Microsoft Windows for Pen Computing includes tablet drivers for input from a handwriting stylus device. An application can use the GetMessageExtraInfo function to distinguish between a mouse-generated event and a stylus-generated mouse event. For more information about GetMessageExtraInfo, see the Microsoft Win32 Programmer's Reference.
Tablet drivers use the SI and DI registers to pass extended event information to Windows. For this reason, mouse drivers should set these registers to zero before a function calls the Mouse_Event function in the Windows USER module. This allows Windows – based and Pen Computing – based applications to distinguish between mouse and tablet events.
See also Mouse_Event