Expects: AX    000dH
Returns: none
Info: This causes the mouse support to work with programs which are
written to support a light pen (e.g., using the PEN function on
BASIC).  The mouse support apparently intercepts INT 10H 04H when
this feature is enabled.

When enabled, a click of either button is taken as a press of the
pen button at current mouse position.

Disable this support via INT 33H 000eH.

- -

INT 33H 000dH: Enable Lightpen Emulation