INFO: MmMapLockedPages Returns Actual Virtual Address In SP4

ID: Q199311

The information in this article applies to:

SUMMARY

In the Windows NT 4.0 Service Pack 3 and earlier versions, MmMapLockedPages returns the base virtual address (a page-aligned address) that maps the locked pages for the range described by the MDL. However, this has been changed in Service Pack 4 and later versions to return the actual virtual address including the byte offset within a page of the buffer described by the MDL.


MORE INFORMATION

In Windows NT 4.0 Service Pack 3 and earlier versions, if the buffer described by the MDL is not page-aligned then you have to add the offset stored in the MDL with the base virtual address returned by the function to get the actual address as shown here: 


VirtualAddress = (PVOID) (((ULONG) MmMapLockedPages(Mdl, Mode)) +  MmGetMdlByteOffset(Mdl));


In Service Pack 4 and later versions, this function has been altered to consider the byte offset stored in the MDL and returns the actual virtual address as shown here: 


VirtualAddress = (PVOID) MmMapLockedPages(Mdl, Mode);


To fix the above mentioned inconsistencies and make the code portable across all versions of Windows NT, use the following: 


VirtualAddress = (PVOID) (((ULONG) MmMapLockedPages(Mdl, Mode)) | MmGetMdlByteOffset(Mdl));


or 


VirtualAddress = (PVOID)(((ULONG)PAGE_ALIGN(MmMapLockedPages(Mdl, Mode))) + MmGetMdlByteOffset(Mdl));


Microsoft has confirmed the pre-Service Pack 4 behavior of this function to be a bug and has fixed it in order to resolve memory addressing issues on future releases of Windows NT; Windows NT 64-bit in particular.


REFERENCES

Q189327

Additional query words: kbDSupport 


Keywords          : kbDDK kbKMode kbNTOS400 kbNTOS400fix kbWinOS2000 
Version           : winnt:
Platform          : winnt 
Issue type        : kbinfo

Last Reviewed: April 8, 1999