## Input/Output Device Driver Reference

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### Request Packet Interface

The file system is the primary component that interfaces with the parallel port device driver. In response to a function call, the file system creates a request packet containing the information required by the parallel port device driver to process the request. The file system then calls the parallel port device driver's strategy entry point, with the registers ES: BX containing the address of the request packet.

The Strategy interface uses the Call/Far return model and must preserve the caller's registers. The strategy entry point routes all IOCtl requests and all Open, Read, Write, Close, and Status requests to the appropriate internal parallel port device driver routines to handle the requests. For a description and format of a request packet, refer to the physical device driver strategy commands, which can be found in the OS/2 Physical Device Driver Reference.