfoForDebug	Hides stolen Heap memory that it uses for itself...
nn::hid::VibrationPlayer::SetCurrentPosition	If position is negative, drop the call.	MiscControlData bit6

nnCompat

Hooked Symbol	Difference
nn::os::GetSystemTickFrequency	Returns 31250000 instead of 19200000.
nn::os::ConvertToTick	Uses conversion ratio 31250000 instead of 19200000.