Microsoft Windows for Pen Computing includes tablet drivers for input from a handwriting stylus device. An application can use the Windows 3.1 GetMessageExtraInfo function to distinguish between a mouse-generated event (the return value is 0L) and a stylus generated mouse event (the return value is non-zero).
Tablet drivers use the SI and DI registers to communicate 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 3.1 USER module. This will allow Windows and Pen-Computing based applications to distinguish between mouse and tablet events.