diff --git a/dlls/ntoskrnl.exe/ntoskrnl.c b/dlls/ntoskrnl.exe/ntoskrnl.c
index 0dc9aaf32f3..6371d85a120 100644
--- a/dlls/ntoskrnl.exe/ntoskrnl.c
+++ b/dlls/ntoskrnl.exe/ntoskrnl.c
@@ -1161,6 +1161,17 @@ PVOID WINAPI MmGetSystemRoutineAddress(PUNICODE_STRING SystemRoutineName)
     return pFunc;
 }
 
+
+/***********************************************************************
+ *           MmQuerySystemSize   (NTOSKRNL.EXE.@)
+ */
+MM_SYSTEMSIZE WINAPI MmQuerySystemSize(void)
+{
+    FIXME("stub\n");
+    return MmLargeSystem;
+}
+
+
 /*****************************************************
  *           DllMain
  */
diff --git a/dlls/ntoskrnl.exe/ntoskrnl.exe.spec b/dlls/ntoskrnl.exe/ntoskrnl.exe.spec
index 7b958528714..849dd518335 100644
--- a/dlls/ntoskrnl.exe/ntoskrnl.exe.spec
+++ b/dlls/ntoskrnl.exe/ntoskrnl.exe.spec
@@ -706,7 +706,7 @@
 @ stub MmProbeAndLockProcessPages
 @ stub MmProbeAndLockSelectedPages
 @ stub MmProtectMdlSystemAddress
-@ stub MmQuerySystemSize
+@ stdcall MmQuerySystemSize()
 @ stub MmRemovePhysicalMemory
 @ stdcall MmResetDriverPaging(ptr)
 @ stub MmSectionObjectType
diff --git a/include/ddk/wdm.h b/include/ddk/wdm.h
index 0f241b6a708..3169153c6c5 100644
--- a/include/ddk/wdm.h
+++ b/include/ddk/wdm.h
@@ -1001,6 +1001,13 @@ typedef struct _KUSER_SHARED_DATA {
     } DUMMYUNIONNAME;
 } KSHARED_USER_DATA, *PKSHARED_USER_DATA;
 
+typedef enum _MM_SYSTEM_SIZE
+{
+    MmSmallSystem,
+    MmMediumSystem,
+    MmLargeSystem
+} MM_SYSTEMSIZE;
+
 NTSTATUS WINAPI ObCloseHandle(IN HANDLE handle);
 
 #define IoGetCurrentIrpStackLocation(_Irp) ((_Irp)->Tail.Overlay.CurrentStackLocation)
@@ -1048,6 +1055,7 @@ ULONG     WINAPI KeQueryTimeIncrement(void);
 
 PVOID     WINAPI MmAllocateNonCachedMemory(SIZE_T);
 void      WINAPI MmFreeNonCachedMemory(PVOID,SIZE_T);
+MM_SYSTEMSIZE WINAPI MmQuerySystemSize(void);
 
 NTSTATUS  WINAPI ObReferenceObjectByHandle(HANDLE,ACCESS_MASK,POBJECT_TYPE,KPROCESSOR_MODE,PVOID*,POBJECT_HANDLE_INFORMATION);