A pen-based portable personal computer can emulate the functions of a standard two-button mouse. In particular the system can emulate standard movement and clicking of a mouse buttons as well as emulate both left and right mouse buttons of a two-button mouse. In order to be compatible with various application programs, the system can also emulate the double clicking of the mouse button. By providing a pen-based computer system which can emulate the functions of a standard two-button mouse, the system is compatible with various application programs available on the market