Thursday, 12 August 2010

Kernel-Mode and User-Mode DLLs

The new memory architecture in Windows CE 6.0 leads us to two types of DLLs: Kernel-Mode DLLs and User-Mode DLLs. We put a Kernel-Mode DLL in the MODULES section of our BIB-File and mark it with a K:


MyDll.dll $(_FLATRELEASEDIR)\MyDll.dll NK SK

If we want to use the DLL in both Kernel- and User-Mode we replace the K with a Q:


MyDll.dll $(_FLATRELEASEDIR)\MyDll.dll NK SQ

MakeImg creates a copy of our DLL named K.MyDll.dll. Each version of the DLL will relocated to the corresponding XIP section in User-Space and in Kernel-Space.

Up to this point the facts are easy and every BSP developer should know about it.

Now it starts to be tricky:

What about LoadLibrary()? What happens if I try to call LoadLibrary(L”MyDll.dll”); from a driver in the Kernel context?

The answer is: LoadLibrary() loads K.MyDll.dll, you don’t have to add a “K.”!

But what if the DLL needs another DLL, e.g. My2ndDll.dll?

It is obvious that My2ndDll.dll must be also a Kernel-Mode DLL, but the name mustn’t be K.My2ndDll.dll.

Alternatively you may move My2ndDll.dll from the MODULES section to the FILES section. Remember to remove any K or Q!

MakeImg cannot resolve all dependencies in this situation:

Warning: Unable to do imports from K.MyDll.dll to My2ndDll.dll - will late bind

But if My2ndDll.dll is on your target, K.MyDll.dll will be loaded successfully.

It’s a good idea to review all DLL dependencies and think about the special functionalities of your DLLs. Some DLLs do kernel-specific things and cannot run in User-Mode, e.g. read from KSeg1 addresses. Some DLLs use application interfaces you won’t move to the Kernel, e.g. MFC or OleAut32.

Tschüß Holger

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