INT 33h,  0Dh (13)       Light Pen Emulation Mode On

    Switch Light Pen Emulation ON in BASIC.

       On entry:      AX         0Dh

       Returns:       Nothing

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    Function 0Dh allows the mouse to emulate a light pen. When the mouse
    emulates a light pen, calls to the PEN function (described in the BASIC
    manual) return the cursor position at  the last "pen down".

    The mouse buttons control the "pen down" and "pen off the screen"
    states. The pen is down when both mouse buttons are down. The pen is off
    the screen when both mouse buttons are up.

    The mouse software enable light pen emulation after each reset.