## Input/Output Device Driver Reference

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### INT 21h Interface

An application running in a DOS session can access the parallel port device driver through INT 21h. The file system creates a request packet from the parameters and calls the parallel port device driver at its strategy entry point. This interface maintains compatibility with DOS applications. As in the Request Packet interface, the parallel port device driver preserves the caller's registers ES:BX and points to the request packet. The strategy entry point follows the Call/Far return model.